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Laser-Writing and Characterization of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers A Thesis Presented to The Department of Electkal and Computer Engineering University of Toronto Jianhao Yang In partial fulfillment of the requirements for the degree of Master of Applied Science January, 1997 O Jianhao Yang, 1997

and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

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Page 1: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Laser-Writing and Characterization of Single-Mode Rib

Waveguides on Planar Germanosilicate Wafers

A Thesis Presented to

The Department of Electkal and Computer Engineering

University of Toronto

Jianhao Yang

In partial fulfillment of the requirements

for the degree of

Master of Applied Science

January, 1997

O Jianhao Yang, 1997

Page 2: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

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Page 3: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

DEDICATED TO MY MOTHER

Page 4: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Laser-Writing and Characterizatioo of Single-Mode Rib Waveguides

on Planar Germanosilicate Wafers

Jianhao Yang

M.A.Sc., 1997

Department of Electrical and Computer Engineering

University of Toronto

For the first tirne, a laser-writing technique has bcen applied to planar

Germanosilicate wafers to fabricate rib waveguides. This approach is an extension of

other laser-witinç processes in polymers and in III-V semiconductors. Single-mode rib

waveçuides with large cross-section size of 8prnx8pm have been fabricated using a

157nm F? excimer laser. The strong absorption of silica materials to 157nm radiation is

key to providing smoothly etched surfaces. Surface scattering !oss of the waveguides

was IdBkm at 635nrn wavclcngth, whilc thc singlc-modc coupling cficicncy from a

single-mode pigrailed fiber was 11%. Computer simulation bascd on the Beam

Propagation Method (BPM) has been applied to guide the waveguide design. The single-

mode conditions and beam profile in the rib waveguide obtained by the simulation are in

csccllcnt agreement with thc expcrimcntal observations.

The thesis presents a comprehensive discussion of optimizïng the waveguide

quality based on several criteria including single-mode guuiding, scattering loss, coupling

rfficiency and confinement factor. Cornparison of this work with that of other research

Page 5: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

groups shows that our rib waveguides are attractive in large cross-section size, single-

mode guiding, relativety large coupling efficiency and single-step fabrication.

The thesis demonstrates that the laser ablation technique can be developed into a

process competing with the Reactive [on Etching ( R E ) technique in fabricating silica rib

waveguides for applications to photonic circuits. Future improvernents of this technique

are also addressed.

Page 6: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

First of ail I deeply thank Professor Peter R. Herman for his numerous valuable

suggestions and financial support during the course of this work.

f would also like to thank: Professor Jirnmy Xu for allowing me using the Bearn

Propagation Method (BPM) software, a ginding machine, and a optical power meter in

his lab; Professor S. Zukotynski for allowing access to a stylus profilorneter and optical

microscope; Dr. Robin Tarn of OLLRC for Iending an XYZ-translater and allowing

access to several fiber-handling equipment; Mr. Fred Neud of the University of Toronto

for allowing me using a polishing machine.

1 was geatly enjoying the fiendship with Keith Beckley. From him, I not only

learned how to operate the Fz excimer laser in the early stage of this thesis, but also

learned the Canadian culture from many of our interesting conversations.

Financial support from the University of Toronto in from OP the University of

Toronto Open Fellowship is acknowledged. I also appreciate Ms. Sarah Cherien for her

coordination of the requirement of this program.

Lastly but not the least, [ thank my wife, Yuhui, for her encouragement and

support during this thesis work.

Page 7: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

TABLE OF CONTENTS

Chapter 1 Introduction ....................................................................................... 1

................................... 1 . I Introduction to Optical Waveguide Technology 1

1.2 OveMew of Waveguide Fabrication Techniques ................................. 3

.............................................................. 1.2.1 Reactive Ion Etching 3

1.2.2 Ion Exchanging Method ......................................................... 4

1 2.3 Direct Wntingj Process .......................................................... .5

............... 1.3 Direct Writing oFSilica Rib Waveguides by Laser Ablation 7

............ 1.4 Purpose and Structure of This Thesis .................................... 9

Chapter 2 Theoretical Studies and Computer Simulation

......................................................................... of Silica Rib Waveguide 17

......................................................................... 1.1 Single-Mode Analysis 12

2.1.1 Theoretical Approximation ............................................... 12

2.1.2 Cornputer Simulation ...... ,. ................ .. ............................. 16

2.2 Optirnization of Confinement Factor ............................ ,., ................... 2 1

Chapter 3 Experirneotal Setup ......................... .. ............................................ 25

3.1 Wavepide Fabrication ...................................................................... 2 5

3.1.1 Expenmental Setup for Laser Ablation ............................ 2 5

3.1.2 Optimization of Edge Resolution .... .. .................................. 28

...................................... 3.1 -3 Waveguide Fabrication Procedure 3 1

3.2 Waveguide Characterization ............................................................... 34

Page 8: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

.................................. 3.2.1 Coupling Light into Rib Waveguides 31

3.2.2 The Loss Measurement and B e m Profile Mcasurement .... 37

.................................................................... Chapter 4 Results and Discussions 40

4.1 Results ................................................................................................. 40

.................................................... 4.1.1 Wavcguidc Cross-Section 40

........................................................ 4.1.2 Surface Scattering Loss 43

41.3 Single-Mode Coupling ........................ ... ......................... 49

4.3 Discussions ................... ,.., ............................................................ 53

4.2.1 Bcam Profiles .................................................................... - 3 3

................. 4.2.2 Optimized Waveguide Width and Etched Depth 55

...................................... 4.2.3 Theory of Surface Scattering Loss -56

........... .......................... 4.3.4 Cornpanson cvith Other Work ... 59

Chapter 5 Conclusions ......................................................................................... 64

5.1 Surnmary ............................... .... ....................................................... 64

........................................................................................ 5.2 Future Work 66

References ................................................ ...,.. ... 68

Page 9: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Chapter 1

Introduction

1.1 Introduction to Optical Waveguide Technology

Data transmission rates of 40Gbps will soon be required in the telecommunication

industry to serve numerous applications including multimedia entertainment. Fiber optic

networks are the best choice for transporting such a high volume of data [Gree93].

Currently, Optical Carrier-192 is being deployed in the Synchronous Optical Network

(SONET) to provide lOGbps data transmission rate for long haul communication, while

alternative fiber systems are under developed to penetrate into the home from Local Area

Networks &AN) and Wide Area Networks (WAN). The rapidly increasing use of optical

technology also demands the development of new optical technology, such as

Wavelength Division Multiplexing, optical switching, and optical interconnection.

Optics not only provides high transmission speed in telecommunication, but also shows

advantages in parallel processing and opticai computing. Many of these areas are based

on the optical waveguide technology. In this sense, optical waveguide technology is

greatly affecting Our daily life.

Low-loss, high-qudity optical waveguides are the basic building block for this

technology, especially for the many photonic devices and photonic circuits under

development. Although optical fiber is a low loss waveguide, it is not suitable to be

Page 10: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

packaged in compact photonic chips. Channel waveguides or rib waveguides in planar

optical matenal are a more practicai choice in packaging photonic devices. However,

existing fiber fabrication technology can not be readily borrowed to fabncate channel

waveguides since a channel or rib waveguide is very different from a fiber. The demands

of photonic packaging and the lack of Iow-cost fabrication technology for optical channel

waveyides has been the basic driver behind the development of new waveguide

fabrication process during the last ten years [Lado96].

The difficulty in developing Iow-cost fabrication process for low loss channelfrib

waveguide is that the fabrication technique usually introduces surface roughness during

the process, causing the transmitted light to suffer higher loss cornpared with standard

single-mode fiber. Surface roughness is the biggest challenge in defining a new

fabrication process.

It is also not clear which material is the best choice for the optical channel or nb

waveguides that could be packaged into photonic or opto-electronic circuits. Materials

such as lithium niobate are impractical as a substrate for board Ievel electronics and

suffers frorn its high dielectric constant and high-cost mart89]. Waveguides based on

polymer materials usuaily provide a lower loss, but advanced devices that are based on

the photosensitivity effect or many opto-electronic effects cannot be fabricated on

polymer materials. Serniconductor materials produce a small guiding cross-section

(compared with standard telecommunication fiber) for channeVrib waveguides due to

their large refractive indices. This mismatch leads to large coupling loss with optical

fiben. Silica material is a natural choice for channeVrib waveguides because it is the

Page 11: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

basic material for many optical devices and compatible with fiber devices. However,

silica is difficult to process because of its large melting temperature and energy bandgap

(8eV). A simple process which can fabricate channeYnb waveguide in silica material and

provides low optical loss is highly desirable.

Some common waveguide fabrication techniques are described in next section.

1.2 Overview of Waveguide Fabrication Techniques

1.2.1 Reactive Ion Etching ( RIE )

Currently, the most successful industrial technique to fabricate silica channel

waveguides is Reactive Ion Etching (RIE). The etching process is a physical-chernical

process whereby silica or doped silica is removed from a surface through bombardment

by ions excited by a plasma. By using a predesigned mask, or pattern, which defines the

unetched area, a channel waveguide or any other complex layout of waveguides and

devices can be produced. Typically, silica reacts strongly with elements like fluorine

under ion bombardment. Ions of the reactive element are created in the plasma, and an

electncal field is used to direct the ions toward the surface to be etched [Lado96].

The RIE process involves several compiicated steps. A typical procedure for

fabricating silica waveguide is as follows: 1.) A silicon layer or an optically flat surface

(quartz, for example), typically a few rnillimeters thick, is used as a wafer. The silicon

wafer is sometimes pre-heated in order to oxidize a layer several microns thick and

facilitate the adhesion of deposited silica layer. 2.) A uniform buffer of pure silica is

deposited on the surface of the wafer to a thickness of about 10jm. 3.) A guiding layer of

Page 12: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

glas with slightly higher refractive index is then deposited on the buffer. Idealiy, the

index difference must match the core-cladding index difference of a standard fiber,

typically 0.3-0.8%. 4.) With a contact mask fabricated by photolithography, the parts of

guiding layer not covered by the mask are etched away by the RIE process descnbed in

the previous paragraph. 5.) The mask is then removed chemicdly to leave a rectangular

cross-sectioned, rib-like pattern on the top of the buffer layer. 6.) Usually a final layer of

silica is deposited as the upper cladding. This is preferred to match the index of the upper

cladding with that of the buffer layer so as to produce symmetric waveguides.

The contact mask is typically fonned by a lithographie processes, borrowed

directly from similar processes in electronics industry. This well-developed technology is

one of the principal advantages of RIE. The mask dimensions are defined by theoretical

analysis for the particular layout of waveguide, and, using computer aided design (CAD)

techniques, the actual mask is fabricated to sub-micron accuracy.

The main disadvantage of RIE is that it is a complex process which involves many

steps and incorporates toxic chernical elements. The process is also costly since the

fabrication plants are very expensive. Further, the method is less flexible than a wnting

process in producing complex waveguide layouts.

1.2.2 Ion-Exchange Technique

The ion-exchange method relies on replacing the ions in a glass substrate with

different ions by a diffusion process. The result is an increase in refractive index without

seriously disrupting the lattice structure. Typically a sodium-doped glas is used as the

Page 13: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

substrate material. A complement mask is pattemed onto the surface by

photolithography. This mask is the reverse of the mask used in RIE as the area not

covered by the mask defines the cores of the waveguide and devices. The sarnple is then

immersed in a liquid salt bath of potassium nitrate at high temperature (-300°C) for a

sufficient time. Potassium ions migrate into the glas rnatrix and replace sodium ions,

which migrate out of the glass into the liquid. This results in an increase of the refractive

index in the uncovered area which forms a core guiding layer. After the ion exchangc

process, the mask is dissolved, leaving a buned channel waveguide in the silica substrate.

The ion-exchange technique is simpler, overall, than RIE. However, the diffusion

process yields a semi-circular cross-sectional guiding profile with the highest index of

refraction near the surface. This non-circular index profile imposes a significant

limitation for many applications due to poor matching with the circula index of

refraction profiles of fi bers [Lad096].

1.2.3 Direct Writing Process

Channel waveguides cm be fabricated without the need for a contact mask by

locally changing the index of the material through its molecular response to focused

particle [Town941 or ultraviolet laser bearns. The direct writing techniques are very

attractive because of their sirnplicity and flexibility for writing complex computer-

controlled patterns.

Particle beam direct writing: In this technique, a proton beam is focused ont0 a

serniconductor such as GaAs and to generate lattice damage, resulting in a region with

Page 14: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

reduced carrier concentration [Huns9 11. The refractive index is slightly larger in the low

carrier concentration region, leading to a buried channel waveguide. The waveguides

produced by proton bombardment usually have large loss, typically 200dB/cm, but the

loss can be reduced to 3dWcm after annealing at temperature below 500°C [Huns9 11.

Currently, laser-based direct writing has drawn a great deal of research effort

[Hart89, Mukh94, Bozh92, Osgo92, Maxw95, Sva1951 to write channel or rib

waveguides. This technique c m be cataioged into several types depending on the laser

wavelength and the waveguide material:

Laser writing of rib wavemiide in polymer: Common polymer materials such as

polyrnethylmethacrylate (PMMA) are widely used and c m produce low-loss rib

waveguides [Hart89, Mukh941. Laser ablation of PMMA films using excimer and Ar ion

lasers have been used to pattern integrated waveguides with typical loss of IdBfcm

[Mukh94, Bozh92]. It was ais0 reported that after filtering the PMMA solution with a

O. lpm pore size, and controlling the laser exposure to below the ablation threshold (laser

ablation usually produces rough surface), the optical loss of PMMA channel waveguide

was reduced to O.OBdB/cm [Mukh94]. This is the lowest loss ever reported for channel

waveguides and represents the state of the art of the polymer waveguide fabrication

technology .

Laser assisted chemicai et ch in^: Laser-assisted chernical etching is an attractive

method for etching smooth profiles in serniconductor materiais such as GaAs Pavi881

and InP [M00fi4]. This technique has been extended to etch single-mode rib waveguides

in GaAs/AiGaAs wafers[Osgo92] with waveguide loss of IdBkm. Passive waveguide

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devices such as a Y-branch was also successfully fabncated using the same techniques

[Osg092].

Laser writine in silica material: For silica material, wavelengths available from

any continuous wave (CW) lasers are not suitable for writing waveguides because the

photon energy is too low compared with the 8eV energy bandgap of silica to significantly

alter the material. Only pulsed ultraviolet lasers are capable of writing waveguides in

silica material. If the laser fluence is below the ablation threshold of silica, for certain

types of doped silica and certain laser wavelengths, the material exposed to laser beam

will change its refractive index [Hi1178]. This photosensitivity effect is a nonlinear

photon material interaction process, which involves permanent modification of point

defects in certain types of silica materials. That induces a permanent change in the

refractive index. Currently, the mechanism of photosensitivity is still not fully

understood. However, the ability of pemanently increasing the refractive index of the

laser exposed area in Ge-doped silica has been used to fabricate novel silica-based

devices such as fiber gratings [Hi1193]. There is also a strong trend to apply this effect to

fabncate silica channel waveguides. For example, single mode channel waveguides have

recently been wntten directly by point-by-point techniques using a focused, 244nm

wavelength laser beam [Svd94].

1.3 Direct Writing of Silica Rib Waveguide by Laser Ablation

If the laser fluence is above the ablation threshold of silica, the materials exposed

to laser bearn will be removed. Laser ablation of optical materials is attractive for many

Page 16: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

applications such as fabricating optical interconnects or diffractive optics, and repainng

photonic circuits. The advantages of laser ablation include simplicity, flexibility, low-

cost and easy control of etching parameters.

This laboratory has studied laser ablation on diverse materials such as polymer,

quartz, and semiconductor at both 193-nm and 157-nm radiation [Herm92]. Laser

ablation of silica material at 157-nm radiation is of particular interest for the following

reason: the bandgap for hised silica is about 8eV, and the bandgap for the crystal siiica is

about 9eV. This large bandgap makes it difficult to ablate silica by typical commercial

excimer laser such as ArF laser (193-nm) or KrF laser (244-nm). On the other hand, FI

excimer laser radiation, with 7.9-eV photon energy close to the silica bandgap, is strongly

coupled into ultraviolet-grade hsed silica through defects generated by the 7.9-eV

photons Werm921. This Iowers the ablation threshold fluence, produces srnoothly-etched

surfaces, avoiding microcracks and laser debris, and provides easy control of etched depth

by the number of laser shots. Recent work [Hem961 shows the ablation threshold of the

interested Ge-doped silica is about 0.36~lcm'.

These attractive properties make 157-nm laser ablation an effective tool in micro-

machining silica materials. It is of panicular interest to apply the technique to define a

single-step fabrication process for silica nb waveguides. In this approach, the UV laser

beam firstly passes through a rectangular aperture mask with a wire in the rniddle, and is

projected by a lens, forming an image in the sarnple surface. The imaged laser beam. with

fluence above the ablation threshold, removes the exposed material. Two trenches are

Page 17: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

produced in the sarnple, leaving the middle unetched regions as a rib waveguide.

Cornputer-controlled target positioning then permits fabrication of a guiding path.

Such a laser-ablation scheme offers several advantages: 1 .) The process is single

step; 2.) No mask is required; 3.) The layout of the waveguide or devices can be

cornputer-controlled by translating to pnnt a programmable photonic circuit.

There is no known report on using the laser ablation technique to write silica

channel or rib waveguides. This is partly because the availability of the 157nm F2

excimer laser, which is the most effective tool to etch silica, is very limited. Although

ultrafast laser pulse is also capable of etching silica [Liu94], there is also no known report

on using the ultrafast laser to write silica nb waveguides. It is the goal of this thesis to

pnnt rib waveguides in silica material using the F2 laser. If such a technique can produce

good quaiity silica waveguides, it could be a competitive fabrication method to the

standard RIE technique.

Although the photosensitivity writing technique is also capable of writing buried

channel waveguides in germanosilicate [Svd94], laser ablation offers additional

advantages such as ease of control of etch depth. More importantly, these two techniques

are indeed complementary. There is a potentiai to combine both laser ablation technique

and photosensitivity technique to fabricate more complicated silica based devices in a

single silica chip.

Page 18: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

1.4 Purpose and Structure of This Thesis

As mentioned earlier, silica material is a natural choice for fabricating channel or

rib waveguides because it is the basic matenal for many optical devices, and because of

its compatibility with fiber and fiber devices. The current standard silica waveguide

fabrication technique (RIE) suffers from disadvantages of numerous processing steps,

high-cost, and use of toxic chernical materiais. Cumntly, there is a great deal of research

work [Hart89, Mukh94, Bozh92, Osgo92, Maxw95, Svd951 in defining simpler

fabrication techniques based on direct laser writing. However, there is no report on

fabricating rib waveguides using laser ablation. The purpose of this thesis project is to

develop a low-cost, single-step fabrication technique for silica rib waveguides based on

laser ablation. Our goals are also to optimize the overall quality of the silica rib

waveguides based on critena such as scattering loss, coupling efficiency from standard a

fiber, and single-mode confinement.

The structure of this thesis is as follows: Chapter 2 provides a theoretical

analysis to define waveguide parameters in meeting requirements of single mode

confinement. Since there is no analytical results for waveguide with trapezoid cross-

section shape, computer simulation is the only tool used to study such waveguide. In

chapter 2, intensive computer simulation will be performed, and the results are then

compared with the theoretical cdculation results.

Chapter 3 describes the experimental setup for the waveguide fabrication. We

start by describing the basic ablation experimental setup. The edge resolution due to

diffraction and lens aberration effect is also optirnized. For the waveguide

Page 19: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

characterization, the most difficult part is the coupling of appreciable light into the

waveguides. A tapered fiber is used to solve this problem. We aiso describe a simple

method to measure surface scattering loss.

Chapter 4 presents the experimental results and compares them with the

theoretical anaiysis in chapter 2. The optimization of waveguide parameters such as

waveguide width and etch depth is important in designing good quality waveguides. We

will discuss the optical physics behind the citena in optimizing these parameten. Also, a

discussion on the mechanism of the surface scattering loss is also included. We will also

compare the beam profiles obtained from both cornputer simulations and experimental

measurement. The chapter is concluded by a cornparison of this thesis work with related

research efforts.

Chapter 5 summarizes the results and achievements of this thesis and provides

suggestions to extend this thesis project.

Page 20: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Chapter 2

Theoretical Studies And Cornputer Simulations of Silica Rib Waveguide

Detail theoretical studies of silica nb waveguide are carricd out in order to define

the single mode conditions. However, due to the nature of laser matenal processing, the

cross-sectional shape of the nb waveguide fabkated by laser ablation technique is not

rectangular. Instead, the cross-section is more similar to a trapezoid shape. This imposes

a difficulty in performing the theoretical analysis. The approach in this chapter is to first

mode1 the laser-ablated waveguide as a rectangular waveguide by analytical means. Then

we perform a computer simulation on the trapezoid waveguide, and compare the resutts

with the rectangular approximation. This approach is used in both defining single-mode

conditions (section 2.1) and optimizing the confinement factor (section 2.2).

2.1 Single Mode Analysis

2.1.1 Theoretical Approximation

Although the theory of a dieiectric waveguide is more than a century old,

application of this theory to a rectangular rib channel was not studied in detailed until the

1980s. It is often assumed that the cross-section dimensions of a three-dimension rib

waveguide must be similar to the dimensions of a single-mode slab waveguide (made

Page 21: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

from the sarne material) in order to support single-mode propagation. This assumption is

incorrect as pointed out by Peterman [Petegl]. The single-mode condition of a rib

waveguide is more restrictive than that of a slab waveguide. For exarnple, for a Ge-doped

silica core layer on a silica subtract with 0.3% index difference, the slab waveguide

thickness must be less than 2 p in order to be single mode at 6 3 5 ~ . This dimension is

rnuch less than the core diameter of a single-mode fiber. Hence the coupling loss from

the fiber to the waveguide is very high. On the other hand, as will be shown in this

section, it is possible to design a large 8pmx8pm cross-section size for a single-rnode rib

waveguide in the same material.

Figure 2.1 shows the cross-section of the rib waveguide used in the theoretical

approximation. The ri5 width is defined as w, the guiding layer thickness is H, and the

etched depth is (H-h).

air I T germanosilicate

silica substrate

silicon wafer

Figure 2.1 ~ectangular rib waveguide for analytical model.

Page 22: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

The refractive indices of the air, the guiding layer, and the substrate layer are no, nl and

nz, respectively, for a specified wavelength. The three-dimensional rib-guide modes are

denoted as TE,, or TM,,, where n = O, 1, 2, . . . , and m = O, 1, 2, . . . . The single-

mode condition is derived by the effective index method, as described below.

First, the wave propagation in a slab waveguide is considered as if the lateral

direction is infinite and the vertical direction is asymmetric. The dispersion relation for

the propagation constant Psiab of this type of waveguide is well-known and can be written

in terms of the effective index of refraction of the guide Nh = P s i a h as [Osgo891

Here, TEo modes are assumed, k = 2irlh is the free space propagation constant at

wavelength h, and h is the thickness of the slab waveguide. Solving Equation (2.1) gives

the effective index of refraction, Nb, as a function of thickness h.

The next step in the effective index method is to view the waveguide structure in

Figure 2.1 from the top and to consider the regions of different thickness in Figure 2.1 as

regions of different effective index of refraction as shown in Figure 2.2.

Figure 2.2 Top view of a channel waveguide with effective indices Nh and NH.

Page 23: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

The stmcture in Figure 2.2 is simply a 1-D syrnmetric waveguide and the anaiytical

results are well-known. The propagation constant Pchmnel and effective index of refraction

ner = Pehmcl/k can be found by:

The single mode condition is defined as [Osgo92]:

Combination of Equations (2.1) and (2.3) can in principle define the single mode

condition by numencal means. Peterman pete9 11 also gives an approximate analytical

expression derived from the effective index method. The results show that the single-

mode condition for the % mode is

where = l / J ~ + l l , / ~ .

The waveguides studied in the thesis are based on the planar waveguides

fabricated by Photonic Integrated Research Inc. (PIEU, SMPWL). The guiding layer is

Ge-doped silica with thickness H = 8 p and refractive index nl = 1.46 12 at h = 0.635 ym.

The substrate layer is silica with thickness of 2 0 p , and refractive index of n2 = 1.4568.

Assuming no = 1.0 for air, the single-mode conditions of Equation (2.3) define the

maximum waveguide width plotted as a function of thickness, H, in Figure 2.3. Regions

above the curves support more than one propagation mode.

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Figure 2.3 shows the mutual dependence of waveguide (w) width and etched

depth (H-h) in maintaining single-mode conditions. A wide rib waveguide can be single-

mode if the etch depth is suficiently shallow. On the other hand, a deeply etched rib can

also be single-mode if the width is sufficiently narrow. For an 8 p thick waveguide. rib

width of less than IOpm are recommended for a 4p.m of etched depth.

Figure 2.3 Single-mode boundary for silica channel waveguides: each . . curve corresponds to different etched depth and defines the maximum wavebide width, W, as a function of thickness, H. Larger waveguide widths support multimode propagation.

2.1.2 Cornputer Simulation

A more realistic profile for the cross-section of the rib waveguide fabncated by

laser ablation is a trapezoid shape. A trapezoid shape that closely reflects the shape of

Page 25: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

laser-etched ribs produced in this study is shown in Figure 2.4. Here width w is defined

as the FWHM of the etched depth, and W, is the width of the sloped surface on the sides

of the trapezoid shape.

H= 8 p f h=5 pm Ge-doped silica

/ 31 20- silica substrate

Figure 2.4 A trapezoid mode1 of a laser etched silica rib waveguide.

To study this waveguide, a computer simulation based on Beam Propagation

Method (BPM) is employed (BPMCAD, National institute of Optics, Canada. 1991).

This version of BPM can not directly simulate a trapezoid waveguide. Therefore, we

divided a 4pm high trapezoid layer into 100 thin layers of 0.04pm thickness as shown in

Figure 2.5. Each layer is a rectangular shape with linearly increasing length of 0.04prn

for each adjacent layer. Thus, the result (Figure 2.5 in next page) closely matches the

trapezoid shape in Figure 2.4.

To snidy how various waveguide cross-sections affect the wave propagation in the

rib waveguide, two BMP simulations on both waveguides with rectangular cross-sections

and waveguides with trapezoid cross-sections were carried out, each having identical

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germanosilicate

Figure 2.5 The trapezoid waveguide is modeled as many rectangular thin layers for the BPM simulation.

values of w = 6 p (FWHM), h=5pnun, and H = 8 p . For die trapezoid waveguide,

wp2km. The propagation step for this simulation is 0.25j~m, the wavelength is 0.635p,

and the propagation distance is 4mrn for both types of waveguides. The input electncai

field is a Gaussian function with 3 p n width (FWHM) and (0.3p.111, 0 . 3 ~ ) off the

waveguide center in order to excite higher modes. Figure 2.6a and 2.6b show the

evolution of the electrical field durhg the wave propagation for both waveguide types.

Figure 2.7 shows the cornparison of the intensity profiles after a 4mm propagation iength.

Page 27: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Figure 2.6 Comparison of wave propagation in rectangular and trapezoid waveguides. a. Electrical field distribution for rectangular waveguide; b. Electrical field distribution for trapezoid waveguide.

-15 -10 -5 O 5 10 15

Distance (pm)

A Y 1 .I

VI E al

(c) = 0 . 8 - u

2 00

Figure 2.7 Comparison of intensity profiles after 4mm of propagation in rectangular and trapezoidal rib waveguides.

2 0.6 - T3

a Q) - N -

.LI - 0.4 - E O O - - a

0.2 1 - O a O

O

- 1 - - O

- O O

0 Rectangular Waveguide Trapezoid Waveguide

- a 1

- - 0 - - O O

*

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From Figure 2.7, we find that the impact of different waveguide cross-section

shape on the lowest-mode wave propagation is very smail. The output intensity profiles

from both types of waveguides are very similar. The 6 . 4 ~ width (FWHM) of the

intensity profiles after propagating 4mm distance are identical for both waveguides. This

is expected since the fields of the lowest-mode is mostly distributed in the center of

waveguides, and the distortion from the waveguide boundary is small. Hence, the wave

propagation of the lowest mode only weakly depends on the cross-sectionai shape of the

waveguides.

In order to test the single-mode boundary by BPM, a second-mode field

distribution generated from a rectangular waveguide was used as an input field to the

trapezoid waveguide to excite a higher mode. If the trapezoid waveguide does not

support the second-mode, the propagating field distribution must then gradually

diminishes with propagation distance. For a given etch depth of the trapezoid waveguide,

the maximum width at which the second-mode is supported defines the single-mode

boundary. For example, for a fixed etch depth of 3pm and a thickness (H) of 6pm,

variation of the waveguide width from 4pm to I O p n showed propagation of a second-

mode for widths exceeding -8f lpm. Therefore, for single-mode propagation, the

waveguide width should be less than -8klp.m. The uncertainty arises because of the

finite propagation distance (-4mm). This BPM simulation was repeated for other etched

depths. Results are shown in Figure 2.7, and compared with the analyticai results in

section 2.1.1. From Figure 2.7, the single-mode condition for a trapezoid waveguide is

very similar to that of a rectangular waveguide.

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O 2 4 6 8 1 O 12

Thickness of Guiding Layer, H (p)

Figure 2.8 Maximum waveguide width for single-mode propagation piotted as a function of waveguide thickness, H, for rectangular waveguides (open circles) and trapezoid waveguides (solid circles). Both rib waveguides are assumed to be etched to one-half of the waveguide thickness. The error bars for the trapezoid rib waveguides anse from the finite propagation distance used in the BPM simulation.

2.2 Optimization of Confinement Factor

The light intensity scattered from the surface of the waveguide due to surface

roughness is proportional to the light intensity at the surface. Therefore, the field

distribution in the optical waveguide affects the surface scattering loss. Intuitively, if

most of the light is confined inside the waveguide with less light near the surface, then

the scattering loss is reduced. Therefore, we need to optirnize the waveguide parameters

such that they not only satisS> the single-mode conditions, but also provide a large

confinement factor.

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Analytical calculation of the confinement factor of a trapezoid waveguide is

difficult. From Figure 2.7, the output fields of the lowest mode after propagation of I mm

distance are very similar for both rectangular and trapezoid waveguides. Therefore, it is

reasonable that the analytical results of the confinement factor derived from a rectangular

waveguide is a good approximation for the confinement factor of a trapezoid waveguide.

The confinement factor r is defined as

where I is the Iight intensity. Iin refers to the intensity within the waveguide and I.., refers

to the iight intensity outside the waveguide. For a rectangular waveguide, the

confinement factor I' is given by [Che0901 :

based on the effective index method described in the previous section, where

y= k( ne$ - N ~ ' ) ' ~

= k wH2 -

Nh and NH are deterrnined from Equation (2.1), and nea is calculated from Equation (2.3).

Numerical solutions of Nh and nerf were obtained by Matlab. Values of Nh for the PIRI

waveguide is listed in Table 2.1 for various ratios of MH, and for H = 8pm, no = 1 .O. ni =

1.46 12, nz = 1.4568. and h = 0.63pm.

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Table 2.1 Effective index of refraction for PIRI waveguide with different etch depths

Figures 2.9 shows the confinement factor plotted as a function of waveguide

width and etched depth. The confinement factor increases rapidly with increasing values

of both parameters. For a fixed etch depth of 4 p , the confinement factor reaches more

than 90% when the waveguide width is 8pm, and increases much more slowly with

further increasing of width. Similarly, for a given waveguide width of IOpm, the

confinement factor is more than 90% when the etched depth is 4pm, and increases much

more slowiy with further increasing of depth. These two plots show that an 8pm of

waveguide width and a 4pm of etched depth are suficient to achieve a 90% confinement

factor. Further increment of both parameters provides little increment of the confinement

factor but makes it more difficult to maintain single-mode conditions.

In conclusion, this chapter provided theoreticai background on choosing

waveguide width and etched depth to achieve single-mode propagation and large

confinement factor. However, other factors such as loss and coupling efficiency are also

important in optimizing the waveguide width and etched depth. Chapter 4 will present a

more complete discussion on choosing these two parameters based on the optimization of

loss. coupling efficiency, confinement factor and single-mode propagation.

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O 1 2 3 4 5 6 7

Etched Depth (pm)

Waveguide Width (p)

Figure 2.8 Confinement factor ploned as a function of waveguide width for an etch depth of 4pm (top), and plotted as a function of etch depth for a waveguide width of 8pm (bottom). Values were determined by the effective index method.

Page 33: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Chapter 3

Experimental Setup and Procedure

In this chapter, the experimental arrangement and procedure for fabricating silica

rib waveguides will be descnbed in detail. We will also describe the experimental setup

for characterizing the nb waveguide, including the coupling of light from a pigtailed fiber

to the rib waveguide, measurement of the scattering loss from the surface of the

waveguide and measurement of the beam profiles of the waveguide output.

3.1 Waveguide Fabrication

3.1.1 Experimental Setup for Laser Micromachining

A carefui study of laser ablation of germanosilicate provides essential information

of etched rates and ablation threshold for controlling etch depth and smcothness of the

rib waveguide. We will briefly describe the experimental setup for the laser ablation

studies and summarize the relevant results in this section.

Figure 3.1 shows a schematic of the experimental setup for the laser ablation and

waveguide fabrication (For laser ablation, there was no wire in the rectangular aperture).

The laser source is a home-built high pressure F2 excimer laser with the following

parameters: the laser wavelength is 157nrn with the spectral linewidth of 0.005nm

[Herm93]; the duration of laser pulse is about lSns (FWHM); and a typical pulse energy

is 40d. The excimer laser was operated at a 1Hz repetition rate in this expenment.

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vacuum chamber

sliding aperture

Rotatable Mirror waveguide 1 (MgF,) sample I 1574 laser beam

wire with 75pm diameter

Figure 3.1 Experimental setup for laser ab1 ation and v

stage

laveguide fabrication.

The laser beam has a typical area of 8mmx 12mm and a 2x 1 O" radian of divergent

angle. The laser beam passes through a rectangular aperture (2mmx3mm) which is

imaged ont0 the germanosilicate target sample by a biconvex lens made of W grade

MgFz (focus length, fi57nm = 86rnm). The laser fluence was controlled by moving the

aperture dong a vacuum sealed glas tube, altering the demagnification factor in this

experiment in a ranges of 6 to 9 times.

The optical system was contained in a vacuum chamber descnbed in detail by

Chen [Chengl]. The vacuum chamber was pumped to 1 0 ' ~ Torr with a turbomolecular

pump (Leybold W / N T 50) before operation of the laser. During experiment, the

Page 35: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

chamber was purged with argon gas at lOSCFH flow rate to rninimize contamination of

the system by air and eliminate optical darnage due to VW photochemical reactions with

hydrocarbon contarninants.

The laser energy was measured with a scintillator plate and diode detector

(Startech VHR-0020-1295, SN-0200) by detecting the refiected W light from a

rotatable mirror as shown in Figure 3.1. The sensitivity of this detector is several micro-

Joule and was cross-calibrated with a Molectron ID500 pyrometer with J25 detector

head. The calibration procedure was straightforward, but required correction for a weak

(-10%) red component in the laser beam and elimination of laser-discharge noise. For a

detailed description, please refer to a technical report in this laboratory [YanggS].

The etching target was placed on a XY-translator (Oriel 16928) offering

transverse motion over a range of O S " with O. l p n resolution.

F? laser ablation of Ge-doped silica was carefully studied in this laboratory

Figure 3.2 Laser ablation etched rate of Ge-doped silica (PIRI SMPWL) at 157nm wavelength plotted as a hnction of laser fluence.

Page 36: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

[Herm96], yielding the etched rates shown in Figure 3.2 as a function of the logarithm of

laser fiuence. A linear representation of the data provides an ablation threshold for the

PIRI waveguide of -0.36 k m t . The etched rates provided by this graph is important in

controlling the etched depth of rib waveguides. For example, to produce a 3pm deep of

trench at 31/cmt, -50 laser shots are needed. Note also that much higher laser fluences

are required to etch germanosilicate with 193nm or longer wavelength lasers, and

surfaces are much rough than that produced by the 157nm laser [Herrn96].

3.1.2 Optimization of Edge Resolution

The experimental setup for fabricating the n b waveguides was similar to the one

shown in Figure 3.1. The only modification was to the aperture. A 75pm diameter

copper wire was added to the rniddle of a 2.5mmx5mrn aperture. The image of this wire

prevents etching of the slab waveguide, leaving a rib sandwiched by two wide trenches.

We usually oriented the wire vertically, and moved the waveguide sample vertically to

produce a Icm long rib waveguide.

Due to diffraction and lens aberration, the side-wall of the rib waveguide was not

sharply defined, resulting in a trapezoid cross-section shape. We define the width of the

sloped edge (W. in Figure 2.4) of the trapezoid shape as the edge resolurion. The

challenge is to minimize diffraction and aberration of lens to increase the edge resolution.

Figure 3.3 shows the schematic arrangement of the optical system. The focal

length of the lens is f , the wire diarneter is Do, and the 157m beam divergent angle is 8.

Other lengths are defined as shown in Figure 3.3. The beam divergent angle after the

Page 37: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

aperture le& waveguide aperture

Figure 3.3 Optical system (left figure) used in the waveguide fabrication expenment. The aperture (right figure) was made of four knife-edges. A copper wire with diameter of 751m was glued to the middle of the rectangular aperture. The aperture was imaged by the lens ont0 the waveguide sample to etch a rib-Iike optical guide.

aperture is hn +8 where h/L includes aperture diffraction. The spherical aberration at the

circle of least confusion in the image plane is then given by Nala941:

Here, a depends on the shape of lens and is 0.404 for the biconvex lens used in this

experimen t.

For diffraction, we first estimate the beam size projected from the mask to the lens

surface as L+(W+8)So. This gives a minimum diffraction feature size of:

To illustrate the dependence of both W, and Wd on the demagnification factor, M, we

substitute S, = MS, and Si = (I+i&Qf in Equations (3.1) and (3.2) and add both Wu and

Wd together to represent the worst case. This results in a theoretical expression for the

edge resolution defining the slope-side-wall width of the trapezoid waveguide:

Page 38: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

The edge resolution depends on the demagnification factor M and the aperture size L, two

parameters to be optirnized. Other parameters in Equation (3.3) are not adjustable. From

equation (3.3), it is easy to see that increasing L will reduce W,. The dependence on M is

more cornplex. Figure 3.4 shows a plot of the edge resolution, W,, as a function of

aperture size for typical demagnification factor in this experiment, with f =86mm, and O=

2x lo5. It can be seen that i t is possible to achieve a 41~m of edge width for M=8 and a

full aperture size of 2L=Smm.

O i 1 I 1 1 1 1 1 1 1 I

O 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Aperture Size (mm)

Figure 3.4 Edge resolution as a function of the full aperture size (2L) for typicai demagnification factors in the waveguide fabrication experiment.

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3.1.2 Waveguide Fabrication Procedure

There are at least three technical problems needed to be carefully solved in the

waveguide fabrication procedure. Before the waveguide fabrication, the rib width and

etch depth must be specified. Many criteria must be considered in choosing these two

numbers as will be discussed in Chapter 4. The final selection then dictates the optimal

demagnification factor and the lens positions. The second step is to make sure the

shadow of the wire in the aperture is exactly aligned in the center of the imaging lem.

Otherwise, lens aberration will make the edge resolution very large and the waveguide nb

will not be symmetnc. A PMMA sample was placed behind the aperture mask and

exposed to the F2 laser radiation, which provided etched patterns that helped position the

mask in a uniform portion of the 157nm beam. The lens was centered in the bearn using

the same technique. Several iterations were needed to adjust the relative positions of wire

and the lens. This was accomplished by observing the sürface features of the rib

waveguides in PMMA samples by a microscope and profilorneter, and selecting the most

symmeuic cross-section profile of the rib.

The third issue is to make sure the image of the wire was precisely on the

waveguide surface (referred to Figure 3.1). In Our experimental setup, the lens

longinidinai position was controlled by a micrometer with O.ûû1" resolution. To find the

best focus position, it was necessary to translate the lens by <0.005" step-size, while

exposing PMMA or waveguide sarnples to 157nm radiation. After several iterations, the

optimal lens position was found when the side-wail widths of the rib waveguides were

rninirnized.

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The waveguide sarnple was attached in the XY-translator by double-sided tape

and moved vertically during laser operation. This translation produced two Icm long

trenches on the surface of slab waveguide resulting a rib waveguide in the middle region.

In order to minimize the surface roughness, the sample was translated each laser shot,

leaving a small ripple at each edge position of the imaged mask.

The step size of each translation on the waveguide of sample was detemined in

following way. Given N laser shots to etch to a required depth, and the wire length, L,

reduced to L, = VM on the sample, the step size is LJV which we denote as Ay. N

depends on the laser fluence and the required etch depth, and determined by the method

described on page 27. The required etch depth must support the single-mode conditions

as described in Chapter 2. Selection of the laser fluence is a trade-off. A large fluence

etches deeper holes each shot, reducing N, but increasing the edge-effect surface

roughness. A lower fluence near the threshold also leads to increasing surface roughness

due to the shot-to-shot variation in laser fluence (-10%) which greatly affects the shot-to-

shot etch depth near the threshold (see Figure 3.1). Selection of the laser fluence then

dictates the lens position (or dernagnification factor) and the nurnber of laser shots, N.

It is necessary to compensate for the slightly different orientation of the vertical

wire and the direction of motion of the translation motor which otherwise leads to a

notched waveguide pattern. The angular offset, typically $ 410°, is compensated by

stepping the horizontal motor position, Ax, ezch time a vertical step is made. Smooth rib

waveguides result when tane = M A Y .

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Before rnicromachining the slab waveguides, the facets of the waveguide sarnpies

were ground by S i c paper, in sequence of 14pm and 5 p grain size, then polished by

A1D3 powder of lpm. To hold the waveguides steady during grinding and polishing, r

holder was used. Typical polishing time is -10 minutes. Longer polishing times lead to

rounded edges on the facets.

The waveguide fabrication then proceeds as follows:

a.) Clean the waveguide facets and surfaces with optical methanol;

b.) Mount the waveguide ont0 the XY-translater such that the laser beam just

misses the beginning facet of the waveguide;

c.) Tum on the excimer laser (1Hz);

d.) Measure the laser energy with the Startech energy meter to confirm that the

appropriate fluence reaches the sample target;

e.) Slowly step the sample until the laser beam is very close (-200km) to rhe

beginning facet of the waveguide. Turn off the excimer laser;

t) Move the sample by Ay verticdiy (typically 10-15pm) and Ax (typically 1 .O-

1 Spm) horizontally;

g.) Fire the laser for only one shot;

h.) Go back to step f, and repeat until the laser beam leaves the opposite facet of

the waveguide;

i.) Measure the laser energy in the middle and at the end of the whole procedure.

The number of repeated translations is the waveguide length divided by Ay, and is

typically 103 for fabricating a k m long waveguide. The motor translation was done each

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step since high precision was required. It typically took three houn to fabricate a lcm

long rib waveguide therefore it is desirable to let a cornputer to do this job in the future.

The laser energy was assumed to be the average of the three measurements taken

during the fabrication process. There was less than a 10% drop in laser energy during the

fabrication, producing uniform rib profiles dong the length of the waveguides.

3.2 Waveguide Characterization

3.2.1 Coupling Light into Wb Waveguide

The rib waveguide studied in this thesis has a cross-section size of about

8pmx8pm, and a numericai aperture of about 0.12 in the vertical direction. It is a

challenge to focus a laser bearn into this small cross-section size while also rnatching the

small numerical aperture.

Figure 3.5 shows the schematic of the expenmental setup for waveguide

characterization. The light coupling is the main concem of this section. The key

component for the coupling is a pigtailed fiber, as shown in the enlarged graph and also in

Figure 3.6. This fiber is the 3M single-mode fiber for 635nm, with a 4p.m core diameter

and 1 2 5 ~ ccldding diarneter (3M FS-SC-3224). The reason for working at 635nm is

simply the convenience of using visible light. In principle, it is straightforward to extend

this work into the 1550nm regime. The pigtailed fiber is about 1.5 meter long, with a

7rnm stripped length at the end. The stripped part of fiber is rnetailized in order to protect

the bared fiber. At the end of the fiber, the facet is rounded with a 10pm radius of

Page 43: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

curviiiure. This results in a semi-sphericül lens ended fiber (SLEF). Figure 3.6 is il

picturc of this pigtailed fiber

v The pigtaiied fibcr

diode Iaser \ waveguide sample

beam splitter i

\ &3b f dete~tor--~+ &

n eyes

Figure 3.5 Schematic of waveguide characterization experirnental setup. Expanded view of pigtailed fiber is shown at top. See text for further details.

Figure 3.6 The pigtailed fiber under a microscope with 20x objective and 10x eye- piece. The diameter of the fiber is 125pm, and the curvature of the end-facet is IOpm.

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The laser source was a AlGaInP laser diode with a multi-quantum well structure

(PDLD L63-3 12-0.5-PH6-lFa). The laser diode was packaged with another piece of 3M

fiber ( FS-SC-3224) so that the laser output could be coupled into the pigtailed fiber.

Some of the key parameters of this fiber coupled laser are summarized here:

wavelength: h = 635nm, single longitudinal mode

optical laser power output: 5mW CW

coupling efficiency from laser diode to fiber: 16%

operation current: 65mA, (85mA maximum)

The output optical power from the fiber was -0.8mW. This fiber was connected to the

pigtailed fiber by a fiber adapter (FCIAPC in Figure 3.5). This introduces about 3dB Ioss,

yielding -0.4mW power after the pigtailed fiber.

The pigtailed fiber was placed in a fiber holder (Thorlab MDT711-125) which

was mounted on a precision XYZ translater (Thorlab MDT602) with -0.5pm position

sensitivity. This high sensitivity is necessary for efficient single-mode coupling between

the pigtailed fiber and the rib waveguide of the small core diameter of -8ym.

The typicai distance from the pigtailed fiber ta the rib waveguide was 10-15pm

when coupling was optimized. In order not to damage the pigtailed fiber, the fiber-to-

waveguide distance was observed by an optical microscope, usuaily before the laser was

turned on.

Three criteria were used to determine when optirnized coupling was achieved by

transIating the pigtailed fiber. a.) At optimized coupling, the scattered light from the

waveguide surface was maximized as observed by a microscope; b.) The far field

Page 45: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

intensity pattern of the rib waveguide output could also indicate the optimum coupling

efficiency. c.) The light coupling was also optimized when the bnghtness of scattered

light from the output facet of the nb waveguide was maximized as observed by a

microscope.

In order to record the near-field intensity profile at the n b waveguide output facet,

we used a lûûx micro-objective just after the waveguide end lacet, and a CCD camera

(Connectix QuickCam) as shown in Figure 3.5. The camera image was sent to a

computer via the serial port and captured with a commercial software (Connetix

QuickPict). Precise positioning of the CCD carnera to image the waveguide end facet

was obtained with the laser source tumed off and the end facet brightly illuminated;

precise XYZ positioning of the micro-objective lens then produced a sharp image of the

waveguide facet. Keeping the same carnera position, we turned on the laser and captured

the optical output intensity profile at the waveguide facet.

3.2.2 Loss Measurement and Beam Profile Measurement

The detection system used to measure the surface scattering loss of the waveguide

is shown in Figure 3.5. A microscope was used to collect light scattering from a smal!

waveguide segment. The objective was typically 50x. and the eye piece was 10x. The

observed waveguide segment was 0.5mrn long when using a SOx micro-objective. This

segment length, denoted as Ad, was the unit length in measuring the scattering loss. The

power of scattering light from this waveguide segment is P, = P u , where P is the

optical power confined in the segment of the nb waveguide. Assuming the waveguide

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loss obeys the exponential law with absorption coefficient cc, and denoting x as the

distance from the front facet to the waveguide segment, one obtains

P,. " p*r\e- (U) ,

where Po is the optical power from the pigtailed fiber, and 7 is the coupling coefficient

from the pigtailed fiber into the single mode light of the nb waveguide. Assuming

isotropic scattering, the actual light power collected by the detector can be wntten as

Pd = P,,@2 / 27r = P,qc e'm(crM)R / 21r (3.5)

where R is the solid angle in which the micro-objective collects light and determined by

the numerical aperture, and 5 represents the percentage of this collected light that reaches

the detector through the prisrn and optical lenses.

The detector (8 18-SL) and the optical power meter (1830-C) are frorn Newport

Corporation. The sensitivity of this power meter is about IOpW. The active area of the

detector is 1.5cmxl.6cm which is relatively large compared with the bearn size

(0.5cmx0.5cm) after the eye piece. Since the collected scattering light is rather weak

(-nW), the detector and the beam paths were sealed from any background light.

Between the detector and the eyepiece, a pnsm (Edmund Scientific, M32,504)

served as a bearn splitter. About 50% of the coilected scattering light passed through the

pnsm and was detected, yielding @ 5 O I . This prism permitted simultaneous detection of

the scattering light power while observing the waveguide segment by eyes. This way, the

microscope-to-waveguide distance can be readily corrected to within the depth of focus

(2.8)c = 1 . 8 ~ for the 50x micro-objective). Simple ray tracing of the microscope system

Page 47: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

showed that deviation of the working distance in such a range (< 3h) did not affect the

optical power of detected light due to blur. This was also confirrned expenmentally.

A field stop was used to filter out scattering light from surface other than

originating from the rib waveguide surface as shown in Figure 3.5. The field stop filter

was placed at the focal plane of eye-piece lens inside the microscope tube. Figure 3.7

shows the shape of the field stop filter.

rib,waveguide (-8 pm wide)

icroscope field of view (0.5mrn 0)

E open field stop - 12pm wide

Figure 3.7 Diagram of the microscope field stop ( 5 0 ~ objective), together with image of a 8pm width rib waveguide. Sizes have been transformed to dimensions on the waveguide surface (50x demagnification). Only scattered light originating within the 12p.m wide field of view was passed by the filter.

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Chapter 4

Results and Discussion

4.1 Results

4.1.1 The Waveguide Cross-Section

In this thesis, four waveguide sarnples are studied in detail. For sample A, the

waveguide width and etched depth are 8pm4pm which is the optimized shape as will be

discussed in detail later. For sample B, C, and D, the width x etched-depths are

12prnx4pm. IOpmx7p1, and 8 p m ~ 6 p , respectively. Laser parameters such as laser

fluence, demagnification factor, and step size for al1 the four rib waveguides, are

surnmarized in Table 4.1 of page 49. The aperture mask used to fabricate these rib

waveguides was described in page 29.

Figure 4.1 shows the endview and topview of a rib waveguide fabricated on a

PIRI slab waveguide (sample A) and observed with a lOOx microscope. The waveguide

width (FWHM) and the etch depth were measured using a calibrated ruler to be 8pm and

4pm, respectively.). The waveguide (sarnple A) was fabricated at 2.8 k m 2 of laser

fluence which, according to Figure 3.2, gives a O.O6Clm/pulse etch rate. The etched

surfaces were exposed to 40 laser pulses for a total etch depth of 2 . 4 p . Diffraction

effects at the rib edge provided deeper trenches resulting in a total etch depth of 4 p .

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- 'ma -

ri b waveguide

1

silicon wafer

Figure 4.3 The top picture is an endview of waveguide sample A. The gray color shows the silica (24pm) coating which aiso includes a 4pm thick germanosilicate guiding layer at the top. The white color represents the silicon wafer. At the top of the germanosilicate layer is the trapezoid-shaped rib waveguide of -8pm FWHM and 3pm etch depth. The bottom edge of the rib is deeper due to edge diffraction of the wire. The bottom picture is the topview of part of the waveguide sample A with 8pm width.

Page 50: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Figure 4.2 Surface profiles of the rib waveguide sample A. Waveguide width ( F m ) was measured to be IOpm and the etch depth was 4ym.

Figure 4.2 shows the surface profiles of this waveguide recorded by a stylus

profilorneter, yielding a 4pm etch depth, a 10ym waveguide width (FWHM) and a 2 p

edge resolution (defined as the width of the sloped side-wall from 10% to 90% of the etch

depth. in Chapter 3, we have calculated the theoretical edge resolution due to diffraction

and lens aberration resolution to be - 4 p when working at a demagnification factor of 8.

which was different from the measured 2p-n in Figure 4.2. There are several reasons

contributing to this discrepancy. Firstly, the calculation in Chapter 3 simply added W.

and Wd (in page 28) which likely over-estimated the resolution feature size; Secondly, the

calculation in Chapter 3 was based on laser bearn intensity profiles and not on etch depth

profiles which follow a logarithm response of fluences (Figure 3.2). The non-linear

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response for fiuences (2.8Ucm2) which was at 8x ablation threshold of PIRI waveguides

favors a sharper feature size; Lastly, surface receiving a laser fluence above the threshold

of PIRI waveguide can remove materiais. Al1 the three factors lead the calculation in

Chapter 3 to over-estimate the side-wall width. This reasoning also implies that the edge

resolution depends on matenals. For PMMA, the edge resolution is about 3pm at the

same demagnification factor. However, the edge resolution is independent of the number

of laser shots; larger number of laser shots etches the deeper trenches, resulting in steeper

slope.

The waveguide width measured by the profilometer (10pm) was larger than that

measured from Figure 4.1 (-8pm). The difference was due to the steep slope of

waveguide side-wall that usually exceeds -45' angle. The profilorneter tip can not touch

the side-wall and gives a wider width than the actual width.

4.1.2 Surface Scattering Loss

Due to the nature of laser ablation, the surface of ablated germanosilicate is not

optically smooth, typically resulting in a 50nm R M S (root of mean square) surface

roughness as determined by the profilometer for a 4 p deep hole. Furthemore, the

fabrication procedure requires a stepping of the sample across the image of the aperture

for each laser shot. This produces a ripple with penod of the stepping size, typicaily 10-

1 5 p . Figure 4.3 is the surface profile of waveguide sample A at the bottom of the

trench near the rib waveguide, and taken parallel to the rib. The ripple period is lOpm,

and the height is -0 .06p, matching the etched rate at 2.81/cm2 laser fluence according to

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Figure 4.3 Surface profile at the bottom of trench and near the rib of waveguide sample A. The ripple height is about 0.06pm wwhh is close to the surface roughness intrinsic to laser ablation, making it difficult to distinguish both types of roughness.

Figure 4.4 Topview of waveguide sample D. The surface ripple has a period of 1Opm and a height of about -O. lpm as determined by a stylus profilorneter.

Page 53: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Figure 3.2. This ripple height is close to the RMS surface roughness due to laser

ablation. Therefore it is difficult to distinguish the surface roughness due to laser ablation

from that due to the stepping process. Figure 4.4 shows a top view of waveguide sample

D for a 2 0 ~ microscope objective. The surface ripple is clearly visible and with a period

of -lOpm, which matches the step-size used for translating the motor.

Figure 4.5 shows the scattered optical power from the nb waveguide surfaces

measured as a function of waveguide position for the four lOmm long waveguides.

Typically from the beginning waveguide facet to 3 or 4mm, the scattering light consists of

both lowest mode and higher mode losses, yielding a steep drop in intensity for this

region; from 3 or 4mm to 8mm, the scattering light is typically due only to the lowest

order mode, if the waveguide is a single-mode waveguide. The power fall-off is slower

in this region, and yields the single-mode scattering loss as shown by the solid lines in

each of the graphs in Figure 4.5. The intensity rise in the last -2mm is due to light

scattered by the waveguide facet, and scattered by backwards propagating light reflected

by the end facet. This allows the microscope to collect part of the facet-scattered or

reflected light even when the waveguide edge is still not within the observed field of the

microscope. The lowest scattering loss was measured to be -4dBkm from sample A.

We repeated the loss measurement four times, and found the uncertainty of the

measurement to be -20%. Several factors contribute to this uncertainty: 1.) surface

debris produced by laser pulses cm causes larger light scattering; 2.) power of the diode

laser can slightly drop during the loss measurement; 3.) coupling between the pigtailed

fiber and the iib waveguide can also drift during the loss measurement.

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O 2 4 6 8 10 12

Distance (mm)

Figure 4.5a Scattering power loss as a function of waveguide position for sarnple A. See the text for a detailed description of the graph.

I I I l u 1 I I , . I I I I I

4 6 8 IO

Distance (mm)

Figure 4.5b Scattenng power loss as a function of waveguide position for sarnple B

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Distance (mm)

Figure 4 . 5 ~ Scattering power loss as a function of waveguide position for sarnple D.

The loss results obtained in Figure 4.5 are summarized in Table 4.1. The lowest

loss of 3.6dBkm was found for sarnple A. The waveguide width of sample D was the

same as that for sarnple Ay but the etched depth is increased by 2pm. Cornparison of the

losses of sarnple A and D shows that deeper etch depth for a rib waveguide has higher

scattenng losses. We will present the explanation in section 4.2.3.

For sample By the loss was particularly high due to the fact that the guide supports

multimode. A careful examination of Figure 4.33 for sample B shows that leveling off of

scattered light power associated with single-mode propagation does not occur. Higher

modes encounter larger loss since the field distributions place larger portion of field at the

rough waveguide boundaries. Hence, it is not recommended to fabricate a waveguide

Page 56: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

with widths larger than - 10pm for a 4 p deep rib, and nsk multi-mode propagation

(also see Figure 2.7).

From the above discussion, when choosing the waveguide parameters to keep the

scattering loss low, the waveguide width should be no larger than IO pm and etched depth

should be no larger than 4 pm. Smaller etched depth may produce lower scattering

losses. This trades against a low coupling efficiency which we will discuss in next

section.

Sample C2 differs from Ci (see Table 4.1) in the number of laser shots per moving

step. Sample C2 was fabricated by five laser shots per step, while CI was fabricated in the

sarne way except by one laser shot per step. The loss for CI was 7.7 dB/crn while the loss

for Cz was 11.2 dB/crn. This reveals a relationship between scattering loss and the

periodical surface ripple caused by stepping process. The ripple height was measured by

a profilonieter to be 0.06pm for Ci and 0 . 3 p for Ct. Therefore, reduction of the ripple

height should reduce the scattering loss. One might think that working at Iow laser

Fiuence will reduce the ripple height. However, lower laser fiuence increases the nurnber

of laser shots required to produce the expected trench depth, and also increasing the etch

depth uncertainty due to +IO% laser energy fluctuations. Optimization is reached when

the final trench depth uncertainty is comparable with the single-shot edge rate that defines

the ripple amplitude.

To make this point clear, supposed Ge work at 2.5~/cm', which yields an etch rate

of -0 .058p depth per laser shot from Figure 3.2. To produce a 4 p etch depth, about

70 laser shots is required. The uncertainty of the etch depth for N laser shots is

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AD, = a ( A F / F) where Do is defined in D = Do log(F I 4 ) (solid line in Figure

3.2) and is 0.072j~m for the PIRI waveguide. The fluctuation of the laser fluence,

LW / F , is about 10%. Putting al1 the numbers together, one gets AD,, = 0.059 p. This

theoretical value already exceeds Our observed O.OSpm (RMS) of surface roughness,

dernonstrating that there is no reason to work below 2.51/cm2 if a 4p.m of etch depth is

required. To reduce the ripple amplitude, other ideas should be developed.

Table 4.

results ol

sample

A

B

Cl

c2

D

4.1.3 Single Mode Coupling

Figure 4.6 shows the optical output at the end facet of waveguide sample A. The

detailed procedure of capturing this picture was described in section 3.2.1. In Figure 4.6,

the output beam profile is an elliptical shape with FWHM dimensions of 6 p m ~ 4 p . No

I Summary of wîveguide pararneters, fabrication pararneters, and rneasured

the four waveguides (M in column 6 is the demagnification factor).

ri b ' width

8p-n

1 2 p

l O p

1 0 p

8 p

etch

depth

4 p

4pm

7pm

7pm

6pm

laser

fluence

2.8J/cm2

2.5J/cm2

2.5Ucrn2

22.~/crn~

2.81/cm2

single

mode

Yes

no

Yes

Yes

Yes

step

size Ay

lOpm

1 2 p

5p.m

2 0 p

l 0 p

' M

8

8

8

8

9

loss

3.6 dB/cm

1 1.4 dB/cm

7.7 dB/cm

1 1.2 &/cm

7.6 dB/cm

coupling

efficiency

1 1.3%

-

35%

-

6%

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node is observed in the beam profile, which indicates single mode confinement. This

observation agrees with the BPM simulation results in Figure 2.7 which predicted that a

rib waveguide with 8pm width and 4pm etched depth supported the single-mode

propagation. Further discussion of the beam profiles is provided in Section 4.2.1.

Figure 4.6 Output beam profile of waveguide sample A. The FWHM of intensity profile was measured to be - 6 . 0 ~ horizontally and - 4 . 0 ~ m verticdly. (A cornparison of the intensity profile to BPM simulation results is given in Figure 4.10.)

The ratio of optical power which is coupled into the single mode of this rib

waveguide to the optical power for the pigtailed fiber was measured to be 1 1%. This is

relatively low. Figure 4.7 shows a Gaussian bearn mode1 to help understand the light

propagation from the fiber tip. Assurning the output from the fiber is a Gaussian beam,

the facet of fiber serves as a lens with radius of curvature R.

radius of curvanire R Wb

Figure 4.7 A Gaussian beam mode1 for the beam propagation from the pigtailed fiber.

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The beam waist is denoted as Wb which is related to the beam divergent angle by

cp = fiAhwb. Figure 4.8 shows the output beam from the pigtailed fiber at the best

focus point. The beam waist (1/e of the peak optical intensity) is estimated to be about

3.5pm, which gives cp = 0.1 16 radians. The beam divergence is controlled by the radius

of curvature R (see Figure 4.7). Larger R gives smaller cp. In principle, a single-mode

fiber with flat end-facet (R = p.) can sufficiently couple the optical power into the rib

waveguide if it is directly attached ont0 the waveguide facet. However, such an

arrangement is not practical since it prevents translating the fiber to achieve optimal

coupling. A finite radius of curvature (R = l O p in our case) is needed so that the focus

point is at some distance (-15pm in our case) from the fiber end, allowing the fiber io be

freely translated while keeping the focus point at the waveguide facet. This eases the

coupling difficulty and permits optimal single-mode coupling.

Figure 4.8 Output beam profiles from the pigtailed fiber at the best focus point. The profile width (l/e of the peak intensity) was estimated to be 3.5prn.

Page 60: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

The low coupling efficiency is not due to the beam size, but due to mismatches in

the bearn divergent angle cp and the numeficd aperture (NA.) of the rib waveguide. The

nb waveguides have a vertical N.A. of O. 11 which is very close to <p. But the laterai N.A.

is ,/N; - N: . From Table 2.2, for an etch depth of 4pm, this N.A. is only 0.0503

which is only one haif of the N.A. apemire, resulting in an asymmetric rib waveguide.

Therefore the ratio of optical power coupled into the rib waveguide is less than

0.0503/0.113 = 44.5%. The coupling effïciency is written as q= q l q z q 3 where

ql is due to the mismatch of lateral numericai aperture and is 44.5%;

qz is the transmission coefficient at the waveguide facet; and

q3 is the percentage of single mode light among the coupled light.

Here, qz is given by (n, - u2 = 95%, where nz is the refractive index of Ge-doped silica. (n, + 1)'

q 3 is less than 50% from the graph in Figure 3.5a. Then the overall coupling efficiency q

for sample A is less than 20%.

To increase ql, a new pigtailed fiber should be designed with larger radius of

curvature such that the divergent angle is small and matches the lateral numerical aperture

of the waveguide. Altematively, we can increase the lateral numerical aperture by

increasing the etched depth. If the etched depth is about 7 p , the conesponding lateral

numericai aperture is 0.105 which is very close to the divergent angle of the present

pigtailed fiber. Consequently the coupling efficiency is substantially improved. This is

shown in Table 4.1, where the coupling efficiency for sample Cl is 35%. Unfortunately,

the scattering loss increases substantially as also shown in Table 4.1.

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4.2 Discussion

4.2.1 Beam Profiles

From the output bearn of the rib waveguide shown in Figure 4.5. one obtains the

intensity profile. It is worthy to compare this measured beam profile with the BPM result

to mess the accuracy of cornputer simulation.

The BPM simulation was based on the trapezoid waveguide model with the

parameters selected to represent sample A. The waveguide width is 8pm, the etch depth

is 4pm, and the edge resolution is 3pm. Figure 4.9 shows the wave propagation along

this waveguide for 4mm length. Note that Figure 4.9 gives the distribution of electrical

field, instead of optical intensity.

We calculated the normalized intensity profile based on the field distribution at

the end of the wave propagation in Figure 4.9, and compared it with the normalized

intensity profile obtained from Figure 4.6. The result is shown in Figure 4.10. The

agreement is very good. The widths (FWHM) are 6.Opm and 6.3pm for the measured

profile and the BPM calculation respectively. Exponential curve fitting of the tail of the

beam profile gives the decay constant y outside the waveguide. We found that y was

0.47pf1 from both the BPM results and the measured profiled. This value can also be

computed analytically from the rectangular waveguide model, as defined by Equation

(2.6). For a rectangular rib waveguide witii 8 p width and 4 p depth, y is calculated to

be 0 . 4 4 ~ " . The agreement is fairiy good.

The consistency among the rectangular waveguide calculation, the BPM

simulation, and the rneasured results in this cornparison of output beam profiles indicates

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thüi tlic BPM simulation givcs vcry rcliable resulis, and can bc a powerlul tool in guiding

tlic design of silica rib waveguidcs.

Figure 4.9 Electrical field distributions obtained from BPM simulation of a trapezoid waveguide mode1 for sample A. Single-mode propagation is clear after 2mm of propagation dis tance.

Distance (pm)

Figure 4.10 Cornparison of beam intensity profile for sample A. The normalized intensity for BPM was obtained by squaring the values of the electrical field obtained in Figure 4.9 at 4.1 mm propagation distance.

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4.2.2 Optimized Waveguide Width and Etch Depth

Based on the results in this chapter, we gain more insight in choosing the

waveguide width and etched depth. This section will summarize the guidelines for

choosing these two parameters. At least four criteria should be kept in mind in

determining these two pararneters:

a. Single mode conditions;

b. Confinement factor;

c. Coupling efficiency (related to lateral numerical aperture);

d. Waveguide loss.

Table 4.2 shows how changes to the etch depth or waveguide width affects the

above four pararneters. Increasing etch depth will increase the coupling efficiency while

also increasing the scattering losses, as shown in Table 4.1. There is a trade-off. Which

critena is more important depends on the type of application. For example, scattering

loss is not a big concem for a relatively short waveguide, then a deeper etch depth is

preferred. Otherwise, the etch depth should not be larger than 4pm for a PIRi waveguide.

Although in this case the mismatch of the laterd N.A. of the rib waveguide to the N.A. of

pigtailed fiber exists, in principle, it can be elirninated if the pigtailed fiber is properly

designed by adjusting the radius of curvature (Figure 4.7). If4pm depth is a good choice,

then to maintain single-mode propagation, the waveguide width should be no larger than

9pn. Further, the width should exceed 7 p for a >80% of confinement factor. This

leads to an optimum choice of 8p.m rib width (FWZIM) and 4pm etch depth.

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Table 4.2 Relationship between four waveguide pararneters and the waveguide width

and etch depth.

4.2.2 Theory of Surface Scattering Loss

parameter

increasing

width

increasing

depth

For silica material based waveguides, surface scattering loss is the dominant loss.

This is in contrast with semiconductor-based waveguides where absorption loss is the

rnost important. A very smooth waveguide boundary of silica-based waveguide can

reduce the loss down to 0.5 dBkm such as in a standard optical fiber. However, silica rib

waveguides show much higher loss that fiber.

Our results shows a 4dB/cm loss of the rib waveguide fabricated in this thesis.

Industry standard for waveguide loss in photonics circuit is about ldB/cm before the

waveguide is considered for serious applications Wart891. To further reduce the loss, we

need theoretical guidance and a better understanding of the mechanism of surface

scattering loss. Unfortunately, there is no simple theory of surface scattenng loss for a ri6

waveguide. Ladouceur and Love [Lad0961 present a theory for scattering loss of buried

channei waveguides that assumes weak guiding where the index difference between the

guiding materid and the cladding material is less than 1%. This is not the case for the rib

single mode

more

difficult

more

difficult

confinement

factor

increasing

increasing

coupling

efficiency

no effect

increasing

loss

increasing

increasing

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waveguides in this thesis because the index difference between the air and the rib is 45%.

Marcuse [Marc821 derived a very general and cornplex theory for surface scattering loss

for slab waveguides. A more simplified version, based on the Rayleigh criterion, was

derived by Tien [Tien'll]. The Rayleigh criterium applies only to the case of long

correlation length of surface roughness. in this section, we attempt to gain some insight

into the scattering loss from Tien's theory.

In Our case, the surface roughness in the sidewall of the rib is much higher than

the surface roughness between the Ge-silica and silica interface or between the Ge-silica

and air interface. If the model of rectangular waveguide is used, then the loss due to the

roughness of the sidewall is estimated by [Tien7 1,Osgo92]:

, cos3 8 / sin 8 a, = 2n,'k2a- w + 2 / y

where sine = N,, / n, and y is the decay constant outside the sidewall. o is the statistical

variance (RMS) of surface roughness, which was measured to be -50nm for waveguide

sample A. The meaning of 8 is shown in figure 4.1 1. Using this model, we obtained

y-0.21m and 0=88.S0. The resulting loss was calculated to be only 0.2dB/cm. This is in

poor agreement with the measured loss of 4dl3lcm for sarnple A.

air 4 *

8p.m Ge-silica *

1

8 air

Figure 4.1 1 Top view of the rib waveguide showing the intemal reflection angle 0.

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The poor agreement can be attributed to the fact that Equation (4.1) only applies

for a slab waveguide. There is no simple way to extend it to calculate the scattering loss

of channel waveguide. To firther complicate the situation, in the vertical direction of the

rib waveguide in this thesis, half of sidewdl is very rough from laser etching and exposed

to air, while the other half is the thin guiding Ge-doped silica layer. It is difficult to

denve a theory of scattering loss from an inhomogeneous sidewall, and probably requires

the effort of a new research project.

Nevertheless, from Equation (4.1) we can still get some qualitative understanding

on the scattering loss. The loss is proportional to d. If the etched depth is increased,

then more of the guiding light will reflect in the sidewall where surface roughness is high.

Effectively, $ increases. Etching deeper also decreases 0 (since the effective index in the

etched region decreases, as seen in Table 2.1) and y. Al1 these changes lead to an

increase of loss, as observed experimentally in section 4.1.2. Furthemore, 0 is the

incident angle in the sidewall and its value depends on the propagating mode. Higher

modes have smaller 8, thereby yielding larger loss according to Equation (4.1). This was

also observed in section 4.1.2.

It is also important to note that Equation (4.1) shows the 1/X2 dependency of

scattering loss since the loss measurement in this thesis was performed at 0.635ym

wavelength. This wavelength scding will reduce the 4dBkm loss at 0.635~ to less than

ldB/cm at the 1.55p.m telecommunication wavelength. In fact, Takato, Yasu and

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kawachi [Taka861 have shown that the surface scattenng loss of a rectangular silica

waveguide fabricated by RIE was 0.3dBkm at 0.6pm and O. ldB/cm at 1 S p .

It is possible to further reduce the scattering loss by reducing etched depth, but the

difference between the lateral numerical aperture and vertical numencal aperture

increases. This will degrade the quality of waveguide since the coupling loss from

standard fiber to such waveguide will increase. Effort to reduce scattering loss should be

directed into reduction of surface roughness, or coating an upper cladding on the

waveguide, as will be further discussed in Chapter 5.

4.2.3 Comparison With Other Work

The drive of this thesis work is to define a simple laser-based waveguide

fabrication technique for silica material. Currently there are a number of research groups

which are also actively developing new fabrication techniques for various optical material

[Hart89, Osgo92, Rich94, Mukh941. Most of them employed direct laser writing, which,

as mentioned earlier, is promising in defining a single-step waveguide fabrication

process. It is valuable to compare our work with the works of other groups.

Krchnavek, Lalk, and Hartman Bart891 used an Ar-ion laser (h = 350nm) to

write channel waveguides using spinsn polymer (Norland 61). The polymer was

pattemed by exposure to the laser. After exposure, the pattern was developed by rinsing

the film in acetone, leaving the unexposed region as a channel waveguide. Strictly

speaking, the process is not single-step. One of the advantage of this approach is that the

writing speed was fast, at 10-400Cun/s writing rate. This is related to that fact that Ar-ion

Page 68: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

laser is a continue-wave laser. The waveguide cross-section was approximately 8x4pm.

The scattering loss was less than IdB/cm after optirnization of the cross-section area.

However, the fabncated waveguides in this report were multimode. Since the report also

showed that the loss decreases as the cross-sectional area increases, it is not clear whether

the loss is as low as ldB/cm for single mode waveguide.

Channel waveguides were also fabricated by Mukhejee, Eapen and Baral

[Mukh94] in polymethylmethacrylate (PMMA) using an intracavity doubled Ar ion laser

(h = 257nm). The loss was greatly reduced to about 0.08dB/cm when the PMMA

solution used was filtered through a OJmm pore. Waveguides based on unfiltered

PMMA solution typically yielded 2dB/cm loss. Reducing the polyrner size of PMMA

decreased the bulk scattering loss to achieve a very low waveguide loss. However, these

waveguides [Mukh94] were also multimode, and the fabrication process required special

treatment of the waveguide material, defining an overall complex laser process.

Although polymer material draws much attention as the guiding material for

channel waveguides, there are some advantages in silica material which can not be

achieved by polymer materiai. For example, the photosensitivity effect is only observed

in silica material. Combining the photosensitivity effect and simple waveguide writing

techniques provides a potential of fabricating complicated photonic circuit in a single

silica chip. This can not be achieved with polymer materîals.

Serniconductor material aiso draws attention as a waveguide material because of

its use as lasers, detectors, and also because it affords a strong electro-optic effect.

Osgood et al. [Osgo921 extended the laser assisted chernical etching technique to

Page 69: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

fabricate rib waveguides in GaAs (refractive index 3.4049) on AlGaAs (refractive index

3.3566) substrate. The laser wavelength was 275nm, and the etchant was

HCL:HN03:H20::4: 1 :50. The resulting waveguide cross-section was 4 p x 1.31m. and

was single mode with a loss of 0.9dB/crn. Several drawbacks existed in these

waveguides. Firstly the cross-section size was too small which causes larger loss in

coupling light from fiber to the waveguide. Secondly the difference between the lateral

numerical aperture and vertical numerical aperture was much higher than the silica rîb

waveguides produced in this thesis work. There were two reasons for this: 1.) the

guiding GaAs layer, has more than twice the refractive index of germanosilicate; 2.) the

etch depth was only 0.25pm and 1 6 of the thickness of the GaAs layer.

Table 4.3 summarizes the cornparison of this work with other work in defining

single-step waveguide fabrication by laser writing techniques. The cornparison shows

that, as the first reported silica rib waveguide fabricated by laser ablation technique, the

rib waveguide in this thesis has already shown advantages such as single-step fabrication,

single mode confinement with large cross-section size, and relatively small difference of

numericd apertures in lateral and vertical direction. The limitation is in the relatively

large surface scattering loss. It is possible to further reduce the loss by reducing the etch,

depth, reducing the surface roughness, and improving the laser energy fluctuation, as will

be discussed in next Chapter.

Page 70: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Table 4.3 Cornparison of this thesis work with other results

1 Report 1 Report 1 Report 1 RIE ( This

Guiding Material

Cross-Section

[Osgo921 GaAs

Single Mode

Single Step

4x 1.3pm

Loss

Yes

[Hart891 Polymer

Yes

0.9dBkm at

1.3pm

Writing Speed I N'A

4 x 8 ~

Table 4.3 also includes a cornparison of this work with the RIE techniques

[Taka861 in fabricating silica channel waveguide. The channel waveguide in Ref.

[Taka861 was a buried channel waveguide with an upper silica cladding which helped to

reduce the scattering loss. The channel waveguide fabricated by RIE process had much

lower loss (O.ldB/cm at 1.5p-n) but suffered from many complex fabrication steps and

high-cost. On the other hand, the rib waveguide fabrication technique in this thesis offers

fast writing speed and is single-step, but suffers from high loss. Both techniques

produced single-mode waveguides with 8prn~8jun of cross-section sizes which were

compatible with those of standard single-mode optical fiber. This property was not

achieved by other three fabrication techniques listed in Table 4.3.

Nukh94J Polymer

No

8 x 3 ~

[Talca861 silica

No

Thesis Ge-silica

8 ~ 8 p m 8~8prn

Yes Yes

Page 71: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

RIE is a large area process and is good for replication of identical photonic

circuits. However, different circuits requires different contact masks which are also

complex to fabricate. Direct laser-writing, while slow, does not require contact masks.

The positioning of sample targets can be cornputer-controlled, offenng much greater

flexibility for printing complex photonic circuits which is especiaily important for small

size applications. The laser-writing speed cm be improved dramatically since the current

laser pulse repetition rate of 1Hz can be increased to 100Hz, yielding 1000pm/s of

writing speed. Construction of a new Ft excimer laser is underway in this laboratory.

In conciusion, the waveguide fabrication technique developed in this thesis has

s hown promising potential to produce high-quality silica rib waveguides, and offers

advantages of single-step and fast writing speed. We are optimistic that the surface

scattering loss cm be brought down to ldB/cm if working at 1.55prn and alter furthet

improvement of surface roughness, as will be funher discussed in next chapter. This

allows small scale applications which only require short waveguide length (several

centirneters) Wart891. In this sense, this laser-ablation-based waveguide writing

technique imposes a practical impact to RIE technique in providing silica channel

waveguides for short-distance applications.

Page 72: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Chapter 5

Conclusions

5.1 Summary of This Thesis

Single-mode rib waveguides have been successfully fabricated on planar

germanosilicate, for the first time, by a laser ablation technique. The laser ablation

technique offers a single-step, loss-cost fabrication process for single-mode silica

waveguide with large cross-section size compatible with standard single-mode fibers.

The waveguide loss was -4dB/cm at 0.63pm wavelength. It is optimistic that the loss c m

be brought down to less than ldB/crn at 1 . 5 5 ~ with further improvement of surface

roughness. This pioneering work on silica rib waveguide fabrication demonstrates that

the laser-ablation-based waveguide wnting technique cm practically compete with the

RIE technique in providing silica channel waveguides for short-distance photonic

applications.

In this study, the edge resolution of the waveguide side walls was optirnized to be

-2pm. This is notable since only a single lens was used in this system. Computer

simulations were carried out to study the impact of the trapezoid waveguide cross-section

on the single mode conditions. We found that the lowest mode propagation is not

seriously effected by the geometric shape of the waveguide cross-section. The single-

mode condition for a trapezoid waveguide was very sirnilar to that of a rectangular

Page 73: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

waveguide. Agreement on the bearn profiles obtained by BPM simulation and

experimental measurement was excellent.

Coupling of light into the rib waveguide was facilitated by a serni-spherical lens

ended fiber (SLEF). The coupling efficiency was found to be only -1 1% for the lowest

loss waveguide owning to the mismatch of the bearn divergent angle and the lateral

numerical aperture of the rib waveguide. The mismatch of numerical aperture can be

eliminated as long as the radius of curvature of the pigtailed fiber is properly designed.

This probiem existed in rnany rib waveguides reponed in literature and is an important

factor to access the overall waveguide quality.

The thesis presenü a comprehensive discussion of optimizing the waveguide

quality based on many criteria including single-mode guiding, scattering loss, coupling

efficiency and confinement factor. Cornparison of this work with other research groups

shows that the rib waveguides fabricated in this thesis are attractive in large cross-section

size, single-mode guiding, relatively large coupling effïciency and single-step fabrication.

The limitation of this thesis work was the relatively large scattering Ioss. Further

research effort is required in order to make the laser-ablation-based waveguide writing

technique compete with RIE in fabricating low loss waveguides.

Page 74: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

5.2 Future work

Future work on laser-writing of rib waveguides should focus on reducing the

optical scattering loss. Several techniques can be employed, as described below.

Redesign of the aperture shown in Figure 3.3, such that the wire is displaced a

srnall distance in front of the rectangular aperture can create a sharp image of the wire on

the waveguide surface while blurring the hard edges of the aperture. This reduces the

surface ripple height due to the stepping process.

To reduce the surface roughness due to laser ablation, two improvement could be

taken. The first is to increase the unifonnity of laser beam. Currently, a new Fz laser is

being constmcted in this laboratory. We expect this laser can produce a more uniform

laser beam to help reduce the surface roughness. The second improvement is to reduce

the laser energy fluctuation from pulse to pulse. Computer control could also help to

compensate for such fluctuation by automatically adjusting the laser energy or the

translating speed of the waveguide sample.

The above approaches still keep the waveguide fabrication as a single-step

process. Additional steps to hirther reduce the surface roughness include chemically

treatment of the silica waveguide after fabrication to srnoothen the roughness; or coating

the waveguide surface with a polyrner materid to reduce the index difference between the

core layer and cladding layer. One advantage for the later case is that the scattering loss

theory is available Eabo96].

Page 75: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

Measuring the waveguide loss at 1 . 5 5 ~ wavelength should also be carried out in

the future, because of the particular importance of this wavelength and the expected

reduction in scattering loss for longer wavelength light [Taka86].

Computer control of stepping process is another improvement recommended in

the future. This will not only reduce the fabrication time, increase the accurecy, but will

also allow more complicated waveguide devices to be printed.

Extension of the fabrication technique developed in this thesis to fabricate devices

such as directional couplers in germanosilicate should be straightfomard, although the

scattering loss need to be reduced first.

Photosensitivity writing is also attractive for future work since there two potential

advantages it can offer: the first is to solve the asyrnmetric vertical and horizontal

numerical aperture problem because the waveguide will be buned and the typical index

change is 0.3% [Maxw94] in Ge-doped silica which matches the index difference

between the core layer and substrate layer in PIRI waveguide (0.3%). Secondly,

photosensitivity may produce a smoother waveguide boundary further reducing the

scattering loss.

Page 76: and Characterization Single-Mode Rib Waveguides … and Characterizatioo of Single-Mode Rib Waveguides on Planar Germanosilicate Wafers Jianhao Yang M.A.Sc., 1997 Department of Electrical

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