Arrayed Waveguide Gratings

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Arrayed Waveguide Gratings

Photonics Integrated Circuits

- Course Project-

Q2 2015/2016

Student:

Ionut-Adrian Hurmuz

Coordinator:

Martijn Heck

Aarhus

University


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Table of Contents

1. Introduction ................................................................................................................................................3

2. AWG Basic Design .................................................................................................................................3

3. Components and Working Principle ............................................................................................5

4. Important Properties and Parameters .........................................................................................6

5. Applications AWG Wavelength Demultiplexers ................................................................7

5.1 Fabrication Platforms Silica-on-Silicon .................................................................................7

5.2 Fabrication Platforms Indium Phosphide (InP) .................................................................8

5.3 Fabrication Platforms: Silicon ........................................................................................................8

5.3 Application ..............................................................................................................................................9

5.4 Basic design and BPM simulation of an InP AWG demultiplexer .......................... 10

6. Conclusion .............................. ............................... ................................. ................................. ................ 13


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Arrayed Waveguide Gratings

1.

Introduction

In the past two decades optical fiber communications has totally changed the way

world communicates and transports information. It is a technological revolution that

has fundamentally transformed the core of telecommunications, its basic science,

and its industry. At the same time the complexity of optical systems is steadily

increasing due to innovations like wavelength-division-multiplexing (WDM) and the

transition from simple point-to point transmission to WDM networking.

Integrated optics was conceived in analogy to electronic integrated circuits to

handle increased systems complexity and to reduce the cost of packaging and of

subsystems. Its early successes included integrated guided-wave wavelength filters

and WDM multiplexers, WDM laser sources such as distributed-feedback (DFB) lasers

providing spectral control, and chips integrating lasers with high-speed modulators.

WDM is the technology for achieving extremely high data rates over fiber-optic

cabling. Also known as Dense Wavelength Division Multiplexing (DWDM),

Wavelength Division Multiplexing is likely to replace Time Division Multiplexing (TDM)

as the standard transmission method for high-speed fiber-optic backbones in the

next few years.

In these systems, signals at different wavelengths are mixed and transmitted through

a single optical fiber, and this technology provides us with a high per-fibertransmission capacity and low communication costs. A wavelength

multi/demultiplexer is a key device in such WDM systems. Research on integrated

optic (de)multiplexers has, since the early 1990s, increasingly been focused on

grating-based and phased-array (PHASAR) based devices, also called arrayed

waveguide gratings (AWG).

The AWG is a transmission grating that consists of multiple channel waveguides of

different lengths. These waveguides are fabricated on a substrate by using planar

lightwave circuit (PLC) technology that includes glass film deposition,

photolithography, and dry etching.The features of AWG multi/demultiplexers are their compact size, stable operation in

the presence of mechanical vibration, high long-term reliability, and mass

production.

2. AWG Basic Design

An AWG functions similar to an optical prism, by imaging the input optical field onto

different spatial output locations based on the wavelength of the incoming light.

Figure 1.1 shows the waveguide layout of the AWG multi/demultiplexer, which is the

same as that of a conventional spectrometer. The concave alignment of the

waveguide ends acts as a lens and the AWG acts as a diffraction grating.


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Accordingly, lights with different wavelengths are focused at corresponding ports

when a wavelength multiplexed light is launched into the input port.

The diffraction angle and wavelength satisfy the following grating equation:

Where:

nc: effective refractive index of channel waveguides in AWG ns: effective refractive index of slab waveguide

d : grating pitch (distance between waveguide ends of AWG)

L: length difference between channel waveguides in AWG

m: diffraction order (natural number)

= 0 is a direction aiming at the center output waveguide.

The wavelength of the light that travels in this direction is called the center

wavelength. As the wavelength changes from the center wavelength, the focal point

moves at a speed of dx/d, which is called the linear dispersion of the grating where

x is a coordinate along the focal line (Figure 1.1.). The linear dispersion is obtained asfollows:

where

and f is the focal length of the waveguide. In order to obtain a high wavelength

resolution, a large linear dispersion is needed. This is realized with a long focal length

f and a small pitch d.

Figure 1.1 AWG Layout

(1)

(2) (3)


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(5)

In an AWG, an extremely large linear dispersion can be obtained, with a high

diffraction order m simply by designing the waveguide length. This is the most

important characteristic feature of the AWG and a wavelength channel spacing of

less than 1 nm is easily obtained in AWG multi/demultiplexers.

This channel-spacing structure is called the free spectral range (FSR). Therefore, the

wavelength range available for WDM is limited to the FSR. The FSR in terms of

frequency is given by the formula below where c is the light velocity in a vacuum.

where c is the light velocity in a vacuum. Using this description a maximum number

of channels (M) can be obtained:

It can also be seen that a large focal length f is required for a higher channel count.

Accordingly, a large number of arrayed waveguides is required to receive all the

diffracted light from the input waveguide.

3.

Components and Working Principle

AWGs consist of three main functional elements: two star couplers and a set of

interconnecting arrayed waveguides, whose optical lengths, given by the product of

their effective indices (ng) and physical lengths, vary by a constant m number of

wavelengths of light at a central wavelength c from one waveguide to another

Equation (1).

The star couplers are generally based on the Rowlandcircle construction, where the

radius of curvature of the input and output waveguide planes is one half of the radius

of inner waveguide array planes. This arrangement ensures that the output light is

focused along the circular output interface with the change in wavelength of the

input signal.

In terms of device principle of operation, the light from the input waveguide radiates

into the first star coupler slab waveguide and excites the modes of the arrayed

waveguides at the star coupler output. After traveling through the AW, the light from

the waveguides is diffracted into the output star slab waveguide, where it

constructively converges in one focal point at the output of the star coupler. This is

accomplished because the path length difference between the arrayed waveguides

results in a relative phase delay in each waveguide, which changes with

wavelength. This results in a rotation of the field phase front in the second slab and a

translation of the location of the focal point as a function of the wavelength.

(4)


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A key for designing AWG components is given by Expression (2) which relates the

star coupler size, array waveguide spacing, and L.

Many important device physical features, such as the access and arrayedwaveguide widths and their minimum separation, will be dictated by the maximum

resolution of the device fabrication process. The arrayed waveguide spacing, da,should be as low as possible, since any light not coupled into the array will contribute

to the device insertion losses. The output waveguide spacing, dr , will directly impact

the crosstalk of the AWG, since the output image at a given spatial location will have

exponential tails that can couple into the adjacent output waveguides, depending

on the distance. The lower bound of this parameter can be determined, based on

the desired crosstalk and receiver waveguide architecture, using a normalized

crosstalk plot like the one in Figure 1.2

4. Important Properties and Parameters

Focusing - obtained by choosing the length difference (L) between adjacent

arrayed waveguides equal to an integer number of wavelengths, measured

inside the array waveguides. Dispersion refers to the dispersion angle ()resulting from a phase difference

between adjacent waveguides.

FSR (free spectral range) - channel-spacing structure that limits the

wavelength range of the design.

Bandwidth - If the wavelength is changed the focal field of the AWG moves

along the receiver waveguides. The frequency response of the different

channels follows from the overlap of this field with the modal fields of the

receiver waveguides.

Channel Crosstalk - may be caused by many mechanisms but in most cases

can be controlled by a proper design of the device; It can be split into:

Figure 1.2 Crosstalk levels and Transmitted Power


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o Receiver Crosstalkcaused by coupling between the receivers through

the exponential tails of the field distributions overlapping fields

o Truncation - due to the finite width of the array aperture. This causes

power to be lost at the input aperture, and at the output aperture thesidelobe level of the focal field will increase.

o Mode Conversion appears if the array waveguides are not strictly

single moded and modes excited at the junctions between straight and

curved waveguides can propagate coherently through the array and

cause ghost images.

o Coupling in the Array: crosstalk can also be incurred by phase distortion

due to coupling in the input and output sections in the arrays.

o Phase Transfer Incoherence: source of crosstalk that results from

incoherence of the phased array due to imperfections in the fabrication

process.

o Background Radiation due to light scattered out of the waveguides at

junctions or rough waveguide edges

Polarization Dependence - Phased arrays are polarization independent if the

array waveguides are polarization independent, i.e., the propagation

constants for the fundamental TE- and TM-mode are equal. Waveguide

birefringence, i.e., a difference in propagation constants, will result in a shift of

the spectral responses with respect to each other, which is called the

polarization dispersion.

5.

Applications AWG Wavelength Demultiplexers

Phased-array wavelength demultiplexers have become key components in modern

WDM systems. PHASARs can be fabricated in a single-mask planar waveguide

technology, which makes them robust and fabrication tolerant and potentially low-

cost. Their main potential lies in applications with moderate crosstalk requirements

and in integration in more complex devices like multi-wavelength receivers and

transmitters.

5.1 Fabrication Platforms Silica-on-Silicon

Silica-on-silicon is the most promising set of materials for integrated optics. It has theadvantages of having a cheap fabrication process and a good compatibility with Si-

based microelectronics. The connection with single mode optical fibers is also cheap

and can be done with low losses. It is good performing technology for passive

devices like the ones based on AWGs, but the research progress leads to active

components such as optical amplifiers. The only major disadvantage is the current

lack of an operating mechanism for high-speed switches (for example, an electro-

optic effect).

As mentioned below, the passive components that can be fabricated using this

technology are currently leading the market. AWG Demultiplexers with crosstalk

figures that reach less than 30 dB are available for commercial use and also with up


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to 80 channels and down to 25 GHz channel spacing. Insertion losses are typically as

low as 5 dB.

5.2 Fabrication Platforms Indium Phosphide (InP)Lasers, photodiodes and waveguides fabricated on InP operate at the optimum

transmission window of glass fiber, which enable efficient fiber communications. The

direct band-gap structure makes it a good solution for electro-optical devices. It has

an extremely low noise figure and can reach frequencies in the terahertz domain.

The disadvantages are represented by its fragility, low breakdown voltage and high

costs - more expensive than GaAs due to starting material costs and smaller wafer

size are used.

Silica-on-silicon technology is closer to maturity than Indium-Phosphide (InP)-based

semiconductor technology, but its applications are restricted to passive and low-speed dynamic functions based on thermo-optic phase shifters.

InP is better suited to more complex functions involving light generation,

amplification, detection and a range of non-linear signal operations. InP can

integrate all these functions on a single chip. Further, InP-based devices are smaller

by one or two orders than silica-based devices which makes them very suitable for

applications in complex integrated circuits.

Their performance is lagging behind silica-on-silicon devices, but it is steadily

improving. Due to their small waveguide core, coupling to fibers is more difficult, and

hence more costly. This makes them less competitive for circuits with a restrictedfunctionality. For more complex circuits, InP will evolve and become a dominant

material.

5.3 Fabrication Platforms: Silicon

The use of silicon has long been established for infrared optics, such as simple lenses

and windows and long-wave detection. There is no doubt about the economic and

technical advantages of silicon and it was inevitable that silicon would be employed

wherever optic fiber is deployed. In terms of optical spectrum advantages, Silicon

has a wide transparent window that extends to infrared. It also has a high heat

conductance and the benefit of low noise, which makes it suitable for high speedintegrated circuits.

Basic components with high performance have already been demonstrated using

silicon wire waveguides. A good example of a standard commercial circuit is the

AWG. Because of material purity and precise geometric control, the silicon AWG has

very low crosstalk and excellent ITU grid registration as shown in figure 5.1. The

absolute insertion loss from glass fiber to a Si AWG is a little higher compared to a

silica AWG. However, when multiple functions are integrated monolithically and fiber

interconnects are not utilized, the Si AWG has an insertion loss advantage.


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Some properties of the silicon waveguides are quite different from the more familiar

silica (SiO2) waveguide. In Silicon, the minimum turning radius is 250 m compared

to 5,000 um in Silica; the Si refractive index is 3.4 compared to Silicas 1.6;

5.3 ApplicationA multiwavelength receiver is obtained by integration of an InP AWG Demultiplexer

with a photodiode array like it is presented in Figure 5.2.

As described in the previous sections, the main advantage of InP-based devices lies

in their potential for monolithic integration of active components, even though they

exhibit higher propagation and fiber-coupling losses. Monolithic integration of an InP

Figure 5.1 1x8 WDM receiver consisting of an AWG

integrated with 8 detector diodes

Figure 5.1 Transmission spectrum of a silicon flat-band AWG


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based demultiplexer allows a reduction in size, low-cost, and functionality for mass

production of more complex devices.

5.4 Basic design and BPM simulation of an InP AWG demultiplexer

In this section, a software simulation will be performed on a detailed example of an

AWG 1x8 Demultiplexer using the WDM PHASAR tool offered by Optiwave. The basic

idea behind these simulations is to break the AWG into three separate components:

an input star coupler, an array of decoupled waveguides and an output star coupler.

After that, numerical solvers like Effective Index Calculator and Beam Propagation

Method can analyze and optimize the devices performance in terms of crosstalk

level, insertion loss, channel spacing, and bandwidth

The 1x8 arrayed waveguide gratingbased demultiplexer needs to be designed in

Indium Phosphide, to operate with the central wavelength of 1550 nm.The optical

channels that need to be multiplexed are spaced by 100 GHz. The waveguide is a

ridge structure which has a 600 nm InGaAsP core, characterized by a refractive

index n1=3.4, in cladding of InP with index n2 = 3.17. Arrayed waveguides are

assumed to be 2 m wideand input and output waveguides are 3 m wide. Figure

5.3 presents a picture of the cross section (transverse plane) of the waveguide to be

used in the phasar array. The thickness of the top and bottom layer is not important to

the calculation, as they are assumed to extend to infinity.

In the following step after designing the multilayer structure, the software calculates

the values of the effective indexes for both core and cladding. This results are:

3.366345 for the core and 3.184621 for the cladding.

At this point waveguide design is complete and the simulation parameters are

required. A propagation length of 15000 m is selected over a mesh of 10000 m in

width. By imposing a maximum crosstalk level specification of -35 dB the minimum

waveguide separation is calculated at 3.3 microns for the I/O waveguides and at 2.4

Fi ure 5.3 InP claddin /InGaAsP core wave uide


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for the phased array. Still at this point a number of 8 output channel is imposed and

based on that and the waveguide and wafer parameters, the Minimum Length of the

Free Propagation Region is calculated to be 325.9 microns. We can now select the

100GHz channel spacing.

The key values of the design are displayed in Figure 5.4:

The grating design is complete (Figure 5.5) and the first simulation performed is a

check on the channel crosstalk level which is found to be at the expected levels,

below -100 dB.

The simulation of the device is computed using BPM, which is an approximation

technique for simulating the propagation of light in slowly varying optical

waveguiding structures.

Figure 5.6 displays the result of BPM. The top left quadrant displays a topographicalview of the optical field and the top right quadrant shows a 3D representation of the

same oprical field. The bottom left quadrant displays a cross-sectional view of the

effective index distribution (in red) and the field distribution (in blue) while the bottom

right quadrant displays a 3D view of the effective index.

Figure 5.4 Key calculated parameters after imposing

the requirements

Figure 5.5 Grating Design


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The simulation is repeated using the wavelength as a scan parameter. 11 iterationsare performed starting from 1.545 nm and going up to 1.555 nm. The figure below

shows the Output Power (dB) for all eight channels scanned.

Figure 5.7 Output Power vs Wavelength

Figure 5.5 Simulation results


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6. Conclusion

Arrayed waveguide gratings represent one of the most important building blocks of

complex photonic integrated circuits. They can be considered as stand-alonepassive components, but their utility as building blocks to realize optical, chip-scale

multiplexers/demultiplexers, cross-connectors and router functions, as well as

different digitally tunable diode laser cavities make them a top research subject.

The highest complexities in optical integration so far have been reported in AWG-

based PICs. From the three fabrication methods presented the one based on Indium

Phosphide has an important growing potential and a more complex spectrum of

functions in terms of active components design. It will lead to a dramatic reduction of

the entry costs for companies that are interested in applying this method to integrate

components. The big challenge that needs to be overcome is represented by thescaling possibilities and in the future, it is expected to be very competitive at small

and medium production levels.

Due to the nature of the production infrastructure the start-up costs are lower

compared to advanced silicon photonics processes, while offering significantly more

functionality. In the long term it is expected that InP-Photonics and CMOS electronics

will merge in a heterogeneous integration technology, where CMOS will provide the

electronic functionality and InP the photonic functionality.

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Arrayed Waveguide Gratings