A TUNABLE LASER USING LOOP-BACK EXTERNAL CAVITY BASED ON DOUBLE RING RESONATORS

M. Ren1,2, H. Cai2, J. F. Tao1,2, J. M. Tsai2, P. Kropelnicki 2, A. B. Randles2, M. Tang2,

D. L. Kwong2 and A. Q. Liu1,2† 1School of Electrical & Electronic Engineering, Nanyang Technological University,

SINGAPORE 639798 2Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research),

11 Science Park Road, SINGAPORE 117685

ABSTRACT

The paper presents a monolithically integrated tunable laser with loop-back external cavity tunable based on double ring resonators. It consists of a gain chip and a double ring resonator structure in a loop-back external cavity, made of silicon photonic waveguides. In the experiment, it measures 40-nm wavelength tuning, > 40 dB side mode suppression ratio (SMSR) and the average output power is approximately -3 dBm. KEYWORDS

Tunable laser, nano photonics, monolithically integrated, thermal-optics. INTRODUCTION

Wavelength tunable laser has been widely used in wavelength division multiplexing (WDM) optical communication system, supplying optical signal source. Compared with conventional distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers, tunable lasers reduces the amount of single wavelength lasers, thus makes network system more flexible and lowers the cost.

DFB arrays and sampled-grating DBR lasers can provide the function of wavelength tuning, which are compact in fabrication to integrate multiple function elements on one chip [1-2]. The wavelength tuning function can be also realized by various external cavity tunable lasers [3-7]. Although the hybrid integrated tunable lasers, especially microelectromechanical system (MEMS) external cavity tunable lasers [3-6] have advantages on large tuning range and low cost, they suffers mode hopping, frequency chirp, low SMSR, and the stability is limited due to movable part. External cavity tunable lasers with optical circuit and broadband light source provide better stability and can be easily integrated with photonics devices. Micro-size ring resonators are believed to be ideal channel-dropping filters for WDM system, which can be integrated with other photonics circuits [8]. Ring resonator based external cavity semiconductor lasers has been successfully fabricated on one chip using III-V materials [9-10]. On the other side, the high optical transmission loss in III-V waveguides and high cost wafer limit the application for photonic integrated system. With the development of silicon photonics technology [11-12], silicon based wafer has also been applied for realizing the function of external cavity [13-14]. However, it is difficult to integrate this passive external cavity with active gain laser onto a single chip. Besides, complex high-reflection

film coating is usually needed in the external cavity to act as mirrors and provide optical feedback.

This paper presents a tunable laser with loop-back external cavity tunable based on double ring resonators. MEMS technology is utilized for monolithically integration. It consists of a gain chip as the internal cavity and a double ring resonator structure in the external cavity. The external photonic circuit is made of silicon photonic waveguides. It advances in high SMSR, narrow linewidth and high stability, which results in potential applications such as wavelength division multiplexing (WDM) networks, signal generation and light source integrated in high density photonic circuits. DESIGN AND THEORY

Figure 1 illustrates the schematic of the external cavity tunable laser.

Figure 1: (a) Schematic illustration of the loop-back external cavity tunable laser based on ring resonators. (b) equivalent scheme of the external cavity laser.

B

A Heater

splitter Gain Chip

SiO2

(a)

(b)

Heater

External Cavity

T3P.129

978-1-4673-5983-2/13/$31.00 ©2013 IEEE 1424 Transducers 2013, Barcelona, SPAIN, 16-20 June 2013

A gain chip, which functions as the internal cavity, is bonded to the silicon wafer. It provides wide-spectrum light source, with its right facet anti-reflected. The external cavity consists of a taper waveguide, an optical splitter (50:50), the two rings with slightly different radii, and a U-shape waveguide connecting these two rings. The taper waveguide acts as a mode size convertor, which matches the mode-size of the light emitted from the Si waveguide with that from the gain chip. The optical splitter, the U shape waveguide and the two ring resonators form a loop-back optical circuit. The loop-back silicon photonic circuit not only selects the single lasting wavelength, but also acts as an optical reflector. The light from the gain chip can be returned by the loop-back circuit when the resonance conditions of both rings are satisfied. No high-reflectivity coating films or reflection mirrors are needed. The ring resonators are used as drop filters and single mode wavelength is selected by Vernier effect. Both the straight waveguide and the ring resonator are channel waveguides with 450-nm width and 220-nm height. A cladding layer of 2-μm SiO2 is deposited on top of the silicon circuit.

Two heaters are fabricated on top of the two ring resonators, respectively. The tuning of the lasing wavelength is realized by thermal-optics method [15-16]. When applying current to the heaters on top of the ring waveguide, the ring waveguide is heated and the effective refractive index is changed, thus the lasing wavelength is tuned. The tuning range is decided by

AA

A B

RFSR

R RλΔ = ⋅

−, (1)

where RA and RB are the radii of the two resonators, respectively. FSRA is the mode spacing of ring resonator A. The equivalent scheme of the loop-back external cavity laser is shown in Fig. 1(b).

0 2 4 6 80

10

20

30

40

50

60

Δg=0.5 dB Δg=1.5 dB Δg=3.0 dB Linewidth

External Cavity Length Lout (mm)

SM

SR

(dB

)

101

102

103

104

Lin

ewid

th (k

Hz)

Figure 2: Numerical analysis of SMSR and linewidth of the external cavity tunable laser as a function of external cavity length and the gain difference.

The SMSR and the linewidth of the external cavity tunable laser are numerically analyzed as shown in Fig. 2. The high wavelength selection ability of the ring resonators makes it possible to achieve both high SMSR and narrow linewidth within 10-mm cavity length. Δg is defined as the

gain difference between the main mode and the side mode. High gain difference is preferred to obtain good performance of SMSR. Long cavity length is preferred for achieving both high SMSR and narrow linewidth. It expects the performance of > 40 dB SMSR and < 100 kHz linewidth under the conditions of 4-mm external cavity length and 1.5-dB gain difference.

The tunable laser is fabricated on the silicon on insulator (SOI) wafer. Both nano-photonics technology and MEMS technology is utilized for the fabrication of the external cavity tunable laser. Fig. 3(a) shows the top-view SEM image of the gain chip bonded on to the silicon chip. Au/Sn solder layer is used as both bonding and conducting material. A high power gain chip is flip-bonded, with the lasing facet on the bottom, in order to optically couple with the silicon taper waveguide. The gain chip and the silicon photonics chip are well alignment to eliminate misalignment. Fig. 3(b) shows the 50:50 photonics waveguide splitter, which is a key element for the realization of the loop-back optical circuit. Fig. 3(c) shows one of the ring resonators in the optical circuit. The light is coupled into and out from the ring resonator via two straight waveguide. It is connected with another ring resonator with a U-shape waveguide.

Figure 3 SEM images of (a) bonded gain laser chip on the silicon chip; (b) optical splitter; (c) one of two ring resonators. RESULTS AND DISCUSSIONS

Figure 4 shows the reflection spectrum of the external cavity. The channel spacing between two neighboring resonant modes is 1.6 nm (200 GHz), which is determined by the effective refractive index and the radius of the ring

Gain chip

Taper waveguide

AR facet

Solder (a)

(b) (c)

Optical splitter Ring resonator

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waveguide. It expects a 10-dB mode power difference between the main mode and the adjacent side mode according to the transform matrix method. In experiment, the mode power difference is 1.8 dB measured from the transmission spectrum. The mismatch comes from the limited resolution of optical spectrum analyzer and the unexpected optical transmission loss in the ring waveguides. Nevertheless, as indicated in Fig. 2, 1.8-dB mode power difference guarantees 40 dB SMSR when the external cavity length is above 4 mm.

1544 1546 1548 1550 1552-80

-60

-40

-20

0

1.6 nm Simulation Experiments

Tran

smis

sion

Pow

er (d

Bm)

Wavelength (nm)

1.8 dB

Figure 4: Reflective spectrum of the wavelength selection function due to the double ring resonators.

1530 1540 1550 1560 1570-60

-50

-40

-30

-20

-10

0

10

Out

put P

ower

(dB

m)

Wavelength (nm)

Figure 5: Experimental results of different lasing wavelengths with in C-band when heater A works.

In experiments, the gain chip works under a constant

pumping current of 200 mA. The effective index of one ring resonator is changed by thermal-optics method. The heating current is increased from 0 to 82 mA, while the voltage is increased from 0 to 0.9 V. A serious of single mode wavelength is acquired as Fig. 5 shows. The SMSR varies within the range of 42-45 dB, measured directly from the optical spectrum analyzer. The output power is approximately -3 dBm with 0.4-dB variation. The variation of SMSR and the output power comes from the gain difference within the gain band. The output wavelength is discontinuous with a space of 1.6 nm.

Figure 6 shows the flexibility of wavelength tuning when both heaters A and B works simultaneously. When

heater B is not activated, and tuning the voltage applied on heater A from 0 to 0.9 V, the output wavelength is tuned from 1529.89 nm to 1566.0 nm, with a channel spacing of 1.6 nm. When applying a constant voltage of 0.3 V to heater B, and tuning the voltage applied on heater A from 0 to 0.9 V, the output wavelength is tuned from 1526.6 nm to 1564.0 nm. The wavelength tuning process is nonlinear with the increasing of voltage. Changing the constant voltage applied on heater B, the achievable wavelength positions can cover the entire gain band of the gain chip, which is above 40 nm. This result indicates that qusi-continuous wavelength tuning is able to be achieved by the cooperation of both heaters.

0.0 0.3 0.6 0.91525

1535

1545

1555

1565

1575

Out

put W

avel

engt

h (n

m)

Voltage (V)

VB=0 V VB=0.3 V

Figure 6: Experimental results of different lasing wavelengths with in C-band when heater A works. CONCLUSTIONS

In summary, a monolithically integrated tunable laser with loop-back external cavity is demonstrated. The output wavelength has demonstrated a tuning range of 40 nm with the output power stably maintained at -3 dBm, SMSR of > 40 dB and linewith of < 100 kHz. The demonstrated tunable laser can be potentially used in WDM systems, signal generation and high density photonic circuits. ACKNOWLEDGEMENTS

The authors would like to acknowledge the support from the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research) under SERC Grant 1021650084. REFERENCES [1] B. Mason, G. A. Fish, S. P. Denbaars and L. A.

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CONTACT

†A. Q. Liu, +65-67904336; eaqliu@ntu.edu.sg.

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