A TUNABLE LASER USING DOUBLE-RING RESONATOR EXTERNAL CAVITY VIA FREE-CARRIER DISPERSION EFFECT

M. Ren1,2, H. Cai2, J. M. Tsai2, W. M. Zhu1, D. L. Kwong2 and A. Q. Liu1†

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 Science Park II, Singapore 117685

ABSTRACT A tunable laser based on double-ring external

resonant cavity is designed, fabricated and tested. The double-ring resonator external cavity consists of a silicon waveguide ring resonator, a p-i-n doped silicon ring resonator, and a superluminescent diode (SLED). The laser is fabricated on a SOI wafer and the wavelength is tuned by injecting electrical currents to p-i-n structures. In the experiment, it measures 45.8 nm wavelength tuning with 110 GHz channel spacing and the average output power is approximately -8 dBm. It advances in high tuning speed, large side mode suppression ratio, and low manufacture cost, such has potential applications in high speed WDM networks. KEYWORDS

Tunable laser, external cavity, double-ring resonator, MEMS. INTRODUCTION

Tunable lasers are employed to reduce inventory of spare lasers with specific wavelengths for cost-effective wavelength division multiplexing (WDM) systems or wavelength-routed systems, and especially for WDM passive optical network systems [1-2]. However, the cost is typically high and its manufacturing process is complex. Micro-electromechanical system (MEMS) technology has been used for external-cavity tunable lasers, which has merits such as low packaging cost and high tuning resolution [3-7]. However, some of the MEMS external cavity structure suffers from mode-hopping and limited wavelength tuning range. In order to increase the tuning range, external cavity tunable laser based on coupled-ring resonator have been fabricated with silica waveguide or polymer structure [8-10]. However, the tuning speed is limited for the method of thermal-optical tuning on polymer waveguide. Recently, modulation speed up to 5 GHz was achieved on a silicon ring resonator via free-carrier dispersion effects [11-12], which provides a potential method for improving the tuning speed of tunable lasers.

This paper presents a tunable laser based on double-ring external resonant cavity in which the output wavelength can be tuned by injecting different electric currents. The theoretical analysis of double-ring resonator is performed to guide the design of the tunable laser. The fabrication processes is developed and experiments is performed to investigate the tuning of the output lasing

wavelength and the output power by varying the current density.

DESIGN AND THEORETICAL ANALYSIS Figure 1(a) illustrates the general schematic of the external cavity tunable laser. It consists of an internal cavity such as the one in the laser diode, and an external cavity with a reflection mirror. The wavelength, which satisfies the resonant conditions of both internal cavity and external cavity, is selected and lased. Fig. 1(b) shows the schematic illustration of the double-ring external cavity tunable laser. The internal cavity is formed by a superluminescent diode (SLED) integrated in the planar lightwave circuit functions. The external cavity consists of the double rings with slightly different radii. The light emitted from the SLED to the double-ring resonator can be reflected back by the high-reflection (HR) coated facet when the resonant conditions of both rings are satisfied. In this way, the output light of the tunable laser is selected and can be tuned by changing the resonant conditions of the rings. Based on the principle of external cavity tunable laser, a double-ring resonator external cavity laser which is tuned by electro-optic effect is designed. Fig. 2(a) shows the structure of the tunable laser realized on the silicon-on-insulator (SOI) wafer. It consists of a silicon waveguide ring resonator (Ring 1), a p-i-n doped silicon

(b)

(a)

Figure 1: Schematic illustration of (a) the external cavity tunable laser; and (b) the double ring external cavity tunable laser.

Output

Output

Mirror

Laser diode External cavity

Internal cavity

SLED

External cavity

External cavity

HR coating

Ring 1

Ring 2

T3P.126

978-1-4577-0156-6/11/$26.00 ©2011 IEEE Transducers’11, Beijing, China, June 5-9, 20111504

ring resonator (Ring 2), and SLED. The radii of Ring 1 and Ring 2 are 160 μm and 157 μm, respectively. The wavelength tuning of the tunable laser is controlled via Ring 2 by injecting electric current in the p-i-n junction. Fig. 2(b) shows the cross-section of this p-i-n junction, which is formed on the rib waveguide of Ring 2 with doping depth of 50 nm. A metal layer is fabricated over P+ and N+ region to control the effective refractive index and change the resonant wavelength.

The wavelength selecting principle of the external cavity is shown in Fig. 3. Due to the slight radii difference between Ring 1 and Ring 2, only one wavelength (λ0), which satisfied the resonant conditions, is pre-selected within the SLED gain region at the initial oprtaing condition of the tunable laser. The effective refractive index of Ring 2 can be tuned through the free-carrier dispersion effect [12] by increasing the injected current density, which changes the resonant condition of Ring 2. Subsequently, the output wavelength is shifted from λ0 to λ1 when the two rings’ transmission peak matches at λ1. The free space range (FSR) of the tunable laser is

determined as

RnFSR

effπλ

2

2

= (1)

where λ is the free-space wavelength, effn is the effective

refractive index of the ring cavity, and R is the radius. The tuning range λΔ is determined as

1 2

2 1

FSR FSRFSR FSR

λ ⋅Δ =−

(2)

where FSR1 and FSR2 are the free space range of Ring 1 and Ring 2, respectively [9].

In this design, FSR1 is kept constant, while FSR2 is tuned by changing the effective refractive index (neff), which can be controlled by the injection current density to the p-i-n junction. Fig. 4(a) shows that neff as the function of the current density. Based on the free-carrier dispersion effect on silicon [13], the change of effn can be determined by

22 18 0.88.8 10 8.5 10effn N Pδ − −= − × Δ − × Δ (3) where NΔ and PΔ are the free electrons and the free holes concentration variations (cm-3), respectively. As the injected current density is increased from 0 to 2.5×105 A/cm2, the effective index of silicon has a change of 10-3 RIU. The output wavelength is tuned by the change of effective index,

eff

eff

nn

δδλλ

= (4)

where δλ is the shift of output wavelength induced by the

shift of effnδ . Fig. 4(b) shows the tuning of the output wavelength by changing the effective index of Ring 2. The output wavelength can be tuned from 1540.0 to 1608.4 nm when effective index is increased from 0 to 3×10-3 A/cm2, with a frequency spacing of 110 GHz (around 0.88 nm).

Figure 3: Wavelength selecting principle of the double-ring external cavity.

(a) (b)

Figure 4: Numerical analysis of (a) the effective index of ring 2 as the function of the current density of the p-i-n junction; and (b) the output wavelength as the function of the effective index of Ring 2.

(a)

Figure 2: Structural illustrations of (a) the double ring external cavity tunable laser on chip; and (b) the cross-sectional view of p-i-n junction.

(b)

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FABRICATION AND EXPERIMENTS Based on the simulation results, the double-ring

external cavity was fabricated on the SOI substrate, and its SEM images are shown in Fig. 5. Fig. 5(a) shows the top-view SEM image of the whole cavity. Both the straight waveguide and the ring resonator are channel waveguides with 450-nm width and 220-nm height. Fig. 5(b) shows the zoom view of the coupling region. The gap between ring cavity and the straight waveguide is 200 nm. The gap value is finely selected by considering both the coupling efficiency and the coupling loss. Fig. 5(c) shows the microscopic photo of Ring 2. The silicon doped with P+ or N+ region are shown in different color under the microscope.

Figure 6 shows the single wavelength lasing output of the tunable laser. A single wavelength at 1581.7 nm is achieved when the current density applied to the p-i-n structure is 1.6×105 A/cm2. The side mode suppression ratio (SMSR) is 22 dB and the full width half maximum of the laser linewidth is approximately 0.09 nm. The output power at 1581.7 nm is -8.1 dBm.

In order to measure the tuning range of the double-ring resonator external cavity, the pumping current of the SLED is fixed at 200 mA, while the injection current is increased from 0 mA. Fig. 7 shows the experimental results of the output wavelength and the power under different injection current densities. When the current density increases from 0 to 1.6 × 105 A/cm2, the output wavelength is tuned from 1535.89 nm to 1581.7 nm, and a tuning range of 45.8 nm is achieved, with a channel spacing of 110 GHz. The tuning resolution of the developed tunable laser is measured as 2.875 × 10-4 nm/A cm-2. The output power is rather maintained approximately at -8 dBm. CONCLUSIONS In summary, a double-ring resonator external cavity tunable laser is demonstrated. The SLED is acted as the internal cavity and the double-ring resonator is acted as the external cavity. One of the ring consists of p-i-n junction, which is used to tuned the effective refractive index of the ring by applying electric current. Subsequently, the resonant wavelength of the tunable laser is changed. The tunable laser has demonstrated a tuning range of 45.8 nm with the output power stably maintained at -8 dBm, and the channel spacing is 110 GHz. The demonstrated tunable laser can be potentially used in WDM systems, signal generation and high density photonic circuits.

Figure 7: Experimental results of output wavelength and power by increasing the injection current density of p-i-n junction from 0 to1.6×105 A/cm2.

Figure 6: Experimental results of single wavelength output at 1581.7nm.

Figure 5: (a) Top-view SEM image of the double-ring external cavity; (b) Zoom view of the coupling region; and (c) Microscopic image of Ring 2.

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CONTACT A. Q. Liu† Email: eaqliu@ntu.edu.sg Tel: (65) 6790-4336 Fax: (65) 6793-3318 URL: http://nocweba.ntu.edu.sg/laq_mems/

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