Three-guide Coupled Rectangular Ring Lasers with Total Internal Reflection Mirrors

Three-guide Coupled Rectangular Ring Lasers with Total Internal Reflection Mirrors Doo Gun Kim *1, Woon Kyung Choi 1, In-Il Jung 1, Geum-Yoon Oh 1, Young Wan Choi 1, Jong Chang Yi 2, and Nadir Dagli 3 1 School of Electronic and Electrical Engineering, Chung-Ang University, 221 Heuksuk-dong, Dongjak-gu, Seoul 156-756, Korea Tel : +82-2-820-5326, Fax : +82-2-822-5326, e-mail : emblemdo@gmail.com 2 School of Electronics and Electrical Engineering, Hong-Ik University, 72-1 Sangsu-dong, Mapo-gu, Seoul 121-791, Korea 3 Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93160, U.S.A. ABSTRACT The lasing characteristics of three-guide coupled ring lasers using the self-aligned total internal reflection (TIR) mirrors were investigated numerically and experimentally. The rectangular laser cavity consists of four low loss TIR mirrors and an output coupler made out of passive three coupled waveguides. Two different lasers having active section lengths of 250 and 350 µm and total cavity lengths of 580 and 780 µm are fabricated. For both devices lasing thresholds of 38 ma is obtained at room temperature and under CW operation. Lasing is predominantly single mode with the side mode suppression ratio better than 20 db. Keywords : rectangular ring laser, total internal reflection mirror, three waveguide coupler, micro ring resonator 1. INTRODUCTION In recent years, significant progress has been made in the development and improvement of cleaved-free ring lasers for photonic integrated circuit (PIC) applications. If unidirectional oscillation can be achieved spatial hole burning effects seen in Fabry-Perot and DFB lasers can be avoided. Furthermore ring cavity offers advantages for Novel In-Plane Semiconductor Lasers VII, edited by Alexey A. Belyanin, Peter M. Smowton, Proc. of SPIE Vol. 6909, 69090D, (2008) 0277-786X/08/$18 doi: 10.1117/12.762030 2008 SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol. 6909 69090D-1

mode locked operation. Because of these attractive features ring lasers have been topics for many different studies. Ring lasers with circular [1], triangular [2], square [3], and disk [4] geometries have been reported. There are two main issues regarding the realization of ring laser for proper operation. The first is the formation of a compact cavity. To form a ring cavity, the optical waveguide has to form a complete loop. This is typically achieved by deep etching so that a ring of a small radius can be fabricated without excessive radiation loss. Deep etching provides strong lateral confinement but sidewall roughness increases the propagation loss. Etching through the active material also creates problems due to excessive surface recombination if the cavity contains an active material. Furthermore, such resonators cannot be made compact by reducing the resonator diameter indefinitely owing to increasing radiation loss. This problem can be solved if a cavity based on total internal reflection (TIR) mirrors is formed. Very compact cavities can be formed by combining TIR mirrors with regular waveguides. The second issue is the efficient coupling between the ring laser cavity and output waveguide. Previous output coupling schemes included laterally coupled Y junctions [1], multimode interference couplers [5], branching grooves [3], and vertical directional couplers [4]. Most of these demonstrations used the same active material for the laser cavity, output coupler, and output waveguide. For such designs the properties of the output coupler also changes as the current applied to the device changes. This difficulty can be eliminated to a large degree by making the output coupler entirely passive, which requires activepassive integration. In this paper, we investigate the properties of rectangular ring lasers having a folded cavity using four TIR mirrors. Active and passive materials are integrated within the folded cavity and output coupler is made out of an entirely passive three-guide coupler [6]. Coupling is done laterally. As a result such lasers can be the key components for high density PIC. 2. DEVICE SIMULATION AND RESULTS Figure 1 shows the top schematic of the three-guide coupled ring resonator. It is formed using four TIR turning mirrors that fold regular straight ridge waveguides into a rectangular cavity. The ridge waveguide on one side of the rectangular cavity contains active material and forms the semiconductor optical amplifier (SOA). The rest of the device is made out of passive material. The whole ring cavity performance was analyzed by using a transmission matrix method incorporating the device parameters extracted from the finite-difference time domain (FDTD) analysis [7]. The 2D FDTD was used to analyze the propagation constant and propagation loss for each subsection combined with effective index method. a i s and b i s indicate the forward and backward scattering parameters at each sub-section in Fig. 1. The transfer matrix in the ring cavity is described by the following equation: a5 a6 a6 = Τ12Τ11Τ10Τ9Τ8Τ7Τ6 = Τ Ring, (1) b5 b6 b6 where a 5 and b 5 are the scattering parameters at the ending side of the ring cavity and a 6 and b 6 those at the starting side. Proc. of SPIE Vol. 6909 69090D-2

The matrices T i are the transfer matrix connecting the adjacent scattering parameters. The formulation of the transfer matrix is straight forward given that the propagation constant and the propagation loss are known. They can be incorporated in the overall phase change, φ i. b 10 b 9 a 10 SOA a 9 b 11 a 11 a 8 b 8 TIR mirror b 12 a 12 a 7 b 7 a 5 a 6 b 5 b 6 a 1 a 2 a 3 a 4 b 1 b 2 Three-guide coupler b 3 b 4 Fig. 1. Top schematic of the ring resonator using four TIR mirrors, an active region, and a three-guide coupler. Second, the coupling matrix form in the directional coupler region can be represented by 2 iφ1 iφ1 1 K e 0 ike 0 a3 a2 a2 2 i 1 i 2 b3 b φ φ 2 0 1 K e 0 ike C11 C12 b2 =, (2) a i 2 2 i 2 6 a φ φ C 5 ike 0 1 K e 0 a 21 C22 5 b 6 b 5 iφ1 2 iφ b 2 5 0 ike 0 1 K e where the coupling constant, K, is defined as K κ 2 2 = sin 2 2 ( κ + δ l ) κ + δ with the coupling coefficient κ, the mode dispersion δ, and the coupling length l. Then, the relationships in the coupler region can be expressed as a3 a 1 2 a 2 = Ring ( Ring ) b C + C Τ I C Τ C 11 12 22 21 = R 3 b Τ ( 2 b. (3) 2 Finally, the overall transfer matrix through a microring resonator can be formulated as a4 a1 a1 = ΤŤ 3 RΤΤ 2 1 T. (4) b4 b1 b1 Figure 2 shows the simulation results for various optical gains in the active region with a SOA length of 250 µm. The total cavity length of 580 µm was used in the calculation. The coupling constant of the directional coupler was assumed to be K=0.7 with the optical loss of passive waveguides at 10 db/cm. The insertion loss in the TIR mirrors Proc. of SPIE Vol. 6909 69090D-3

was about 0.7 db [8]. The lower curves in Fig. 2 show the optical filter characteristics when the optical gain in the active region is smaller than 28 cm -1. When the optical gain in the active region is sufficiently high, the microring resonator behaves as an optical amplifier. The upper curves in Fig. 2 show the optical amplification behavior. Thus, current injection into the active region for optical gain above 28 cm -1 would result in lasing in the microring cavity structure. 8 Transmission (db) 6 4 2 0-2 -4 40 cm -1 32 cm -1 28 cm -1 36 cm -1-6 24 cm -1-8 20 cm -1 1.555 1.556 1.557 1.558 1.559 1.560 Wavelength (µm) Fig. 2. Calculated optical transmission spectrum of the microring cavity as a function of the optical gain. 3. DEVICE DESCRIPTION AND FABRICATION Figure 3 (a) shows the cross sectional profile of the active and passive waveguides. They are 3 µm wide ridge waveguides etched 1.8 µm deep. The core of the waveguides is 0.35 µm thick quaternary material with photoluminescence peak of 1.4 µm. It is grown on 1.8 µm thick n InP on an n+ InP substrate. On top of the core there is 1.8 µm thick p InP capped by 0.1 µm thick p+ InGaAs contact layer. Active waveguide contains 0.1 µm thick multi quantum well (MQW) region on top of the core. MQW has 7 wells and 8 barriers [9]. The well and barrier widths are 75 and 80 Å, respectively. Since TIR mirrors are used in forming the cavity there is no need to use deeply etched curved waveguides to reduce the radiation loss. Hence there is no excess scattering loss and no need to etch through the active material. Active waveguide is the same as a standard ridge waveguide laser structure. Furthermore the area of the laser can be minimized by making the sides of the rectangle short. This helps to increase the integration density significantly. However this requires low loss mirrors to keep the round trip loss low, which can be achieved by proper design and processing. This typically requires a self aligned process and a vertical and smooth mirror etch. In our previous work we demonstrated such mirrors with excess loss of about 0.7 db per mirror [8]. Proc. of SPIE Vol. 6909 69090D-4

These mirrors were also used in compact ring resonator band stop filters [10]. Another design criterion is to have deterministic coupling that does not change depending on operating conditions. Since output section is made out of passive material a simple directional coupler could be sufficient. But in practice this is hard to realize due to the presence of TIR mirrors. If the waveguide inside the resonator and output waveguide are placed in closed proximity to allow coupling, deeply etched edge of the mirror will perturb the output waveguide generating undesired reflections. This can be reduced by increasing the gap of the coupler but coupling reduces significantly. We found a compromise solution using three coupled waveguides as shown in Fig. 1. This approach separates the resonator and output waveguides significantly hence the mirror edge is not a problem. However, coupling between three waveguides is still less than the coupling in a directional coupler of the same waveguide width, gap, and length. But this is not a major problem since a small amount of output coupling is desirable to keep the round trip cavity loss low. In our design we kept the gap between the waveguides 1.5 µm due to practical considerations. The only price paid is the loss of the power remaining in the middle waveguide. Although this is a small loss, reflections from the ends of the middle waveguide can interfere with the operation of the laser. We tapered the ends of the middle waveguide to reduce back reflections and help the radiation of the light at the end of this waveguide into substrate. Simulations indicate that loss of this arrangement is less than the loss obtained using two mirrors to bend the output waveguide in and out of the coupler. The other possibility is to bend the output waveguide using S shaped bends. But low loss requires rather long bends defying the advantage of compactness due to TIR mirrors. The details of the fabrication are reported in [8] and [10]. The scanning electron microscope picture of a corner of the device showing three guide coupler and two self aligned mirrors is shown in Fig. 3 (b). The sidewalls of etched mirrors show excellent uniformity and smoothness. 1.8 µm Active waveguide 3 µm 0.1 µm p + InGaAs 1.8 µm p InP 1.8 µm Passive waveguide 3 µm 0.1 µm p + InGaAs 1.8 µm p InP 500 Å nid InP 500 Å p InP 500 Å nid InP 250 Å Q(1.22 µm) 1015 Å QW 100 Å nid InP 3500 Å Q(1.4 µm) 500 Å nid InP 100 Å nid InP 3500 Å Q(1.4 µm) 1.8 µm n InP 1.8 µm n InP n + InP Substrate n + InP Substrate (a) (b) Fig. 3. (a) Cross sectional profiles of the active and the passive waveguides used in the experiments, (b) Scanning electron microscope picture of the three-guide coupled microring resonator. Proc. of SPIE Vol. 6909 69090D-5

4. EXPERIMENTAL RESULTS AND DISCUSSION Figure 4 shows the current versus light output of a ring laser diode under CW operation for a total cavity length of 580 µm and for a SOA length of 250 µm. The lasing threshold current at 20 C is around 38 ma. The output spectrums at different bias currents are also shown in the inset of Fig. 4. The adjacent peaks are due to other resonator modes and their separation is about 1.12 nm. This agrees well with the free spectral range of the resonator calculated using λ = λ 2 /(nl) with λ = 1.55 µm, L = 580 µm and n = 3.7. The side mode suppression ratio (SMSR) is 21 db at 1.3 times threshold. Output Power ( µw) 3.0 2.5 2.0 1.5 1.0 0.5 Intensity (dbm) -20-30 -40-50 -70-30 -40-50 -70-54 -57 55 ma 45 ma 35 ma -63-66 1540 1550 1560 1570 1580 1590 Wavelength (nm) 0.0 0 10 20 30 40 50 60 70 80 SOA Current (ma) Fig. 4. Light output versus current characteristics of a ring laser diode with a total cavity length of 580 µm and SOA length of 250 µm. The inset shows the output spectrum at different bias currents. Figure 5 shows the CW current versus light output for another ring laser diode for a total cavity length of 780 µm and an active waveguide length of 350 µm. In this case the lasing threshold current at 20 C is again around 38 ma. The output spectrums at different bias currents are also shown in the inset of this figure. The separation between adjacent peaks is about 0.847 nm which agrees well with the FSR of the resonator calculated using the above formula and values except for L=780 µm. The SMSR is 23 db at 1.8 times threshold. There is a slight change in the threshold current when the active waveguide length and total cavity length are increased. Increased active waveguide length reduces the effective loss coefficient of the TIR mirrors and increases the optical gain. But Proc. of SPIE Vol. 6909 69090D-6

increased cavity length increases the loss of the passive waveguide sections and makes the coupler longer. Increased coupling length decreases the transmission through the three-guide coupler and round trip cavity loss increases. Output Power (µw) 10 8 6 4 2 Intensity (dbm) -20-30 -40-50 -70-30 -40-50 -70-58 -62-64 50 ma 45 ma 35 ma -66 1540 1550 1560 1570 1580 1590 Wavelength (nm) 0 0 10 20 30 40 50 60 70 SOA Current (ma) Fig. 5. Light output versus current characteristics of a ring laser diode with a total cavity length of 780 µm and SOA length of 350 µm. The inset shows the output spectrum at different bias currents. 5. CONCLUSIONS We have fabricated and characterized novel rectangular ring lasers containing active and passive sections. The output coupler was made out of passive three coupled waveguides and rectangular cavity was formed using four low loss TIR mirrors. Two types of lasers with active section lengths of 250 and 350 µm were fabricated. The total cavity lengths of these lasers were 580 and 780 µm respectively. Lasing thresholds for both types were 38 ma at room temperature and under CW operation and lasing spectra were predominantly single mode with SMSR better than 20 db. The power loss of a single TIR mirror was found to be about 0.5 db. Such low loss TIR mirrors enabled lasers with very small footprint. As such it enables a platform for compact PIC. 6. ACKNOWLEDGEMENT This work was partially supported by Seoul R&BD program and ERC OPERA (R11-2003-022). Proc. of SPIE Vol. 6909 69090D-7

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