Integrated MM! optical couplers and optical switches in Silicon-on-insulator technology Jinzhong Yu, Hongzhen Wei, Xiaofeng Zhang, Qinfeng Yan, and Jinsong Xia State Key Laboratory on Integrated optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, PRC. Email: jzyured.semi.ac.cn ABSTRACT Integrated multimode interference coupler based on silicon-on-insulator has been become a kind of more and more attractive device in optical systems. Thin cladding layers (<1.Opin) can be used in SO! waveguide due to the large index step between Si and SiO2, making them compatible with the VLSI technology. Here we demonstrate the design and fabrication of multimode interference (MMI) optical couplers and optical switches in 501 technology. Keywords: 501, Multimode Interference, Optical coupler, optical switch, Integrated Optics 1. INTRODUCTION Optical couplers and switches are the key devices in the integrated optics. In recent years, multimode interference (MMI) couplers, based on the self-imaging effect, are rapidly gaining popularity because of the advantage of such as low-loss, compact size and large fabrication tolerance21. Couplers based on MMI structure have been realized in various materials such as InP, SiO2, and GaAs and excellent performance has been reported35. Silicon-on-insulator (SO!) technology has shown to be a promising technology for guided wave photonic devices operating in the infrared (2>1.2im). In this paper, we present the design and fabrication of integrated MM! optical couplers and switches in SO! technology. 2. DESIGN AND FABRICATION OF STRAIGHT MMI COUPLER IN SOI An MMI coupler is based on the self-imaging property of a multimode waveguide:an input field profile is reproduced in single or multiple images at periodic intervals along the propagation direction of guide. According to the different position of input waveguide, and corresponding, different mechanisms, there are three kinds of MM! structures: general Multimode Interference (GMMI), Paired Multimode Interference (PMMI) and Symmetric multimode interference (SMMI)61. Tab. 1 Comparison of three MMI structures Structure of MM! GMMI PMMI SMMI Position of input waveguide We/(N +1) We/3 and 2We!3 At the center of the multimode waveguide Length of multimode waveguide 3L, /N LJJN 3L, /(4N) 4flrW Lg (1) Optical Switching and Optical Interconnection, Lih-Yuan Lin, Shulian Zhang, Editors, Proceedings of SPIE Vol. 4582, APOC 2001, Beijing, China (2001) 2001 SPIE 0277-786X/01/$15.00 57
rxe 1ttT = (a) iitjjt TJ 1 oir (b) Fig. I Schematic structure of2x2 and 1x4 straight MMI coupler iitpr,uid Fig. 2 The output near-field images ofthe 1X4 (a) and 2X2 (b) couplers Schematic diagram of 1x4 SMMI and 2x2 PMMI couplers are shown in Fig. 1, where N is 4 and 2 respectively. MMI coupler consists of a (two) single mode input waveguide(s), a planar straight multimode waveguide and four (two) single-mode output waveguides. Tapered waveguides between the single accesses and the multimode waveguide were used to reduce the junction loss. In our design, the width of the straight multimode waveguide is 40 jim. Thickness of the core layer is lojim and the etching depth of the rib SOl waveguide is 4jim. Correspondingly, the length ofthe 1x4 SMMI and 2x2 PMMI couplers is 1356.8pm and 36l8pm, respectively. The conventional Si process technology is used to fabricate the MMI splitter. The light from the fiber at wavelength X=l.3im was coupled into the rib waveguides through the cleaved facet of an input guide. The light emerging from the output waveguides was projected onto the light sensitive area of an infrared CCD using a microscope objective 20x. Then the image was displayed on the TV monitor with a total 200 magnification of that at the cleaved output facet. The images on TV were taken by a digital camera. Output near-field of 1x4 and 2x2 couplers were shown in Fig.2 (a) and (b) respectively. The result shows that the devices we fabricated realize the function of 1x4 splitter and 3dB coupler. 3. DESIGN AND FABRICATION OF TAPERED MMI COUPLER IN SO! For the straight MMI couplers, the length is in proportion of the square of the width of the multimode waveguide. Generally, the width of the multimode waveguide in straight MMI is determined by the number of ports and the space between different ports. Increasing the number of ports by a factor of two requires a corresponding double of the MMI region width and a quadrupling of the device length. In order to reduce the length of the multimode waveguide, the tapered MMI was proposed'71'81. The structure of tapered 2x2 (a) and 4x4 (b) MMI couplers are illustrated in Fig.3, where w0 is the width of the MMI section at z=o and w1 is the width at ZLmm/2. The width is parabolically tapered according to: 58 Proc. SPIE Vol. 4582
w(z) = w1 + (w0 )(L11/ z)2 /(Ln?m/)2 (2) where z is the direction of propagation. The access waveguides are tilted to keep the phase tilt of input image W14 (a) 1' 41 Wn WI Fig. 3 The schematic structure of the 2X2 (a) and 4X4 (b) tapered MMI coupler (b) (a) (h (c) Fig. 4 The output near-field images of the tapered 2X2 (a) and 4X4 (b) and 1X2 (a)mmi couplers approximately along a coordinate system that is conformal with respect to the end walls of the tapered MMI region. The tilt angle, 6, from the propagation axis can be obtained by: 6 = tan'(4yp/lmmj) (3) where y is the transverse waveguide position from the center and = w1/ is the normalized width variation. The / W0 Proc. SPIE Vol. 4582 59
length of the tapered section can be obtained by comparing tapered MMI with straight MM!, = (4) Wg W I wtan I + Wj+Wg (5) 2(Wo+Wg)(Wi +Wg) 2Jw0 w1(w1 +Wg)A Wg n) (6) is the Goos-Hähnchen broaden width and A is the free space wavelength, n,, n, are the effective index of the MMI section and lateral confinement layer respectively. Corresponding to the SMMI, we proposed the untracompact 3-dB tapered MMI coupler, as shown in Fig.4, where w0 is the width of the MM! section at z=o and w1 is that at ZLm,ni. Thewidth is parabolically tapered according to w(z) = w0 +(w1 w0)z2 /Lnmi (7) where, z is the direction of the light propagation and Lmmj is the length of the tapered MMI section. The input waveguide is located at the center of the start wall of the tapered MM! section. Length of the tapered multimode waveguide is where L 8 (8) = 41lrWe (9) 3aA and w + wtan 1 lw wo 2(w0 + Wg)(Wi + Wg) 2Jw1 w0(w0 + Wg) I 1 w0 + wg (10) Fig. 5 Schematic structure of 2x2 MMI-MZI thermo-optical switch Table 2 shows the design parameters of the tapered 2x2, 4x4 and 1x2 MM! couplers. 60 Proc. SPIE Vol. 4582
Tab. 2 the design parameters of tapered MM! couplers Structure 2x2 4x4 1x2 wp 40 50 14.4 p 24 0.4 0.4 Lmmi 5767 4265 389 i] Thickness oftop silicon layer in SOl 10 10 5 Etching depth 4 4 2 Standard Si processing was used to fabricate the devices. For the 2x2 and 4x4 tapered MM!, the 501 wafer was obtained by Seperation-by-Implanted-Oxigen (SIMOX) technology. For the 1x2 tapered MM!, 501 wafer was obtained by bonding and back etching technology. The devices were then thinned and cleaved for the test purpose. Fig.5 shows the near-field output of the fabricated 2x2, 4x4 and 1x2 tapered couplers. Compared with the straight counterpart, the length ofthe MMI section was reduced greatly. Fig. 6 The time response of2x2 MMI-MZI thermo-optical switch 4. DESIGN AND FABRICATION OF 501 OPTICAL SWITCHES Mach-zehnder interferometer (MZI) is one of the basic structures of optical switch9. In this section, we describe the design and fabrication of MMI-MZI optical switch based on 501. Fig. 6 shows the structure of MMI-MZI thermooptical switch based on SO!. The MZI switch consists of two 2x2 paired MM! couplers, two phase shift arms and the input/output waveguides. For 501, in addition to the plasma dispersion effect, the high thermo-optical coefficient offers a good possibility for influencing the optical wave. A temperature of only 1K will be sufficient for a refractive index change of 2x104( A. =1.3/mi). And the thermo-optical coefficient at 1.55/im is = 9.48 x 1-5 + 3.47 x 1 T 1.49 x 1 0' T2 + 3T The light from the fiber at wavelength?=l.55um was coupled into the rib waveguides through the cleaved facet of an input guide directly. The light emerging from the output waveguides was also coupled into a fiber to measure the output light power. To measure the sivitching time, light from the output waveguide was coupled into a Ge detector and the signal was detected by a oscillograph. For the fabricated device, in cross-state, the crosstalk is 22dB, and the extinction ratio is 13. 1dB. And in the bar-state, the cross-talk is 12dB. The switching time of the switch is less than 2Ops, as shown in Fig.7. Proc. SPIE Vol. 4582 61
5. CONCLUSION In conclusion, we have designed and fabricated several kinds of MMI optical couplers and optical switches in silicon-on-insulator technology. The general straight MMI can server its function well. The proposed tapered MMI couplers can reduce its length greatly while keeping uniform output. Also we demonstrated the 2x2 MMI-MZI thermooptical switch and the switching time is less than 2Ops which is faster than the thermo-optical switch based on other materials such as polymer and silica. ACKNOWLEDGEMENT This work was supported by the National Science Foundation of China under grant No 69896260 and 69990540. REFERENCES [1] M. A. Fardad and M. Fallahi, "Sol-Gel multimode interference power splitters", IEEE photonics Technol. Lett., 1999, 11(6): 697-699 [2] Mohammad R. Paiann and Robert I. MacDonald, "Design of phased-array wavelength division multiplexers using multimode interference couplers", Appl. Opt.,l 997,36(21):5097-5108 [3] Pierre A. Besse, Maurus Bachmann, H. Melchior etal., "Optical bandwidth and Fabrication tolaerance of Multimode Interference Couplers", J. of Lightwave Technol., 1994, l2(6):1004-1009 [4] R. M. Jerkins, J. M. Heaton, D. R. Wight etal., "Novel lxn and NxN integrated optical switches using self-imaging multimode GaAs/AIGaAs waveguides", Appi. Phys. Lett. 1994, 64(6):684-686 [5] Q. Lai, M. Bachmann, W. Hunziker et al., "Arbitrary ratio power splitters using angled silica on silicon multimode interference couplers", Electronics Letters, 1996, 32(17):1576-1577 [6] Lucas. B. Soldano and Erik C. M. Pennings, "Optical Multi-mode Interference Based on self-imaging:principle and application"s,.1. Of Lightwave Technol, 1995 l3(4):615-627 [7] David S. Levy, Robert Scarmozzino, York M. Li, and Richard M. Osgood, Jr., "A new design for ultracompact multimode interference-based 2x2 couplers," IEEE Photon. Technol. Lett., Vol. 10, pp. 96--98,1998 [8] David S. Levy, Robert Scarmozzino, and Richard M. Osgood, Jr., "Length reduction of tapered NxN MMI devices," IEEE Photon. Technol. Lett., Vol. 10, pp. 830--832, 1998. [9] Treyz G. V., "Silicon Mach-Zehnder waveguide interferometers operating at 1.3/mi", Electron. Lett., 1991, 27(2):118. 62 Proc. SPIE Vol. 4582