Improved 8-channel silicon mode demultiplexer with grating polarizers Jian Wang, Pengxin Chen, Sitao Chen, Yaocheng Shi, and Daoxin Dai* State Key Laboratory for Modern Optical Instrumentation, Centre for Optical and Electromagnetic Research, Zhejiang Provincial Key Laboratory for Sensing Technologies, Zhejiang University, Zijingang Campus, Hangzhou 310058, China. * [email protected]
Abstract: An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~−20dB in a ~100nm bandwidth. ©2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (030.4070) Modes; (060.4230) Multiplexing; (230.5440) Polarization-selective devices.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
D. J. Richardson, J. M. Fini, and L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photon. 7(5), 354–362 (2013). J. Sakaguchi, B. Puttnam, W. Klaus, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, K. Imamura, H. Inaba, K. Mukasa, R. Sugizaki, T. Kobayashi, and M. Watanabe, “305 Tb/s space division multiplexed transmission using homogeneous 19-core fiber,” J. Lightwave Technol. 31(4), 554–562 (2013). K. S. Abedin, J. M. Fini, T. F. Thierry, B. Zhu, M. F. Yan, L. Bansal, F. V. Dimarcello, E. M. Monberg, and D. J. DiGiovanni, “Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber,” Opt. Lett. 39(4), 993–996 (2014). S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle, Jr., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). A. M. J. Koonen, H. Chen, H. P. A. van den Boom, and O. Raz, “Silicon photonic integrated mode multiplexer and demultiplexer,” IEEE Photon. Technol. Lett. 24(21), 1961–1964 (2012). N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, K. Tsujikawa, M. Koshiba, and F. Yamamoto, “Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission,” Opt. Express 21(22), 25752–25760 (2013). A. Li, J. Ye, X. Chen, and W. Shieh, “Low-loss fused mode coupler for few-mode transmission,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), OTu3G.4. D. Dai, “Silicon mode-(de)multiplexer for a hybrid multiplexing system to achieve ultrahigh capacity photonic networks-on-chip with a single-wavelength-carrier light,” in Asia Communications and Photonics Conference, OSA Technical Digest (online) (Optical Society of America, 2012), ATh3B.3. T. Uematsu, Y. Ishizaka, Y. Kawaguchi, K. Saitoh, and M. Koshiba, “Design of a compact two-mode multi/demultiplexer consisting of multi-mode interference waveguides and a wavelength insensitive phase shifter for mode-division multiplexing transmission,” J. Lightwave Technol. 30(15), 2421–2426 (2012). J. B. Driscoll, R. R. Grote, B. Souhan, J. I. Dadap, M. Lu, and R. M. Osgood, “Asymmetric Y junctions in silicon waveguides for on-chip mode-division multiplexing,” Opt. Lett. 38(11), 1854–1856 (2013). J. Xing, Z. Li, X. Xiao, J. Yu, and Y. Yu, “Two-mode multiplexer and demultiplexer based on adiabatic couplers,” Opt. Lett. 38(17), 3468–3470 (2013). D. Dai, J. Wang, and Y. Shi, “Silicon mode (de)multiplexer enabling high capacity photonic networks-on-chip with a single-wavelength-carrier light,” Opt. Lett. 38(9), 1422–1424 (2013). H. Qiu, H. Yu, T. Hu, G. Jiang, H. Shao, P. Yu, J. Yang, and X. Jiang, “Silicon mode multi/demultiplexer based on multimode grating-assisted couplers,” Opt. Express 21(15), 17904–17911 (2013).
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12799
14. L.-W. Luo, N. Ophir, C. P. Chen, L. H. Gabrielli, C. B. Poitras, K. Bergmen, and M. Lipson, “WDM-compatible mode-division multiplexing on a silicon chip,” Nat. Commun. 5, 3069 (2014). 15. J. Wang, S. He, and D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photon. Rev. 8(2), L18–L22 (2014). 16. K. Rollke and W. Sohler, “Metal-clad waveguide as cutoff polarizer for integrated optics,” IEEE J. Quantum Electron. 13(4), 141–145 (1977). 17. D. Dai, Z. Wang, N. Julian, and J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). 18. M. Z. Alam, J. S. Aitchison, and M. Mojahedi, “Compact and silicon-on-insulator-compatible hybrid plasmonic TE-pass polarizer,” Opt. Lett. 37(1), 55–57 (2012). 19. Y. Huang, S. Zhu, H. Zhang, T. Y. Liow, and G. Q. Lo, “CMOS compatible horizontal nanoplasmonic slot waveguides TE-pass polarizer on silicon-on-insulator platform,” Opt. Express 21(10), 12790–12796 (2013). 20. C. H. Chen, L. Pang, C. H. Tsai, U. Levy, and Y. Fainman, “Compact and integrated TM-pass waveguide polarizer,” Opt. Express 13(14), 5347–5352 (2005). 21. S. Lin, J. Hu, and K. B. Crozier, “Ultracompact, broadband slot waveguide polarization splitter,” Appl. Phys. Lett. 98(15), 151101 (2011). 22. D. Dai, Z. Wang, and J. E. Bowers, “Ultrashort broadband polarization beam splitter based on an asymmetrical directional coupler,” Opt. Lett. 36(13), 2590–2592 (2011). 23. J. Wang, D. Liang, Y. Tang, D. Dai, and J. E. Bowers, “Realization of an ultra-short silicon polarization beam splitter with an asymmetrical bent directional coupler,” Opt. Lett. 38(1), 4–6 (2013). 24. X. Guan, H. Wu, Y. Shi, L. Wosinski, and D. Dai, “Ultracompact and broadband polarization beam splitter utilizing the evanescent coupling between a hybrid plasmonic waveguide and a silicon nanowire,” Opt. Lett. 38(16), 3005–3008 (2013). 25. D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D. Van Thourhout, P. Bienstman, and R. Baets, “Grating couplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys. 45(8A), 6071–6077 (2006). 26. G. Roelkens, D. Van Thourhout, and R. Baets, “High efficiency silicon-on-insulator grating coupler based on a poly-Silicon overlay,” Opt. Express 14(24), 11622–11630 (2006). 27. Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, “Highly efficient nonuniform grating coupler for silicon-on-insulator nanophotonic circuits,” Opt. Lett. 35(8), 1290–1292 (2010). 28. D. Dai, J. Wang, and S. He, “Silicon multimode photonic integrated devices for on-chip mode-divisionmultiplexed optical interconnects,” Prog. Electromagn. Res. 143, 773–819 (2013).
1. Introduction In order to satisfy the increasing demand for optical interconnect capacity, various advanced multiplexing techniques have been developed and applied for optical fiber communication networks. It is well known that wavelength-division-multiplexing (WDM) is one of the most successful multiplexing techniques. In recent years, the capacity of WDM-based optical communication is going to be saturated and the spatial-division-multiplexing (SDM) technique [1] is re-activated by introducing multi-core fibers [1–3] as well as few-mode fibers [4–7]. The SDM channels carrying different data share the same wavelength and thus only one laser diode (LD) with a fixed wavelength is needed. When combining the WDM and SDM technologies, the total channel numbers can be increased significantly to improve the link capacity. The SDM technique also helps to reduce the system cost because fewer laser diodes are needed. The SDM technology is also becoming attractive for optical interconnects in data centers which are bandwidth hungry and cost sensitive. It might be more convenient to upgrade an existing network or install a new network with the SDM technology for short-distance data communications in comparison with long-distance optical networks because the installation of new fiber infrastructure over a long distance is very complicated [8]. Particularly, for photonic networks-on-chip, optical signals propagate along planar optical waveguides and the control of light propagation / conversion are not difficult with some specific on-chip waveguide structures, which makes it be promising to introduce on-chip SDM technology and one of the potential applications is the interconnect for the multi-processor or interchips. In comparison to the multi-core SDM, the multimode SDM [8] provides a way to be more concise and compact photonic integrated circuits (PICs) as only one multimode bus waveguide is included and the guided-modes carrying different data are overlapped spatially in the multimode bus waveguide. The conciseness of the PICs will help make the circuit design easy and convenient. It is even
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12800
possible to realize a promising approach of combining the multi-core and multimode multiplexing technologies in the future in order to achieve ultra-high link capacity. In a multimode SDM system, low-loss and low-crosstalk mode (de)multiplexer is one of the most important devices. Various structures have been proposed to realize mode (de)multiplexer [8–15]. In [9], a two-channel mode multiplexer is proposed by using a complicated structure with multimode interference couplers and phase shifters. However, this design is inconvenient and inflexible to be extended for more mode-channels. A mode multiplexer with more flexibility is using adiabatic mode-evolution couplers and the popular structures include adiabatic Y-branches [10] as well as adiabatic directional couplers [11]. With this design, one can achieve low-loss and low-crosstalk mode (de)multiplexer at the expense of large footprints. Alternatively, the mode (de)multiplexers based on cascaded asymmetrical directional couplers (ADCs) [8, 12] have a small footprint and high scalability for more mode channels. In our previous paper, a 4-channel mode multiplexer with low loss and low crosstalk has been demonstrated theoretically and experimentally for TM polarization [12]. Grating-assisted ADCs have also been used to realize mode multiplexers [13], which however has a relatively large footprint due to the weak coupling of grating structures. When combining ADCs with microring resonators, a WDM-compatible multimode SDM technology has been realized [14]. More recently, we have proposed and realized an 8-channel hybrid demultiplexer enabling the multimode SDM and polarization-division-multiplexing (PDM) technologies simultaneously by utilizing ADCs and a polarization beam splitter (PBS) [15]. According to the experimental results demonstrated in [15], we note that the dominant crosstalks for the TM0, TM1, TM2 and TE3 mode-channels are from some orthogonal polarization modes. In this paper, we give a comprehensive analysis for the crosstalk in the 8-channel hybrid demultiplexer and propose a solution to significantly reduce the polarization crosstalk by introducing optical polarizers at the end of each port. An improved 8-channel hybrid demultiplexer with low crosstalk is demonstrated by introducing TE-type or TM-type grating polarizer at the output ends. The grating polarizer also serves as an efficient fiber-chip coupler. Our experimental results show that the maximum crosstalk of the demonstrated 8-channel hybrid multiplexer is reduced greatly from −11dB to be ~−20dB in a ~100nm wavelength bandwidth. 2. Structure and analysis
Fig. 1. Schematic configuration of an 8-channel hybrid demultiplexer [15].
Figure 1 shows the 8-channel silicon hybrid demultiplexer including a three-waveguide PBS and six cascaded ADCs [15]. The PBS is used to separate the fundamental modes for TE and TM polarizations (TE0 and TM0) while the six ADCs are designed to demultiplex the high-order modes of both polarizations (i.e., the TE1, TE2, TE3, TM1, TM2, and TM3 modes). According to the phase matching condition, the parameters for the PBS and the ADCs are chosen optimally, as shown in Ref. [15]. The TE3, TE2, TE1, and TE0 modes are dominantly output from ports O1~O4 respectively while the TM0, TM1, TM2, and TM3 modes are dominantly output from ports O5~O8, respectively.
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12801
Fig. 2. The transmission responses at the eight output ports for the 8-channel hybrid demultiplexer shown in Fig. 1 (without any polarizer at the output ports) (a) O1 (TE3 mode channel), (b) O2 (TE2 mode channel), (c) O3 (TE1 mode channel), (d) O4 (TE0 mode channel), (e) O5 (TM0 mode channel), (f) O6 (TM1 mode channel), (g) O7 (TM2 mode channel), and (h) O8 (TM3 mode channel) when all of the used modes are launched at the left of the bus waveguide, respectively.
Figures 2(a)–2(h) show the calculated transmission responses at the eight output ports (O1~O8) of the designed hybrid demultiplexer, respectively when all of the modes (TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 modes) are launched from the input port of the bus
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12802
waveguide at the left, respectively. Here a three-dimensional finite-difference time-domain (3D-FDTD) method is used for the calculation. One should note that some higher-order modes might be cut-off as the bus waveguide is tapered down. Therefore, the crosstalk from the cut-off higher-order modes to the following output ports will be negligible. For example, when the TM3 mode is launched from the input port of the bus waveguide, it will be dominantly dropped by the first ADC (which is designed for the TM3 mode) and received by the O8 port (as shown in Fig. 2(h)) while there is a small part of the TM3 mode power resident in the bus waveguide (due to the fabrication deviations). This resident power is still carried by the TM3 mode and propagates forward. When it goes through the following adiabatic taper, the bus waveguide will become too narrow to support the TM3 mode and the resident power carried by the TM3 mode becomes radiated. In this case, little crosstalk will be introduced to the following ports. Therefore, here the transmissions to some output ports are too low to be shown in Figs. 2(a)–2(h). From Figs. 2(a)–2(h), it can be seen that the crosstalk from orthogonal polarization modes is dominant for some output ports. For example, the dominant crosstalk for the TM2 mode-channel (port O7) comes from the TE3 mode-channel and the polarization crosstalk is ~−11dB @1550nm, as shown in Fig. 2(g). The reason is that the second-stage ADC designed for the TM2 mode does not have a significant phase-mismatch between the TE3 mode in the bus waveguide and the TE0 mode of the access waveguide. Consequently, some part of power carried by the TE3 mode is dropped to port O7 by the second-stage ADC designed for the TM2 mode. From Fig. 2(a) we also note that the crosstalk from the launched TM2 mode in the bus waveguide to port O1 (which is for the TE3 mode channel) is much lower (20dB when the etching depth varies from 60nm to 80nm. In experiment, the etching depth can be controlled accurately #208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12804
by slowing down the etching rate. In our fabrication process, the etching rate is slowed down to ~2nm/s and the deviation of the etching depth is less than ± 6nm in our lab. For the measurement, a tunable laser (Agilent 81940A) is used as the light source and a powermeter (Agilent 8163A) is used at the terminal to monitor the output. Single-mode fibers are aligned with a 10° incident angle to couple light to/from the chip. The polarization state of input light is adjusted by a polarization controller. When measuring the 8-channel hybrid multiplexer, the light is launched from an input port Ii (i = 1,…, 8) and the transmission responses at the output ports (O1~O8) are measured one by one. Figures 5(a)–5(h) show the measured transmission responses at a fixed output port Oi (i = 1,…, 8), respectively, when light is launched from any of the input ports (I1~I8). Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip. Since the output powers at some non-major output ports are beyond the power range of our powermeter, the corresponding transmission responses are not shown in Figs. 5(a)–5(h). As expected, Figs. 5(a)–5(d) show that the TE(4-i) mode in the bus waveguide is excited dominantly and dropped to output port Oi when TE-polarized light is launched from input port Ii (i = 1, 2, 3, 4). Similarly, from Figs. 5(e)–5(h), it can be seen that the TM(i−5) mode in the bus waveguide will be excited dominantly and dropped to output port Oi when TM-polarized light is launched from input port Ii (i = 5, 6, 7, 8). The measured excess losses around 1560nm are about 3.1dB, 2.2dB, 3.5dB, 0.2dB, 0.7dB, 2.1dB, 1.5dB, and 1.4dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. The loss is mainly from the insufficient cross-coupling due to the fabrication deviation. From Figs. 5(a)–5(h), it can also be seen that the present 8-channel mode demultiplexer with grating polarizers has low crosstalk. The crosstalk is defined as usual to be the difference between the powers at a fixed output port (Oi) when light is launched from the major input port (Ii) and another input port (Ij, j≠i). The dominant crosstalks (around 1560nm) for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. The accumulated crosstalk from all the other non-major mode channels are −20.5dB, −20.2dB, −16.6dB, −29dB, −33.1dB, −38.3dB, −16.9dB, and −20.6dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. And the crosstalk is insensitive to the wavelength over a broad band from 1520nm to 1620nm. One sees that the present hybrid demultiplexer with grating polarizers has much lower crosstalk than the previous hybrid demultiplexer without grating polarizers demonstrated in [15]. For example, for our previous hybrid demultiplexer without grating polarizers, there is a −11dB crosstalk from the TE3 mode channel to the TM2 mode channel (see Fig. 2(g)). In contrast, for the present hybrid demultiplexer with grating polarizers, the crosstalk from the TE3 mode channel to the TM2 mode channel is not observed. The present improved hybrid demultiplexer with low-crosstalk helps the realization of a mode-multiplexed multi-channel optical interconnect link in the future. It is also possible to use any other on-chip polarizer or PBS [16–24] to filter out the polarization crosstalk of the hybrid multiplexer particularly when the mode (de)multiplexer is integrated monolithically with other functionality elements (e.g., the sources and photodetectors) on the same chip. For the present mode (de)multiplexer with grating polarizers, it might be useful for the case of hybrid integration with some active photonic devices (e.g., photodetectors) bonded on the chip since the grating polarizer can simultaneously serve as an efficient coupler between the passive and active components.
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12805
Fig. 5. The normalized transmission responses with respect to the straight waveguide on the same chip at the eight output ports (a) O1; (b) O2; (c) O3; (d) O4; (e) O5; (f) O6; (g) O7; (h) O8 when all of the input ports from I1 to I8 are launched. Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip.
4. Conclusion In summary, we have demonstrated an improved 8-channel hybrid demultiplexer with the assistance of TE-type and TM-type grating polarizers (which also serve as fiber-chip couplers).
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12806
The experimental results have shown that the grating polarizers with high extinction ratios filter out the polarization crosstalk and the crosstalk of the 8-channel hybrid demultiplexer is reduced greatly. The dominant crosstalks (around 1560nm) from the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. And the crosstalk is insensitive to the wavelength from 1520nm to 1620nm. On-chip polarizers or PBSs can also be used to filter out the polarization crosstalk of the hybrid demultiplexer when needed. Acknowledgments This project was partially supported by a 863 project (No. 2011AA010301), the Nature Science Foundation of China (No. 11374263), Zhejiang provincial grant (Z201121938), the Doctoral Fund of Ministry of Education of China (No. 20120101110094).
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12807
Abstract: An improved 8-channel silicon mode demultiplexer is realized with TE-type and TM-type grating polarizers at the output ends, and these gratings serve as fiber-chip couplers simultaneously. The present 8-channel silicon mode demultiplexer includes a three-waveguide PBS (for separating the TE0 and TM0 modes) and six cascaded ADCs (for demultiplexing the high-order modes of both polarizations). The grating polarizers with high extinction ratios are used to filter out the polarization crosstalk in the 8-channel hybrid multiplexer efficiently and the measured crosstalk for all the mode-channels of the improved 8-channel mode multiplexer is reduced greatly to ~−20dB in a ~100nm bandwidth. ©2014 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (030.4070) Modes; (060.4230) Multiplexing; (230.5440) Polarization-selective devices.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
D. J. Richardson, J. M. Fini, and L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photon. 7(5), 354–362 (2013). J. Sakaguchi, B. Puttnam, W. Klaus, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, K. Imamura, H. Inaba, K. Mukasa, R. Sugizaki, T. Kobayashi, and M. Watanabe, “305 Tb/s space division multiplexed transmission using homogeneous 19-core fiber,” J. Lightwave Technol. 31(4), 554–562 (2013). K. S. Abedin, J. M. Fini, T. F. Thierry, B. Zhu, M. F. Yan, L. Bansal, F. V. Dimarcello, E. M. Monberg, and D. J. DiGiovanni, “Seven-core erbium-doped double-clad fiber amplifier pumped simultaneously by side-coupled multimode fiber,” Opt. Lett. 39(4), 993–996 (2014). S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R. J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle, Jr., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). A. M. J. Koonen, H. Chen, H. P. A. van den Boom, and O. Raz, “Silicon photonic integrated mode multiplexer and demultiplexer,” IEEE Photon. Technol. Lett. 24(21), 1961–1964 (2012). N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, K. Tsujikawa, M. Koshiba, and F. Yamamoto, “Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission,” Opt. Express 21(22), 25752–25760 (2013). A. Li, J. Ye, X. Chen, and W. Shieh, “Low-loss fused mode coupler for few-mode transmission,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), OTu3G.4. D. Dai, “Silicon mode-(de)multiplexer for a hybrid multiplexing system to achieve ultrahigh capacity photonic networks-on-chip with a single-wavelength-carrier light,” in Asia Communications and Photonics Conference, OSA Technical Digest (online) (Optical Society of America, 2012), ATh3B.3. T. Uematsu, Y. Ishizaka, Y. Kawaguchi, K. Saitoh, and M. Koshiba, “Design of a compact two-mode multi/demultiplexer consisting of multi-mode interference waveguides and a wavelength insensitive phase shifter for mode-division multiplexing transmission,” J. Lightwave Technol. 30(15), 2421–2426 (2012). J. B. Driscoll, R. R. Grote, B. Souhan, J. I. Dadap, M. Lu, and R. M. Osgood, “Asymmetric Y junctions in silicon waveguides for on-chip mode-division multiplexing,” Opt. Lett. 38(11), 1854–1856 (2013). J. Xing, Z. Li, X. Xiao, J. Yu, and Y. Yu, “Two-mode multiplexer and demultiplexer based on adiabatic couplers,” Opt. Lett. 38(17), 3468–3470 (2013). D. Dai, J. Wang, and Y. Shi, “Silicon mode (de)multiplexer enabling high capacity photonic networks-on-chip with a single-wavelength-carrier light,” Opt. Lett. 38(9), 1422–1424 (2013). H. Qiu, H. Yu, T. Hu, G. Jiang, H. Shao, P. Yu, J. Yang, and X. Jiang, “Silicon mode multi/demultiplexer based on multimode grating-assisted couplers,” Opt. Express 21(15), 17904–17911 (2013).
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12799
14. L.-W. Luo, N. Ophir, C. P. Chen, L. H. Gabrielli, C. B. Poitras, K. Bergmen, and M. Lipson, “WDM-compatible mode-division multiplexing on a silicon chip,” Nat. Commun. 5, 3069 (2014). 15. J. Wang, S. He, and D. Dai, “On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode- and polarization-division-multiplexing,” Laser Photon. Rev. 8(2), L18–L22 (2014). 16. K. Rollke and W. Sohler, “Metal-clad waveguide as cutoff polarizer for integrated optics,” IEEE J. Quantum Electron. 13(4), 141–145 (1977). 17. D. Dai, Z. Wang, N. Julian, and J. E. Bowers, “Compact broadband polarizer based on shallowly-etched silicon-on-insulator ridge optical waveguides,” Opt. Express 18(26), 27404–27415 (2010). 18. M. Z. Alam, J. S. Aitchison, and M. Mojahedi, “Compact and silicon-on-insulator-compatible hybrid plasmonic TE-pass polarizer,” Opt. Lett. 37(1), 55–57 (2012). 19. Y. Huang, S. Zhu, H. Zhang, T. Y. Liow, and G. Q. Lo, “CMOS compatible horizontal nanoplasmonic slot waveguides TE-pass polarizer on silicon-on-insulator platform,” Opt. Express 21(10), 12790–12796 (2013). 20. C. H. Chen, L. Pang, C. H. Tsai, U. Levy, and Y. Fainman, “Compact and integrated TM-pass waveguide polarizer,” Opt. Express 13(14), 5347–5352 (2005). 21. S. Lin, J. Hu, and K. B. Crozier, “Ultracompact, broadband slot waveguide polarization splitter,” Appl. Phys. Lett. 98(15), 151101 (2011). 22. D. Dai, Z. Wang, and J. E. Bowers, “Ultrashort broadband polarization beam splitter based on an asymmetrical directional coupler,” Opt. Lett. 36(13), 2590–2592 (2011). 23. J. Wang, D. Liang, Y. Tang, D. Dai, and J. E. Bowers, “Realization of an ultra-short silicon polarization beam splitter with an asymmetrical bent directional coupler,” Opt. Lett. 38(1), 4–6 (2013). 24. X. Guan, H. Wu, Y. Shi, L. Wosinski, and D. Dai, “Ultracompact and broadband polarization beam splitter utilizing the evanescent coupling between a hybrid plasmonic waveguide and a silicon nanowire,” Opt. Lett. 38(16), 3005–3008 (2013). 25. D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D. Van Thourhout, P. Bienstman, and R. Baets, “Grating couplers for coupling between optical fibers and nanophotonic waveguides,” Jpn. J. Appl. Phys. 45(8A), 6071–6077 (2006). 26. G. Roelkens, D. Van Thourhout, and R. Baets, “High efficiency silicon-on-insulator grating coupler based on a poly-Silicon overlay,” Opt. Express 14(24), 11622–11630 (2006). 27. Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He, “Highly efficient nonuniform grating coupler for silicon-on-insulator nanophotonic circuits,” Opt. Lett. 35(8), 1290–1292 (2010). 28. D. Dai, J. Wang, and S. He, “Silicon multimode photonic integrated devices for on-chip mode-divisionmultiplexed optical interconnects,” Prog. Electromagn. Res. 143, 773–819 (2013).
1. Introduction In order to satisfy the increasing demand for optical interconnect capacity, various advanced multiplexing techniques have been developed and applied for optical fiber communication networks. It is well known that wavelength-division-multiplexing (WDM) is one of the most successful multiplexing techniques. In recent years, the capacity of WDM-based optical communication is going to be saturated and the spatial-division-multiplexing (SDM) technique [1] is re-activated by introducing multi-core fibers [1–3] as well as few-mode fibers [4–7]. The SDM channels carrying different data share the same wavelength and thus only one laser diode (LD) with a fixed wavelength is needed. When combining the WDM and SDM technologies, the total channel numbers can be increased significantly to improve the link capacity. The SDM technique also helps to reduce the system cost because fewer laser diodes are needed. The SDM technology is also becoming attractive for optical interconnects in data centers which are bandwidth hungry and cost sensitive. It might be more convenient to upgrade an existing network or install a new network with the SDM technology for short-distance data communications in comparison with long-distance optical networks because the installation of new fiber infrastructure over a long distance is very complicated [8]. Particularly, for photonic networks-on-chip, optical signals propagate along planar optical waveguides and the control of light propagation / conversion are not difficult with some specific on-chip waveguide structures, which makes it be promising to introduce on-chip SDM technology and one of the potential applications is the interconnect for the multi-processor or interchips. In comparison to the multi-core SDM, the multimode SDM [8] provides a way to be more concise and compact photonic integrated circuits (PICs) as only one multimode bus waveguide is included and the guided-modes carrying different data are overlapped spatially in the multimode bus waveguide. The conciseness of the PICs will help make the circuit design easy and convenient. It is even
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12800
possible to realize a promising approach of combining the multi-core and multimode multiplexing technologies in the future in order to achieve ultra-high link capacity. In a multimode SDM system, low-loss and low-crosstalk mode (de)multiplexer is one of the most important devices. Various structures have been proposed to realize mode (de)multiplexer [8–15]. In [9], a two-channel mode multiplexer is proposed by using a complicated structure with multimode interference couplers and phase shifters. However, this design is inconvenient and inflexible to be extended for more mode-channels. A mode multiplexer with more flexibility is using adiabatic mode-evolution couplers and the popular structures include adiabatic Y-branches [10] as well as adiabatic directional couplers [11]. With this design, one can achieve low-loss and low-crosstalk mode (de)multiplexer at the expense of large footprints. Alternatively, the mode (de)multiplexers based on cascaded asymmetrical directional couplers (ADCs) [8, 12] have a small footprint and high scalability for more mode channels. In our previous paper, a 4-channel mode multiplexer with low loss and low crosstalk has been demonstrated theoretically and experimentally for TM polarization [12]. Grating-assisted ADCs have also been used to realize mode multiplexers [13], which however has a relatively large footprint due to the weak coupling of grating structures. When combining ADCs with microring resonators, a WDM-compatible multimode SDM technology has been realized [14]. More recently, we have proposed and realized an 8-channel hybrid demultiplexer enabling the multimode SDM and polarization-division-multiplexing (PDM) technologies simultaneously by utilizing ADCs and a polarization beam splitter (PBS) [15]. According to the experimental results demonstrated in [15], we note that the dominant crosstalks for the TM0, TM1, TM2 and TE3 mode-channels are from some orthogonal polarization modes. In this paper, we give a comprehensive analysis for the crosstalk in the 8-channel hybrid demultiplexer and propose a solution to significantly reduce the polarization crosstalk by introducing optical polarizers at the end of each port. An improved 8-channel hybrid demultiplexer with low crosstalk is demonstrated by introducing TE-type or TM-type grating polarizer at the output ends. The grating polarizer also serves as an efficient fiber-chip coupler. Our experimental results show that the maximum crosstalk of the demonstrated 8-channel hybrid multiplexer is reduced greatly from −11dB to be ~−20dB in a ~100nm wavelength bandwidth. 2. Structure and analysis
Fig. 1. Schematic configuration of an 8-channel hybrid demultiplexer [15].
Figure 1 shows the 8-channel silicon hybrid demultiplexer including a three-waveguide PBS and six cascaded ADCs [15]. The PBS is used to separate the fundamental modes for TE and TM polarizations (TE0 and TM0) while the six ADCs are designed to demultiplex the high-order modes of both polarizations (i.e., the TE1, TE2, TE3, TM1, TM2, and TM3 modes). According to the phase matching condition, the parameters for the PBS and the ADCs are chosen optimally, as shown in Ref. [15]. The TE3, TE2, TE1, and TE0 modes are dominantly output from ports O1~O4 respectively while the TM0, TM1, TM2, and TM3 modes are dominantly output from ports O5~O8, respectively.
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12801
Fig. 2. The transmission responses at the eight output ports for the 8-channel hybrid demultiplexer shown in Fig. 1 (without any polarizer at the output ports) (a) O1 (TE3 mode channel), (b) O2 (TE2 mode channel), (c) O3 (TE1 mode channel), (d) O4 (TE0 mode channel), (e) O5 (TM0 mode channel), (f) O6 (TM1 mode channel), (g) O7 (TM2 mode channel), and (h) O8 (TM3 mode channel) when all of the used modes are launched at the left of the bus waveguide, respectively.
Figures 2(a)–2(h) show the calculated transmission responses at the eight output ports (O1~O8) of the designed hybrid demultiplexer, respectively when all of the modes (TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 modes) are launched from the input port of the bus
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12802
waveguide at the left, respectively. Here a three-dimensional finite-difference time-domain (3D-FDTD) method is used for the calculation. One should note that some higher-order modes might be cut-off as the bus waveguide is tapered down. Therefore, the crosstalk from the cut-off higher-order modes to the following output ports will be negligible. For example, when the TM3 mode is launched from the input port of the bus waveguide, it will be dominantly dropped by the first ADC (which is designed for the TM3 mode) and received by the O8 port (as shown in Fig. 2(h)) while there is a small part of the TM3 mode power resident in the bus waveguide (due to the fabrication deviations). This resident power is still carried by the TM3 mode and propagates forward. When it goes through the following adiabatic taper, the bus waveguide will become too narrow to support the TM3 mode and the resident power carried by the TM3 mode becomes radiated. In this case, little crosstalk will be introduced to the following ports. Therefore, here the transmissions to some output ports are too low to be shown in Figs. 2(a)–2(h). From Figs. 2(a)–2(h), it can be seen that the crosstalk from orthogonal polarization modes is dominant for some output ports. For example, the dominant crosstalk for the TM2 mode-channel (port O7) comes from the TE3 mode-channel and the polarization crosstalk is ~−11dB @1550nm, as shown in Fig. 2(g). The reason is that the second-stage ADC designed for the TM2 mode does not have a significant phase-mismatch between the TE3 mode in the bus waveguide and the TE0 mode of the access waveguide. Consequently, some part of power carried by the TE3 mode is dropped to port O7 by the second-stage ADC designed for the TM2 mode. From Fig. 2(a) we also note that the crosstalk from the launched TM2 mode in the bus waveguide to port O1 (which is for the TE3 mode channel) is much lower (20dB when the etching depth varies from 60nm to 80nm. In experiment, the etching depth can be controlled accurately #208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12804
by slowing down the etching rate. In our fabrication process, the etching rate is slowed down to ~2nm/s and the deviation of the etching depth is less than ± 6nm in our lab. For the measurement, a tunable laser (Agilent 81940A) is used as the light source and a powermeter (Agilent 8163A) is used at the terminal to monitor the output. Single-mode fibers are aligned with a 10° incident angle to couple light to/from the chip. The polarization state of input light is adjusted by a polarization controller. When measuring the 8-channel hybrid multiplexer, the light is launched from an input port Ii (i = 1,…, 8) and the transmission responses at the output ports (O1~O8) are measured one by one. Figures 5(a)–5(h) show the measured transmission responses at a fixed output port Oi (i = 1,…, 8), respectively, when light is launched from any of the input ports (I1~I8). Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip. Since the output powers at some non-major output ports are beyond the power range of our powermeter, the corresponding transmission responses are not shown in Figs. 5(a)–5(h). As expected, Figs. 5(a)–5(d) show that the TE(4-i) mode in the bus waveguide is excited dominantly and dropped to output port Oi when TE-polarized light is launched from input port Ii (i = 1, 2, 3, 4). Similarly, from Figs. 5(e)–5(h), it can be seen that the TM(i−5) mode in the bus waveguide will be excited dominantly and dropped to output port Oi when TM-polarized light is launched from input port Ii (i = 5, 6, 7, 8). The measured excess losses around 1560nm are about 3.1dB, 2.2dB, 3.5dB, 0.2dB, 0.7dB, 2.1dB, 1.5dB, and 1.4dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. The loss is mainly from the insufficient cross-coupling due to the fabrication deviation. From Figs. 5(a)–5(h), it can also be seen that the present 8-channel mode demultiplexer with grating polarizers has low crosstalk. The crosstalk is defined as usual to be the difference between the powers at a fixed output port (Oi) when light is launched from the major input port (Ii) and another input port (Ij, j≠i). The dominant crosstalks (around 1560nm) for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. The accumulated crosstalk from all the other non-major mode channels are −20.5dB, −20.2dB, −16.6dB, −29dB, −33.1dB, −38.3dB, −16.9dB, and −20.6dB for the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels, respectively. And the crosstalk is insensitive to the wavelength over a broad band from 1520nm to 1620nm. One sees that the present hybrid demultiplexer with grating polarizers has much lower crosstalk than the previous hybrid demultiplexer without grating polarizers demonstrated in [15]. For example, for our previous hybrid demultiplexer without grating polarizers, there is a −11dB crosstalk from the TE3 mode channel to the TM2 mode channel (see Fig. 2(g)). In contrast, for the present hybrid demultiplexer with grating polarizers, the crosstalk from the TE3 mode channel to the TM2 mode channel is not observed. The present improved hybrid demultiplexer with low-crosstalk helps the realization of a mode-multiplexed multi-channel optical interconnect link in the future. It is also possible to use any other on-chip polarizer or PBS [16–24] to filter out the polarization crosstalk of the hybrid multiplexer particularly when the mode (de)multiplexer is integrated monolithically with other functionality elements (e.g., the sources and photodetectors) on the same chip. For the present mode (de)multiplexer with grating polarizers, it might be useful for the case of hybrid integration with some active photonic devices (e.g., photodetectors) bonded on the chip since the grating polarizer can simultaneously serve as an efficient coupler between the passive and active components.
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12805
Fig. 5. The normalized transmission responses with respect to the straight waveguide on the same chip at the eight output ports (a) O1; (b) O2; (c) O3; (d) O4; (e) O5; (f) O6; (g) O7; (h) O8 when all of the input ports from I1 to I8 are launched. Here these transmission responses are normalized by the transmission of a straight bus waveguide (w = 2.363μm) with TE-type or TM-type grating polarizers at both ends on the same chip.
4. Conclusion In summary, we have demonstrated an improved 8-channel hybrid demultiplexer with the assistance of TE-type and TM-type grating polarizers (which also serve as fiber-chip couplers).
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12806
The experimental results have shown that the grating polarizers with high extinction ratios filter out the polarization crosstalk and the crosstalk of the 8-channel hybrid demultiplexer is reduced greatly. The dominant crosstalks (around 1560nm) from the TE3, TE2, TE1, TE0, TM0, TM1, TM2, and TM3 mode channels are about −20.8dB, −20.3dB, −18dB, −29.3dB, −36.5dB, −40.6dB, −17.7dB, and −20.9dB, respectively. And the crosstalk is insensitive to the wavelength from 1520nm to 1620nm. On-chip polarizers or PBSs can also be used to filter out the polarization crosstalk of the hybrid demultiplexer when needed. Acknowledgments This project was partially supported by a 863 project (No. 2011AA010301), the Nature Science Foundation of China (No. 11374263), Zhejiang provincial grant (Z201121938), the Doctoral Fund of Ministry of Education of China (No. 20120101110094).
#208520 - $15.00 USD Received 18 Mar 2014; revised 6 May 2014; accepted 11 May 2014; published 19 May 2014 (C) 2014 OSA 2 June 2014 | Vol. 22, No. 11 | DOI:10.1364/OE.22.012799 | OPTICS EXPRESS 12807
