A novel configuration for realizing a polarization-insensitive optical switch on silicon-on-insulator of 340nm-thick top-silicon layer is demonstrated, using submicron sized waveguides. The device is based on the Mach-Zehnder interferometer structure. By carefully designing the 3dB couplers and the delay-line waveguides in the device, it is possible to achieve a similar switching behavior for all polarizations. Theoretical analyses indicate that extinction ratios of better than −25dB and insertion losses of better than −0.6dB can be obtained simultaneously for transverse-electric and transverse magnetic polarized modes in the whole C-band. Experimental results also confirm the polarization-insensitive property of the proposed optical switch. Extinction ratios of about −15dB are measured for both polarizations.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Photonic integration based on silicon-on-insulator (SOI) platform has shown a great potential in optical interconnect applications . The high refractive-index-contrast of the SOI structure leads to waveguide dimensions in hundreds of nanometers and bends in radius of several microns, which renders an SOI photonic chip ultra-high integration density. However, the same reason also results in a high polarization dependency. The very different properties of the transverse-electric (TE) and transverse-magnetic (TM) polarized modes in a device built on SOI make it difficult to interface with other polarization-insensitive systems, such as ordinary single-mode-fiber based networks. Polarization diversity scheme, using polarization splitter-rotators, has been introduced to possibly solve this issue [2–4]. However, it comes at a cost of almost doubling the device count on a chip [2, 5]. The pursuing for internal polarization-insensitive devices on SOI is continuously attracting research efforts in the recent years. Some basic building blocks capable of polarization independency, such as fiber couplers , 3-dB splitters [7–10], have been proposed. Yet, more functionalities have to be brought up in order to realize a true polarization-insensitive photonic chip on SOI.
In this paper, a novel configuration for a polarization-insensitive optical switch is introduced on silicon. Although this type of device has been demonstrated using rib waveguides , such a shallowly-etched waveguide structure is not suitable for sharp bends and dense integration. Here, the proposed polarization-insensitive switch is built on an SOI wafer of 340nm-thick top-silicon layer using submicron sized wire waveguides. The device is based on the well-known Mach-Zehnder interferometer (MZI) structure . A polarization-insensitive 3dB coupler made of multi-mode interferometer (MMI) structure is employed as broadband power splitters in the MZI structure. Balanced delay lines are employed to ensure a broadband response. The waveguide structure of the delay lines is also tuned to exhibit an identical phase shift for both polarizations. The device performance is first analyzed theoretically, and then the optimized structure is fabricated. Thermo-optical tuning with integrated micro-heaters is employed to actuate the switch. Characterization indicates that the present device can switch both polarizations simultaneously between the two output ports with an extinction ratio of about −15dB in the C-band.
Figure 1 shows the basic structure of an MZI structure and its building blocks. The whole device is built on an SOI wafer with top-silicon layer thickness of h = 340nm. This classic device can act as an optical switch if the optical length of one delay line can be tuned. Therefore, the light, when incident from, e.g., the input port 1, can be switched between the output port 3 or 4. There are two basic building blocks for this MZI structure, i.e., the 3dB couplers and the delay-line waveguides. In order to obtain a polarization-insensitive MZI switch, it is then crucial to make these two building blocks polarization insensitive.
A polarization-insensitive 3dB coupler can be achieved using MMIs , adiabatic tapered couplers , bent directional couplers , or cascaded directional couplers . In this paper, the MMI structure is adopted, due to its compact size and broad-band responses. As sketched in Fig. 1(b), the structural parameters of an optimized MMI are win = 750nm, g = 500nm, wmmi = 2.0μm, and Lmmi = 15.5μm. All the silicon patterns are fully etched and a silicon oxide top cladding layer is used. The spectral responses of the two output ports of this MMI device are plotted in Fig. 2, which is obtained through a mode expansion and matching algorithm. One can find that the input optical power is nearly equally distributed to the two output ports for both TE and TM polarizations in the whole simulated 100-nm wavelength band. Besides power, the phase difference between the two outputs of the MMI is also important, since this phase difference, added up by that from the delay lines, eventually determines how the interference would happen at the output ports of the whole MZI. As shown in Fig. 2(b), the TE and TM polarizations exhibit a similar phase difference of ~π/2, which is also relatively flat in the whole simulated band. Besides static responses, it is also important to look at the delays of pulsed optical signals at different polarizations going through a device, i.e., the differential group delay (DGD). A significant DGD would increase the power penalty of the device. Figure 2(c) shows the dispersion relations of effective refractive indices neff and group indices ng for the fundamental TE and TM modes in the MMI section. The difference of ng for the two polarizations is about 0.5. Taking into account the size of the MMI, the DGD of the present MMI is about 25fs, which is negligible as compared to the bit period of a modulated signal at, e.g., 40Gb/s. With the above analyses, one can conclude that the designed MMI-based 3dB coupler indeed presents a polarization-insensitive characteristic.
Another important building block for an MZI switch is the delay line, which essentially is a straight waveguide as shown in Fig. 1(c). Intuitively, it is difficult to make such an SOI waveguide polarization insensitive, since the effective refractive indices neff of the TE and TM modes normally exhibits a large difference from each other as shown in Fig. 3(a). Yet, only at one circumstance when the width wd of the waveguide equals to the height h, i.e., the waveguide becomes a square, the two polarized modes would show the same behavior. Theoretically, such a square waveguide structure can be adopted to build a polarization-insensitive MZI switch . However, in practice, it is very difficult to maintain such a condition. Tiny fluctuations in dimensions, shapes, or cladding materials resulted from fabrications would make the waveguide deviate from a perfect square. Even if a point, where the effective refractive indices of the TE and TM modes are the same, could be found, the mode hybridization at this point due to a slight asymmetry of the waveguide structure would result in unwanted coupling, and hence cross-talks, between the two polarized modes . Here, a novel approach to achieve polarization-insensitive delay lines is proposed.
It is known that the output property of an MZI is determined by the phase differences Δϕ of the light passing through the two delay-line waveguides, which are expressed as:Eq. (1) reduces to:Fig. 1(a), is considered. The change rates of the effective refractive indices with respect to a local temperature rise (dneff/dT) is analyzed and also shown in Fig. 3(a). Apparently, there exists a point where the condition is fulfilled. Furthermore, when wd is between 360nm to 400nm, the dneff/dT curves become relative flat and their difference for the two polarizations is maintained within 0.4%. Figure 3(b) presents the dispersion relations of neff and ng for the fundamental TE and TM modes with wd = 380nm. According to ng values shown here, the DGD of the delay line is calculated to be about 40fs, assuming the length of the delay line is 120μm (see below). Taking into account that of the MMI discussed above, the DGD of the whole MZI device would be in total less than 100fs. This is again a negligible value, thanks to the compact size of the device.
The performance of the MZI switch using the proposed delay-line design is further studied. The power transmission through one of the output port of the switch at different local temperature rise ΔT on one delay line is plotted in Fig. 4(a). In order to demonstrate solely how the delay line would affect the switching performance, perfect 3dB couplers are assumed in this scenario. One can find that the TE and TM modes almost follow the same response curve. A zoom-in plot around the dip position shows that the switching extinction ratios are always better than −45dB for wd within 360nm-400nm. Such a 40nm tolerance can be easily maintained with modern fabrication technology for silicon photonics. Combining the above analyses, the performance of the whole MZI switch using the MMI-based 3dB couplers and the tuned delay-line waveguides can be obtained, as shown in Fig. 4(b). Here, the MMI parameters are same as those in Fig. 2, and the width of the delay-line waveguides wd is 380nm. One can find that the proposed MZI structure indeed shows a polarization-insensitive switching performance. Extinction ratios of better than −25dB and insertion losses of better than −0.6dB can be achieved simultaneously for TE and TM polarized modes in the whole C-band.
3. Experiment and characterization
The designed MZI switch device is then fabricated on an SOI wafer with 340nm-thick top-silicon layer and 2μm-thick buried oxide layer. Some pictures of one finished sample are shown in Fig. 5. The length of each delay-line waveguide beneath the metal heaters is 120μm. On-chip polarization beam splitters (PBSs), using a bent directional coupler , are adopted at the input and output ports of the MZI structure for separating the TE and TM polarized modes, which are then guided to grating couplers for light input from, or output to, fibers. After fabricating the silicon patterns, the chip was covered by a thin spin-on-glass layer, and then followed by a chemical vapor deposition of SiO2. The total SiO2 layer thickness is about 700nm. In the final step, micro-heaters were fabricated on top of the delay-line waveguide of the MZI structure. The performance of fabricated samples is characterized on a temperature-controlled stage using fiber and metal probes for optical and electrical signal access.
First, characteristics of the switch at different driving powers on the heaters were measured as shown in Fig. 6(a). As expected, the two output ports 3 and 4 show complementary responses. The difference in driving powers ΔPh between the two dips of the response curves corresponds to a π change of the phase difference in the delay-line waveguides. One can read from this Fig. 6(a) that = 14.57mW and = 14.85mW, which are very close to each. This proves that the delay-line structure in the MZI does exhibit a polarization-insensitive tuning behavior. Yet, the absolute dip positions for two polarizations still do not coincide. In principle, one of the dip positions for the response curves at output port 3 should be at 0mW, provided that the MZI is strictly balanced. However, due to fabrication variations, there would still exist some imbalance in the two delay-line waveguides, which would result in a shift of the dip position from 0mW, as well as a deviation between the dips of the two polarized modes as shown in Fig. 6(a). This imbalance can be minimized through a better fabrication control or using a separate tuning section in the delay lines for compensation. Nevertheless, about −20dB extinction ratios can be obtained simultaneously for the two polarizations in the present device. The wavelength responses were further measured at Ph = 4.2mW and 18.2mW, which corresponds to the two working conditions of the switch. As shown in Figs. 6(b) and 6(c). Extinction ratios of about −15dB can be achieved in the C-band at both output ports for both polarizations. The insertion losses are about −2.5dB, which is relatively high probably due to poor performance of the fabricated MMI structure. This can be improved using more robust designs for the polarization-insensitive 3dB coupler, e.g., adiabatic tapers .
We have demonstrated, experimentally for the first time, a polarization-insensitive optical switch on the SOI platform, which is based on the conventional MZI structure. The proposed MZI structure consists of MMI-based couplers for polarization-insensitive power splitting and optimized waveguide structures for polarization-insensitive delay lines. The performance of the proposed optical switch has first been simulated, and then the optimized structure was fabricated and characterized. Polarization-insensitive switching behavior has been confirmed in both simulated and measured results. Extinction ratios of the about −25dB have been obtained theoretically, and experimentally about −15dB, for both TE and TM polarizations in the C-band. Although thermo-optical tuning was employed to actuate the switching in the present structure, the same principle can be extended for other actuation approaches, e.g., carrier injection using a PN junction for a higher switching speed [11, 12].
National Nature Science Foundation of China (61675069); Guangzhou Science and Technology Program (201707010444); Guangdong Science and Technology Program (2017A010101023).
References and links
1. W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert, B. Luysseart, J. Van Campenhout, P. Bienstman, and D. Van Thourhout, “Nanophotonic waveguides in silicon-on-insulator fabricated with CMOS technology,” J. Lightwave Technol. 23(1), 401–412 (2005). [CrossRef]
2. T. Barwicz, M. R. Watts, M. A. Popović, P. T. Rakich, L. Socci, F. X. Kärtner, E. P. Ippen, and H. I. Smith, “Polarization-transparent microphotonic devices in the strong confinement limit,” Nat. Photonics 1(1), 57–60 (2007). [CrossRef]
4. L. Liu, Y. Ding, K. Yvind, and J. M. Hvam, “Silicon-on-insulator polarization splitting and rotating device for polarization diversity circuits,” Opt. Express 19(13), 12646–12651 (2011). [CrossRef] [PubMed]
5. K. Tanizawa, K. Suzuki, K. Ikeda, S. Namiki, and H. Kawashima, “Non-duplicate polarization-diversity 8 × 8 Si-wire PILOSS switch integrated with polarization splitter-rotators,” Opt. Express 25(10), 10885–10892 (2017). [CrossRef] [PubMed]
6. M. Pu, L. Liu, H. Ou, K. Yvind, and J. M. Hvam, “Ultra-low-loss inverted taper coupler for silicon-on-insulator ridge waveguide,” Opt. Commun. 283(19), 3678–3682 (2010). [CrossRef]
7. D. Dai and S. He, “Optimization of Ultracompact Polarization Insensitive Multimode Interference Couplers Based on Si Nanowire Waveguides,” IEEE Photonics Technol. Lett. 18(19), 2017–2019 (2006). [CrossRef]
8. J. Xing, Z. Li, Y. Yu, and J. Yu, “Design of polarization-independent adiabatic splitters fabricated on silicon-on-insulator substrates,” Opt. Express 21(22), 26729–26734 (2013). [CrossRef] [PubMed]
9. X. Chen, W. Liu, Y. Zhang, and Y. Shi, “Polarization-insensitive broadband 2 × 2 3 dB power splitter based on silicon-bent directional couplers,” Opt. Lett. 42(19), 3738–3740 (2017). [CrossRef] [PubMed]
10. X. Deng, L. Yan, H. Jiang, W. Pan, B. Luo, and X. Zou, “Polarization-Insensitive and Broadband Optical Power Splitter With a Tunable Power Splitting Ratio,” IEEE Photonics J. 9(3), 4501609 (2017). [CrossRef]
11. S. Nakamura, S. Yanagimachi, H. Takeshita, A. Tajima, T. Hino, and K. Fukuchi, “Optical Switches Based on Silicon Photonics for ROADM Application,” IEEE J. Sel. Top. Quantum Electron. 22(6), 185–193 (2016). [CrossRef]
12. L. Lu, L. Zhou, Z. Li, X. Li, and J. Chen, “Broadband 4×4 nonblocking silicon electrooptic switches based on Mach-Zehnder interferometers,” IEEE Photonics J. 7(1), 7800108 (2015). [CrossRef]
13. L. Jia, H. Zhou, T.-Y. Liow, J. Song, Y. Huang, X. Tu, X. Luo, C. Li, Q. Fang, M. Yu, and G. Lo, “Analysis of the polarization rotation effect in the inversely tapered spot size converter,” Opt. Express 23(21), 27776–27785 (2015). [CrossRef] [PubMed]
14. 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). [CrossRef] [PubMed]