Broadband Polarization-Insensitive Thermo-Optic Switches on a 220-nm Silicon-on-Insulator Platform

We demonstrate a polarization-insensitive 2 × 2 thermo-optic switch on a 220-nm silicon-on-insulator platform. This device is based on a balanced Mach-Zehnder interferometer with arms consisting of square-cross-section waveguides. In simulations, the Mach-Zehnder switch (MZS) exhibits extinction ratios (ERs) > 9.5 dB for the transverse electric (TE) and transverse magnetic (TM) polarized modes from 1500 nm to 1600 nm. Experimental results verify the broadband operation of the MZS: switching the two polarizations with ERs > 10 dB over an 85-nm wavelength range. Across the C-band (1530 <inline-formula><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 1565 nm), the fabricated MZS has insertion losses of 0.2 <inline-formula><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 4.3 dB and ERs > 16 dB (14 dB) at the cross (bar) port for both the TE and TM polarizations. The polarization-dependent losses are around 2 dB within a 100-nm wavelength range, showing the low polarization diversity of the MZS. The proposed MZS is a promising building block for implementing on-chip polarization-division-multiplexed optical interconnects.


I. INTRODUCTION
T HE arrival of the Big Data era has driven data centers to increase their transmission capacity using high-bandwidth interconnect towards high switching speed and low energy consumption. Optical switches have the potential to achieve sustainable growth of traffic bandwidth because the power consumption of data switching is low when the data is aggregated [1]. Switches on the silicon-on-insulator (SOI) platform are attractive for high integration density because they benefit from strong light confinement in silicon, and low manufacturing cost thanks to the mature complementary metal-oxide-semiconductor technology (CMOS) fabrication process. In addition, compared to other material platforms like silicon nitride [2] and lithium niobate [3], Manuscript  silicon photonics allows lower energy consumption in thermooptic switching due to its higher thermo-optic coefficient (TOC) (∼ 1.8 × 10 −4 K −1 at 1550-nm wavelength) and heat conductivity (∼ 149 W/mK) [4]. Various silicon thermo-optic switches employing microring resonators [5], [6], [7] or Mach-Zehnder interferometers (MZIs) [8], [9], [10] have been demonstrated.
In this work, a polarization-insensitive 2 × 2 thermo-optic MZS is demonstrated on the 220-nm SOI platform. To ensure a broadband response, the MZS consists of a balanced MZI structure adopting polarization-insensitive 3-dB adiabatic couplers [46] as the power splitters/combiners for the delay lines. The cross-section of the waveguide in the delay line is 220 × 220 nm 2 . Employing square-cross-section waveguides ensures the same TOCs and therefore identical phase shifts for both polarizations. Considering the low tuning efficiency of a 220-nm-width waveguide, the phase shifter is carefully designed to reduce the power consumption. In simulation, the MZS exhibits a 10-dB-ER bandwidth of 85 nm spanning from 1515 nm to 1600 nm for both polarizations. Furthermore, we experimentally characterize the performance of the fabricated MZS, and obtain ERs of approximately 15 dB for both polarizations with polarization-dependent losses (PDLs) of approximately 2 dB over the C-band. Notably, at the wavelength of 1550 nm, the ERs are better than 19 dB (17 dB) and 26 dB (29 dB) at the cross (bar) port for the TE and TM modes, respectively.

II. DESIGN AND SIMULATION
A schematic of the proposed polarization-insensitive MZS is shown in Fig. 1. To realize dual-polarization input or output, the MZS and polarization-sensitive grating couplers (GCs) are connected by a polarization beam combiner (PBC) or two polarization beam splitters (PBSs) [47]. A balanced MZI structure with identical phase shifters is adopted in the proposed MZS. Adiabatic couplers [46], based on mode-evolution theory [48], are used as 3-dB splitters/combiners for the phase shifters on the MZI arms. The top and cross-section views of the phase shifter are illustrated in Fig. 2(a) and (b), respectively.

A. Phase Shifter Optimization
The main challenge to realize polarization-insensitive phase shifters is the discrepancy in TOCs of the TE and TM modes in SOI waveguides. With different TOCs, the phase shifts induced by applying the same power for the two polarizations are inconsistent. The calculated TOCs (∂n eff /∂T ) of the TE and TM polarizations at 1550-nm wavelength are shown in Fig. 3. For a 450-nm-width single-mode waveguide, the TOC of the TE mode is almost twice that of the TM mode. Fig. 3 shows  that the discrepancy between the TOCs of the two polarizations shrinks as the waveguide width is reduced. For a 220-nm-height waveguide, the TOC curves of the TE and TM modes intersect when the width is 220 nm, due to the symmetry of the waveguide and similar field distributions of the TE and TM modes. The same TOCs correspond to identical phase shifts under the same temperature change for both polarizations. Therefore, waveguides with a cross-section of 220 × 220 nm 2 are adopted in the phase shifters.
Adopting square cross-section waveguides allows for polarization independence at the expense of the low TOCs. The TOC of the 220-nm-width waveguide is about 1/6 (1/3) of that of the 500-nm-width waveguide, a widely used single-mode SOI waveguide [49], for the TE (TM) mode. Therefore, to achieve  the same phase shift, the 220-nm-width waveguide consumes more power than the 500-nm-width waveguide. The power consumption needs to be reduced, because the high energy cost and heat dissipation prohibit the application of switches in highly integrated systems [16], [50], [51]. Hence, we optimize the design of the metal heater by investigating the relationship between the power consumption for a π phase shift, P π , and the width of the metal, W . Also, the IL introduced by the metal heater is taken into consideration. The calculated P π and ILs as functions of width of the metal heater are shown in Fig. 4. As the width decreases, the P π decreases due to less heat dissipation. For a narrower heater, the IL for the TM mode introduced by the metal absorption is also lower, whereas that for the TE mode which is less than 1% of its counterpart changes minimally. Hence, the width of the metal heater is chosen to be 3 μm, which is the minimum feature size permitted by the foundry.
To further reduce the IL of the switch, we design the taper connecting the square waveguide and the rest of the MZS. As shown in Fig. 2(a), the 220-nm waveguides are only employed in the phase shifter, and 450-nm waveguides are used to connect other components of the MZS. Linear tapers are introduced between the two waveguides to reduce the reflection and mode mismatch losses. Fig. 5 shows the calculated ILs of the taper for the TE and TM modes with varied taper length, based on three-dimensional finite-difference time-domain (3D-FDTD) simulation. The IL decreases monotonically with the taper length. The TE mode exhibits higher ILs than the TM mode, due to it is more sensitive to the lateral disturbance. Considering the compactness of the MZS, the taper length is chosen to be 45 μm, within the constraint that the ILs remain less than 0.05 dB for both polarizations. Moreover, there is a clearance section inserted between the phase shifter and the taper to further reduce the polarization diversity of the MZS, as illustrated in Fig. 2(a). The polarization independence will degrade if non-square waveguides with different TOCs for TE and TM modes are heated during tuning. Unfortunately, the heated region is not limited to the waveguide beneath the metal heater. Therefore, it is essential to determine the range of the area which is heated and elongate the square waveguide accordingly. We investigate the temperatures of the waveguides underneath the edge of the metal heaters with different lengths. The lengths of extended square waveguides, i.e., clearance lengths, needed to cool down to 301 K are also calculated. The thermal and optical simulations are performed and shown in Fig. 6. The results show that the shorter the heater, the higher the boundary temperature, and therefore a longer clearance is needed. To reduce the IL, the length of the metal heater is chosen to be 100 μm, with a corresponding clearance length of 35 μm.
We have optimized the phase shifter in terms of the polarization independence, IL, and power consumption at a wavelength of 1550 nm. The phase shifter is determined to be 100-μm long, and composed of the 220-nm-width waveguide and 3-μm-width metal heater, with P π = 146.3 mW at 1550-nm wavelength.

B. Broadband Operation
In this section, all the components of the MZS are investigated individually over a 100-nm wavelength range. Then the broadband performance of the MZS is evaluated using a circuit level simulation.
First, the wavelength dependency of the phase shifter is explored, including phase shifts and ILs. As shown in Fig. 7, the TE and TM polarizations have the same phase shift at P π (@1550 nm), verifying our discussion above. The phase shift of a 100-μm-length waveguide varies from 1.4π to 0.7π when the wavelength changes from 1500 nm to 1600 nm. The phase shifter is larger than P π for wavelength <1550 nm, due to stronger mode confinement; the phase shift is lower than P π for wavelength >1550 nm, due to weaker mode confinement. This variation reduces the ER of the MZS as the wavelength deviates   Fig. 8. In general, the ILs for the TE and TM polarizations both increase with respect to the increased wavelength, due to weaker mode confinement and larger spatial overlap between mode and metal heater. Notably, the IL for the TM mode is around two orders of magnitude higher than that for the TE mode, which is consistent with the results in Fig. 4. This difference can be attributed to a high metal absorption for the TM polarization, due to a large spatial overlap between the mode and the metal heater as seen in the simulation. Comparing the results of the initial state and the tuned state at P π (@1550 nm), the IL is reduced after tuning. This is because, at P π = 146.3 mW, the effective index of the waveguide increases with increasing temperature, leading to higher optical confinement and a lower loss. The main problem with the loss difference is that in most cases, only one of the two phase shifters will be used in the switching operation. Consequently, there is less loss introduced in the heated arm, resulting in an imbalance between the two arms. This imbalance reduces the ER and limits the broadband operation of the MZS.
Next, the ILs of other components of the MZS are investigated from 1500 nm to 1600 nm. The taper between the 220-nm square waveguides and 450-nm routing waveguides shows an increasing IL with wavelength, as indicated in Fig. 9. For the adiabatic couplers [46], the power imbalances are 1.2 dB for  the TE polarization and 0.2 dB for TM polarization over the wavelength range from 1500 nm to 1600 nm.
After analyzing all the components individually, we conduct circuit simulations using Lumerical Interconnect to evaluate the broadband performance of the MZS. Noting that the PBC and PBS are only used to facilitate the characterization of the MZS, we focus on the simulation results of the MZS without them. As shown in Fig. 10, over the whole C-band, the ERs are larger than 8.9 dB (12.5 dB) at the bar port and 10.4 dB (20.0 dB) at the cross port for the TE (TM) mode. The 10-dB-ER wavelength ranges are [1513, 1600] nm for the TE mode and [1500, 1600] nm for the TM mode at the cross port. Overall, a broadband operation of switching the TE and TM polarization simultaneously has been shown in the simulation.

III. FABRICATION AND CHARACTERIZATION
The device is fabricated on a standard 220-nm SOI wafer by Applied Nanotools Inc. The silicon layer of the device is patterned using a 100 keV electron-beam lithography (EBL) technology, followed by an inductively coupled plasma-induced reactive ion etching process. The TiW thin film for metal heater and aluminum thin film for metal routing is deposited above a 2.2-μm plasma-enhanced chemical vapor deposition (PECVD) oxide cladding using electron beam evaporation. 300-nm SiO2 passivation is deposited on top of them as a protective layer. The footprint of the device is 1015 × 682 μm 2 . The optical microscope image of the fabricated MZS and the layout are displayed in Fig. 11.
For the characterization, a Yenista TUNICS 1200S-HP Cband tunable laser is adopted as the light source. Between the laser and the device under test, a polarization controller (PC) is inserted to adjust the injected light to be TE or TM polarized. Broadband GCs [52] are employed to guide the TE or TM polarized light between the fiber array unit (FAU) and the corresponding ports of the PBC/PBS connected to the MZS. The output light collected by the FAU is fed into the power meter, a Yenista CT400 optical component tester. Two back-to-back PBSs connected with two GCs are placed near the MZS for normalization.
In the experiment, an initial calibration power of 37.0 mW (ΔP = 0 mW) is needed to tune the MZS to the on-state at the bar port and the off-state at the cross port, compensating for the random phase error introduced by the imbalance between the phase shifters. This imbalance is caused by the fabrication error in the waveguides and can be minimized using better fabrication control. At the bar (cross) port, from the on-state (off-state) to off-state (on-state), corresponding to a π-phase shift, the switching power is ΔP = 152.78 mW, consistent with the simulated results. The high value of P π can be attributed to the low TOC of the 220-nm-width waveguide compared to wider single-mode waveguides of switches in the literature [8], [9], [10]. Aside from the aforementioned design of the metal heater, the power consumption could be further reduced by adopting suspended structures [53], [54] in the phase shifters to improve the thermal isolation. Fig. 12 shows the measured transmissions for both polarizations from 1500 nm to 1600 nm at the bar port and the cross port. The transmissions of back-to-back PBSs and GCs are normalized. The MZS switches the TE and TM modes with ERs of 19.40 dB (17.19 dB) and 26.46 dB (29.88 dB) at the cross (bar) port at the wavelength of 1550 nm, respectively. The difference in the ERs of the two polarizations can be mainly attributed to the different maximum splitting ratio imbalances of the adiabatic coupler for the TM mode (0.2 dB) and the TE mode (1.2 dB) [46]. Over the C-band, the MZS exhibits ERs >14.06 dB at the bar port and >15.91 dB at the cross port for both the TE and TM modes. At the bar (cross) port, the 10-dB-ER bandwidth is 85.3 nm (75.8 nm) for the TE mode, and the ERs are better than 26 dB (18 dB) for the TM mode, from 1500 nm to 1600 nm. The experimental results of the ERs and bandwidths are summarized in Table I. Overall, the experimental results are consistent with the simulated results and verify the broadband operation capacity of the MZS. The deviation between the dips of simulated and measured transmissions could be attributed to the fabrication variations in the waveguide width and/or thickness. It is also observed that there are ripples in the power transmissions. This can be partially explained as the ripples in GC's response due to the fabrication imperfection. Furthermore, the fabrication-variation-caused mismatch of the ripples between  Fig. 13. Polarization-dependent loss (PDL) for the difference in the transmissions of the TE and TM modes at P π at the cross port.
GCs in the calibration set and in the MZS results in stronger ripples in the normalized transmissions.
The ILs of the MZS are 0.2∼4.3 dB over the C-band, which are high due to the sidewall roughness of the narrow waveguide in the phase shifter region. These high ILs can be further reduced with improved fabrication process. Fig. 13 shows the PDLs of the MZS, which are the transmission differences between the TE and TM polarizations at the cross port when P π is applied. From 1500 nm to 1600 nm, the absolute values of the measured PDLs are around 2 dB. Our results indicate that the switching performances of the MZS for the TE and TM modes are approximately equivalent.

IV. CONCLUSION
In conclusion, we have demonstrated a broadband, polarization-insensitive thermo-optic MZS on the 220-nm SOI platform. The balanced MZI employs symmetric phase shifters consisting of square waveguides and clearances, to maintain the polarization independence. The tuning efficiency and the IL of the phase shifters are also improved by optimizing the width of the metal heater. Broadband polarization-insensitive switching operation has been shown both in simulation and experimentally. The fabricated MZS exhibits ILs of 0.2∼4.3 dB and ERs > 14 dB over the C-band, and a 10-dB-ER bandwidth of 85 nm (1515∼1600 nm), for both the TE and TM polarizations. The power consumption of the MZS can be further reduced by using a suspended structure to improve the thermal isolation. The extinction ratio of the switch can also be improved by utilizing couplers with a more balanced splitting ratio. The proposed MZS can be scaled to a multiport switch in a multi-stage manner, and connect different modules in an optical interconnect network employing PDM technology.