Introduction
With the exponential growth in data traffic, there is an increasing demand for efficient and compact photonic integrated circuits (PICs) in optical communication networks. Silicon (Si) optical switches play a crucial role in PICs due to their exceptional capabilities in optical path routing and compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes. This article presents a novel design for a power-efficient 2 x 2 Si Mach-Zehnder interferometer (MZI) optical switch utilizing a compact slow-light phase shifter.
Traditional Approaches and Challenges
Numerous MZI-based Si optical switches utilizing either thermo-optic (TO) or electro-optic (EO) effects have been proposed. TO switches, assisted by the large thermo-optic coefficient of Si, can achieve compact phase shifters of approximately tens of micrometers. However, they require relatively long switching times and high power consumption at the milliwatt level. On the other hand, Si EO switches utilizing carrier-injection type phase shifters with a p-i-n junction can achieve higher operating speeds but typically require longer phase shifters of hundreds of micrometers and power consumption in the milliwatt class.
Carrier-depletion type phase shifters with a p-n junction are extensively utilized in high-speed and power-efficient Mach-Zehnder modulator designs. However, they require long phase shifters of a few millimeters. Therefore, developing a power-efficient Si EO switch with a compact phase shifter is crucial.
Novel Design Approach
Two-dimensional (2D) photonic crystal waveguides (PCWs) have immense potential for developing compact PICs due to their strong light-matter interaction caused by the slow-light effect, resulting from their high group index (ng). This study proposes a power-efficient Si MZI optical switch designed using a compact slow-light carrier-injection type EO phase shifter based on a novel 2D PCW in a perturbed kagome lattice on a silicon-on-insulator (SOI) platform.
As shown in Figure 1, the photonic crystal is constructed from hexagonal-arranged unit cells in a perturbed kagome lattice. A line defect is introduced at the center of the PCW with a width of √3b, where b is the side length of the unit cell. Additionally, a lattice shift (w) of 50 nm is introduced. The PCW is designed on a 220 nm thick Si membrane using an air-bridged structure.
Compact Slow-Light Phase Shifter Design
Figure 2(a) presents a 3D schematic and top view of the slow-light phase shifter. To achieve efficient light coupling with a Si strip waveguide, a step-taper PCW design is applied to the input/output regions of the PCW phase shifter. The lengths of the taper and the main PCW are as short as 3.63 μm and 10.44 μm, respectively, due to the high ng (~ 50) of the perturbed kagome lattice PCW. The resulting total length of the phase shifter (L) is 17.7 μm, comparable to one of the most compact TO phase shifters.
The defect width of the taper is optimized, as shown in Figure 2(b). A maximum transmission of -5.11 dB is confirmed when wstep is 90 nm. Figure 2(c) shows the transmission spectra of the phase shifter with and without utilizing the step-tapered PCW. Compared to the phase shifter without the step-tapered regions, a transmission improvement of more than 9 dB is confirmed in the 3.3-nm wavelength range with high ng and low group velocity dispersion (GVD).
MZI Switch Design and Performance
Figure 3(a) and (b) depict schematic views of the designed MZI switch and a cross-sectional view of the phase shifter, respectively. A p-i-n junction structure is introduced in both arms to maintain the balance of the transmission loss. A forward bias voltage (Vbias) is applied to one arm of the switch. The calculated I-V curve is shown in Figure 3(c).
Figure 3(d) shows the calculated switching characteristics at a wavelength of 1555 nm. A distinct switching operation is observed as Vbias increases to a half-wave voltage (Vπ) of 1.033 V. A much lower product of Vπ and L (VπL) of 1.83 × 10-3 V·cm is obtained compared with reported EO and TO switches, indicating a compact and power-efficient design.
A low current of 296 μA is obtained at a Vbias of 1.033 V, as shown in Figure 3(c), leading to a low switching power of 305 μW. Figure 3(e) illustrates the transmission spectra for both output ports (port 3 and 4) of the switch at Vbias of 0 and 1.033 V. The switch exhibits an enough extinction ratio (ER) of 13 dB or greater within the 3.3-nm wavelength range with high ng and low GVD.
Based on the carrier density response in the I-region to the electrical step signal, the EO response at port 3 of the MZI switch is calculated using Von and Voff of 1.033 and 0 V, respectively, as depicted in Figure 3(f). A switching operation is achieved with an on and off time of 0.6 and 8.9 ns, respectively.
Conclusion
In this study, a 2 x 2 Si MZI optical switch utilizing a compact slow-light carrier-injection type EO phase shifter is numerically studied. Assisted by the novel slow-light PCW in the perturbed kagome lattice with a high ng of 50, the phase shifter is designed in a compact size of 17.7 μm and requires an extremely small VπL of only 1.83 × 10-3 V·cm. An ER of 13 dB or more within the 3.3-nm wavelength range with a low switching power of 305 μW is obtained through numerical calculation. Moreover, a switching operation is achieved with an on or off time of 8.9 ns or shorter, demonstrating the potential for power-efficient and high-speed optical switching applications.
Reference
[1] D. Li, W. Yang, T. Kakitsuka, and K. Takahata, "Power-efficient 2 × 2 Si Mach-Zehnder interferometer optical switch utilizing compact slow-light phase shifter," Graduate School of Information, Production and Systems, Waseda University, Fukuoka, Japan, 2024.
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