Optical Beam Steering of Large-Scale Silicon Photonics Optical Phased Arrays

Introduction

Optical phased arrays (OPAs) are at the core of solid-state light detection and ranging (LiDAR) systems, which have numerous applications in optical sensing. Silicon photonics-based OPAs have emerged as crucial components for solid-state LiDAR due to their low power consumption, compact form factor, and ability to generate highly focused, low-divergence beams. However, achieving a wide steering angle range typically requires a tunable laser with a large wavelength tuning range, which can be challenging to realize with photonic integration.

In this article, we will explore a novel approach to achieving wide-angle beam steering using large-scale integrated 16x64 silicon photonics OPAs with small-range tunable lasers and 1x16 optical switches.

Device Structure

The demonstrated device comprises sixteen sets of 1x64 sub-OPAs (sOPAs) integrated onto a single silicon photonics chip, as shown in Figure 1. Each sOPA incorporates an apodized sub-wavelength grating (SWG) structure to diffract light toward the chip surface, with the grating pitch varying from sOPA to sOPA to emit light at different angles.

Fig. 1. (a) Schematic diagram of 16x64 OPA chip.

The chip adopts not only grating pitch variation but also two propagation directions to achieve a wide steering angle along the grating direction. The sixteen sOPAs are addressed with a 1x16 optical switch implemented with 4-stage Mach-Zehnder interferometer (MZI) switches and thermal phase shifters.

The OPA chip, designed on a silicon-on-insulator (SOI) platform, comprises 1024 phase shifters, 1024 grating antennas, 1008 multi-mode interference (MMI) couplers, and 15 optical switches, all integrated into a 5.25x5.25 mm² chip area. Figure 2(a) shows a photo of the fabricated OPA chip.

Figure 2: (a) Photo of the fabricated chip and (b) measurement setup.

Experimental Results

The OPA chip was packaged on a circuit board and connected to a 64-channel digital-analog converter (DAC) for controlling the phase shifters. Near-field (NF) and far-field (FF) patterns were measured using an IR camera mounted on an adjustable fixture, as shown in Figure 2(b).

Figure 3(a) displays the rectangular NF pattern when the input light is directed to the 9th sOPA, while Figure 3(b) shows the superimposed NF patterns as the input light is switched over all 16 individual sOPAs, verifying the successful operation of the 1x16 optical switch.

Figure 3: (a) Near-field pattern of a single sOPA (#9), and (b) superimposed near-field patterns of switching to 16 sub-OPAs.

The FF pattern along the grating direction of the 7th sOPA at a wavelength of 1560 nm is shown in Figure 4. Optical speckles occur along the array direction due to phase fluctuations during fabrication, highlighting the need for phase error correction (PEC) to suppress these speckles.

Figure 4: Far-field of #7 sOPA at 1560 nm along the grating direction.

Figure 5 summarizes the beam steering angles for each sOPA, verifying that the OPA chip can cover a steering angle range of -19° to 21°, totaling 40°. This wide steering angle range can be achieved with a tunable laser having a wavelength tuning range of only 16 nm, thanks to the design of the multiple sOPAs and optical switches.

Figure 5: Beam steering angles for each sOPA.

Conclusion

The demonstrated 16x64 OPA with 1x16 optical switches represents a significant advancement in achieving wide-angle beam steering with small-range tunable lasers. By integrating multiple sOPAs with varying grating pitches and optical switches, a 40° steering angle range can be covered with a tunable laser having a wavelength tuning range of only 16 nm. This design simplifies the requirements for integrated tunable lasers, reducing the complexity of control hardware and phase error correction procedures.

With the potential to double the steering angle range to 80° by utilizing a 32-nm tunable laser, this approach paves the way for more feasible and cost-effective implementation of integrated OPAs in solid-state LiDAR and other optical sensing applications.

Reference

[1] C.-Y. Chung, H. K. Njoto, W.-X. Chen, W.-C. Peng, T.-H. Lee, Y.-H. Chen, Y.-C. Hong, S.-L. Lee, "Optical Beam Steering of 16x64 Optical Phased Arrays with Small-range Tunable Lasers," Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, 2024.