Silicon integrated multimode Optical Phased Array (OPA) for parallel beam steering

Silicon integrated Optical Phased Arrays (OPA) have been widely studied for wide and accurate beam steering applications, taking advantage of the high power handling capability, the stable and precise optical beam control, and the CMOS fabrication compatibility to realize low-cost devices. Both one-dimensional and two-dimensional silicon integrated OPAs have been demonstrated, and beam steering over a large angular range with versatile beam patterns have been achieved. However, existing silicon integrated OPAs are based on single mode operation, tuning the phase delay of the fundamental mode amongst phased array elements and generating a single beam from each OPA. Whilst generating more beams for parallel steering are feasible by using multiple OPAs integrated on the same silicon circuit, the device size, complexity as well as power consumption increase substantially. To overcome these limitations, in this research, we propose and demonstrate the feasibility of designing and using multimode OPA to generate more than one beam from the same silicon integrated OPA that can be steered in different locations to scan the target area in a parallel manner. The overall architecture, multiple beam parallel steering operation principle, and key individual components are designed and discussed. Results show that with the simplest two modes operation, the proposed multimode OPA design principle can realize parallel beam steering to reduce the total number of beam steering required over the target angular range and the power consumption by almost 50%, whilst minimizing the device size by more than 30%. When the multimode OPA operates with a larger number of modes, a larger number of beams can be generated, and the required total number of beam steering, the power consumption, and the size are further improved. In addition, we have investigated key multimode supported components (e.g., mode multiplexer and converter, phase shifter, and emitter) required for the multimode OPA to achieve parallel beam steering. We have proposed a novel mode multiplexer and converter (MUX-C), one of the key components of the multimode OPA, based on asymmetric adiabatic taper approach, which has a low insertion loss of 0.32 dB and a broad bandwidth of 75 nm with a compact size. Besides, the proposed multimode phase shifter is capable of providing different phase shifts to signals with different modes. It achieves up to π phase shift for the TE0 and TE1 modes, consuming 25 mW and 23.7 mW power, respectively.  It achieves a low thermal crosstalk of <-25 dB at a lateral distance of 21 µm. Besides, the designed multimode emitter can steer two different beams in two directions, forming parallel beam steering. The designed 1×4 emitter array with 1 μm element spacing and 0˚-150˚ phase delay can steer TE0 mode at 12˚and TE1 mode at 30˚ with negligible sidelobes and having a compact die area of 5.8 × 320 μm2. Therefore, all the designed multimode components contribute greatly to achieve the parallel beam steering with the improved performance of the proposed multimode OPA.