Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications, such as lidar, radar, and artificial intelligence. Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility, and thus makes it possible to develop large-scale programmable optical signal processors. The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors. In this paper, we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches. The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers (MZCs), four Ge/Si photodetectors, four channels of thermally-tunable optical delaylines. Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step. Particularly, these waveguide spirals used here are designed to be as wide as 2 μm, enabling an ultralow propagation loss of 0.28 dB/cm. Meanwhile, these MZCs and MZSs are designed with 2-μm-wide arm waveguides, and thus the random phase errors in the MZC/MZS arms are negligible, in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly. Finally, this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities, including tunable time-delay, microwave photonic beamforming, arbitrary optical signal filtering, and arbitrary waveform generation.

All-optical silicon-photonics-based LiDAR systems allow for desirable features in scanning resolution and speed, as well as leverage other advantages such as size, weight, and cost. Implementing optical circulators in silicon photonics enables bidirectional use of the light path for both transmitters and receivers, which simplifies the system configuration and thereby promises low system cost. In this work, to the best of our knowledge, we present the first experimental verification of all-passive silicon photonics conditional circulators for monostatic LiDAR systems using a nonlinear switch. The proposed silicon nonlinear interferometer is realized by controlling signal power distribution with power-splitting circuits, allowing the LiDAR transmitter and receiver to share the same optical path. Unlike the traditional concept requiring a permanent magnet, the present device is implemented by using common silicon photonic waveguides and a standard foundry-compatible fabrication process. With several additional phase shifters, the demonstrated device exhibits considerable flexibility using a single chip, which can be more attractive for integration with photodetector arrays in LiDAR systems.

Low-loss and compact optical waveguides are key for realizing various photonic integrated circuits with long on-chip delay lines, such as tunable optical delay lines, optical coherence tomography, and optical gyroscopes. In this paper, a low-loss and compact silicon photonic waveguide spiral is proposed by introducing broadened Archimedean spiral waveguides with a tapered Euler S-bend. A 100-cm-long waveguide spiral is realized with a minimal bending radius as small as 10 μm by using a standard 220-nm-thick silicon-on-insulator foundry process, and the measured propagation loss is as low as 0.28 dB/cm. Furthermore, the present waveguide spirals are used to realize a 10-bit tunable optical delay line, which has a footprint as small as 2.2mm×5.9mm and a dynamic range of 5120 ps with a fine resolution of 10 ps.

A novel wavelength-selective 2×2 optical switch based on a Ge2Sb2Te5 (GST)-assisted microring-resonator (MRR) is proposed. The present GST-assisted MRR consists of two access optical waveguides and an MRR coupled with a bent GST-loaded silicon photonic waveguide. The 2×2 optical switch is switched ON or OFF by modifying the GST state to be crystalline or amorphous. In particular, the microring waveguide and the bent GST-loaded waveguide are designed to satisfy the phase-matching condition when the GST is crystalline. As a result, the MRR becomes highly lossy and the resonance peak is depressed significantly. On the other hand, when it is off, there is little coupling due to the significant phase mismatching. Consequently, one has a low-loss transmission at the drop port for the resonance wavelength. In this paper, the simulation using the three-dimensional finite-difference method shows that the extinction ratio of the designed photonic switch is ～20dB at the resonance wavelength, while the excess losses at the through port and drop port are 0.9 dB and 2 dB. In particular, the resonance wavelength changes little between the ON and OFF states, which makes it suitable for multichannel wavelength-division-multiplexing systems.

Radio frequency (RF) switches are essential for implementing routing of RF signals. However, the increasing demand for RF signal frequency and bandwidth is posing a challenge of switching speed to the conventional solutions, i.e., the capability of operating at a sub-nanosecond speed or faster. In addition, signal frequency reconfigurability is also a desirable feature to facilitate new innovations of flexible system functions. Utilizing microwave photonics as an alternative path, we present here a photonic implementation of an RF switch providing not only the capability of switching at a sub-nanosecond speed but also options of frequency doubling of the input RF signals, allowing for flexible output waveforms. The core device is a traveling-wave silicon modulator with a device size of 0.2mm×1.8mm and a modulation bandwidth of 10 GHz. Using microwave frequencies, i.e., 15 GHz and 20 GHz, as two simultaneous RF input signals, we experimentally demonstrated their amplitude and frequency switching as well as that of the doubled frequencies, i.e., 30 GHz and 40 GHz, at a switching frequency of 5 GHz. The results of this work point to a solution for creating high-speed RF switches with high compactness and flexibility.

An ultrahigh-Q silicon racetrack resonator is proposed and demonstrated with uniform multimode silicon photonic waveguides. It consists of two multimode straight waveguides connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified Euler curves, and a bent directional coupler is used to achieve the selective mode coupling for the fundamental mode and not exciting the higher-order mode in the racetrack. In this way, the fundamental mode is excited and propagates in the multimode racetrack resonator with ultralow loss and low intermode coupling. Meanwhile, it helps achieve a compact 180° bend to make a compact resonator with a maximized free spectral range (FSR). In this paper, for the chosen 1.6 μm wide silicon photonic waveguide, the effective radius Reff of the designed 180° bend is as small as 29 μm. The corresponding FSR is about 0.9 nm when choosing 260 μm long straight waveguides in the racetrack. The present high-Q resonator is realized with a simple standard single-etching process provided by a multiproject wafer foundry. The fabricated device, which has a measured intrinsic Q-factor as high as 2.3×106, is the smallest silicon resonator with a >106Q-factor.