Lorentz reciprocity is a direct consequence of Maxwell equations governing the propagation of light in passive linear media with symmetric permittivity and permeability tensors. Here, we demonstrate the first active optical isolator and circulator implemented in a linear and reciprocal material platform using commercial Mach-Zehnder modulators. In a proof-of-principle experiment based on single-mode polarization-maintaining fibers, we achieve more than 12.5 dB isolation over an unprecedented 8.7 THz bandwidth at telecommunication wavelengths, with only 9.1 dB total insertion loss. Our architecture provides a practical answer to the challenge of non-reciprocal light routing in photonic integrated circuits.

We propose a multilayer silicon nitride (SiN) -on-silicon photonic integrated circuit (PIC) platform with a monolithic laser at the C-band. A tapered edge coupler and a meta-structure-based interlayer directional coupler in the platform were designed to realize low-loss broadband laser-to-chip 3D coupling with small footprint. The coupling length of the interlayer directional coupler and the gap between different SiN layers were optimized as 12.7 µm and 1.4 µm. We measured the 1-dB-drop optical operation bandwidth of greater than 76 nm and the coupling loss of 6.1 ± 0.1 dB at 1550 nm for the interlayer directional coupler. The hybrid integration was demonstrated as a proof of concept for monolithic integration of light sources. The butt-coupling loss of 3.7 ± 0.1 dB between an on-chip DFB laser and a SiN edge coupler at 1549.48 nm was achieved. This approach opens the possibility of employing monolithic laser in the silicon photonics platform.

We propose a bi-layer 5-tip edge coupler in a multilayer silicon nitride-on-silicon (SiN-on-Si) waveguide platform. The coupler is used for the integration between a monolithic 1550 nm laser and a single-mode SiN waveguide. The simulated coupling efficiency is 92.8%. The vertical 1-dB-loss misalignment tolerance is as large as 0.5 μm. Broad 1-dB-drop bandwidth (1338 nm to 1700 nm) and small footprint (total length: 38.2 μm) are achieved simultaneously. A broadband bi-layer SiN-Si adiabatic transition cascaded to the edge coupler is designed to couple the laser power into a single-mode Si waveguide at an efficiency of 90.6%. Low-computation-cost electromagnetic numerical simulation and optimization strategies are applied to improve the reverse design of the complex couplers.

A novel Microstructure PIN photodiode (MPIN-PD) using the ray-optic theory of light trapping effect is presented and analyzed numerically. The MPIN-PD is proposed from an ordinary large-diameter PIN-PD by introducing an obconical shape surrounded by a V-groove trench. The photon-material interaction is increased by using the microstructure in the PIN-PD. It can control light and cause syntony in the absorption layer, reducing the junction capacitance of the PD, and suppressing the tradeoff between the efficiency and bandwidth. Thus, both 3- dB bandwidth and quantum efficiency can be improved simultaneously. The parameters of the MPIN-PD, including the depth and apex angles of the micro-structure, the distance between the cone and V-groove trench are carefully studied to estimate their positive effects of the performance of the MPIN-PD. The transmission line model is exploited for the specific structure to design the electrodes’ distribution.

A cascade array structure and depleted layer thickness design method of uni-traveling-carrier photodetector (UTC-PD) for terahertz applications is proposed. The cascade array structure can increase the 3dB bandwidth from 203 GHz to 255 GHz compared to the high-speed single UTC-PD with a diameter of 3 μm by effectively reducing the overall device capacitance. The 3dB bandwidth can be further improved to 267 GHz by thickness optimization. The combination of cascade array structure and depleted layer thickness optimization can significantly improve the saturation output power of the device at 100, 200 and 300 GHz, which enables the design of cascade UTC-PD array to be applied in terahertz (THz) band.

We report the phase shift of two types of photodetectors for a variation of bias voltage and optical power. One type is PIN-PD, and the other is a modified uni-traveling carrier photodetector (MUTC-PD). In PIN-PD, the phase change shows a photocurrent-dependent variation of about 15 degrees at 3 GHz. It exhibits good stability over a wide bandwidth as the bias voltage changes. By comparison, the phase shift of the MUTC-PD is more sensitive to the bias voltage and the photocurrent. The continuous tunable range of phase shift in MUTC-PD reaches 50 degrees at 10 GHz. It is demonstrated that the adjustable phase shift mainly results from the transit time and the impedance dynamic effects in response to the optical power and bias voltage.