Ambient light immunity of a frequency-modulated continuous-wave (FMCW) LiDAR chip.

The interference between a frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) and other LiDARs or sunlight was theorized, considering the spatial overlap, frequency overlap, and intensity ratio. It has been concluded that the interference probability between LiDARs can be lower than a safety standard value for autonomous vehicles when the number of the resolution points of a single LiDAR is increased sufficiently and that the interference with incoherent sunlight does not occur. Due to the coherent detection of FMCW, such ambient light immunity is much better than time-of-flight LiDAR. The dependence of the interference on the wavelength range, sweep bandwidth, and sweep period was also observed experimentally using a silicon (Si) photonics FMCW LiDAR chip incorporating slow-light grating beam scanners. It was shown that the interference can be suppressed by increasing the number of resolution points and changing their common parameters moderately. Regarding the contamination of sunlight, unwanted beam shift due to heating was observed, although it will be suppressed simply by wavelength filtering.

All-optic control using a photo-thermal heater in Si photonics.

We propose and demonstrate a simple all-optic control for Si photonics using a photo-thermal heater. The control light is absorbed in a heavily doped control waveguide and the signal light phase is tuned through thermal diffusion in a signal waveguide adjacent to but not optically coupled with the control waveguide. We designed and fabricated Mach-Zehnder- and microring-type devices requiring 17 (π-phase shift) and 4 (switching between resonance and non-resonance with 6 dB extinction) mW of control power, respectively. We confirmed that the heating efficiency of all-optic control exceeded that of an electrical heater placed above the signal waveguide.

Step-like beam scanning in a slow-light grating beam scanner for a FMCW LIDAR.

We have developed a nonmechanical beam scanner equipped with a Si photonics slow-light grating toward an on-chip frequency-modulated continuous-wave light detection and ranging (FMCW LIDAR) device. An optical beam is scanned thermo-optically, but it is also shifted sensitively to the frequency modulation, which is inconvenient for FMCW LIDAR. In this study, we canceled this shift and obtained step-like beam scanning with synchronized thermo-optic signals, which was confirmed in space-time-domain beam observations. The step-like scanning allows finer angular resolution of the range profile.

Space-time-domain observation of high-speed optical beam scanning in a thermo-optic Si photonic crystal slow-light beam scanner.

We developed a thermo-optically controlled nonmechanical optical beam scanner using a Si photonic crystal slow-light waveguide with a diffraction grating to achieve on-chip light detection and ranging (LIDAR). This Letter applies pre-emphasis signals to the thermo-optic control, and the cutoff frequency increases to 500 kHz. Observing the beam scanning in the space-time domain showed that the turn-on and turn-off times of the scanner for a rectangular drive voltage were 10 µs and reduced to 2.7 µs when the pre-emphasis signals were optimized. This new, to the best of our knowledge, result enables a frame rate of 29 fps for 12,800 resolution points in LIDAR.

Slow-light-based variable symbol-rate silicon photonics DQPSK receiver.

We report a silicon DQPSK receiver whose symbol rate can be varied by a tunable one-bit delay line including an all-pass micro-ring slow-light device. It also consists of Si-wire waveguides with spot-size converters, optimized splitters/couplers, heater-controlled Mach-Zehnder attenuators and phase shifters, 90° hybrid with a low-loss crossing and balanced Ge photodiodes, all of which are fabricated by using CMOS-compatible process. Demodulation was confirmed at symbol-rates of 7.4 - 9.0 Gbaud, corresponding to bit-rates of 14.8 - 18.0 Gb/s.

Continuously tunable slow-light device consisting of heater-controlled silicon microring array.

We experimentally demonstrate a tunable slow-light device consisting of all-pass Si microrings. A compact device of 0.014 mm2 footprint is fabricated by using CMOS-compatible process, and its center wavelength, bandwidth and delay are continuously tuned by integrated heaters. The tuning range is 300 ps at fixed wavelengths with a 1 nm bandwidth. Eye opening of 40 Gbps non-return-to-zero signals is observed at up to a 150 ps delay and a 4 bit buffering capacity is confirmed, which corresponds to a spatial buffering density of 0.29 kbit/mm2.