From a paper by Anton Lukashchuk, et al in Nature Communications.
Widely employed in the early 2000’s time-of-flight sensors, which measure the arrival time of reflected pulses, relied on available legacy 900 nm diode lasers and silicon detectors. Another type of LiDAR is frequency-modulated continuous wave (FMCW) LiDAR5,6, which maps the distance and velocity of an object to frequency. This method, an optical analogue of coherent RADAR, utilizes optical self-heterodyne detection of a frequency-chirped continuous-wave light reflected from a target with its replica, that serves as the local oscillator (LO).
In contrast to the time-of-flight approach, coherent ranging allows for instantaneous velocity measurement via the Doppler frequency shift, quantum noise limited detection enabled by heterodyne detection with sufficient LO power, eye-safe operation at low average powers, immunity to ambient light sources, and low-bandwidth receiver electronics (100s of MHz) capable of providing cm-level resolution mainly dependent on frequency excursion of the transmitted chirp. However, the cost and bulky size of individual LiDAR components and their assembly still preclude the wide adoption of ranging sensors.
Frequency-modulated continuous wave LiDAR, in particular, requires multiple building blocks, including a frequency-agile laser, driving electronics, scanning optics, passive components (grating couplers, switching network), and detectors. A variety of recent work attempted to integrate coherent LiDAR components on chip. Martin et al. demonstrated a silicon photonic circuit with integrated detectors, waveform calibration and switching network for passive beam scanning capable of 60 m coherent ranging at 5 mW output power7.
A number of recent works employed optical phased array (OPA) technology to achieve 2D passive scanning8,9,10. Poulton et al. demonstrated a nearly centimeter scale OPA aperture with 8192 elements achieving 100o × 17o field of view3. Rogers et al. developed a focal plane array (FPA) 3D LiDAR on a silicon chip with photonic-electronic monolithic integration of a 512-pixel coherent receiver array1. Further large scaling of FPA pixels employing switchable MEMS grating antennas was reported by Zhang et al.11.
For the complete paper on coherent laser ranging CLICK HERE.
