Intensity-Modulated Direct-Detection Doppler LiDAR Using Pseudo-Random Code and Optical Direct Down-Conversion of Modulation Frequency for Range and Speed Measurement of a Hard Target

Intensity-modulated direct-detection Doppler LiDAR (IM-DDDL) with pseudo-random code is demonstrated for range and speed measurement of a hard target. This IM-DDDL has key features of 1) long-time coherent signal accumulation without requirement of narrow-linewidth laser source, 2) high receiving sensitivity with optical direct down-conversion of modulation frequency, 3) ranging using pseudo-random code, and 4) Doppler shift detection at baseband frequency. These features are demonstrated in preliminary experiments with fiber-based circuit at 1550 nm wavelength which has eye-safety. The employment of fiber-based configuration contributes to compactness and reliability of the instrument. Ranging function using pseudo-random code is suitable for prevention from interference between multiple LiDARs. The condition for detecting Doppler frequency shift is also shown. A moving stroke of a target within a measurement time needs to be enough larger than the wavelength of modulation frequency. Process for future practical use and application to volumetric target sensing are additionally investigated.