dc.description.abstract | Compared to pulsed Time-Of-Flight (TOF) radar, Frequency Modulated Continuous Wave (FMCW) radar offers unique advantages, such as instantaneous velocity information of (multiple-) objects, elimination of blind time during operation and has a numerous application in the industry, such as velocity measurement for automotive or military sensing, Hand Gesture Recognition (HGR) and non-contact Vital-Sign Monitoring (VSM). In particular for the VSM application where the vital signs are routinely monitored by medical professionals and health care providers, extremely high velocity sensitivity in FMCW radar detection system is highly desirable with minimum influence of phase noise on the small Doppler frequency shift (< 1 Hz) in the received Radio-Frequency (RF) signals. However, obtaining a real-time 4D (3D + velocity) image based on the FMCW radar scheme with a compact antenna size remains a challenge. One of the effective solutions to achieve this goal is FMCW LiDAR, which combines the FMCW radar architecture with additional Electrical-to-Optical (EO) and Optical-to-Electrical (OE) conversion modules. 4-D images can be obtained at the optical wavelength of 1.55 µm by using a miniaturized FMCW LiDAR module with compact internal optics inside. Moreover, these LiDAR images usually exhibit much better angular resolution in both azimuth and elevation than those of FMCW radar. In addition, when we boost the central frequency from RF to optical wave, high velocity sensitivity can also be expected. Ultra-high velocity sensitivity (~ nm/sec) has been demonstrated in the commercial available laser doppler vibrometers [LDV]. Nevertheless, in contrast to 4-D FMCW LiDAR, which is driven by a wavelength sweeping laser, the 3-D profile with absolute distance information can’t be obtained by the interference signal from a static laser in LDV. The use of a wavelength sweeping laser with an instantaneous linewidth as narrow as that of a high-performance static laser is a major challenge in achieving comparable velocity sensitivity between 4-D FMCW LiDAR and LDV. This is because a larger linewidth can result in a reduced measurement range and lower sensitivity to small Doppler frequency shifts. To address this challenge, the concept of the SILO (Self Injection-Locked Oscillator) has been adapted and applied to semiconductor lasers. The SILO technique uses a low-frequency electrical signal to stabilize the optical frequency of the laser, which can result in a significant reduction in linewidth. However, even with the use of SILO, wavelength sweeping lasers can still suffer from larger phase noise and nonlinearity than those of a swept RF source on the transmitter side of the FMCW radar. This can limit the ability of the FMCW LiDAR to resolve small Doppler frequency shifts for high velocity sensitivity performance. In order to simultaneously reduce the phase and amplitude noises in the FMCW LiDAR receiver end, we illustrate how the receiving end of this FMCW LiDAR can provide high-resolution 3-D + instantaneous velocity (4-D) images with an extremely high velocity sensitivity by combining a self-injection-locked oscillator (SILO) and high-performance avalanche photodiode (APD), which simultaneously has high-responsivity and high saturation current, to minimize the phase and amplitude noise, respectively. Compared with using the p-i-n PD receiver, the LiDAR system with APD one can provide a much better quality of 4-D images with a higher velocity sensitivity due to the enhancements in contrast ratio of each voxel. Besides, to further improve the sensitivity for sensing velocity in our 4-D image, our sweeping laser is driven by a pre-programmed waveform to linearize the optical chirp waveform and separately measure the depth and velocity of object at different time slots (hybrid waveform). Consequently, the SILO system produces a higher velocity sensitivity, resulting in a considerably superior quality of 4-D images of a slow-moving (~ 5µm/sec) object to those of reference one with conventional RF receiver. To the best of our knowledge, the velocity sensitivity achieved here (~ 5 µm/sec with 4-D image) is the highest among all the FMCW LiDARs that have been reported so far such as with slow-light grating structure (75 mm/sec, 400 mm/sec) on silicon photonic platform, or photonic crystal (19 mm/sec) beam scanners and phase-diversity coherent detection (1500 mm/sec).Due to ability of the SILO-based receiving end to reduce phase noise in the down-converted baseband signals, a minor Doppler frequency shift can thus be detected with exceptional sensitivity. The reduction of phase and intensity noise in our FMCW LiDAR system opens up new possibilities for the next generation of 4-D LiDARs. | en_US |