A LiDAR system typically consists of a waveform generator and a laser, which emits light to gather range or velocity data. Although theoretically any light source could be used, lasers have become the standard due to their precision. The laser can be a single unit, a seeded laser, or a master oscillator with amplifiers. LiDAR systems can be either monostatic (using a single aperture for transmission and reception) or bistatic (using separate apertures for each).
Diagram of Lidar
LiDAR operates by transmitting light through a medium, usually the atmosphere, to reach a target. It can also function in space, water, or human tissue, though water and tissue absorb light, limiting the effective range. In water, blue and green wavelengths are best for imaging, while red light penetrates human tissue better. Medical LiDAR applications typically use a tomographic imaging mode, capturing scattered light from multiple angles to create an image of the tissue.
After bouncing off the target, the light returns to the receiver through the medium. If a lens is used to focus the returning light, the system converts the captured data into an image. This combination of range detection and imaging makes LiDAR a versatile tool for various applications.
LiDAR systems can capture the full field (phase and intensity) of the light at the aperture (pupil plane) to generate high-resolution images using Fourier transformation. This technique allows for synthesizing larger pupil-plane apertures. To measure the phase in addition to intensity, LiDAR systems use a local oscillator (LO), which beats against the return signal. This method, known as coherent LiDAR, enables precise phase detection.
Optical wavelengths are too high in frequency for direct phase measurement, so the LO is used to detect the beat frequency, allowing measurement of both the phase and intensity of the return signal. Coherent LiDAR can accurately measure distance and velocity by determining the phase changes. The timing signal from the transmitter enables range measurement through the calculation of time-of-flight for the laser pulse. The data captured by the detector is digitized and processed to generate detailed information like velocity or vibration frequency using Doppler shift.
