LiDAR detection modes are crucial for processing return signals and obtaining valuable data. There are two primary LiDAR detection modes: direct detection and coherent detection.
In direct detection, when light hits the optical detector, it generates a response proportional to the square of the light intensity. This mode is ideal for simple, high-intensity detection.
In coherent detection, the return signal interacts with a sample of the emitted signal, known as the local oscillator (LO). The detector responds to the beat frequency, which is the difference between the return signal frequency and the LO frequency. This process is described by the equation:
I=2EsigELOcos[−j(ωsig−ωLO)]
In temporal heterodyne detection, the LO frequency is offset slightly to help determine target velocity and reduce noise. When the LO has no offset, the method is referred to as homodyne LiDAR.
In spatial heterodyne detection (or digital holography), the LO and return signal are slightly misaligned on the detector to create interference patterns, which are then analyzed by a detector array to extract the phase and amplitude information. If the tilt between the signals is too large, the fringes form too rapidly, causing a loss in clarity.
Figure below demonstrates how the beat frequency for temporal heterodyne detection generates measurable data through interference fringes.
