SPIE Journal Paper | 23 February 2024
KEYWORDS: Heterodyning, Sensors, Signal detection, Signal intensity, Beam diameter, Optical engineering, LIDAR, Partial coherence, Doppler effect, Quantum detection
In a coherent Doppler wind lidar detection system, heterodyne efficiency is an extremely important parameter that reflects the matching degree between local oscillator (LO) light and signal light. More generally, assuming that both the LO light and signal light are partially coherent beams based on the Gaussian–Schell model, we have derived an analytical expression for the heterodyne efficiency in the presence of tilt aberration. The expression indicates that the heterodyne efficiency not only depends on the tilt aberration but also is influenced by the beam parameters, misalignment angle, detector diameter, and wavelength. Specific examples of the variation in heterodyne efficiency with these factors are given in the simulation section. The numerical results show that if high heterodyne efficiency is required, the intensity width of the LO light does not have to be large when the intensity width of the signal light is determined; besides, setting ws/wLO=0.5 is appropriate, where ws and wLO are the intensity width of signal and LO, respectively. Even if LO is a partially coherent beam, the heterodyne efficiency can still reach the global maximum of 0.86, and setting δs/δLO=0.6 is suitable, where δs and δLO are the coherence width of the signal and LO, respectively. As expected, the bigger the δs is, the higher the heterodyne efficiency is. For the same heterodyne efficiency, the tilt aberration can be larger slightly when the misalignment angle is small. We also find that the detector diameter plays a significant role in enhancing the heterodyne efficiency, and it is beneficial to choose a small detector diameter. Besides, the tilt aberration in the system should be less than 0.5λ. Moreover, we show that the rate of decrease in heterodyne efficiency with the tilt aberration is different for different misalignment angles. We also have demonstrated that the longer the wavelength, the higher the heterodyne efficiency for system with the same tilt aberration.
