High-Sensitivity Heterodyne Detection Based on Optical Orbital Angular Momentum and Algorithm Restoration

Heterodyne detection of weak signals is an important detection method for radar velocity measurement at long distances. However, its measurement precision of velocity is often affected by background light or interference light. Unfortunately, the frequency filtering method cannot filter out background noise, the frequency of which is the same as of the signal light, so the precision cannot be further improved. In this paper, we propose a heterodyne detection system based on optical orbital angular momentum (OAM) modulation to eliminate the effect of noise at the same frequency. Meanwhile, we use Geiger-mode avalanche photodiode (GM-APD) at the receiving end to improve the response sensitivity of the signal light by photon counting method. Then we consider the influence of atmospheric turbulence on the signal light and theoretically derive the expression of heterodyne efficiency at this time. Subsequently, we apply the Gerchberg–Saxton (GS) phase restoration algorithm to the receiving end to improve the velocity measurement precision. The simulation results show that the GS algorithm can improve the velocity measurement precision; when iteration number of GS algorithm is 200 and the structure constant Cn2 of refractive index of atmospheric turbulence is 1 ∙ 10⁻¹⁴ m^−2/3, the velocity measurement precision is improved by 63.0% compared with the original value.