Simplified Homodyne Detection for FM Chirped Lidar

Abstract

The investigation of global warming requires more sensitive altimeters to better map the global ice reserves. A homodyne detection scheme for FM chirped lidar is developed in which dechirping is performed in the optical domain, simplifying both the optical and the RF circuits compared to heterodyne detection. Experiments show that the receiver sensitivity approaches the quantum limit and surpasses the performance of direct and heterodyne detection. In addition, the required electrical bandwidth of the photodiode and receiver RF circuitry are both significantly reduced, facilitating the use of large area photodetector arrays. A field trial using a 5"-aperture diameter telescope and a 370-m target range verified the sensitivity estimation and demonstrates the feasibility of this technique. The problem of homodyne carrier fading is addressed by incorporating a phase diversity receiver using a 90-degree optical coupler. Finally, an outline of the future direction of research is given.