Optical fiber preamplifier in lidar detection

A ratio for direct detection and heterodyne detection has been developed for an in-house 1.06-μm lidar system at Wright Laboratory. A signal-to-noise ratio formula has also been developed for direct detection and heterodyne detection with a rare earth doped optical fiber preamplifier incorporated into the lidar system. With the fiber preamplifier in the system, the return signal is amplified by a power gain factor G, but the fiber preamplifier also adds a noise term to the signal-to-noise ratio as a result of the spontaneous emission inherent in this type of optical amplification. This noise that is added by the spontaneous emission limits the benefit that the fiber preamplifier will have on system sensitivity. The spontaneous emission noise will have a different impact on the direct detection scheme than it will on the heterodyne detection scheme because of the presence of the local oscillator in the heterodyne case. The effect of the spontaneous emission is examined in detail for both detection schemes. An experimental setup for incorporating the fiber preamplifier into the lidar system is given, and results of sensitivity comparisons are discussed.