Optical-fiber preamplifiers for ladar detection and associated measurements for improving the signal-to-noise ratio

In an effort to increase achievable postdetection signal-to-noise ratios (SNRs) of continuous-wave, 1-μm all-solid-state ladar systems, a prototype rare-earth-doped optical-fiber amplifier has been included in the optical return signal path of both a heterodyne and a direct-detection ladar system. We provide numerical predictions for SNR increases according to our previously developed theory. We also detail our experimental efforts and provide the results of SNR measurements for four distinct cases: direct ladar detection with and without a fiber amplifier, and heterodyne ladar detection with and without a fiber amplifier. Experimentally measured increases in SNRs for ladar systems incorporating an optical-fiber amplifier are then compared with our earlier predictions. Specifically, we have found that for direct detection with a fiber amplifier in place, the predicted SNR increase is 42.0 dB, and we have measured an increase of 36.5 dB. Similarly, for heterodyne ladar detection with a fiber amplifier, the predicted SNR increase is 3.8 dB, and we have measured an increase of 8.0 dB.