A 1.5 micron wavelength KTP OPO Lidar system has been developed and used to study the sensitivity and the system signal-to-noise ratio for hard target and atmospheric aerosol lidar returns. Optimization of the receiver system was studied that included the use of different sizes, types of detectors, and the effect of laser beam quality factor, M2, on the telescope overlap function. Detectors that were studied included several different sizes of InGaAs APDs, direct photodetectors, a transferred-electron intensified photodiode (TE-IPD), and a PMT. The influence of the diffraction or beam quality factor (M2) of the OPO laser was studied and found to have a significant influence on the overlap of the transmit and receiver field of view. This overlap function is also influenced by the size of the lidar detector since a large M2 value can overfill the detector/telescope field of view so that small (high speed) detectors may be subject to a large reduction in the lidar signal. The size of the photodiodes and APDs used in the initial OPO lidar experiments were of the order of 0.1 mm to 2 mm, while the TE-IPD and the PMT are larger in acceptance area. Some initial experimental measurements with the photodetectors and APD, and projected theoretical comparisons with the TE-IPD and PMT detectors have been made.