Abstract: The KTP OPO was studied for frequency conversion of Nd-doped lasers to longer wavelengths and can be used as the basis for eye-safe lidar systems operating in the 1 550 nm wavelength region. One of the major advantages of OPO based on KTP and its isomorphs is the ability to operate with non-critical phase matching (NCPM). The large acceptance angle for NCPM permits efficient OPO operation even with multi-transverse-mode pump lasers. The operation efficiency of KTP optical parametric oscillators (OPO) pumped by multimode Nd:YAG lasers was recently demonstrated. The highest pulse energy(400 mJ )and highest average-power signal output(12 W) of signal output at 1571 nm from a singly-resonant OPO with a 1 J, 30 Hz, Nd:YAG pump laser were obtained. The results provide a clear demonstration that