Abstract In this thesis, a single longitudinal mode OPO output pumped by a 1064 nm nanosecond laser is proposed. The single mode is achieved by grazing incidence OPO with an etalon inserted in the OPO cavity. The bandwidth of OPO output is measured by directing signal into a scanning Fabry-Perot, which is controlled by Labview program. The output single mode’s bandwidth is about 210 MHz and can last 30 minutes without feedback control; with feedback control, it can last an hour. The actual single mode’s power is around 20 mW while the total OPO output power is 50~60 mW, others come from the oscillation between etalon and the scanning mirror in the cavity. To extract the single mode’s power, we either reduce the background power to as low as possible but never to zero, or an external etalon might have to be added. The pump power is 2.3 W and the single mode output is 20 mW. The power conversion efficiency is 0.87%.