Generation of tunable narrow-band midinfrared radiation in a type I potassium niobate optical parametric oscillator

We have demonstrated a type I phase-matched KNbO3 optical parametric oscillator (OPO) pumped by the fundamental of a Q-switched Nd:YAG laser. The wavelengths of the OPO’s signal and idler radiation are tuned continuously in the spectral regions of 1.45–2.01 and 2.27–4.0 μm, respectively. With a 7.8-mm-long crystal the energy density at threshold is 65 MW cm−2 for 9-ns-long laser pulses and a transmission of the outcoupling cavity mirror of 10%. At pump pulse energies of 82 mJ (two times above threshold) the OPO generates pulses of up to 6 mJ at the signal wave and 5.4 mJ at the idler wave. This output corresponds to a total conversion efficiency of 14%. The measured spectral width of the signal wave increases from 2.5 nm (at 1.56 μm) to more than 30 nm (at 1.97 μm). Injection seeding with the idler radiation of a 355-nm-pumped narrow-band β-barium borate OPO reduced the spectral width of the KNbO3 OPO output to a single longitudinal mode.