Abstract

The synchronous two-photon excitation of 3-D sodium level has been carried out using the output of KDP optical parametric oscillator (OPO), and second-harmonic of a pulsed Nd:YAG laser with passive mode-locking and negative intra cavity feedback. Nd:YAG laser with three-stage amplifier system was delivering 60-80 pulses with a total energy of 60-80 mJ at 1 Hz repetition rate. Two light beams were used for excitation of the 3-D sodium level: the one, which was the signal wave of the KDP OPO (the wavelength λ_p1 = 962 nm, spectral width Δν_p1 = 2,5-3,5 cm^-1, energy of the train of 30-35 pulses E_p1 = 0,1-0,4 mJ, pulse duration τ_p1 = 12-18 ps, and the peak intensity of collimated beam I_p, = 0,04-0,16 GW/cm²), and the second-harmonic of the fundamental beam (λ_p2 = 532 nm, Δν_p2 = 0,8-0,9 cm^-1, E_p2 = 10-20 mJ, τ_p2 = 15-20 ps, and I_p2 = 2-4 GW/cm²). The second-harmonic train was used for KDP OPO as well as for sodium OPO pumping. The length of nonlinear medium in sodium vapour cell was 15 cm. When sodium vapour density was 10¹³-10¹⁷cm^-3, the four-photon parametric luminescence in the 589-nm (signal wave) and 820-nm (idler wave) regions was observed. To achieve the positive feedback for four-photon parametric gain the ring cavity, formed mainly by Al-coated mirrors, was used.

© 1994 IEEE