A near Fourier-transform limited optical parametric oscillator/amplifier system as a pulsed source for high resolution spectroscopy

Abstract

Abstract:We use optical heterodyne and nonlinearoptical spectroscopic techniques to measure the near Fourier-transform limited output from a narrowband tunable injection-seeded, pulsed optical parametric oscillator/amplifier system that has a controllable frequency chirp of <10 MHz.We have demonstrated single-longitudinal-mode (SLM) operation of an injection-seeded, tunable optical parametric oscillator (OPO) based on quasi-phasematched KTiOPO 4 (PPKTP) [1]. Our most recent system [2] employs a SLM Nd:YAG laser to pump the PPKTP crystal with much longer (~27-ns) 532-nm pulses, and with injection-seeding at ~842 nm by a cw SLM tunable diode laser (TDL). This generates narrowband, continuously tunable output radiation with 25-ns pulse duration. The optical layout of our system is shown in Fig. 1. Optical heterodyne techniques [3] have been used to control the OPO's output frequency chirp and to optimize its SLM operation. Investigation of the longpulsed system [2] shows that at low pump energies (<2.5 times the free-running threshold), the narrowband tunable OPO output is SLM and its frequency chirp can be <10 MHz, much less than the transform-limited optical bandwidth (~17.5 MHz). Fig. 2 shows that the chirp (variation of the instantaneous frequency f inst during each OPO pulse) can be minimized by matching the output wavelength of the injection-seeded OPO to that of a free-running (unseeded) OPO with a given PPKTP crystal temperature.