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Complicated spectrotemporal processes are associated with the generation of signal and idler output pulses on a nanosecond timescale in an injection-seeded optical parametric oscillator (OPO). The mechanisms of such spectrotemporal dynamics are revealed by numerical simulation, including innovative modeling of instantaneous-frequency profiles and frequency chirp. These simulations are in satisfactory agreement with optical-heterodyne measurements of output from a nanosecond-pulsed OPO system that is based on periodically poled KTiOPO4, pumped at 532 nm by a Nd:YAG laser and injection-seeded at a signal wavelength of ∼842 nm. Frequency chirp in narrowband signal output pulses from such an OPO system has previously been observed to depend on phase mismatch between the pump, signal, and idler waves, and also on the pump-pulse energy. Our simulations accurately predict this behavior and yield realistic estimates of the frequency chirp, optical bandwidth, and spectral purity of the signal output pulse as it evolves, including effects that are not readily observed directly. This approach provides insight into instrumental conditions that facilitate continuously tunable, single-longitudinal-mode operation of such a pulsed OPO system, with optical bandwidth as close as possible to the Fourier-transform limit.

We present a new technique for high-resolution pulsed spectroscopy that employs optical heterodyne detection to determine the instantaneous frequency of individual optical pulses, together with a frequency-binning method to generate high-precision spectra. We further demonstrate that active tuning of the light source is not necessary if the inherent frequency jitter of the source spans the spectral region of interest. This heterodyneassisted approach to coherent pulsed spectroscopy (CHAPS) is applied in real time by using output from a pulsed, injection-seeded optical parametric oscillator–amplifier (OPO–OPA) system, the optical bandwidth of which is characterized via two-photon excitation of the 6S–8S transition in cesium. The resulting sub-Doppler measurements demonstrate the utility of CHAPS as a high-resolution spectroscopic technique and confirm that the pulsed OPO–OPA system operates very close to the Fourier-transform limit. © 2006 Optical Society of America OCIS codes: 020.4180, 040.2840, 120.6200, 190.4970, 230.4320, 300.3700, 300.6320, 300.6410.

An injection-seeded optical parametric oscillator (OPO) based on periodically poled KTiOPO4 is pumped at 532 nm by relatively long (~27-ns) pulses from a specially constructed Nd:YAG laser. This pulsed OPO system generates continuously tunable, single-longitudinal-mode output at signal wavelengths near 842 nm, which is suitable for high-resolution spectroscopy. Optical-heterodyne measurements show that chirp in the instantaneous frequency of the pulsed OPO signal output radiation increases linearly as the seed frequency is detuned from the free-running (unseeded) OPO frequency. The frequency chirp can be maintained below 10 MHz, which is substantially less than the Fourier-transform-limited optical bandwidth (17.5 MHz full width at half-maximum for a 25-ns OPO signal pulse) and is insensitive to variation of the fluence of the pump radiation. The effects of detuning the OPO cavity length from resonance with the seed frequency and the onset of partially seeded OPO operation are also investigated.

We evaluate ways to analyze optical-heterodyne measurements of frequency chirp in pulsed, single-longitudinal-mode output from lasers (or other coherent light sources) that operate on nanosecond time scales. The instantaneous frequency is extracted from the beat signal generated between a continuous-wave reference beam and the output of the pulsed source. Three analysis techniques are tested: Fourier-transform, direct curve fitting, and electronic mixing. We use synthetic beat waveforms based on actual experimental parameters to evaluate the three methods and apply these chirp-measurement techniques to an injection-seeded optical parametric oscillator system.

We describe techniques for laser spectroscopy in the vacuum-UV (VUV) spectral region that combine high spectral resolution with high absolute accuracy. A nearly transform-limited nanosecond laser source at 120 nm is constructed with difference-frequency mixing. This source is used to perform the first, to our knowledge, Doppler-free VUV measurement. We measure the inherently narrow 11S–21S two-photon transition in atomic helium with a spectral resolution of 7 parts in 108 (180 MHz), the narrowest line width so far observed at such short wavelengths. Careful measurements of optical phase perturbations allow us to determine the absolute frequency of the line center to a fractional uncertainty of 1 part in 108. Improvements now in progress should reduce this uncertainty to 2 parts in 109.

The interference between two different quantum-mechanical pathways for four-wave mixing in sodium vapor has been observed. The four-wave mixing was enhanced by two-photon resonance with the 5s state, for one pathway, and the 4d state, for the other. The phase of the interference could be varied continuously by varying laser frequencies, and constructive and destructive interference effects were studied. A model based on third-order perturbation theory gives a good description of the features observed.