Knowledge Bank

Ideally, pulse-amplified cw lasers combine the high peak powers of a nanosecond pulsed laser with the resolution and controllability of a continuous laser. In practice, optical phase perturbations in the amplifier chain can produce substantial chirping and shifting of the laser frequency, sometimes by $hundredsofMHz.1$ Recently, our group has demonstrated that additional frequency shifts occur when optical harmonic generation is performed with imperfect phase matching. We describe recent efforts to understand the sources of both types of perturbations, and to reduce them by redesigning the optical system. The most significant improvement is accomplished by tailoring a laser dye mix that has an excited-state susceptibility near zero at the operating wavelength. Dramatic improvements have been obtained at wavelengths near 605 nm, required for our precise multiphoton measurements of transitions to the $EF(2s\backslash sigma)\; \{^\{1\}\{\backslash Sigma^\{+\}\}\_\{g\}$ state in molecular hydrogen. Optical heterodyning is used to explicitly measure the time-dependent optical phase. The average frequency shift is now typically 1 MHz (previously it was about 20 MHz), and the chirp is constrained to a range of about 10 MHz (previously about 150 MHz). The result is a temporally smooth pulse, eight nanoseconds in duration, with a bandwidth very close to the theoretical transform limit. This has allowed us to overcome significant difficulties that were encountered in the final stages of data analysis of our previous measurements of two-photon $EF\leftarrow X$ intervals in $H2$, HD, and $D22$, so the accuracy now exceeds one part in $10g$. The same techniques will find applications not only for other high-resolution measurements, but also for spectroscopic methods that use optical phase information, such as frequency-modulation spectroscopy in the far ultraviolet.