Search

Coherent anti-Stokes Raman spectroscopy (CARS) is a form of nonlinear-optical spectroscopy that is useful for spectroscopic sensing of high-temperature media. Earlier research1 evaluated optical parametric oscillator (OPO) devices as an alternative to dye lasers in providing tunable coherent light for this purpose. Further development of the OPO CARS theme follows demonstrations2,3 of how to control the optical bandwidth and output wavelengths of a pulsed OPO with a passive ring cavity, injection-seeded by a single-mode external-cavity diode laser (ECDL). We have succeeded in injection seeding a single pulsed OPO at two separate optical frequencies that fall within its intrinsically broad optical bandwidth, using a pair of ECDLs. This OPO-based strategy enables single-shot, two-colour multiplex CARS measurements4 that can be used for combustion diagnostics, as has previously been demonstrated with dye-laser sources.5

The performance and reliability of optical parametric oscillators (OPO) has improved markedly with the availability of high-quality pump laser sources and of new nonlinear optical materials, such as β-barium borate (BBO).1,2 This yields enhanced OPO conversion efficiency and output power over a wide tuning range. For instance, we have shown3 that a BBO OPO, pumped by pulsed 355-nm radiation from a single-mode injection-seeded Nd: YAG laser, can attain a total (signal + idler) output pulse energy of ≥100 mJ throughout the tuning range 430 nm −2.0 µm, with maximum conversion efficiency >60%.

The performance of optical parametric oscillators (OPOs) has recently improved markedly through the use of injection-seeded single-mode pump laser sources and of new nonlinear optical materials, such as β-barium borate (BBO).1 With a BBO OPO pumped by such a source (delivering 315 mJ per 5-ns pulse at 355 nm), we attain a total (signal + idler) output pulse energy of ≥100 mJ throughout the tuning range 430 nm–2.0 µm, with maximum conversion efficiency >60% at a signal wavelength of 485 nm. This is achieved with a large BBO crystal (7 × 7 × 15 mm3, 30° cut for type I phase matching). Continuous tunability of the signal and idler wavelengths is readily achieved2 by injection seeding of the OPO cavity, at either the signal or idler wavelength, with narrowband radiation from a low-intensity tunable laser, such as a dye or diode laser. The output wavelengths are controllable over a wide tuning range (> 100 cm-1) with minimal adjustment of the angular setting of the BBO crystal, resulting in remarkably simple spectroscopic operation.

The technique of time‐resolved infrared–ultraviolet double resonance (IRUVDR) spectroscopy is used to characterize the rate and mechanism of state‐to‐state rotational energy transfer (RET) in D2CO/D2CO collisions. The investigations employ CO2‐laser irradiation to prepare a D2CO molecule in the v4=1, (J,Ka) =(18,11) rovibrational level of its X 1A1 electronic ground state. Vapor‐phase collisions with other D2CO (v=0) molecules then induce RET, with IRUVDR‐monitored quantum‐number changes ΔJ for the state‐selected molecule ranging between +3 and −7. Kinetic modeling of the resulting experimental data shows that the inelastic cross sections for such J‐changing rotational relaxation can be described adequately by simple scaling laws based on the rotational energy change ‖ΔE‖ for the state‐selected molecule, with a power‐gap fitting law proving marginally superior to an exponential‐gap fitting law. The range of ‖ΔJ‖ monitored in these experiments is sufficiently extensive to discredit a simple propensity‐rul...