Sergei N. Smetanin

Four-wave parametric processes in multicascade SRS conversion of neodymium laser radiation into an eye-safe region

Four-wave parametric processes in multicascade SRS conversion of neodymium laser radiation into an eye-safe region are studied in relation to the phase mismatch of parametric coupling, which is determined by the refractive index dispersion of the SRS medium. The numerical simulation showed that the generation of multicascade SRS at a particular wavelength in the IR region can be even more efficient than the single-cascade conversion if the parametric coupling between the first Stokes and anti-Stokes components is weak and higher Stokes components are strongly parametrically coupled with each other. This situation is observed in the eye-safe IR region, where the refractive index dispersion of solid-state SRS media is minimal, which allows one to optimize the SRS conversion by choosing an SRS medium and its length.

Determination of the stimulated raman scattering threshold for a pump pulse of arbitrary width

A theoretical solution to the problem of determining the stimulated Raman scattering (SRS) threshold has been found within the undepleted pump approximation for a pump pulse of arbitrary width, which distinguishes it from the known solutions for the limiting cases of very short (highly transient SRS) and very long (quasi-steady-state SRS) pump pulses with respect to the oscillation dephasing time of the SRS medium. The general formula of the theoretical estimate of SRS threshold, in dependence of not only the pump radiation intensity and the SRS interaction length but also the pump-pulse width, is obtained based on the found solution. The theoretical estimate of the SRS threshold has been shown to be in good agreement with the experimental results on the excitation of picosecond SRS in crystals, which justifies the new express method for estimating the SRS gain in experimental measurements of the picosecond SRS threshold.

Self-Q-switching of a loop laser cavity with a self-pumped four-wave phase-conjugate mirror in an active laser medium

Self-Q-switching of a loop laser cavity with a self-pumped four-wave phase-conjugate mirror in an active laser medium with the use of a Faraday isolator or a passive Q-switch is studied theoretically. It is found that, in contrast to passive Q-switching of an ordinary cavity, a passive Q-switch in a loop phase-conjugate cavity serves as an auxiliary unit that increases the self-Q-switching efficiency by gain gratings in the active laser medium, since it decreases the difference in the intensities of waves that write a phase-conjugate mirror in the active medium, which is similar to the use of a Faraday isolator. However, the passive Q-switch operates alternating the highly efficient writing of a phase-conjugate mirror in the closed state with the highly efficient generation of a giant laser pulse in the open state, which stabilizes the period of repetitive laser pulses.

Parametric Raman crystalline anti-Stokes laser at 503 nm with collinear orthogonally polarized beam interaction at tangential phase matching

Picosecond Parametric Raman crystalline anti-Stokes laser at 503 nm with collinear beam interaction of orthogonally polarized Raman components in calcite at tangential phase matching under 532 nm 20 ps laser pumping is investigated. For the first time the highest anti-Stokes (503 nm) conversion efficiency of 30 % close to the theoretical limit of about 40 % at overall optical efficiency of the anti-Stokes generation of up to 3.5 % is obtained. The highest anti-Stokes 503 nm pulse energy was 1.5 uJ.

Parametric Raman anti-Stokes laser at 503 nm with phase-matched collinear beam interaction of orthogonally polarized Raman components in calcite under 532 nm 20 ps laser pumping

Lasers based on stimulated-Raman-scattering process can be used for the frequency-conversion to the wavelengths that are not readily available from solid-state lasers. Parametric Raman lasers allow generation of not only Stokes, but also anti-Stokes components. However, practically all the known crystalline parametric Raman anti-Stokes lasers have very low conversion efficiencies of about 1 % at theoretically predicted values of up to 40 % because of relatively narrow angular tolerance of phase matching in comparison with angular divergence of the interacting beams. In our investigation, to widen the angular tolerance of four-wave mixing and to obtain high conversion efficiency into the antiStokes wave we propose and study a new scheme of the parametric Raman anti-Stokes laser at 503 nm with phasematched collinear beam interaction of orthogonally polarized Raman components in calcite under 532 nm 20 ps laser pumping. We use only one 532-nm laser source to pump the Raman-active calcite crystal oriented at the phase matched angle for orthogonally polarized Raman components four-wave mixing. Additionally, we split the 532-nm laser radiation into the orthogonally polarized components entering to the Raman-active calcite crystal at the certain incidence angles to fulfill the tangential phase matching compensating walk-off of extraordinary waves for collinear beam interaction in the crystal with the widest angular tolerance of four-wave mixing. For the first time the highest 503-nm anti-Stokes conversion efficiency of 30 % close to the theoretical limit of about 40 % at overall optical efficiency of the parametric Raman anti-Stokes generation of up to 3.5 % in calcite is obtained due to realization of tangential phase matching insensitive to the angular mismatch.

Generation of 120 ps, 1168 nm anti-Stokes pulses from the all-solid-state, self-mode-locked, parametric Raman CaCO3 laser with intracavity pumping by 1338 nm Nd:YAG laser

A novel, all-solid-state, self-mode-locked, collinearly phase-matched, parametric Raman Nd:YAG/CaCO3 laser at 1168 nm anti-Stokes wavelength is reported. We have achieved parametric Raman conversion into the 1565 nm Stokes and 1168 nm anti-Stokes components and self-mode-locking using single Kerr-lens and Raman-active CaCO3 nonlinear crystal inside the cavity of the diode side-pumped Nd:YAG laser generating at 1338 nm. Collinear phase matching of equally polarized Stokes-anti-Stokes coupling was self-organized due to zero dispersion of the CaCO3 crystal at the fundamental laser wavelength of 1338 nm. We demonstrate possibilities of the Stokes and anti-Stokes picosecond pulse shortening and separation of few and even only one 120 ps ultra-short 1168 nm anti-Stokes pulse from the self-modelocked laser pulse train because of fast and spatially uniform depletion of pumping of intracavity Raman conversion without using any electro-optical device. We have obtained high energy output of up to 1.1 μJ in the single intensive anti-Stokes 120 ps ultra-short pulse which was up to 6.8 % from overall output radiation pulse train energy.

All-solid-state, synchronously pumped, ultrafast BaWO4 Raman laser with long and short Raman shifts generating at 1180, 1225, and 1323 nm

A lot of attention is currently focused on synchronously pumped, extra-cavity crystalline Raman lasers generating one or two Stokes Raman components in KGW or diamond Raman-active crystals, and also generating additional components of stimulated polariton scattering in lithium niobate crystal having both cubic and quadratic nonlinearities. In this contribution we report on generation of more than two Stokes components of stimulated Raman scattering with different Raman shifts in the all-solid-state, synchronously pumped, extra-cavity Raman laser based on the Raman-active a-cut BaWO4 crystal excited by a mode-locked, 220 nJ, 36 ps, 150 MHz diode sidepumped Nd:GdVO4 laser generating at the wavelength of 1063 nm. Excitation by the pumping radiation polarized along the BaWO4 crystal optical axis resulted in the Raman generation with not only usual (925cm – 1), but also additional (332cm – 1) Raman shift. Besides the 1180-nm first and 1323 nm second Stokes components with the Raman shift of 925cm – 1 from the 1063nm fundamental laser wavelength, we have achieved generation of the additional 1227 nm Raman component with different Raman shift of 332cm – 1 from the 1180nm component. At the 1227 nm component the strongest 12-times pulse shortening from 36ps down to 3ps was obtained due to shorter dephasing time of this additional Raman line (3ps for the 332-cm – 1 line instead of 6.5ps for the 925cm – 1 line). It has to be also noted that the 1225 nm generation is intracavity pumped by the 1179 nm first Stokes component resulting in the strongest pulse shortening close to the 332cm -1 line dephasing time (3ps). Slope efficiency of three Stokes components generation exceeded 20%.