Ruslan V. Chulkov

Observation of Raman conversion for 70-fs pulses in KGd(WO4)2 crystal in the regime of impulsive stimulated Raman scattering.

Raman conversion of femtosecond pulses in a solid-state medium is reported. Stimulated Raman scattering (SRS) was observed in an impulsive SRS regime when the pump pulses duration was comparable with the period of Raman-active vibration. The generation of super broadband radiation, which is a distinguishing feature of impulsive SRS, is shown.

Bessel beam transformation by anisotropic crystals

Transformation of Bessel beams by biaxial and uniaxial crystals is investigated experimentally and theoretically. Experimental observations show beam symmetry changing and formation of complex intensity patterns, depending on the orientation of the crystal. These patterns can appear as a regular system of peak intensities. Results of numerical calculations support the experimental findings.

Spectral shift of Raman generation band in a Bessel pump beam

A spectral shift in the first Stokes generation band has been observed at stimulated Raman scattering in a Bessel light beam. Under nanosecond pulse excitation in barium nitrate crystal, the Stokes emission propagating along the Bessel beam axis has been found to be frequency shifted toward the blue side by 3.7 GHz with respect to the Stokes emission propagating along the surface of a cone. Numerical data have shown that the observed frequency shift is directly related to the contribution of the real part of the Raman susceptibility to the refractive index. This contribution causes an increase in the Stokes gain out of exact Raman resonance owing to induced focusing, which inhibits diffraction spread of generated light and thus provides its better spatial overlap with the pump beam.

Generation of eye-safe radiation in passively Q-switched laser with three Stokes intracavity Raman conversion

Raman conversion is one of the methods for obtaining pulsed laser radiation into the eye-safe (near 1.5 µm) spectral region. Well developed lasers on Nd doped crystals generate radiation around the 1.06 and 1.3 µm wavelengths. Both passive and active Q-switchings are widely used for pulse generation at 1.06 µm wavelength. For generation at 1.3 µm wavelength an active Q-switch is primary applied, because of low efficiency of passive absorbers in this spectral range. Starting at 1.06, or 1.3 µm wavelengths Raman conversion in crystal media with Raman shifts from 900 to 1000 cm−1 allows to reach the eye-safe region by three, or one Raman shifts. Such possibilities were demonstrated at the flash lamp pumping for milliJoule level of laser pulses. For laser systems with longitudinal diode pumping when pulse energy is in the microJoule range the one shift Raman conversion of the 1.3 µm laser radiation is usually used with active Q-switch laser operation. Meanwhile, a possibility to obtain the eye-safe radiation from a passively Q-switched all solid-state laser system with Raman conversion and longitudinal pumping is highly attractive due to constitutive simplicity, compactness and good beam quality.

2 kW threshold quasi-cw narrowband all-solid-state Raman laser generating in UV, visible and near IR ranges for spectroscopy

This contribution reports on experimental study of a barium nitrate based Raman laser system pumped with the second harmonic of a quasi-cw Nd:YAG laser. Five Stokes components generated and frequency doubled cover a spectral range of 280-740 nm with an average power from 10 to 800 mW and a spectral width of 0.2 cm in the visible range. Pulsed lasers generating tunable or multiwave radiation have wide applications in Raman spectroscopy. For the successful use, the developed lasers should meet such requirements as continuous tunability or multiwave operation in a wide spectral range, narrow linewidth, sufficient output power, low divergence of the laser beam, simplicity in operation and low cost. All-solid state lasers meet these requirements rather well. Especially Raman lasers using stimulated Raman scattering (SRS) of light in crystals can be promising laser sources for the spectroscopic applications. Recently, we have developed a cheap and simple Raman laser based on the wellknown barium nitrate crystal which can generate nanosecond narrowband (0.25 cm in IR) continuously tunable radiation in the ranges of 190-1800 nm [1,2]. The repetition rate of this laser was equal to 10 Hz. However, for many applications in spectroscopy, especially in Raman spectroscopy, it is necessary to use laser pulses with higher repetition rates, so-called quasi-cw radiation. High repetition rate can substantially improve the conditions of measurement of Raman spectra and shorten the time for this measurement. Recently, some studies have been performed to develop quasi-cw Raman lasers generating radiation in IR and visible ranges [3-6]. In these studies, comparatively high cost diode-pumped Raman lasers were used. To develop a cheap quasi-cw Raman laser and to extend the spectral range of its generation to UV region we have performed our studies on a barium nitrate based Raman laser system pumped with the second harmonic (SH) radiation of quasi-cw flash-lamp-pumped Nd:YAG laser and on frequency shift of Raman laser radiation using the second harmonic generation (SHG). The results of these studies are presented in this report. The optical scheme of the developed laser source is shown in Fig. 1. For pumping the Raman laser the linear polarized SH radiation at 532 nm from a commercially available quasi-cw (1 kHz) Nd:YAG laser with acousto-optic modulation (model LF2210, SOLAR TII) was used. The pumping pulse width was equal to 110 ns (FWHM). The pumping laser beam passed through an optical isolator (half-wave plate, polarizer and quarter-wave plate) to block back-scattered radiation or smoothly change the pump beam power and its polarisation between linear and circular and then the beam was focused with a lens inside a Raman laser resonator. The Raman laser consisted of two spherical mirrors and a barium nitrate crystal of 70 mm length between them. Spherical mirrors were used to compensate partly the thermal lens effect in the crystal due to the dissipation of Raman excitation to heat. Also, the crystal was mounted in a special cage for axial symmetric heat removing. Our previous studies using two-beam time-resolved z-scan in barium nitrate showed that a considerable thermal lens is induced in it due to SRS of nanosecond laser pulses [7]. Using z-scan data and measurement with the help of a collimated He-Ne laser beam propagating through a Raman laser we could determine the optical power of the thermal lens at 1