Petr G. Zverev

We report on single and dual-wavelength laser performance of a diode-side-pumped neodymium-doped YAG crystal in the range of 1.3–1.44 µm with sw

Dual- and triple-wavelength diode-side-pumped electro-optically Q-switched Nd:YAG laser at 1318.8 nm, 1338.2 nm and 1356.4 nm with an intracavity etalon was demonstrated. The spectral composition of the generation was controlled by the rotation of the intracavity YAG etalon with a thickness of 100 µm. The experimental values of the tilt angle of the etalon for the laser generation with dual- and triple-wavelength output correspond well to the calculated values. When pumped by the electrical pulses with an energy of 8.7 J, triple-wavelength laser pulses with an energy of 96 mJ, a pulse duration of 37 ns, a peak power up to 2.6 MW and a repetition rate of 100 Hz were obtained. The proposed method of using an intracavity etalon allows to switch the spectral composition of the output radiation of Nd:YAG laser at ⁴F_3/2 → ⁴I_13/2 transition between single-, dual- and triple-wavelength modes.

Two nonlinear processes, namely, two-photon absorption (TPA) and stimulated Raman scattering (SRS) in Na₂W₂O₇ crystal at a wavelength of 523.5 nm, are studied theoretically and experimentally depending on the polarization of the incident picosecond laser radiation. It was found that, due to the anisotropy of the band gap structure, the TPA coefficient at 523.5 nm strongly depends on the pump radiation polarization. For radiation along the c axis with polarization parallel to the a axis, the TPA coefficient was measured to be 1.28 cm GW⁻¹, and no SRS process was registered. However, for polarization parallel to the b axis, no measurable TPA was detected and SRS with a gain of 6.5 cm GW⁻¹ was observed. To the best of our knowledge, this is the first demonstration of the competition of two nonlinear processes in one crystal depending on its orientation.

Comparative characteristics of spontaneous and stimulated Raman scattering in Pb(MoO₄)_1−x(WO₄)_x single crystals (x = 0, 0.5, 0.8, and 1.0) including both the high (ν₁) and low (ν₂) frequency internal anionic group vibrations have been investigated. It was found that among these crystals, Pb(MoO₄)_0.2(WO₄)_0.8 is the most suitable for multi-wavelength Raman laser in several Raman modes simultaneously. This is caused by the optimal relative Mo/W content corresponding to the most efficient coherent combination of the (MoO₄)²⁻ and (WO₄)²⁻ vibrations enhancing the output radiation characteristics of the synchronously pumped Pb(MoO₄)_0.2(WO₄)_0.8 Raman laser. Oscillation of up to twelve, closely spaced SRS components in a range of 1128–1360 nm and the strongest pulse shortening down to 1.16 ps in comparison with not only PbMoO₄ and PbWO₄ but also with all the earlier investigated nominally pure scheelite-like tungstate and molybdate crystals has been achieved.

Comparative investigation of stimulated Raman scattering (SRS) characteristics in the YVO₄, GdVO₄, and Ca₃(VO₄)₂ orthovanadate crystals at both low and high frequency anionic group vibrations is presented. It was found that GdVO₄ is the most perspective for SRS generation on both the ν₁ stretching and ν₂ bending modes of internal anionic group vibrations with the strongest SRS pulse shortening under synchronous picosecond pumping. It is as a result of GdVO₄‘s widest linewidth (17cm⁻¹) of the homogeneously broadened scheelite-type component of the bending ν₂ Raman line that led to the strongest SRS pulse shortening down to the dephasing time of the widest (scheelite-type) Raman mode at the secondary intracavity short-shifted SRS conversion. It allowed us to achieve SRS pulses with sub-picosecond duration under tens-of-picoseconds pumping due to the strongest 42-fold pulse shortening. Using the Ca₃(VO₄)₂ crystal with essentially wider Raman lines (~50cm⁻¹) did not allow us to generate SRS pulses shorter than 1 ps. It can be explained by inhomogeneous broadening of the Raman lines in Ca₃(VO₄)₂ because of its structural disordering. Using the measured SRS pulse duration, the homogeneous broadening of the inhomogeneously broadened bending Raman line of Ca₃(VO₄)₂ was estimated to be ~9cm⁻¹. Among the orthovanadate crystals, the YVO₄ crystal with the highest Raman gain and with homogeneously broadened Raman lines allowed us to realize the most efficient SRS lasing and SRS pulse shortening truly down to inverse half-width of the bending Raman line.

Active media based on particles of solid-state laser materials distributed in liquid or gas may provide a new solution to the problem of heat removal from laser media. We propose active media based on particles of LiF crystal with F₂ color centers, as well as of Nd³⁺-doped crystals and glasses. Mixtures of index-matched liquids with glass and crystal particles, methods of their preparation, and their optical characteristics are considered. The laser characteristics of the active media under excitation by nanosecond laser pulses are compared. A “slurry laser” based on a dense mixture of LiF:F₂ microparticles with an index-matched liquid is demonstrated. Lasing was obtained in small neodymium pentaphosphate crystals (NdP₅O₁₄) and unpolished bulk pieces of concentrated neodymium phosphate glass in immersion liquids.

Active media composed of the particles of solid-state laser-active material immersed in the liquid are the subject of research since this approach can provide new solutions to the problem of heat removal from the laser medium. Investigation of the active medium on microparticles of LiF crystal with F₂ color centers is presented. The technique of the preparation and characterization of the index-matched slurry-like mixture was developed. Powders of LiF crystal particles with F₂ color centers with size from 50 to 1000 μm immersed in various liquids and their mixtures were tested. Laser radiation of a compact cavity containing two flat dichroic mirrors and a glass cuvette with the LiF:F₂ slurry with grain size of 500 ÷ 1000 μm and the mixture of heptane and vaseline oil was obtained under Coumarin 120 dye laser pumping. The pulses of the laser radiation with the beam divergence of 2 ÷ 5 mrad at the wavelength of about 680 nm, duration of 10 ns and energy of 83 μJ were obtained under pulsed pumping with the energy of 1.34 mJ. The real and slope efficiencies of the LiF:F₂ slurry laser reached 6.2% and 12.9% correspondingly.

The frequency shift of the Raman modes in strontium tungstate (SrWO₄) was investigated in the temperature range from 15 to 295 K. The experimental temperature dependence of the shift was analyzed using both the lattice dynamical calculations and the lattice perturbative approach. We found that the quartic anharmonic term of the first-order perturbation and the cubic term of the second-order perturbation, as well as the thermal expansion, contribute to the temperature shift of the highest-frequency A_g(ν₁) mode. The values of the temperature sensitivity of the frequency shift of the Raman modes at room temperature were measured, which is important for developing high-power crystalline Raman lasers and frequency shifters.

Comparative investigation of characteristics of spontaneous and stimulated Raman scattering (SRS) in different alkali-earth tungstate and molybdate crystals at both high and low frequency anionic group vibrations is presented. It has been found that, among these crystals, the SrMoO₄ and SrWO₄ crystals are the most perspective for SRS generation on both stretching and bending modes of internal anionic group vibrations with the strongest SRS pulse shortening under synchronous laser pumping because of not only highly intense stretching mode Raman line for efficient primary extra cavity long-shifted SRS conversion but also the widest bending mode Raman line for the strongest SRS pulse shortening down to the inverse width of the widest Raman line (~1 ps) at secondary intracavity short-shifted SRS conversion. The strongest 26-fold pump pulse shortening down to 1.4 ps at the Stokes component with the combined Raman shift in the synchronously pumped extra cavity SrMoO₄ and SrWO₄ Raman lasers has been demonstrated. It was found that synchronously pumped cascade SRS with combined Raman shift is more efficient in the SrWO₄ crystal because the bending mode Raman line is more intense relative to the stretching mode Raman line than that in SrMoO₄.

The efficient oscillation of LiF:F ₂ ⁻ color center laser pumped by a compact LD-pumped Nd:YVO₄ acousto optically Q-switched laser with 30 kHz pulse repetition rate was demonstrated. The broadband oscillation with 75 μJ pulse energy and 37 kW peak power with the slope efficiency 20% was obtained. The average output power as high as 230 mW was reached. The narrow line tunable from 1.10 to 1.29 μm laser radiation with 10% conversion efficiency in the maximum of the tuning curve was achieved under pumping with 1.6 W average pump power.