Milan Frank

Multi-Stokes ultrashort pulse shapes and their relative positions of synchronously pumped stimulated Raman scattering (SRS) on combined primary and secondary vibrational modes in a BaWO₄ crystal are investigated. An original method of its simultaneous measurement with the help of a streak camera has been developed. The structure of SRS pulses at the pulse shortening effect down to the pulse duration, close to the dephasing time of the secondary Raman mode of the BaWO₄ crystal, is registered and analyzed for the detuning of the Raman laser cavity length.

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.

In this Letter, a mixed P b ( M o O 4 ) 0.2 ( W O 4 ) 0.8 as a new, to the best of our knowledge, active medium with optimized content for a synchronously pumped multiwavelength Raman laser with a combined frequency shift is presented. The unique structure of this crystal resulted in oscillations at 12 closely spaced output wavelengths in a spectral range from 1128 to 1360 nm. The strongest pulse shortening in comparison with nominally pure scheelite-like crystals has been achieved.

For the first time to our knowledge, the operation of a synchronously pumped ultrafast Raman laser that uses a PbMoO₄ crystal as the active medium has been demonstrated. We achieved efficient Raman conversion in PbMoO₄ from pumping 1063 nm into 1171 and 1217 nm, respectively, at single and combined frequency shifts on stretching and bending Raman modes. The output pulse energy (up to 160 nJ) and peak power (up to 11 kW) of the output picosecond radiation is the highest among all-solid-state synchronously pumped Raman lasers published to date. The strongest pulse shortening at 1217 nm down to 1.4 ps was obtained that is close to the bending mode dephasing time.

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.

Recently optical capillaries modified by Bragg reflection mirrors applied on the inner walls have been investigated for transmitting radiation of MIR lasers. Such capillaries include Bragg and omniguide fibers, holley fibers, or silica Kagome like fibers. Although OmniGuide fibers are commercially available and have been used for delivery of radiation of CO₂ fibers at 10.6 μm, novel types of hollow-core fibers are still investigated for MIR applications. In this paper a novel approach for the preparation of capillary optical fibers for MIR region is presented. This approach employs the application of thin layers of arsenic sulfide glass and acrylate polymer from their solutions onto the inner wall of silica capillary. Arsenic sulfide forms high-index and polymers the low-index parts of reflection mirrors. By controlling optical thicknesses of such layers, Bragg mirrors can be obtained. In experiments, input solutions of arsenic sulfide in n-propylamine and UV-curable acrylate in acetone were prepared. Such solutions were applied by dip-coating method on glass slides in order to obtain samples of single layers and multilayer coatings for the determination of thicknesses and refractive indices. Acrylate layers were UV cured and arsenic sulfide layer were thermally treated at 80°C. By passing columns of the input solutions through a silica capillary with a hole diameter of 80 μm and a length of 50 cm multilayer coatings on the inner capillary wall were prepared. The column velocity for each solution was controlled as a main factor influencing the layer thickness. Applied layers were UV cured or thermally treated under a nitrogen flow through the capillary. Coatings of three pairs of the high- and low-index layers were fabricated. Single layers and multilayers applied on planar substrates were characterized by transmission spectroscopy and by optical microscopy. Attenuation coefficients of internally coated capillary fibers of 10-20 dB/m were determined at a wavelength of 1940 nm.

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.

Synchronously-pumped combined-shift GdVO₄ Raman laser was investigated in dependence on the crystal length (16-40 mm). The shortest and longest crystals allowed to achieve the shortest pulse of 860 fs or the highest slope efficiency of 49.5%, respectively.

Synchronously-pumped Raman lasers based on YVO4, GdVO₄, and Ca3(VO₄)₂ crystals generating on both stretching and bending Raman modes with more than 30 times pump pulse shortening down to 0.8-1.2 ps are demonstrated.