For the first time to our knowledge, a single-phase solid solution Sr(MoO_4) 0.8 (WO_4) 0.2 was used as an active medium of a Raman laser. Using the high-intensity synchronous picosecond pumping satisfying the condition of phase capture of the parametric Raman interaction on the second vibrational mode made it possible to oscillate six components of Raman radiation with a combined frequency shift on the first (888 cm -1 ) and second (327 cm -1 ) vibrational modes in the wavelength range of 1194-1396 nm. Oscillation efficiency of the multiwavelength Raman laser radiation was as high as 10%. By detuning the Raman laser cavity length, the pulse shortening down to 6 ps for the Raman laser radiation components with the combined frequency shift was obtained, which is an order of magnitude shorter than the pumping pulse duration (64 ps). Keywords: stimulated Raman scattering, single-phase solid solution, vibrational mode, synchronous pumping.

Highly transient stimulated Raman scattering in a SrMoO 4 crystal on both stretching (888cm - 1 ) and bending (327cm - 1 ) Raman modes under ultrafast double-pulse pumping by orthogonally polarized pump pulses at 1030 nm with a controllable chirp and a different delay between them was investigated.

For the first time, to the best of our knowledge, we demonstrate highly transient, multiwavelength, single-pass Raman generation with combined frequency shifts on two Raman modes of an SrMoO4 crystal with high total Raman conversion efficiency of up to 48% in conditions of competition with self-phase modulation (SPM). A 58-mm-long SrMoO4 crystal was used as the active medium under pumping by the 1030-nm, 40-µJ laser pulses with controllable dispersive stretching in a range of 0.25-6 ps at negative and positive chirping. The pump pulse chirping was optimized for both high- and low-frequency Raman shifts on the primary (888 cm-1) and secondary (327 cm-1) Raman modes of the crystal. At the optimal conditions, four Stokes components of stimulated Raman scattering (SRS) radiation with high- and low-frequency Raman shifts at the wavelengths of 1066, 1134, 1177, and 1261 nm were efficiently generated.

Graphene-mode-locked solid-state lasers usually have a low optical efficiency (<10%) or a low output average power (<1W) due to losses in the graphene saturable absorber and the thermal load of the laser crystal. We demonstrate what we believe to our knowledge is an improved design for a highly efficient, powerful, diode-end-pumped Nd:YAG laser mode-locked with graphene. Internal losses of the laser cavity were minimized by using a high-quality monolayer graphene saturable absorber applied directly on the cavity end mirror. In comparison to other graphene-mode-locked solid-state lasers, the record high optical-to-optical and slope efficiencies of 19.1 and 24.5%, respectively, in a continuous wave mode-locking regime were achieved. The highest output average power was as high as 1.7 W for the ultrashort pulse repetition rate of 82 MHz at the beam quality of M 2 =1.3×1.4.

The spectroscopic properties of cobalt ions in Ca 3 (VO 4 ) 2 crystals doped via thermal diffusion and during Czochralski growth process are studied. The spectroscopic properties of M1 and M2 Co 2+ optical centers are presented. A narrow fluorescence line at 1170 nm is attributed to trivalent Coions.

For the first time to our knowledge, a single-phase solid solution Sr(MoO4)0.8(WO4)0.2 was used as an active medium of a Raman laser. Using the high-intensity synchronous picosecond pumping satisfying the condition of phase capture of the parametric Raman interaction on the second vibrational mode made it possible to oscillate six components of Raman radiation with a combined frequency shift on the first (888 cm–1) and second (327 cm–1) vibrational modes in the wavelength range of 1194-1396 nm. Oscillation efficiency of the multiwavelength Raman laser radiation was as high as 10%. By detuning the Raman laser cavity length, the pulse shortening down to 6 ps for the Raman laser radiation components with the combined frequency shift was obtained, which is an order of magnitude shorter than the pumping pulse duration (64 ps).

Activation of calcium orthovanadate crystals by cobalt ions was carried out by introducing cobalt oxide into the melt during Czochralski growth process or during high-temperature annealing of nominally pure Ca3(VO4)2 single-crystal using Co3O4 powder as a solid-phase diffusant. The maximum concentration of the Co ions in the matrix is achieved in the case of the diffusion doping process. For the concentration series of Ca3(VO4)2:nCo crystals, the local structure was investigated and the crystal chemical composition was determined. Polarized absorption spectra of doped crystals are obtained. The influence of the atmosphere and the annealing temper-ature on the redistribution of the intensities of the characteristic absorption bands in the visible and near-IR spectral regions is shown. Luminescence parameters of the doped crystals were studied.

The generation of a crystalline anti-Stokes parametric Raman laser with collinear phase matching under intracavity synchronous pumping by a 1064-nm passively Q-switched longitudinally mode-locked YAG:Nd3+ laser has been studied. A CaMoO4 crystal is chosen as an active medium. This crystal has optimal birefringence for matching Stokes–anti-Stokes parametric Raman four-wave interaction at a phase-matching angle of 71° and an angular phase-matching width of 4°, which is insensitive to angular phase mismatch. The conditions for generating a single ultrashort anti-Stokes pulse at a wavelength of 973 nm with a duration of 9 ps and energy of up to 9 μJ are determined.

Theoretical and experimental studies were conducted on providing conditions for the generation of multiple radiation components with a small wavelength spacing in a crystalline synchronously pumped SRS laser with combined frequency shift on high-frequency and low-frequency vibrational modes of an SRS-active crystal. A theoretical analysis has shown an important role of four-wave parametric Raman interactions on the low-frequency vibrational mode of the crystal provided the conditions of coherence and of nonlinear phase capture of such interactions are satisfied. For the first time, SRS generation was carried out at five closely spaced wavelengths of 1194, 1242, 1294, 1336, and 1396 nm in a SrMoO 4 crystal under synchronous pumping by a high intensity picosecond YAlO 3 :Nd 3+ laser at a wavelength of 1079 nm satisfying the condition of nonlinear phase capture.

In this work, the high-temperature diffusion doping method was used for introduction of active cobalt ions into calcium orthovanadate Ca3(VO4)2crystals. Experimental samples were made from a nominally pure CVO single crystal obtained by the Czochralski method. The high-temperature diffusion conditions have been optimized to obtain doped crystals of optical quality during annealing in open and closed zones. Diffusion coefficients of cobalt ions (D) were calculated for various conditions: annealing time 24 - 48 hours; temperature range 1150-1300°C; diffusants - oxide compounds of calcium, cobalt and vanadium: Co3O4, Ca10Co0.5(VO4)7 and Ca3(VO4)2:2wt.%Co3O4; diffusion direction is parallel or perpendicular to the CVO crystal optical axis. The calculated values of the diffusion coefficient varied between 2.09·10-8—1.58·10-7 cm2/s. The activation energy of the diffusion process was determined to be 2.58±0.5 and 2.63±0.5 eV for the [001] and [100] directions, respectively. The maximum cobalt concentration in doped CVO crystals was 2·1020 cm–3. The absorption spectrum of diffusion-doped Ca3(VO4)2:Co samples demonstrates the presence of absorption bands characteristic for Co2+ and Co3+ ions. It was shown that the intensity ratio of the characteristic absorption bands varies depending on the crystal doping method. The optical anisotropy of the crystal increases with dopant concentration increase.

Calcium orthovanadate Ca-3(VO4)(2) crystals doped with manganese ions have been fabricated by high-temperature diffusion doping. Nominally pure CVO plates were prepared from the Czochralski grown single crystal. They were used as an effective matrix for diffusion impurity ions from solid phase (Mn2O3 powder) to solid phase (CVO single crystal). The high-temperature diffusion conditions were optimized to obtain the doped crystals of optical quality under using open and closed annealing zones. Diffusion coefficients of manganese ions were calculated for different conditions (temperature interval 1150-1300 degrees C, annealing time 24 h, 48 h, directions [001], [100]) of diffusion doping of CVO samples. The diffusion coefficient D varied from (3.7 +/- 0.6) . 10 8 cm(2)s(-1) at 1150 C up to (1.3 +/- 0.2) . 10(-7) cm(2)s(-1) at 1300 C. The diffusion activation energy was determined to be 1.7 +/- 0.4 eV and 3.3 +/- 0.4 eV for the processes based on Fick's laws during [100] and [001] direction, respectively. The ultimate solubility of manganese in CVO crystals at T = 1300 degrees C was 3 . 10(20) cm(-3). The absorption spectrum of diffusion doped Ca-3(VO4)(2):Mn samples is characterized by a combination of absorption bands specific for Mn2+, Mn3+ and Mn5+ ions. It was indicated that Mn5+ concentration in the diffusion doped CVO:Mn is higher than in CZ-grown Mn-doped materials.

Calcium orthovanadate Ca-3(VO4)(2) crystals with the whitlockite structure as nominally pure as doped with manganese ions have been grown by the Czochralski method from melt in air in the [100] direction. The single crystals of optical-quality were obtained for 0.05-2 wt% Mn2O3 doping level in the melt. X-ray diffraction analysis was carried out. Analyze of the parameters of diffraction peaks indicates the microstructure improving for manganese doped crystals. The absorption spectrum of doped Ca-3(VO4)(2):Mn is characterized by a combination of absorption bands specific for Mn2+, Mn3+ and Mn5+ ions, it was also proved by spectroscopic measurements. Two fluorescence spectra were observed under 670 nm excitation: narrow fluorescence line peaking at about 1180 nm, which quite well fits the position of Mn5+ ions and broad band peaking at 1250 nm, which quite well fits the position of Mn3+ ions. Fluorescence in the visible range was observed in Ca-3(VO4)(2):Mn crystal. The specific domain structure of ferroelectric Ca-3(VO4)(2) crystal was revealed by the method of selective chemical etching depending on crystal composition and thermal treatment. The anisotropic character of the optical properties of the material was estimated.

The spectroscopic properties and lifetimes of various Ho3+ optical centers at the two-micron I-5(7)-> I-5(8) laser transition were studied in CaF2 crystals at cryogenic temperatures (15 K, 77 K) using resonant selective excitation of holmium ions. For the first time, the excitation and luminescence spectra of three different optical centers with I-5(7) level lifetimes of similar to 11 ms, similar to 18 ms, and similar to 26 ms were identified. The 18 ms lifetime optical center was ascribed to tetragonal one, the center with 26 ms lifetime is most likely a cubic one, and the center with a lifetime of 11 ms is a clustered optical center.

Different Ho 3+ ion optical centers in CaF 2 and SrF 2 crystals were separated and their spectroscopic properties at the 5 I 8 ↔ 5 I 7 laser 2.1 μm transition were investigated at 14K. For the first time room temperature lasing is obtained in Ho:SrF 2 crystal under resonant (1948nm) pumping.

Collinear phase matching of the Stokes ↔ anti-Stokes interaction for Raman-active crystals with different birefringence was studied theoretically as well as experimentally. It was shown that collinear phase matching of the Stokes ↔ anti-Stokes interaction in low-birefringent crystals can be insensitive to angular mismatch if a phase matching angle is higher than 60°. We have developed and experimentally realized an extracavity parametric Raman anti-Stokes laser based on a low-birefringent SrWO4. Cyan 507-nm anti-Stokes conversion from green (532 nm) pump radiation of a 5-ns, 1-mJ second harmonic Nd:YAG laser has been obtained. Laser setup with a single beam excitation made it possible to use an output face of the SrWO4 crystal as an output coupler because of wide (6°) angular tolerance of collinear phase matching that resulted in an increase of slope efficiency of anti-Stokes generation higher than 3% at the anti-Stokes energy output of a 10-µJ level.

The series of Tm3+/Ho-3(+) co-doped vanadates based on Ca-3(VO4)(2) was obtained by Czochralski method. The trivalent doping ions were introduced into the melt in the form of oxides without any additional compensating impurities. The spectroscopic investigations showed anisotropic character of optical properties of the material. For optimal output mirror reflectivity of 94% the oscillation spectrum for holmium ions was measured to be centered at about 2070 nm with oscillation spectrum width of about 20-nm width at half maximum. Slope efficiency 5% for Ho3+ ions under 792 nm thulium pumping was obtained. Broadband tuning of Ho3+ ions oscillation wavelength within 2020-2115 nm spectral range was obtained in selective cavity with SiO2 filter.

The optically homogeneous colorless bismuth germanate Bi4Ge3O12 (BGO) crystals (nominally pure and doped with Dy-3+) were grown by Czochralski method along [1 1 1] directions. Optimal bulk crystallization rate was estimated for nominally pure crystals of 0.7 cm(3)/h and 0.1 cm(3)/h for doped materials. Influence of annealing conditions on BGO real structure and properties was shown. The colored ("reddish") samples were obtained under special treatment. Absorption and IR luminescence spectra were measured for as-grown, annealed and doped samples. The peculiarities of colored BGO crystal and samples doped with Dy3+ ions were demonstrated.

The spectroscopic properties of Tm3+ ions in Ca-3(VO4)(2) crystal were investigated and diode pumped tunable lasing was demonstrated. The maximum absorption and emission cross-sections for E perpendicular to c at F-3(4) H-3(6) 2 mu m transition were calculated to be similar to 1 x 10(-20) cm(2) and similar to 1.3 x 10(-20) cm(2) respectively. Noticeable H-3(4) F-3(4) H-3(6) F-3(4) cross-relaxation was observed resulting in a slope laser efficiency up to 15% at similar to 1960 nm under 792 nm laser diode pumping. Broadband tuning of the oscillation wavelength within the 1850-2070 nm spectral range was realized.