Property Research On Some Tunstate And Vanadate Raman Laser Crystals

Stimulated Raman scattering (SRS) is a third-order nonlinear optical process which occurs in gases, liquids and solids. In the last several years, much attention has been paid to SRS in bulk crystals due to the demand of new type solid-state lasers. Raman shifting of crystals extends the spectral coverage of solid-state lasers substantially. It can generate some specific wavelengths, including yellow laser around 580nm and eye-safe laser around 1.5μm. Applications of such lasers include rangefinder, lidar measurements, medical treatment, laser guide stars, and spectroscopy, etc.As one of the earliest nonlinear optical process, SRS was discovered by Woodbury E J and Ng W K in 1962. Since then a large number of materials have been found that can realize efficient SRS. In the early years, most investigation was focused on gases and liquids, such as H₂, N₂, CS₂, Nitrobenzene, etc. For solid-state mediums, the first SRS experiments were made on diamond,α-sulphur, and natural calcite single crystals. Since 1980s, several new artificial crystals were reported as promising candidates for efficient Raman shifting, including Ba(NO₃)₂, BaWO₄, etc. They have many advantages, such as good thermal and mechanical properties, a narrow line width of the vibration modes, chemical stability, and especially high concentration of active Raman centers.From the reports in the literature, we found that most researchers pay their attentions to stimulated Raman scattering (SRS) properties of the single sample or improving some SRS properties of the samples. Due to their complexities, there have been no perfect theories about the SRS properties for the certain crystal. By experiments, it has been found that the SRS properties of the crystals are influenced by the wavelength, the pulse width and polarization of the pump power and the species, structure, dimension and the cutting direction of the crystals. We believe that there are a lot of theoretical and applied values, including the new phenomena, theories, Raman crystals and Raman laser equipments, to investigate some Raman crystals in the same condition. In this paper, we investigated tens of crystals for 7 kinds and compared them with each other in the same condition.By the material advantages of Institute of Crystal Materials, Shandong University, the SrWO₄,BaWO₄,YVO₄,GdVO₄,YbVO₄ and Nd:LuVO₄ crystals with high qualities and large size are grown by the Czochralski method. Using different dimensions and cutting directions of the crystals listed above, some new phenomena and rules are found which are valuable for the crystals in the application and researching the new solid-state Raman laser including the yellow and eye-safe laser.The main contents of this dissertation are as follows:1. (1)The Raman spectra of SrWO₄ crystal with different configurations were investigated and the observed Raman spectra were registered according to the group theory calculation. (2) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm, 30ps) was investigated in different SrWO₄ crystal samples. For the first Stokes line, the steady-state gain coefficient of SrWO₄ crystal was calculated to be 15.5±0.4- cm/GW. In our experiment, as much as five Stokes lines (559.23nm, 589.61nm, 623.49nm, 661.50nm, 704.44nm) and three anti-Stokes lines (506.97nm, 484.34nm, 463.65nm) were observed, the SRS thresholds for the first to the fourth Stokes line were measured, and the total conversion efficiency was as high as 62%. (3) In a 355nm pumping single-pass configuration, as much as three Stokes lines (366.44nm,379.25 nm,392.98nm) were observed, and the steady-state gain coefficient was calculated to be 49.09cm/GW. (4)The highly efficient extra-cavity Raman laser was realized based on SrWO₄ crystal. The maximum total conversion efficiency was obtained to be 73% in the experiment. The maximum conversion efficiency of 1324nm Raman laser was obtained to be 19.3% in the experiment.2. (1) The Raman spectra of BaWO₄ crystal with different configurations were investigated and the observed Raman spectra were registered according to the group theory calculation. (2) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm, 30ps) was investigated in different BaWO₄ crystal samples. For the first Stokes line, the steady-state gain coefficient of BaWO₄ crystal was calculated to be 19.6±0.7 cm/GW. In our experiment, as much as five Stokes lines (559.64nm, 590.26nm, 624.42nm, 662.76nm, 706.18nm) and three anti-Stokes lines (463.42nm, 484.24nm, 507.04nm) were observed, and the total conversion efficiency was as high as 78%. (3) In a 355nm pumping single-pass configuration, as much as three Stokes lines (366.52nm,379.40 nrm 393.22nm) were observed, and the steady-state gain coefficient was calculated to be 51.8±2.7cm/GW. (4)The highly efficient extra-cavity Raman laser was realized based on BaWO₄ crystal. The maximum total conversion efficiency was obtained to be 58% in the experiment. The maximum conversion efficiency of 1325nm Raman laser was obtained to be 23% in the experiment. (5) Using an external couple cavity, efficient 1.5μm eye-safe Raman laser is obtained based on BaWO₄ crystal. The largest output energy is 8.5mJ, corresponding a conversion efficiency of 47%.3. (1) The Raman spectra of YVO₄ crystal with different configurations were investigated and the observed Raman spectra were registered according to the group theory calculation. (2) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm, 30ps) was investigated in different YVO₄ crystal samples. For the first Stokes line, the steady-state gain coefficient of YVO₄ crystal was calculated to be 15.7±0.8cm/GW. In our experiment, as much as two Stokes lines (558.6nm, 587.8nm) and one anti-Stokes lines (508.0nm) were observed, and the total conversion efficiency was as high as 52%. (3) In a 355nm pumping single-pass configuration, as much as two Stokes lines (366.03nm,378.37 nm) were observed, and the steady-state gain coefficient was calculated to be 20.3±0.6 cm/GW. (4) The highly efficient extra-cavity Raman laser was realized based on YVO₄ crystal. The maximum total conversion efficiency was obtained to be 51.4% in the experiment. The maximum conversion efficiency of 1313nm Raman laser was obtained to be 12.3% in the experiment.4. (1) The Raman spectra of GdVO₄ crystal with different configurations were investigated and the observed Raman spectra were registered according to the group theory calculation. (2) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm, 30ps) was investigated in different GdVO₄ crystal samples. For the first Stokes line, the steady-state gain coefficient of GdVO₄ crystal was calculated to be 26.6±0.2cm/GW. In our experiment, as much as three Stokes lines (557.98nm, 586.86nm, 618.92nm) and one anti-Stokes lines (508.01nm) were observed, and the total conversion efficiency was as high as 43%. (3) In a 355nm pumping single-pass configuration, as much as two Stokes lines (365.9nm,378.1 run) were observed, and the steady-state gain coefficient was calculated to be 114±9cm/GW. (4) The extra-cavity 1311nm Raman laser was realized based on GdVO₄ crystal. The maximum conversion efficiency was obtained to be 2.7% in the experiment.5. (1) The Raman spectra of YbVO₄ crystal with different configurations were investigated and the observed Raman spectra were registered according to the group theory calculation. (2) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm,'30ps) was investigated in different YbVO₄ crystal samples. For the first Stokes line, the steady-state gain coefficient of YbVO₄ crystal was calculated to be 17.8±0.2cm/GW. In our experiment, as much as five Stokes lines (558.47nm, 587.92nm, 620.67nm) and three anti-Stokes lines (507.58nm) were observed, and the total conversion efficiency was as high as 37%. (3) In a 355nm pumping single-pass configuration, as much as one Stokes lines (366.11nm) were observed, and the steady-state gain coefficient was calculated to be 28.9±0.4 cm/GW. (4) The extra-cavity 1315nm Raman laser was realized based on YbVO₄ crystal. The maximum conversion efficiency was obtained to be 3.5% in the experiment.6. (1) The Raman spectra of Nd:LuVO₄crystal with different configurations were investigated and the observed Raman spectra were registered according to the group theory calculation. (2) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm, 30ps) was investigated in different Nd:LuVO₄ crystal samples. For the first Stokes line, the steady-state gain coefficient of Nd:LuVO₄ crystal was calculated to be 15.67cm/GW. In our experiment, one Stokes lines (558.92nm) were observed. 7. (1) In a single-pass configuration, the stimulated Raman scattering (SRS) of pico-second pulse (532nm, 30ps) was investigated in different LiIO₃ crystal samples. For the first Stokes line, the steady-state gain coefficient of LiIO₃ crystal was calculated to be 30.3cm/GW. In our experiment, as much as three Stokes lines (556.07nm, 582.30nm, 611.76nm) and one anti-Stokes lines (509.57nm) were observed, and the total conversion efficiency was as high as 56%. (2)The highly efficient extra-cavity Raman laser was realized based on LiIO₃ crystal. The maximum total conversion efficiency was obtained to be 27% in the experiment.The main innovations of this dissertation are summarized as follows1. For the first time, at the same experimental conditions, a systematic research on SRS properties is given to vary crystals, vary cut directions, and vary lengths. Based on magnitude experimental data the comparison and evaluation of 7 crystals are fulfilled, the configuration optimums are realized. The final conclusion is: BaWO₄ is the best Raman shifting material for all wavelength range, and GdVO₄ is the best base material for self-Raman laser.2. For the first time, a 355nm laser is used to pump multi-crystals, new wavelength of 379nm, 393nm is obtained. The Raman threshold and gain coefficient are measured. GdVO₄ is found to be a new material which has high Raman gain at ultra-violet wavelength range, whose gain coefficient is two times larger than that of BaWO₄. Although 355nm has approach the cut edge of transmittance, but considering the Raman gain is so high, GdVO₄ still has good application foreground at blue and violet wavelength range.3. For the first time, the ultra-cavity work style pumped by 1064nm laser is used for multi-crystals, and efficient 1.3μm second Stokes Raman lasers are obtained. When the BaWO₄ crystal is 50mm in length, the maximum output energy is 0.69mJ and the highest conversion efficiency is 23%. This result is better than the data reported aboard.4. For the first time, efficient 1.5μm eye-safe Raman laser is obtained by an external couple cavity. The BaWO₄ crystal is anti-reflective coated, and the pump beam is compressed but not focused to make good use of the length of Raman media. We obtained 8.5mJ 1.5μm eye-safe Raman laser, and the highest conversion efficiency reaches 51%. As we have known, all these data are the best result up to the present.5. For the first time, the Raman threshold and gain coefficient are measured for high order Stokes components of multi-crystals. SrWO₄ is measured to the fourth order, BaWO₄ is measured to the fourth order, and LiIO₃ is measured to the third order. In this way, when some special order Stokes component is required to output, the pump energy can be estimated correctly, and the pump laser source and experimental setup can be prepared properly. Such work has reference value when high order Stokes Raman lasers are designed.6. For the first time, the SRS properties of YbVO₄ crystal pumped by 532nm laser are researched in detail. At 532nm pump condition the threshold and gain coefficient for the first and the second Stokes lasers are measured. The variation of output energy and conversion efficiency for the first Stokes component is investigated. The conversion efficiency for the first Stokes component reaches 17%, and the total conversion efficiency for all Stokes components reaches 37%, which are better than those data reported abroad. The ultra-cavity Raman laser pumped by 1064nm is realized for YbVO₄ crystal. The damage threshold of YbVO₄ crystal is measured. Furthermore, as we have known, it's the first time that the SRS threshold and gain coefficient are measured for LuVO₄ crystal.The work in this dissertation has fulfilled and consummated SRS theory and experimental methods, and applies new proofs for the design and improvement of Raman lasers. It will exert active affection for the evaluation of Raman crystal, as well as the making of new Raman lasers.