Stimulated Raman Scattering (SRS) is important method of laser frequency conversion. Optical frequency-comb is a special kind of SRS which output multiple Stokes beam simultaneously, and lasers with mutiple wavelength have broad applications. In this paper, the optical frequency-comb generated by SRS of CO<sub>2</sub> is presented and the spectral range covers from 0.4 μm to 1.5 μm. Research also indicates that the characteristics of optical frequency-comb depends on the wavelength of pumping laser. For instance, the SRS photon conversion efficiency pumped by 1064 nm laser is high at 1248nm, 1510nm but that pumped by 532 nm laser is high at 574nm, 624nm 683nm. The different features are compared and analyzed by the use of the mechanism of four-wave mixing and the change of SRS gain coefficient with Stokes wavelength.
SRS (Stimulated Raman Scattering) is a very effective method to expand the spectrum range of high power laser, especially in the regime of near IR and middle IR. In this paper SRS of high pressure H<sub>2</sub> and D<sub>2</sub> with MPC (multiple-pass cell) configuration were reported. Relation of (FS1) first forward Stokes and (BS1) first backward Stokes has been analysis. The process of gain of FS1 was explained. Experimental results also indicated the second Stokes was also generated. D<sub>2</sub> SRS of the fundamental output of Nd:YAG laser generates the second Stokes light of 2.92 m. The lasers with wavelength of 2.9 μm have broad applications. Finally, multiple-pass SRS was better for complete conversion of pump laser.
Achieving population inversion through multi-photon cascade pumping is almost always difficult, and most laser medium work under 1-photon excitation mechanism. But for alkali atoms such as cesium, relatively large absorption cross sections of several low, cascading energy levels enable them properties such as up conversion. Here we carried out research on two-photon excitation alkali fluorescence. Two photons of near infrared region are used to excite alkali atoms to n <sup>2</sup> D<sub>5/2</sub>, n <sup>2</sup> D<sub>3/2</sub> or higher energy levels, then the blue fluorescence of (n+1)<sup> 2</sup> P<sub>3/2</sub>,(n+1) <sup>2</sup> P<sub>1/2</sub>→n <sup>2</sup> S<sub>1/2</sub> are observed. Different pumping paths are tried and by the recorded spectra, transition routes of cesium are deducted and concluded. Finally the possibility of two-photon style DPALs (diode pumped alkali laser) are discussed, such alkali lasers can give output wavelengths in the shorter end of visual spectroscopy (400-460 nm) and are expected to get application in underwater communication and material laser processing.
Sodium based excimer-pump alkali laser (Na-XPAL) is expected to be an efficient method to generate sodium beacon light, but the information about the spectroscopic characters of Na-XPAL remains sparse so far. In this work, we utilized the relative fluorescence intensity to study the absorption spectrum of blue satellites of complexes of sodium with different collision partners. The yellow fluorescence of Na D<sub>1</sub> and D<sub>2</sub> line was clearly visible. After processing the fluorescence intensity and the input pumping laser relative intensity, we obtained the Na-CH<sub>4</sub> system’s blue satellites was from 553nm to 556nm. Meanwhile, we experimentally demonstrated the Na-Ar and Na-Xe system’s wavelength range of blue satellites. Also, it was observed that the Na-Xe system’s absorption was stronger than the other two systems.
Oxygen molecules existed in pairs under liquid condition, the radiation from vibrational ground state of <sup>1</sup> Δ state to the first vibrational excited state of <sup>3</sup> ∑ state was electronic dipole moment transition allowed, and a photon with wavelength of 1580 nm was emitted. In our experiment, dye laser with wavelength of 581 nm, 634 nm, 764 nm was used to excite liquid oxygen to different excited states, while a tunable OPO was used as the seeder laser, and the small signal gain was measured to be 0.23 cm<sup>-1</sup>, 0.3 cm<sup>-1</sup> and 0.076 cm<sup>-1</sup> respectively. The small signal gain (pump by photon of 634 nm) was significantly higher than that of common solid state lasers and chemical lasers. When the fundamental output of a Q-switched Nd:YAG laser was used as the pump source, the corresponding small signal gain was 0.12 cm<sup>-1</sup>. The profiles of small signal gain form 1579.2 nm to 1580.8 nm were also presented. These results were consistent with theoretical calculation. The high positive gain indicated that the liquid oxygen was a potential medium for high energy laser. A comprehensive parameter optimization was still necessary in order to improve the mall signal gain.
<sup>1 </sup>Δ<sub>g </sub>oxygen was the active medium of chemical oxygen iodine laser (COIL), the concentration and distribution of <sup>1</sup> Δ<sub>g</sub> oxygen was important for the output power and beam quality. However, the current test technique, such as fluorescence detection method, absorption spectrum method could not get accurate <sup>1</sup> Δ<sub>g</sub> oxygen information, due to the interference from the iodine fluorescence or the rigorous request of the laser source and optics and detection elements. The anti-stokes Raman spectrum of <sup>1 </sup>Δ<sub>g</sub> oxygen was regarded as a potential technique to obtain desirable signal, and the coherent anti-stokes Raman scatter (CARS) was the most feasible technique to get better signal to noise ratio (SNR). In this paper, we reported a broadband nanosecond coherent anti-stokes Raman scatter (CARS) detecting system built up for the detection of the concentration and distribution of O<sub>2</sub>( <sup>1 </sup>Δ<sub>g</sub>) in COIL：The second harmonic of a Nd: YAG pulse laser was separated into two parts, one part was used to pump a broadband nanosecond dye laser to generate light of 578-580 nm, which covered both stokes lines of O<sub>2</sub> ( <sup>1</sup> Δ<sub>g</sub>）and O<sub>2</sub> (<sup>3 </sup>∑）; The other part was combined with dye laser output by a dichroic mirror, and then introduced into the detection region of COIL through a focus lens. CARS signals for O<sub>2</sub>（<sup>1</sup> Δ<sub>g</sub>）and O<sub>2</sub> （<sup>3</sup> ∑）have different wavelengths, and their intensity was proportional to the square of the concentration of O<sub>2</sub>（<sup>1 </sup>Δ<sub>g</sub>） and O<sub>2</sub>( <sup>3</sup> ∑). By changing the focus spot of pump and stokes laser, the concentration distribution of O<sub>2</sub>（<sup>1</sup> Δ<sub>g</sub>） and O<sub>2</sub>（<sup>3</sup> ∑）at different position could be obtained.
Stimulated Raman Scattering (SRS) is an effective means of laser wavelength conversion. Hydrogen is an excellent Raman medium for its high stimulated Raman gain coefficient and good flowability which can rapidly dissipate the heat generated by SRS process. In this paper we reported the H<sub>2</sub> SRS in multiple-pass cell pumped by the fundamental frequency output of a Q-switched Nd: YAG laser. Two concave reflection mirrors (with 1000 mm curvature radius and 50 mm diameter) were used in our experiment, both mirrors with a hole near the edge and were positioned to form co-center cavity, therefore the laser could repeatedly pass and refocus in the Raman cell to achieve a high SRS conversion efficiency and reduce SRS threshold for pump laser. By changing the pass number (1～17) of optical path in the Raman cell and the pump power(0～2.5MW), the Stokes conversion efficiency is optimized. Experimental results indicated that the Raman threshold was 0.178MW and the highest photon conversion efficiency was 50 %.
The experimental study of the amplification of stimulated Raman scattering (SRS) in high purity H<sub>2 </sub>gas was demonstrated employing a Q-switched Nd:YAG laser at 1064 nm as the pump source. A part of the 1064 nm pump light (20% in energy) was focused into the first H<sub>2</sub> gas cell to generate the backward first Raman Stokes light (BS1), which is taken as the Raman seed light. The BS1 seed light combined to the residual pump light were focused into the second H<sub>2 </sub>gas cell to get the amplification of the S1 1900 nm infrared Raman light. In this study, the maximum quantum conversion efficiency of the S1 light was estimated to be 76%. Under the condition of the same pump energy, especially for the low pump energy (lower than 40 mJ), the quantum conversion efficiency of the S1 light with the Raman seed light was significantly increased comparing to the single focus geometry (without the Raman seed light).