2 KW THRESHOLD QUASI-CW NARROWBAND ALL-SOLID-STATE RAMAN LASER GENERATING IN UV, VISIBLE AND NEAR IR RANGES FOR SPECTROSCOPY A. Vodchits1, R. Chulkov1, D. Busko1, V. Lisinetskii1, A. Grabtchikov1, W. Kiefer2* and V. Orlovich1 1
B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, F. Skaryna Ave. 68, Minsk 220072, Belarus; E-Mail:
[email protected] 2 Institut für Physikalische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany; E-Mail:
[email protected] Keywords: quasi-cw Raman laser, barium nitrate, Stokes component, second harmonic generation Abstract: This contribution reports on experimental study of a barium nitrate based Raman laser system pumped with the second harmonic of a quasi-cw Nd:YAG laser. Five Stokes components generated and frequency doubled cover a spectral range of 280-740 nm with an average power from 10 to 800 mW and a spectral width of 0.2 cm-1 in the visible range. Pulsed lasers generating tunable or multiwave radiation have wide applications in Raman spectroscopy. For the successful use, the developed lasers should meet such requirements as continuous tunability or multiwave operation in a wide spectral range, narrow linewidth, sufficient output power, low divergence of the laser beam, simplicity in operation and low cost. All-solid state lasers meet these requirements rather well. Especially Raman lasers using stimulated Raman scattering (SRS) of light in crystals can be promising laser sources for the spectroscopic applications. Recently, we have developed a cheap and simple Raman laser based on the wellknown barium nitrate crystal which can generate nanosecond narrowband (0.25 cm-1 in IR) continuously tunable radiation in the ranges of 190-1800 nm [1,2]. The repetition rate of this laser was equal to 10 Hz. However, for many applications in spectroscopy, especially in Raman spectroscopy, it is necessary to use laser pulses with higher repetition rates, so-called quasi-cw radiation. High repetition rate can substantially improve the conditions of measurement of Raman spectra and shorten the time for this measurement. Recently, some studies have been performed to develop quasi-cw Raman lasers generating radiation in IR and visible ranges [3-6]. In these studies, comparatively high cost diode-pumped Raman lasers were used. To develop a cheap quasi-cw Raman laser and to extend the spectral range of its generation to UV region we have performed our studies on a barium nitrate based Raman laser system pumped with the second harmonic (SH) radiation of quasi-cw flash-lamp-pumped Nd:YAG laser and on frequency shift of Raman laser radiation using the second harmonic generation (SHG). The results of these studies are presented in this report. The optical scheme of the developed laser source is shown in Fig. 1. For pumping the Raman laser the linear polarized SH radiation at 532 nm from a commercially available quasi-cw (1 kHz) Nd:YAG laser with acousto-optic modulation (model LF2210, SOLAR TII) was used. The pumping pulse width was equal to 110 ns (FWHM). The pumping laser beam passed through an optical isolator (half-wave plate, polarizer and quarter-wave plate) to block back-scattered radiation or smoothly change the pump beam power and its polarisation between linear and circular and then the beam was focused with a lens inside a Raman laser resonator. The Raman laser consisted of two spherical mirrors and a barium nitrate crystal of 70 mm length between them. Spherical mirrors were used to compensate partly the thermal lens effect in the crystal due to the dissipation of Raman excitation to heat. Also, the crystal was mounted in a special cage for axial symmetric heat removing. Our previous studies using two-beam time-resolved z-scan in barium nitrate showed that a considerable thermal lens is induced in it due to SRS of nanosecond laser pulses [7]. Using z-scan data and measurement with the help of a collimated He-Ne laser beam propagating through a Raman laser we could determine the optical power of the thermal lens at 1 346
kHz repetition rate. Our data show that the power of this lens corresponds to a negative lens with a focusing length of tens of cm. This value correlates rather well with the data in Ref. [8]. The radius of curvature of the Raman laser resonator mirrors and the configuration of heat remover were adjusted in such a way as to minimize the influence of the thermal lens. Also,the focusing lens and the resonator length have been optimized to obtain maximum output power. Radiation generated by the Raman laser was collimated and dispersed with a prism to different Stokes components for further frequency conversion using SHG in a KTP crystal. 532 nm, 1 kHz
Fig. 1. Optical scheme of experiment: PB- polarizer; Lf, Lc, Lfs, Lcs – focusing and collimated lenses; M1, M2 – mirrors of Raman laser resonator; RC – Raman crystal in a special mount; DP- dispersive prism We could generate up to 5 Stokes components at 563.4, 598.7, 638.7, 684.5, and 737.4 nm simultaneously applying resonator mirrors with special reflections. For optimising the output to a definite component it was necessary to use the different mirrors. The maximum average power of the first and second Stokes components reached up to 1 W and of other components up to several hundreds of mW applying a pump power of 3.7 W before the Raman laser. SHG of the Stokes components allowed us to obtain quasi-cw radiation at 281.7, 299.4, 319.4, 342.3, and 368.7 nm with average powers of 10 mW and higher. For narrowing the spectrum of radiation a Fabry-Perot etalon was applied inside the Raman laser resonator. In this case we could narrow the spectral linewidth to 0.2 cm-1 in the visible range. Acknowledgements: Financial support from the International Science and Technology Center (ISTC) (project B-898) is highly acknowledged. References: 1. A. Vodchits, I. Mishkel, V. Orlovich, W. Kiefer and P. Apanasevich, in Proceedings of SPIE Vol. 5137 Advanced Lasers and Systems, edited by G. Huber et al. (SPIE, Bellingham, WA, 2003) pp. 100-107. 2. A.V.Kachinski, V.A.Orlovich, A.A.Bui, V.D.Kopachevsky, A.V.Kudryakov, W.Kiefer. Opt. Comm., 218, 351 (2003) 3. H. Pask and J. Piper, IEEE J. Quant. Electr. 36, 949 (2000). 4. H. Pask, S. Myers, J. Piper, J. Richards and T. McKay, Opt. Lett. 28, 435 (2003). 5. R. Mildren, M.Convery, H. Pask, J. Piper and T. McKay, Opt. Express 12, 785 (2004). 6. J. Simons, H. Pask, P. Dekker and J. Piper, Opt. Communs. 229, 305 (2004). 7. A. Vodchits, V. Kozich, P. Apanasevich and V. Orlovich, in Proceedings of SPIE Vol. 4751 ICONO 2001: edited by K. Drabovich et al. (SPIE, Bellingham, WA, 2002) pp. 355-359. 8. H. Pask, J. Blows and J. Piper, Opt.Soc.Am. TuB15-1 (2001) 276.
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