Kilohertz raman lasers based on bno and kgw crystals for a wide spectral range

Abstract

UDC 621.375.826+548 Efficient Raman lasers based on Ba(NO 3 ) 2 and KGd(WO 4 ) 2 crystals which produce nanosecond pulses with repetition rates of 1 and 4 kHz at 26 wavelengths between 280 and 1600 nm are developed and studied. A maximum SRS conversion efficiency of 25% is obtained. It is found that the divergence of the beams generated by the Raman laser can be significantly reduced by using quasi-Bessel pump beams and an unstable telescope cavity.Introduction. Pulsed UV, visible, and near infrared laser sources are used in spectroscopy, medicine, ecology, and other areas. The repetition rate of the pulses from existing sources usually is at most a few tens of Hertz, which limits their range of applications and efficiency. Thus, there is considerable interest in developing higher pulse repetition rates, especially in the kilohertz range. We believe that a comparatively simple and effective approach to solving this problem may be to create lasers based on the stimulated Raman scattering (SRS) of light in Raman active crystals (Raman lasers) [1][2][3][4][5][6][7][8].An efficient visible and UV Raman laser based on a barium nitrate Ba(NO 3 ) 2 (BNO) crystal with a pulse repetition rate of 1 kHz has been developed previously [3]. In this paper we describe a study of Raman lasers based on KGd(WO 4 ) 2 (KGW) crystals with a pump pulse repetition rate of 1 kHz and lasers based on BNO and KGW crystals operating at higher pulse repetition rates (4 kHz). The Raman lasers are pumped by the second harmonic of an acousto-optically Q-switched YAG:Nd laser (λ = 532 nm). Excitation both by gaussian and by quasi-Bessel beams is studied, and the output of a Raman laser with an unstable telescope cavity is examined. Lasing in the near IR based on a BNO crystal excited by the fundamental of the YAG:Nd laser (λ = 1064 nm) with a pulse repetition rate of 1 kHz is also studied.Experimental Setup. The optical layout of the experiment is shown in Fig. 1. A YAG:Nd laser (model LF2210, SOLAR TII, Minsk, Belarus) was used as the pump. It is flashlamp pumped and has an acousto-optical Q switch [3]. The laser generates nanosecond pulses at λ = 1064 and 532 nm with a repetition rate of 1-15 kHz. The maximum average output power for a 1 kHz repetition rate is 10 W at λ = 1064 nm and 7 W at λ = 532 nm. The laser beams are multimode with a relatively smooth intensity distribution over their transverse cross sections.BNO crystals with lengths of 6.5 cm and KGW crystals with lengths of 5 cm were used for the Raman conversion. These crystals have strong Raman-active vibrations and comparatively weak nonlinear refraction [9][10][11][12][13]. The ends of the crystals were antireflection coated at the pump wavelengths. They were mounted in a high-Q optical cavity formed by plane and (or) spherical mirrors with different reflectivities for the pump and SRS Stokes components. The input mirror for the pump radiation was highly transparent and the output mirror was highly reflective. This ensured two passes of the pump radiation through the scattering cry...