Stimulated Raman Scattering in Yttrium, Gadolinium, and Calcium Orthovanadate Crystals with Single and Combined Frequency Shifts under Synchronous Picosecond Pumping for Sub-Picosecond or Multi-Wavelength Generation around 1.2 µm

Abstract

Comparative investigation of stimulated Raman scattering (SRS) characteristics in the YVO4, GdVO4, and Ca3(VO4)2 orthovanadate crystals at both low and high frequency anionic group vibrations is presented. It was found that GdVO4 is the most perspective for SRS generation on both the ν1 stretching and ν2 bending modes of internal anionic group vibrations with the strongest SRS pulse shortening under synchronous picosecond pumping. It is as a result of GdVO4‘s widest linewidth (17cm−1) of the homogeneously broadened scheelite-type component of the bending ν2 Raman line that led to the strongest SRS pulse shortening down to the dephasing time of the widest (scheelite-type) Raman mode at the secondary intracavity short-shifted SRS conversion. It allowed us to achieve SRS pulses with sub-picosecond duration under tens-of-picoseconds pumping due to the strongest 42-fold pulse shortening. Using the Ca3(VO4)2 crystal with essentially wider Raman lines (~50cm−1) did not allow us to generate SRS pulses shorter than 1 ps. It can be explained by inhomogeneous broadening of the Raman lines in Ca3(VO4)2 because of its structural disordering. Using the measured SRS pulse duration, the homogeneous broadening of the inhomogeneously broadened bending Raman line of Ca3(VO4)2 was estimated to be ~9cm−1. Among the orthovanadate crystals, the YVO4 crystal with the highest Raman gain and with homogeneously broadened Raman lines allowed us to realize the most efficient SRS lasing and SRS pulse shortening truly down to inverse half-width of the bending Raman line.

Measurement of Multi-Stokes Ultrashort Pulse Shapes of Synchronously Pumped Stimulated Raman Scattering on Combined Vibrational Modes in a BaWO4 Crystal

“…It was in BaWO 4 that stimulated Raman scattering (SRS) with a combined frequency shift of ν 1 + ν 2 was previously observed for the first time [4]. After that, similar effects were studied for many other crystals having a lower Raman gain, including KGd(WO 4 ) 2 , GdVO 4 , YVO 4 , SrWO 4 , SrMoO 4 , Ca 3 (VO 4 ) 2 , PbMoO 4 , and Pb(MoO 4 ) 0.2 (WO 4 ) 0.8 [5][6][7][8][9][10][11]. These effects were experimentally realized in schemes of Raman lasers with a high-Q optical cavity compensating a lower Raman gain at the secondary Raman mode rather than at the primary Raman mode.…”

“…The interest in such synchronously pumped crystalline Raman lasers is due to the observed phenomenon of the strong shortening of SRS pulses with a combined frequency shift down to the dephasing time τ2 = 1/(πcΔν2) of the secondary Raman mode. Therefore, the SRS pulse duration becomes shorter than the primary Raman mode dephasing time τ1 = 1/(πcΔν1), and it amounts ~1 ps and shorter under pumping by 36 ps pulses [8][9][10][11]13,14]. This phenomenon so far has only a qualitative explanation as a combination of several optical effects.…”

“…and then in other crystals [8][9][10][11]. The interest in such synchronously pumped crystalline Raman lasers is due to the observed phenomenon of the strong shortening of SRS pulses with a combined frequency shift down to the dephasing time τ2 = 1/(πcΔν2) of the secondary Raman mode.…”

“…The scheme of simultaneous streaking of two light sources used earlier, for example, for electron-beam diagnostics studies [21], was refined here for the characterization of a one multiwavelength Raman laser. To perform this, the output radiation of the Raman laser was split into Stokes components (1198 nm and 1248 nm) by a reflective diffraction grating (8); a system of mirrors (9)(10)(11), which formed a delay line (∆t delay = 150 ps); and a semi-transmitting mirror (12), which combined the Stokes components into one beam for measurement by the PS-1/S1 streak camera (13) with a time resolution of 1.5 ps, developed at the Prokhorov General Physics Institute of the Russian Academy of Sciences, Russia [22].…”

“…In the case of ultrashort pulse pumping, the method [12] of synchronous pumping by repetitive laser pulses with a repetition period synchronized with the Raman laser cavity round-trip time was used. Synchronously pumped SRS with the combined frequency shift was realized for the first time again in a high-gain BaWO 4 crystal using linear [13] and ring [14] optical cavities, and then in other crystals [8][9][10][11]. The interest in such synchronously pumped crystalline Raman lasers is due to the observed phenomenon of the strong shortening of SRS pulses with a combined frequency shift down to the dephasing time τ 2 = 1/(πc∆ν 2 ) of the secondary Raman mode.…”