Simultaneous Two-Stokes Raman amplification in crystals
Final Report Abstract
The main goal of the project was to investigate simultaneous Raman amplification of 1st and 2nd Stokes waves by stimulated Raman scattering in crystals. For this purpose, a solid-state laser system was developed which simultaneously emits two narrow-band laser lines of different wavelengths with ns pulse duration, high pulse energy and at the same time low beam propagation factor M2. The developed laboratory system consists of a commercial, frequency-doubled ns Nd:YAG pump laser, which simultaneously pumps two solid-state Raman lasers for the first Stokes line and the second Stokes line, respectively, and a solid-state Raman amplifier, which should simultaneously amplify both the first and the second Stokes laser radiation. The two Raman lasers serve as pulsed seed lasers, and their radiation is simultaneously coupled into the Raman amplifier together with the pump beam where they are amplified by means of stimulated Raman scattering. Two different crystalline materials have been investigated as active media for the Raman lasers and the amplifier, barium nitrate and potassium gadolinium tungstate (KGW). Since the seed lasers are pumped with visible light (λ = 532 nm) of the frequency-doubled Nd:YAG laser, the first and second Stokes lines of the Raman lasers are also located in the visible spectral range, for barium nitrate at 563 nm and 598 nm, and for KGW at 558 nm and 588 nm, respectively. A frequency doubling of these lines produces corresponding line pairs in the UV-B range near 300 nm, which are then suitable e.g. for measuring ozone height profiles in Earth’s atmosphere. Based on detailed model calculations concerning the Raman laser resonators with optical design programs (ZEMAX, CODE V), and measurements on differently designed resonators, the choice fell on stable resonators with a length of 8 cm, consisting of a spherical in-coupling mirror, a flat out-coupling mirror, and one Raman-active crystal, which is positioned inside the resonator close to the output mirror. The focus of the spherical mirror is also lying close to the output mirror and ensures a high intensity in the Raman crystal. This system provides seed laser pulses of the first and second Stokes line with tuneable pulse energies in the range between a few µJ up to mJ, respectively, which are then amplified. The beam quality of the seed laser pulses is confirmed by M2 values near to 1.5. Experiments concerning the amplification process led to the result that the first Stokes signals are well amplified with a factor of 51, almost retaining the high beam quality of the seed laser. However, investigation of simultaneous amplification of the 2nd Stokes seed laser pulses led to unsatisfactory results. An amplification factor near 5 obtained so far is much smaller than expected. The reason may be an insufficient pulse overlap between the ps pulse trains of the 1st and 2nd Stokes waves generated by self-mode locking, the envelopes of which form the corresponding ns seed laser signals.
Publications
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Raman lasers for trace gas detection. XXII International Symposium on High Power Laser Systems and Applications, 41. SPIE.
Zesch, Christoph; Schrader, Sigurd; Prosposito, Paolo; Lux, Oliver & Eichler, Hans-Joachim
