Abstract

The development of the longitudinal modes in resonators with SBS-mirrors is investigated as a function of the length of the start resonator cavity and the SBS-based resonator cavity. The Eigenmodes and the optimal operation properties are presented.

Mirrors based on stimulated Brillouin scattering (SBS) are useful for phase conjugation and passive Q-switching in laser resonators. Especially the beam quality in high average output power solid state lasers can be increased up to the theoretical diffraction limit. Average output powers of 27 W with beam qualities of better than 1.2 * DL were reported [1]. For further improvements, the detailed investigation of the temporal development of longitudinal and transverse modes of such resonators with phase conjugating SBS-mirrors is of interest.

It was reported earlier, that a resonance condition should be fulfilled for the cavity length (resonance length) with respect to the Brillouin frequency of the SBS material to obtain a stable operation of the laser [2]. With every round-trip a frequency shift occurs due to the reflection of the laser light at the SBS mirror. Thus several frequencies appear during the Q-switch pulse duration [3]. In this paper the influence of the Brillouin shift on the longitudinal mode structure and the Q-switch process is investigated in detail.


Therefore the lengths of the SBS- and start resonator were varied. The measurements were performed with SF₆, CO₂ and Xenon as SBS-material. The temporal pulse structure was observed with a photodiode (PD) whereas the frequency spectrum of the output pulses was observed with a Fabry-Perot-Interferometer (FPI) as shown in figure 1. The temporal pulse structure was analyzed further by a Fourier Transformation (FFT). The start resonator was stable for every setting and the fundamental mode volume remained constant.

The longitudinal mode structure is mainly a function of the SBS-resonator length. The Brillouin-shift occured in the FPI-patterns only. The resonator frequencies of the SBS-resonator can be seen in the FFT of the Q-switch pulses (Fig. 2). Pulse stability and output energy do not depend on the SBS-resonator length. Furthermore length tuning of the start resonator is necessary for stable SBS-Q-switch operation (Fig. 3).

References:

[2] M. Ostermeyer, A.Heuer, R. Menzel, Conference on Lasers and Electro-Optics 1996, Vol. 9, OSA Technical Digest Series, p. 447

[2] A. Kummrow, R. Menzel, D. Schumann and H. J. Eichler, International Journal of Nonlinear Optical Physics, 2, (1993) 261

[3] B. Barrientos, V. Aboites and D. Damzen, J. Optics (Paris), 26, (1995), 97

Figure captions:

Fig. 1: Experimental setup

Fig. 2: FFT of a Q-switch pulse of a 156 cm SBS-Resonator with SF₆ ( c/2L = 96 MHz )

Fig. 3: Length tuning of the start resonator with SF₆