Passive Q-switching of fiber lasers with use of a dynamic SBS silica fiber mirror

Abstract

standard 2 x 2 sinple-mode fused fiber couDler this was attributed to not being able to prevent v with nominally 5050 coupling ratio at 1.06 pm, and a loop consisting of 9 m of singlemode fiber at 1.06 )*m fusion-spliced to the coupler output arms. Three meters of fiber in the loop were coiled around a piezoelectric cylinder (PZT8) that had dimensions of 1-mm thickness, 5-cm outer diameter, and 1-cm height. The voltage applied to the piezoelectric was used to modulate the overall birefringence of the fiber loop. Phase modulation effects were avoided by placing the fiber coil right at the center of the 9-m loop3; included in this loop was an all-fiber polarization controller that was used to adjust the polarization, and therefore the reflectivity, under cw fiber laser operation. The fiber laser consisted of 6 m of Nddoped single-mode, double-clad fiber; a dielectric dichroic mirror was attached to one end of it (reflectivity 299.5% at 1060 nm and 50.5% at 807 nm) and the other end was coupled through a linear polarizer to the Sagnac output mirror. Cavity dumping (Fig. 1) was observed by application of a harmonic voltage to the piezoelectric element, at a frequency of 2 MHz, which matched a mechanical resonance of it. The voltage amplitude and the polarization controller were adjusted to achieve near 100% modulation of the Sagnac mirror reflectivity. Notice that one pulse per modulation cycle was emitted in a train of cavity-dumped pulses that had a peak power equal to twice the cw power, and pulse widths of 160 ns. This is close to optimum performance, which would predict pulse widths equal to the laser cavity round-trip-time of approximately 150 m4 Q-switching was achieved by first adjusting the polarization controller to give the minimum reflectivity of the Sagnac mirror under cw operation. Then, a voltage impulse with an amplitude of 30 V and a duration of 6 p s was applied to the piezoelectric element. The observed output Q-switched pulses of the laser (Fig. 2) had an energy of 125 nJ and pulse widths of 550 ns, when pumping with 440 mW of pump power. Notice that Q-switching performance is far from optimum, as one would expect to achieve pulse energies comparable to 1 )*I, the saturation energy of the fiber laser; lasing before the voltage impulse is applied. Furthermore, to eliminate low amplitude relaxation oscillations of the fiber laser after the Q-switched pulse is emitted, caused by undesirable mechanical oscillations of the piezoelectric element, it was necessary to modulate the pump with 4-ms pulses as shown in Fig. 2. *Departamento de Fisica Aplicada, Universidad de Valencia, Dr. Moliner 50 461 00, Budassot (Valencia) Spain 1. J. D. Cao et al., CLE0'93 Tech. Digest, paper CFJ3, p. 622 (Baltimore, Md.). 2. L. A. Zenteno, J. Lightwave Technology