Abstract

We have studied the gain and noise characteristics of a Brillouin amplifier, both when it is used to amplify weak signals and when it is used for phase conjugation. The amplifier consists of a 20-cm long cell filled with TiCl₄ which is pumped by a 25-ns pulse from a near diffraction-limited injection seeded Nd:YAG laser. The Brillouin gain is given by G_B = exp(gIpL), where I_p is the input pump intensity; and gains in excess of 10⁹ have been measured for signals of 500 fJ. The observed signal-to-noise ratio is 85:1; within a factor of 16 of the theoretical limit. In the absence of a signal, the output noise is observed to vary as N_out = N_oG_B/g, where N_o is a spontaneous scattering term equivalent to an input noise signal. When an aberrator is placed in front of the beam, the intensity of noise which is conjugate to the pump beam is enhanced by a factor J > 2, i.e., N_out pc = N_oG^J/Jg. The enhancement factor depends on the speckle in the pump beam, and observed values of J > 3 are larger than predicted by theory.

© 1989 Optical Society of America