Noise in x⁽³⁾ and photorefractive amplifiers

A comparison of the noise characteristics of x⁽³⁾ and photorefractive coherent amplif iers reveals basic differences in their dependence on operating parameters. Unlike all types of x⁽³⁾ amplif iers, which are shown to have a well-def ined optimum working point in the region of the self-stimulated scattering threshold, the photorefractive amplif ier can be made increasingly quieter by lowering the pump power. This is demonstrated by use of highly doped Co:BaTiO₃ in a tight-focus ref lection grating geometry. It is shown that scattering from inhomogeneities in the crystal is the major limiting noise source and is signif icantly higher than predictions resulting from fundamental considerations such as random space-charge noise. An extremely high smallsignal gain of 107 was measured with this crystal and geometry. To our knowledge this is the highest photorefractive gain reported to date.