LOW-LIGHT-LEVEL, QUANTUM-NOISE-LIMITED AMPLIFICATION IN A STIMULATED RAMAN AMPLIFIER

We have investigated the noise characteristics and the low-light-level imaging capabilities of a stimulated Raman amplifier. Diffraction-limited Stokes beams as well as Stokes beams carrying spatial structure have been amplified with a collimated pump with a Fresnel number in the range 3-17. Both direct imaging and Fourier-transform imaging of the Stokes signal through the Raman amplifier were used. Results indicate that approximately 1-4 photons per spatial mode are needed to produce images with a signal-to-noise ratio of 1. Amplification of the Fourier transform of the object effectively provides spatial filtering of the noise and gives the best signal-to-noise ratio at the lowest input levels for our experiments.