Abstract

We have previously reported experimental results of Raman amplification of a low light level seed Stokes beam in the transient limit.¹ Here we present results of measurements done using a 10 ns seed-injected, single spectral mode, Nd:YAG laser. The 532 nm frequency-doubled output has a duration of 8.5 ns and is used both to generate a 4.5 ns duration seed-Stokes pulse at 683 nm and to pump the 1 m long Raman amplifier. The seed-Stokes beam is spatially filtered and apertured at the Airy minimum and is collimated before the amplifier at a size corresponding to a Fresnel number adjustable over the range of 5-30. Likewise, the spatial mode of the pump is optimized to be as smooth as possible and is collimated and the plane of the doubling crystal is imaged into the Raman amplifier with Fresnel number of 2-15. For unseeded operation, the typical random patterns characteristic of the quantum-noise initiated, self-generated Stokes light are observed. We find that for a Fresnel number 2 pump beam and a gain of 10⁷, the output of the amplifier is controlled with a signal to noise ratio of about 15 when the seed-Stokes energy is about 170 aJ (560 photons). When the pump beam diameter is increased to give a Fresnel number of 15, similar control of the output is obtained with a seed-Stokes level of 7 fJ (2.3 × 10⁴ photons) at a gain of 10⁶. Further experiments will be conducted to identify the influence of Fresnel number and Raman gain on the seed level needed to exert control over the spatial modes of the output Stokes.

© 1990 Optical Society of America