Quality of the Stokes beam in stimulated Raman scattering

Stokes beam quality as obtained by stimulated Raman scattering is investigated numerically using various pump-beam profiles. It is found that the quality of the Stokes beam, expressed by M², is often roughly the same as the pump beam. For pump beams with Gaussian-Hermite modes, TEM₀₀ and TEM₀₁, the Stokes beam has the same-phase relation and M² as the pump beam. However, the TEM₁₁ pump beam, which has an M² of 3.0, gives a Stokes beam with a higher M², namely 3.7, and a distorted phase relation. For a Gaussian-Schell model beam, the M² of the Stokes beam is equal to that of the pump if the Stokes has the same wavelength as the pump; if the ratio of the Stokes and the pump wavelength increases, then the M² of the Stokes beam decreases. Experimental verification was obtained using a pump beam with an M² of 8.7 and a wavelength of 1.06 μm. The generated Stokes beam at 1.54 μm had an M² of 5.8, which agrees well with the numerical M² of 6.1