Intensity fluctuations and spatial coherence of a single polarized component of a saturating amplified-spontaneous-emission source

Intensity fluctuations of the amplified-spontaneous-emission (ASE) output of a saturated 3.51-μm xenon-helium cw gas-laser amplifier have been characterized by measurements of the first seven normalized cumulants of the intensity probability distribution function (IPDF) for a single linear polarization. ASE output at all amplifier lengths measured (60 to 200 cm) displays some degree of gain-saturation distortion of the negative exponential IPDF expected for linearly amplified spontaneous emission. At long lengths (heavy saturation) the higher cumulants [initially $\mathrm{}(n-1\mathrm{})!$ for the $n$th normalized cumulant] are substantially reduced to order unity. However, the normalized variance (second cumulant) is only slightly reduced and appears to be approaching a long-length limiting value of approximately 0.45. The output shows a high degree of spatial coherence at both long and short lengths studied.