Internal thermal noise in optical frequency converters

We present a detailed analysis of noise due to the frequency-converted internal thermal radiation in a proustite crystal pumped by a ruby laser and adjusted for noncritical phase matching with a 10.6-μm signal. A three-dimensional expression is developed for the wave-number mismatch Δk, and phase-matching surfaces are plotted, which make clear the relative importance of phase matching and system filters in determining the spectral and spatial characteristics of the frequency-converted noise. Comparison of theory and experiment establishes that a crystal absorption coefficient of 0.22 cm−1 at 10.6 μm will account for the observed noise level. This compares favorably with a direct measurement of 0.4 cm−1 for the crystal loss coefficient. Good agreement is found between experiment and predictions of the noise temperature dependence.