Laser Amplifier Noise at 3.5 Microns in Helium-Xenon

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Abstract
The noise added to the amplified signal in laser amplifiers results from spontaneous emission in the laser medium. A measurement of the intensity of the spontaneous emission noise has been performed at the high‐gain 3.508‐μ line in a dc‐excited He‐Xe discharge. Measurements were made with the laser amplifier operating under normal low‐level signal conditions as well as under high‐level, saturated conditions. It is found that the measured values of the noise power agree closely with the expressions derived for a four‐level excitation system using a plane wave propagation model. The low‐level measurement yields a population inversion of 2.6±0.3. For an amplifier with an unsaturated gain of 17 dB and a saturated gain of 1 dB, the increase in the noise output power is only 7 dB over that of an equivalent unsaturated 1‐dB amplifier. The measurement of the noise power under saturation gave a ratio of the saturated to the unsaturated upper level population of 0.4. The measurements indicate that the 3.508‐μ line has good noise properties. The experiments and the theoretical analysis apply to a one‐dimensional geometry in which variations in the transverse direction are negligible.