Intensity Fluctuations in the Output of cw Laser Oscillators. I

Using linearized rate equations, we have calculated the intensity fluctuations expected in the output of four-level cw laser oscillators. Intrinsic quantum effects have been included by restricting the eigenvalues of photon and atomic population operators to discrete integral values and by using equivalent noise sources. The total output fluctuations increase monotonically with output power; the relative fluctuations decrease monotonically, and at high output levels inversely as the output power squared. The low-frequency fluctuations differ in that the relative fluctuations increase with output below threshold (noise amplifier region) but decrease sharply above threshold (dynamic saturation region). Curves typical of gas, solid, and diode lasers are illustrated. The theory agrees well with preliminary experimental results.