Influence of laser mode amplitude and laser mode phase fluctuations on spectral noise in CARS

• A model for calculating the noise in nonlinear optical processes arising from (nonquantum) stochastic fluctuations (mode beating) in and between the lasers participating in the nonlinear interaction has been previously developed. Assuming that all individual laser mode phases and laser modes are (statistically) independent (Gaussian statistics) and that the nonlinear interaction can be expressed as a perturbation expansion in the participating fields, deterministic expressions for the expected intensity and higher order moments for the signal were obtained in a straightforward way. The authors have explicitly evaluated the deterministic expressions, obtaining closed-form expressions for the SNR in coherent anti-Stokes Raman scattering (CARS) as a function of laser mode spacing, laser and Raman bandwidths and the duration of the anti-Stokes pulse. Those results qualitatively agreed with their experimental results. The theoretical analysis further indicates that the (nonquantum) noise in nonlinear optical processes from multimode lasers can be separated into terms (explicitly evaluated) arising from shot-to-shot fluctuations in the laser mode amplitudes and terms arising from shot-to-shot fluctuations in the individual laser mode phases.

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