Optimized operating parameters for modulation transfer spectroscopy as a laser-locking mechanism

Modulation Transfer Spectroscopy (MTS) enables a wideband laser lock that suppresses frequency noise and improves laser stability. Here, we present a theoretical analysis for optimum MTS laser parameters on the 87Rb F=2 -> F'=3 transition and verify the analytic solutions experimentally. We optimize the lock by utilizing a figure of merit that considers both slope and peak-to-peak amplitude. Our model uses the two-level atom approximation and considers a power-broadened FWHM linewidth to account for intensity, which is one of our three optimizable operating parameters. We analytically find the optimum parameters that maximize our figure of merit, which in turn maximizes the stability of the frequency locking system. We analyze our model up to 18 times the saturation intensity and determine the optimum parameters within the region where our model best agrees with our experimentally derived absorption signals.