Optimization of polarization spectroscopy signal for laser-frequency stabilization

We experimentally generate polarization spectroscopy (error) signals corresponding to the D2-line hyperfine transitions, of 87Rb, and of 85Rb, and show that the strongest error signals correspond to the closed hyperfine transitions (oscillator strength ), and , respectively. We make the generated error signal robust to fluctuations in external parameters by finding optimum values for the vapor cell temperature and pump-probe intensities at two different beam diameters. We further employ these optimized error signals to directly (without the need for frequency/phase modulation) lock the laser frequency—reducing the rms drift/linewidth from MHz to kHz, when measured over a min duration. In addition, by comparing theoretically calculated and experimentally measured error signals for the pump intensities ranging from (where, mW cm−2 is the two-level saturation intensity for the Rb D2-line transitions), we discuss the applicability and limitations of existing numerical and analytical approaches based on a full multi-level rate equation model for polarization spectroscopy.