Laser frequency stabilization via bichromatic Doppler-free spectroscopy of an ⁸⁷Rb D₁ line

We demonstrate a bichromatic Doppler-free spectroscopy of an ⁸⁷RbD₁ line by using a dual-frequency, counterpropagating laser field with orthogonal linear polarizations. A reversed Doppler-free resonance dip is observed in the dual-frequency scheme, and a significant improvement of frequency discrimination curve is acquired due to the coherent population trapping (CPT) effect. The influence of the static magnetic field and laser intensity on the spectroscopy is studied in both single- and dual-frequency schemes. After locking the laser frequency to the ⁸⁷RbD₁ line in the dual-frequency stabilization scheme, the beat note fractional frequency stability is at the level of 7×10^-12 at 1 s integration time. This technique can be used in various applications, such as CPT atomic clocks, laser spectroscopy, quantum optics, and laser-cooling experiments.