Robust laser frequency locking by dispersion of atoms in a transverse magnetic field

A robust laser frequency locking scheme to an atomic transition is proposed and experimentally demonstrated. In this scheme, dispersion (real part of electric susceptivity) of the Zeeman-shifted atoms is employed as an error signal for feedback, whereas in the DAVLL (dichroic atomic vapor laser lock, Appl. Opt. {\bf 37}, 3295(1998)), absorption (imaginary part of electric susceptivity) is. Magnetic field transverse to the laser beam is applied to the atoms. The error signal of the proposed scheme provides a wide locking range, depending on the parameters and the energy level structure. Experimental demonstration with transition between $5d$ $^2D_{3/2}$ and $6p$ $^2P_{1/2}$ of Ba$^+$ is carried out for a grating-feedback external-cavity laser diode at 650 nm.