Laser frequency locking by direct measurement of detuning
  We present a new method of laser frequency locking in which the feedback signal is directly proportional to the detuning from an atomic transition, even at detunings many times the natural linewidth of the transition. Our method is a form of sub-Doppler polarization spectroscopy, based on measuring two Stokes parameters ($I_2$ and $I_3$) of light transmitted through a vapor cell. This extends the linear capture range of the lock loop by up to an order of magnitude and provides equivalent or improved frequency discrimination as other commonly used locking techniques. 
A. Ratnapala, C. J. Vale, A. G. White, M. D. Harvey, N. Heckenberg and   H. Rubinsztein-Dunlop