Frequency locking of tunable diode lasers to a stabilized ring-cavity   resonator
  We demonstrate a technique for locking the frequency of a tunable diode laser to a ring-cavity resonator. The resonator is stabilized to a diode laser which is in turn locked to an atomic transition, thus giving it absolute frequency calibration. The ring-cavity design has the principal advantage that there is no feedback destabilization of the laser. The cavity has a free-spectral range of 1.3 GHz and $Q$ of about 35, which gives robust locking of the laser. The locked laser is able to track large scans of the cavity. 
Ayan Banerjee, Dipankar Das, Umakant D. Rapol, and Vasant Natarajan