THE BIREFRINGENT CAVITY - LASER FREQUENCY STABILIZATION WITH A NOVEL TUNING SCHEME

Long term stability of a large frequency difference between two lasers is assured by locking them to the same cavity containing an electro-optic crystal. This crystal provides tunability by permitting the optical path length for one polarisation direction in the reference cavity to be altered by applying a dc voltage. This represents the novel feature of this scheme and provides a convenient, easy to implement alternative to modulation techniques. The addition of a second crystal into the cavity provides thermal compensation and assures stability of the difference frequency. In the present case, a tuning range of almost-equal-to 1 GHz is demonstrated and is sufficient to cover one reference cavity mode spacing. However, when higher voltages are applied and the cavity mode spacing is reduced, a tuning range of more than 10 GHz should be available. The observed long term stability of almost-equal-to 1 MHz represents at least a factor of ten improvement over commercial reference cavities and is now commensurable with the short term linewidth. This is adequate for experiments involving optical pumping via single photon or Raman type transitions and in slow scan spectroscopy.