Fig. 1. The principle of operation of the birefringent etalon demonstrated in an extra-cavity set-up. The input light is polarized at a slight angle to one of the optic axes of the quarter-wave etalon. An intensity component α² is directed along axis 1 and a component β ² along axis 2. The frequency of the laser or the tilt angle of the etalon are chosen such that the α ² component is close to a reflection minimum for the etalon. At exact resonance the reflection of the component along axis 1 vanishes and the reflected light is linearly polarized along axis 2 (indicated by green arrow). Away from exact resonance the reflection is elliptically polarized with opposite helicity for frequencies above and below resonance (indicated by red and blue ellipses). A quarter-wave plate is inserted with its axes aligned with those of the etalon. The transmitted light is now linearly polarized. The polarization is along axis 2 at exact resonance and changes clockwise and counter-clockwise respectively above and below resonance. This linear polarization is analyzed with a polarizing beamsplitter, which is rotated by 45° with respect to the axes of the analyzing waveplate. On resonance an equal amount of light is transmitted to both detectors while the split is asymmetric for frequencies above and below resonance.

Fig. 2. The calculated signal S for an etalon with (a) quarter-wave retardation and varying reflectivities R and (b) a 20% reflectivity and a retardation varying from λ/8 to 3 λ/8. The inset in (a) shows the dependence on the reflectivity of the gradient through the zero-crossing.

Fig. 3. Experimental results obtained with an uncoated waveplate in an extra-cavity configuration as shown in Fig. 1. The sum and difference signals from the two detectors as well as the ratio of the difference and sum are shown as a function of the laser wavelength. The solid curves shown with the sum and difference signals are sinusoidal fits to the data, which are expected to provide good fits to the experimental data for a low reflectivity etalon. The solid curve shown with the ratio data is the theoretical prediction for an etalon with a 4% reflectivity.

Fig. 4. Experimental results for a 25% reflecting etalon inserted in the cavity of a VECSEL. The ratio signal S defined by Eq. 6 is derived from the measured outputs of the polarization analyzer and shown as function of etalon tuning. The discontinuities correspond to longitudinal laser mode jumps.