| 摘要 |
<p>In a ring laser (12) with an intracavity saturable absorber (47) and a birefringent cavity (41-44; 50), a bias is established whose frequency is less than the axial mode spacing. Bias drift may be nulled on the basis of bias error signals obtainable directly from the output beams (27; 67, 68). In a first embodiment the plurality of birefringent elements (50) comprises a waveplate (95) and a rotator (97), and the optical property measured to derive the bias error signal is the state of polarization of either output beam. In a second embodiment the plurality of birefringent elements (50) comprises two misaligned waveplates (105, 107) that are nominally half-wave plates. In the second embodiment, the bias error signal may be derived by two techniques. The first technique is based on polarization. Cavity birefringence is modulated (124) in a prescribed way; the signal generated by polarization-detection apparatus (125) contains modulated components, which are then demodulated (127) to provide the desired bias error signal. The second technique is based on frequency. Cavity birefringence is altered by means of a sequence of pulsed interrogation signals (137), and a difference frequency measurement is made for each step of the sequence to obtain a bias error signal.</p> |
