| 摘要 |
In a ring laser with an intracavity saturable absorber and a birefringent cavity, 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. In a first embodiment the plurality of birefrigent elements comprises a waveplate and a rotator, 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 comprises two misaligned waveplates that are nominally half-waveplates. In the second embodiment, the bias error signal may be derived by two techinques. The first technique is based on polarization. Cavity birefringence is modulated in a prescribed way; the signal generated by polarization-detection apparatus contains modulated components, which are then demodulated 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, and a difference frequency measurement is made for each step of the sequence to obtain a bias error signal.
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