| 摘要 |
An improved frequency locking circuit suitable for indirectly locking an optical frequency f0 to a radio frequency f1 or of locking the radio frequency to the optical frequency. A beam of optical frequency f0 is modulated by a compound signal which is the sum of a signal at frequency f2 and an FM subcarrier at frequency f1 that is phase modulated at frequency f3 to produce a phase modulated beam. The phase modulated beam is filtered by a filter having a transfer function having a characteristic frequency ff. A pair of control signals are generated that are proportional respectively to the amplitudes of two components of the filtered signal at frequencies f2, f3. These control signals are separately used in a pair of servo loops to separately establish fixed values of f0/ff and f1/ff. By using an optical cavity of the filter, the frequency locking circuit may be used for measuring the refractive index of a gas. The circuit may also be used for calibrating a multi-mode filter.
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