Opto-electronic Oscillator - Operation

Operation

Most OEOs utilize the transmission characteristics of a modulator together with a fiber-optic delay line to convert light energy into stable, spectrally pure RF/microwave reference signals. Light from a laser is introduced into an E/O modulator, the output of which is passed through a long optical fiber and detected with a photodetector. The output of the photodetector is amplified and filtered and fed back to the electric port of the modulator. This configuration supports self-sustained oscillations, at a frequency determined by the fiber delay length, the bias setting of the modulator, and the band pass characteristics of the filter. It also provides for both electric and optical outputs. The conditions for self-sustained oscillations include coherent addition of partial waves each way around the loop and a loop gain exceeding losses for the circulating waves in the loop. The first condition implies that all signals that differ in phase by some multiple of 2π from the fundamental signal may be sustained. Thus the oscillation frequency is limited only by the characteristic frequency response of the modulator and the setting of the filter, which eliminates all other sustainable oscillations. The second condition implies that, with adequate light input power, self-sustained oscillations may be obtained without the need for the RF/microwave amplifier in the loop.