Optoelectronic oscillators (OEOs) that incorporate carrier-suppression subsystems for reduction of close-to-carrier phase noise are undergoing development. The carrier-suppression phase-noise-reduction technique has previously been applied to microwave oscillators. The close-to-carrier phase noise in a microwave oscillator consists mainly of a component that is generated in an amplifier in the oscillator feedback loop and that has a spectral amplitude proportional to 1/f, where f is the difference between the frequency of interest and the carrier frequency. In the case of an OEO, one can use the carrier-suppression technique to reduce not only the component of phase noise generated in the amplifier but also the component of phase noise associated with laser relative-intensity noise.
The figure schematically depicts a double-loop OEO with a carrier-suppression bridge for reduction of phase noise. The long oscillator loop includes a polarizing beam splitter (PBS), a long optical fiber wound into a coil, a half-mirror Faraday polarization rotator, a photodetector (PD1), a radio-frequency (RF) amplifier, a band-pass filter, a voltage-controlled phase shifter (VCP), and an electro-optical modulator. The short oscillator loop contains only a short optical fiber and does not include a VCP or a PBS.
The polarization of the light entering the PBS is adjusted so that all of this light passes through the PBS and into the long optical fiber. At the output end of the long optical fiber, part of the light passes through the Faraday half-mirror and travels on to PD1; another part of the light is reflected by the Faraday half-mirror with a polarization that, everywhere in the long optical fiber, is orthogonal to the polarization of the light traveling forward. The orthogonality minimizes the interaction between the forward-going and reflected light beams, thereby reducing noise.
The carrier-suppression bridge functions as follows: The reflected light in the long oscillator loop is further reflected by the PBS and thereby made to enter a photodiode (PD3), which detects RF amplitude modulation of the light. Another RF signal is obtained from the RF output port of the short oscillator loop, then processed through a variable attenuator (VA) and variable phase shifter (VP1), then made to interfere with the RF output of PD3. The VA and VP1 are adjusted so that output port 1 of the bridge passes minimum power while output port 2 of the bridge passes maximum power.
The signals from the two ports are amplified, and the amplified signal at port 1 is further processed by another variable phase shifter (VP2) to set the phase difference between the two signals at either 0 or Π radians. The signals are then mixed with each other in a balanced mixer. As explained in more detail below, the mixer output is processed into an error signal that is fed back to VCP to control the frequency of oscillation.
The long optical fiber in the long oscillator loop acts as both a high-Q (where Q is the resonance quality factor) storage component for the oscillator and as a frequency-discriminator component for the carrier-suppression bridge. The bridge acts as a frequency discriminator in that it converts the frequency jitter of the OEO into amplitude jitter in the output of the bridge. The role of the mixer is to detect this amplitude jitter. The output of the mixer is amplified, filtered, and fed back to VCP to reduce the frequency jitter (and thus the phase noise) of the OEO.
This work was done by Steve Yao, John Dick, and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory.
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