The application of a photonic automatic gain control system, over a 3 km loop of single-mode fiber for the 100 m Green Bank Telescope in order to correct the effects of polarization state changes, is described. Commercially available photodetector conversion gains are polarization mode dependent, which produce microwave gain changes on the order of 10−2. With the selection of the least polarization sensitive photodetector, the gain changes were reduced to less than the measurement limit of 10−4. The implementation of the control loop with a digital feedback loop is detailed, along with measured results and correction limit explanations. Effects of temperature on the quadrature point and corrections for small temperature deviations with a direct current bias circuit is also described.

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