Automatic Tuning of a Superconducting Cavity Using Optical Feedback

The resonant frequency of a superconducting cavity has been photodielectrically controlled by a feedback loop which contains an optical path. The cavity is an 870-MHz lead-plated quarter-wave reentrant structure. A 19 000 Ω·cm silicon wafer terminates the quarter-wave stub. The loaded Q is approximately 105 indicating the losses in the semiconductor dominate the superconducting surface resistance. The high Q provides a large phase error between the input and output voltage when the driving oscillator drifts or is modulated away from the very stable resonant frequency of the cavity. The error is processed by a wide-bandwidth phase detector that drives a gallium arsenide diode to follow the frequency excursion of the oscillator by photodielectric tuning. The 9000 Å light is the optical feedback signal which corrects the phase error. The system response typically provides a frequency deviation of ±50 kHz at a 0.2 MHz rate. Higher cavity frequency and purer semiconductor samples extend the deviation limits and shorter free-carrier lifetimes increase the speed of response.