All-optical microwave feedback oscillator with atomic cell resonator

We report the demonstration of an all-optical self-sustained cell-based microwave feedback oscillator. In this scheme, a microwave resonance, optically induced in a buffer-gas filled vapor cell resonator through coherent population trapping (CPT), is detected by a fast photodiode, amplified, and used to drive back, through a frequency divider, a Mach–Zehnder electro-optic modulator in a sustaining loop configuration. The total gain and phase of the system was measured in an open-loop configuration with a network analyzer. In good agreement with values predicted by the Leeson effect, the CPT feedback oscillator demonstrates an absolute phase noise of −24 dB rad²/Hz at 1 Hz offset frequency, compatible with a fractional frequency stability of 8 × 10⁻¹² at 1 s, and a phase noise floor of −112 dB rad²/Hz, limited by the low microwave power available at the photodiode output. The amplitude noise of the oscillator shows a comparable noise floor and, for offset frequencies lower than 200 Hz, a 1/f dependence, due to the presence of the frequency divider in the loop.