Photonic microwave oscillator with reduced phase noise at small offsets from the carrier frequency

We have developed a transportable laboratory sample of a photonic microwave oscillator (PMO) based on an optical frequency standard (He–Ne/CH₄ laser, λ = 3.39 μm) and an optical frequency divider (femtosecond Er-doped fiber laser, λ = 1.55 μm). The radiation spectrum of the oscillator is a comb of frequencies in the range from 1 to 10 GHz. A relative Allan deviation is demonstrated to be less than 1×10^–14 at averaging times of 1 – 1000 s for spectrum components with frequencies of 1.0 and 9.2 GHz. The choice of frequencies is determined by the prospect of using the designed PMO as a master oscillator in Cs/Rb fountain-type time and frequency standards. At averaging times from 1 to 200 s, the resulting frequency instability is lower than that of commercial hydrogen masers. The spectral power density of phase noise for the specified frequencies at offsets of less than 100 Hz from the carrier frequency is significantly lower than that of the best electronic and optoelectronic oscillators. The designed oscillator seems useful for problems of fundamental time and frequency metrology, development and phase calibration of precision microwave equipment, coherent radar detection, monitoring, and communications, as well as in cases requiring relatively long (10^–2 – 10³ s) accumulation of a weak signal in the region of small carrier frequency offsets and/or synchronization holding of distributed sources.