Methane-Based Photonic Microwave Oscillator With Stability Better Than 10−14 at 1–103 s Averaging Times

An improved version of the transportable photonic microwave oscillator based on the He–Ne/CH
₄
optical frequency standard (OFS) (
$\lambda $
=
$3.39 ~\mu \text{m}$
) and the femtosecond Er fiber laser optical-to-microwave frequency divider (
$\lambda $
=
$1.55 ~\mu \text{m}$
) has been developed. Stable continuous operation of the generator was achieved for several days without operator intervention, and the relative Allan deviation below
$1\times10$
⁻¹⁴
was demonstrated for averaging times 1–10
³
s at output frequencies of 1.0 and 9.2 GHz. The choice of frequencies is determined by the implementation of the methane-based photonic microwave oscillator as an interrogation oscillator for Cs (Rb) fountain. The obtained relative frequency instability at averaging times of 1–200 s is lower than that of the best commercial H-masers. Compared to transportable photonic microwave oscillators based on an ultralow expansion Fabry–Perot (FP) cavity, the oscillator described in this article has an order of magnitude less drift and can be attractive for different tasks of coherent radio photonics where a relatively long-term accumulation of weak signals is required.