Methane microwave optical master oscillator for fountain references

We have demonstrated a microwave master oscillator operating on optical principles and based on a He – Ne/CH₄ optical frequency standard (λ = 3.39 μm) and femtosecond fibre laser system (λ = 1.54 μm). The output signal spectrum of the oscillator has the form of an equidistant frequency comb in the range 60 MHz to 10 GHz with a 60-MHz step. Comparison of the frequencies of two master oscillators shows that the short-term output frequency instability for the comb components in the range 0.8–1.5 GHz is under 1 × 10^-14 at an averaging time of 1 s. We have tested microwave signals from the oscillator using apparatus in the reference facility at the State Time and Frequency Service, All-Russia Research Institute of Physical and Radio Engineering Measurements. A signal at a nominal frequency of 100 MHz synthesised from one comb component was compared to signals from two hydrogen masers with enhanced short-term stability in the reference facility. The short-term frequency stability of the synthesised signal has been shown to be twice better than the stability of the masers and to be limited by the intrinsic instability of the commercial synthesiser of a 100-MHz nominal frequency. Our experiments confirm that the proposed microwave optical master oscillator is potentially attractive for use in systems with increased requirements for short-term frequency stability, in particular in time and frequency fountain references.