Subcompact hydrogen maser atomic clocks

Recent progress in compact hydrogen maser atomic frequency standards or clocks is reviewed. After a brief description of the principle of a hydrogen maser, the techniques used to realize high-performance compact hydrogen masers are discussed. Two approaches in compact hydrogen maser design are presented. The first approach uses active or atomic resonance sustained maser oscillation. The design uses cavity Q enhancement to overcome the intrinsic higher losses in a compact cavity. The versatility of the cavity design enables masers of various sizes to be realized. The complete package for the smallest oscillating compact maser measures 17.8×30.5×43.2 cm, weighs 19.5 kg, and has a measured stability of 4.14×10^-14 for an averaging time τ=400 s, with a τ^-1/2 dependence for τ<10⁵ s. The second approach uses a passive maser. The design uses the atomic resonance as a narrow-bandpass amplifier. Using a dielectric loaded cavity, a package size of 26.7×66.0×45.5 cm with a weight of 30 kg and a stability of 1.0-3.0×10^-12 τ^-1/2, for 1<τ<10⁵ s, has been attained. Compact masers of both designs have demonstrated frequency drift rates of 1×10^-15 per day or less