A new method to reduce frequency-temperature coefficient of sapphire-loaded cavities for compact hydrogen masers

To reduce the size and weight of the hydrogen maser atomic clocks, some useful attempts and related research results about sapphire-loaded cylindrical cavities for hydrogen masers were reported by the Beijing Institute of Radio Metrology and Measurement. The fractional frequency stability of the order of 10^??15 over 10 000 seconds can be realized. However, because of a large frequency-temperature coefficient in a single sapphire bulb in the cavity, further improvement of the stability in the compact hydrogen clock was restricted. In this work, we chose several small single-crystal chips of SrTiO₃ with a large negative frequency-temperature coefficient to compensate the sapphire microwave cavity. Based on the theoretical calculation, the frequency-temperature coefficient in the TE011 mode of a sapphire cavity associated with several small chips of SrTiO₃ can be greatly reduced. A sapphire-loaded cavity and 8 compensated chips of SrTiO₃ were prepared, and a combined cavity was simulated by finite element method and measured by experiments. When quality factor was kept above 40 000, the frequency-temperature coefficient can be reduced to about 1/5 of its starting value. The experimental results agree very well with the calculation and simulation. Furthermore, this new method was applied in the compact hydrogen maser. Because of the decrease of temperature frequency shift, the hydrogen maser stability at medium- and long- term averaging time from 1 s to 10⁵ s has an obvious improvement compared with the our previous results.