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

In order to reduce the size and weight of the hydrogen maser atomic clocks, some useful attempts and related research results about sapphire loaded cylindrical cavity for the hydrogen maser were reported in the Beijing Institute of Radio Metrology and Measurement. The fractional frequency stability of the order of 10^-15 over ten thousand seconds can be realized. However, due to 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 cavity. The permittivity of SrTiO₃ is 300 along the [001] crystallographic direction at 50degC, which is about thirty times magnitude larger than that of sapphire (that is 9.4 along the z-axis of sapphire single crystal) at the same temperature. Thus, through theoretical calculation, finite element method simulation, and experiments, we have successfully performed the compensation of frequency-temperature coefficient in sapphire cavity using small SrTiO₃ rings for chips. Based on theoretical calculation, the frequency-temperature coefficient in the TE₀₁₁ mode of a sapphire cavity associated with two small rings of SrTiO₃ can be reduced obviously. For instance, at 50degC, when the thickness of the SrTiO₃ ring is 5 mm, the frequency-temperature coefficient can be reduced about five times, and quality factor can be kept at above 40000 synchronously. A sapphire loaded cavity and eight small compensated chips of SrTiO₃ were prepared and a combined cavity was simulated by finite element method and measured by experiments. The simulation and the experimental results agree very well. In addition, experimental results show that frequency-temperature coefficient in eight small SrTiO₃ chips adhere- d to one endcap of sapphire bulb is much better than that in top and bottom of sapphire symmetrically.