Development of commercial fountain clocks in NIM

Abstract

A new Rb fountain clock has been built in NIM aiming to operate semi-continuously and achieve an excellent long-term instability for time keeping. A double metal microwave cavity with a thermal expansion self-compensating mechanism is used for Ramsey interrogation to reduce the sensitivity to ambient temperature fluctuations and increase the operating temperature range. With an optimized design, the thermal coefficient of the resonance frequency has been reduced to less than 10 kHz/°C. This represents an improvement of more than an order of magnitude compared to a copper cavity. The optical system with two independent frequency stabilized lasers with automatic re-locking system is located on a 400×600 mm optical breadboard with special designed optical mounts to ensure stable output light powers, which ensure to resume operating without any realignment of optical system after transportation. The clock has been operated for more than a year without any failure. A long-term instability of 3.7×10−16 was achieved compared with NIM5 Cs fountain clock. By implementing feedback of the frequency difference to the Voltage Controlled Crystal Oscillator within a servo-loop, the fountain clock can generate a standard frequency output, the instability of which can enter 10−16 order at 40000 seconds.