Abstract: An atomic maser comprises a walled enclosure (76) defining a resonant cavity (4), and a storage bulb (8) for storing a medium capable of stimulated emission. The storage bulb is a sapphire cylinder (70) that is open at both ends and has two closure members (71,72) for closing the open ends. The closure members are made substantially of titanium and have a coating (73,74) of polyfluoroethene formed on their inner surfaces. At least one of the closure members (71,72) forms part of the walled enclosure.

Abstract: An atomic maser includes a circuit (25) for effecting a shift in the frequency of a signal injected into a resonant cavity (4) so as to generate alternately a first and a second signal portion having respectively a frequency greater than and less than the natural frequency of stimulated emission, the first and second signal portions having a periodicity corresponding to a predetermined interrogation period. Circuitry (31,32,33,51,52) is responsive to a signal detected in the cavity (4) to produce an error signal representing the difference between the amplitude of the detected signal in response to the injection of the first and second signal portions. Further circuitry (53,54) adjusts the resonant frequency of the cavity in response to the error signal. A pulse generator 26 and a mixer 28 periodically apply both the first and second signal portions for a limited duration only during the interrogation period.