Patent GB-1350038-A (GB)
Title: Method for tuning the oscillation frequency of the resonant cavity of a maser oscillator
IPC: H01S1/00
Filed: 1972-07-27
Abstract: 1350038 Masers AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE 27 July 1972 [6 Aug 1971] 35182/72 Heading H1C Mismatch between the resonant cavity and the stimulated emission transition frequency of a hydrogen or rubidium beam maser is determined by periodically modulating the maser beam oscillation intensity, and comparing the phase of the modulated output with that of a reference oscillator, any resultant phase difference due to mismatch being used to correct the resonant cavity tuning. A maser is shown in Fig. 2 in which an atomic hydrogen beam 6 is produced from a source 2 comprising a discharge tube supplied with molecular hydrogen. The beam passes through a hexapole magnetic lens 8 which gives rise to three Zeeman levels, the atoms in energy state F = 0, and in energy state F = 1, quantum number m F  = -1 being deflected to the maser easing 10, and the atoms in energy states F=0, m F =0 or +1 being focused into a storage cell 12 within a cylindrical resonant cavity 14. The maser output is obtained from a loop probe 20, and the cavity is adjustably tuned to frequency hv corresponding to transitions between energy states F = 1, m F  = 0 and F = 0 by the reverse bias applied to a semiconductor diode 24. Modulation may be effected by gas flow intensity control, or preferably by the provision of mutually perpendicular fixed and alternating magnetic fields applied to the focused beam 6 by coils 26 and 30, Fig. 3. The fixed field causes further line splitting, while the alternating field is of such frequency, e.g. 1À4 mc/s, that the population of state F=1, m F  = 0 is periodically changed by sub level transitions from the F = 1, m F = +1 state, a consequential modulation of the maser output intensity being obtained. A frequency regulating system is shown in Fig. 5 in which the maser 52 is controlled by a square wave modulator 54 and its output applied to a quartz oscillator 50 which repeats the phase variation of the maser after a time interval. A phase comparison is made between the output of oscillator 50 and an auxiliary quartz oscillator 58 by a phasemeter 56, the auxiliary oscillator being frequency controlled through a filter 60 by the phasemeter output. Additionally the maser resonant cavity is tuned by the output from the phasemeter which passes through a demodulator 62 and a filter 64. In an alternative embodiment, Fig. 6, the maser 68 has the same control components of a modulator 70, demodulator 72 and filter 74. The maser output (1420 MHz) is however combined at a frequency mixer 40 with the output of a quartz oscillator 32 (5 MHz) which is frequency increased (1400 MHz) at a synthesizer 36. The difference frequency in the mixer (20 MHz) is applied through an amplifier 42 and a frequency synthesizer 44 (20 to 5À75 MHz) to a phasemeter 46 which compares its phase with that of the oscillator output which is changed to the same frequency (5À75 MHz) by a synthesizer 38. Any detected phase difference is applied to the demodulator 72 and through a low pass filter 48 to the oscillator 38.