Patent GB-875242-A (GB)
Title: Atomic stabilized frequency source
IPC: H03L7/26
Filed: 1959-02-20
Abstract: 875,242. Automatic frequency control systems. VARIAN ASSOCIATES. Feb. 20, 1959 [Feb. 21, 1958], No. 6027/59. Class 40(5) [Also in Group XL(b) ] An optical absorption system, for example for use as a standard frequency source or "atomic clock" comprises a vessel containing alkali vapour which is placed in a magnetic field, a source of optical radiation arranged to effect optical pumping of the atoms of the vapour whereby they become aligned in said magnetic field, a source of radio frequency energy at the hyperfine resonance transition frequency of said atoms to produce such hyperfine transition of the atoms in the vessel and means for detecting the optical radiation after it has passed through the vessel, the detecting means serving to measure changes in the intensity of the radiation due to the absorption of energy therefrom at the hyperfine resonance. In Fig. 1, the absorption vessel 11 contains metallic sodium in equilibrium with its vapour and argon acting as a buffer gas, and is positioned in a cavity resonator 13 contained in an oven kept at 120‹-145‹C. Lamp 21 forms the source of optical radiation and the aligning magnetic field H o  is the earth's field, a radio frequency generator 29 being coupled to the cavity resonator to provide a radio frequency magnetic field and thereby induce the hyperfine transitions of the sodium atoms. A resonance of the sodium atoms occurs and results in a weakening of the light detected by a photo-cell 26, the output from which is amplified and displayed on an oscilloscope 28. The radio frequency of generator 29 is frequency modulated by a modulator 32 which also provides the time base for the oscilloscope so that the point of maximum hyperfine transition can be swept through periodically. The spectral frequency of the energy quanta separating the hyperfine levels is substantially constant and independent of the strength of (weak) field H o . A stabilized oscillator or "atomic clock" is described in Fig. 4 (not shown) in which a phase detector compares the output of the photo-cell and that of a sweep generator, the D.C. error output signal of the phase detector being dependent on the extent to which the resonance output signal from the absorption vessel is off maximum resonance, and tuning the radio frequency generator to maximum hyperfine resonance frequency, e.g. through a reversible D.C. motor. In order to compensate for any change in spectral frequency due to possible variations in H o , a second radio frequency generator 38 in addition to the one referred to with respect to Fig. 4 is provided in the embodiment of Fig. 5, the output of generator 38 being coupled to coils 39 in vessel 11 to induce a radio frequency field of the Larmor frequency of the sodium atoms. An additional sweep generator 41 coupled to coils 42 is provided to produce a modulation of H o  and therefore a sweep through the Larmor frequency. Photo-cell 26 detects the optical absorption in vessel 11 due to both the hyperfine resonance and the Larmor frequency transitions and the output from amplifier 27 is fed to a second phase detector 43 the reference signal for which is obtained from sweep generator 41. The output of detector 43 is a D.C. error signal the sign of which depends on the side of maximum resonance on which the Larmor resonance signal is and the amplitude of which depends on the amount of resonance, and the output is fed to a bias coil 44 producing a correcting magnetic field for H o ; detector 37 and sweep generator 36 being used as before to adjust the radio frequency generator. The correcting information for the radio frequency generator 29 may be obtained without the use of frequency or phase modulation by varying the pressure of the buffer gas (Fig. 6). Absorption vessel 51 is separated into two parts 52, 53 having different buffer gas pressures (100 m.m. Hg, 90 m.m. Hg) two points of maximum resonance being therefore produced, which are detected by separate photo-cells 54, 55. The outputs from said cells are connected to a difference amplifier comprising a pair of triodes 57, 58, the potentials on the grids of said triodes varying with the light signals on the photo-cells, any difference in the light signals producing a D.C. output which tunes generator 29. In order to avoid errors due to arbitrary changes in light intensity on the photo-cell, two pairs of photo-cells are used in Fig. 7 (not shown) which are under continuous test by amplitude modulating the light from the optical source by a low amplitude signal, so that, if all four photocells are equal, meters connected thereto will read zero. Alternatively, one photo-cell only may be used which detects the light from each half vessel alternately.