Patent GB-974389-A (GB)
Title: Optically driven spin precession
IPC: G01R33/26
Filed: 1962-03-14
Abstract: 974,389. Photo-electric nuclear magnetic resonance apparatus. VARIAN ASSOCIATES March 14, 1962 [March 14, 1961], No. 9854/62. Headings G1A and G1N. [Also in Division H3] Apparatus for producing precessing spin polarizations in an assemblage of quantum systems comprises means for irradiating the assemblage with optical pumping radiation and means for modulating the radiation intensity at a frequency in the vicinity of the resonance frequency of the precessions. Cell 1 contains an alkali metal vapour or metastable helium gas and is irradiated by lamp 2, through modulator 3 and circular polariser 4. At any instant the light produces a spin polarisation perpendicular to the constant magnetic field H 0 , which polarisation immediately begins to precess about the field direction at the Larmor frequency w. Modulation of the beam intensity at the Larmor frequency prevents cancellation of the effect over a complete cycle, resulting in a net spin polarisation. The spins may be coupled to an r.f. coil or microwave cavity, or absorption or scattering of optical radiation by the spins may be detected. If H 0  is swept through the valve at which the Larmor frequency equals the modulation frequency the transmitted light detected by photocell 7 goes through a peak due to preferential population of the non-absorbing magnetic sub-levels at resonance. Magnetometers. In Fig. 2, helium lamp 11 is excited by generator 13 which is amplitude modulated at 1. 4 mc/s by generator 18. Helium atoms in cell 14 are raised to a metastable level by r.f. generator 16. Generator 18 is frequency modulated at 60 c/s by modulator 19, so that if H 0  deviates from the reference value corresponding to the frequency of generator 18 the photo-cell produces a signal at the frequency of modulator 19, and of magnitude and phase corresponding to the deviation of H 0 . The signal is compared in phase with a reference signal from the modulator to produce a D.C. signal tending to maintain the frequency of generator 18 at resonance. The value of the D.C. signal, or the frequency of generator 18, indicates the field intensity. In a further system modulation of the transmitted light at frequency 2w is detected. The frequency of the photo-cell current is halved and used to modulate the light intensity, maintaining a self-sustained resonance frequency oscillation, the frequency of which is a measure of the magnetic field. Another arrangement maintains the spins at forced resonance at frequency f 2  by means of an r.f. magnetic field, while the assemblage is irradiated with light which is intensitymodulated at frequency f 1 . The photo-cell detects frequency |f i -f 2 | and this signal is mixed with f 1  or f 2  to reproduce f 2  or f 1 , respectively, to maintain the precession in regenerative feed-back manner. A modification (Fig. 6), operating as a widerange magnetometer, has an oscillator 64 providing a frequency f 3  which is compared in phase detector 63 with the photo-cell output of frequency |f 1 -f 2 |. The output of detector 63 varies f 1  to maintain |f 1 -f 2 | equal to f 3 . The field H 0  is indicated by frequency f 1  or the signal from detector 63. Two lamps may be driven in push-pull by an oscillator at the precession frequency and their beams passed through opposite-handed circular polarisers and into the gas cell. The spin polarisation will then build up every half cycle. The use of additional lamps is also referred to. Modulation of the light may be by means of a mechanical chopper, Kerr cell, a gas cell at resonance, or a modulated magnet effecting Zeeman splitting, or by modulation of the lamp excitation power source. The use of field-independent hyperfine transitions in establishing a frequency-standard system is also described. In Fig. 3 the hyperfine transition at 252 mc/s in potassium-41 is used. Crystal oscillator 25 drives lamp 27 at 126 mc/s to cause intensity modulation at twice this frequency. If oscillator 25 is frequency modulated at a low frequency by modulator 32, photo-cell 31 produces a signal at this frequency which is phase compared with modulator 32 in detector 34 to produce an output frequency stabilizing oscillator 25.