Electricity: measuring and testing – Particle precession resonance – Using optical pumping or sensing device
Patent
1997-10-17
2000-04-25
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Using optical pumping or sensing device
324305, G01V 300
Patent
active
060548525
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods and devices for obtaining a signal by the principle of optically pumped magnetometers.
Optically pumped magnetometers have been known since the beginning of the sixties and since then have found increasing application in devices for measuring magnetic fields. With these magnetometers, for instance, the earth's magnetic field can be determined with exceptionally high accuracy. The basic theory and mode of operation of optically pumped magnetometers is known as such, the basic principle being always that a light beam of known polarisation and direction passes through a medium, usually caesium, but also potassium, helium or rubidium, thereby producing an output frequency, which corresponds to the amplitude of the local magnetic field.
In recent decades prior art has disclosed the development of various kinds of optically pumped magnetometers, the simplest embodiment being a self-oscillating Cs magnetometer in which a light beam tuned to a resonance line of caesium passes through an absorption cell filled with caesium and is incident on a photocell. The output signal of the photocell is amplified, phase shifted 90.degree. and applied to the coil surrounding the absorption cell. In its tuned condition such a self-oscillating magnetometer generates a frequency f proportional to the surrounding magnetic field H: broad resonance line so that the generated frequency fails to correspond to the actual field value exactly.
This is why so-called tandem magnetometers (see FIG. 1) have been developed in which a self-oscillating magnetometer is coupled to a further narrow-band optical magnetometer. One such tandem magnetometer is disclosed, for example, by the paper of J. H. Allen et al. in J. Geomagn. Geoelectr. 24, pages 105-125 (1972). This magnetometer produces, however, a magnetic disturbance field and both sensors need to be arranged far removed from each other which is a big disadvantage. When, by contrast, an arrangement having a conventional voltage-controlled oscillator (VCO) is employed as demonstrated, for example, in the publication of E. B. Alexandrov et al. in OPTICAL ENGINEERING, April 1992, Vol. 31, No. 4, this results in a restricted dynamic response and problems in the case of fast-changing magnetic fields.
On the basis of this prior art it is thus the object of the present invention to define methods and devices for obtaining a signal by the principle of optically pumped magnetometers which can be implemented or operated at reduced electronics expense using a compact magnetometer arrangement producing no magnetic disturbance field.
Basically the invention involves using a broadband self-oscillating magnetometer as a voltage-controlled oscillator (VCO). In this embodiment of the invention the broadband self-oscillating portion of a tandem magnetometer is reconfigured so that this portion has the function of a voltage-controlled oscillator (VCO) in making use of a voltage-controlled frequency shift to tune the frequency of the self-oscillating magnetometer within a range. In a further embodiment of the invention, instead of making use of a conventional VCO in the arrangement according to Alexandrov et al. a broadband self-oscillating magnetometer is employed which operates like a VCO. The invention permits the configuration of a compact disturbance-free sensor.
The invention will now be described in more detail with respect to the Figures in which:
FIG. 1 is a tandem magnetometer in accordance with prior art;
FIG. 2 is one embodiment of the present invention; and
FIG. 3 is a further embodiment of the present invention.
FIG. 1 shows a tandem magnetometer in accordance with prior art in which a self-oscillating Cs magnetometer 10 is coupled to a narrow-band K absorption cell 20, the object of the complete arrangement being to measure the strength of the surrounding magnetic field H. In principle the output frequency of the self-oscillating magnetometer is multiplied according to the gyromagnetic ratio of the resonance used (for Rb, K:2, for He:8) and applied to the high-fre
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"Narrow Line Rubidium Magnetometer for High Accuracy Field Measurements" by Allen et al. (J. Geomag. Geoelectr. 24) p. 105-125; Dec. 1972.
"Optically pumped atomic magnetometers after three decades" by Alexandrov et al.; Optical Engineering/Apr. 1992 vol. 31, No. 4, pp. 711-717.
Optical Engineering, Bd. 31, 1992; Seiten 711-717, XP000265429; E.B. Alexandrov, V.A. Bonch-Bruevich: "Optically Pumped Atomic Magnetometers After Three Decades" in der Anmeldung erwahnt siehe Seiten 714-715, Paragraph 4.2 und Fig. 6.
Journal of Geomagnetism & Geoelectricity, Bd. 24, 1972; Seiten 105-125, XP000576475 J.H. Allen, P.L. Bender: "Narrow Line Rubidium Magnetometer for High Accuracy Field Measurements" in der Anmeldung erwahnt siehe Seite 107, Absatz 2--Seite 109, Absatz 1; Abbildung 2; siehe Seiten 109-122, Paragraphen 2.1 und 2.2; siehe Seiten 113 unde 114, Paragraph 4.
"Modulation of a Light Beam by Precessing Absorbing Atoms" by H.G. Dehmelt; (Physical Review, vol. 105, No. 6, Mar. 15, 1975, pp. 1924 and 1925).
"Principles of Operation of the Rubidium Vapor Magnetometer" by Arnold L. Bloom; Jan. 1962, vol. 1, No. 1, Applied Optics, pp. 61-68.
"Optical Detection of Magnetic Resonance in Alkali Metal Vapor" by Bell et al. (Physical Review, vol. 107, No. 6, Sep. 15, 1957, pp. 1559-1565).
Arana Louis
Geo-Forschungszentrum Potsdam
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