SelfOscillating Magnetometer Utilizing Optically Pumped 4HeRobert E. Slocum, P. Clayton Cabiness Jr., and Stephen L. Blevins Citation: Review of Scientific Instruments 42, 763 (1971); doi: 10.1063/1.1685225 View online: http://dx.doi.org/10.1063/1.1685225 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/42/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Self-oscillating rubidium magnetometer using nonlinear magneto-optical rotation Rev. Sci. Instrum. 76, 126103 (2005); 10.1063/1.2136885 Nd:LNA laser optical pumping of 4He: Application to space magnetometers J. Appl. Phys. 64, 6615 (1988); 10.1063/1.342042 Frequency shifts of selfoscillating magnetometer with cesium vapor J. Appl. Phys. 45, 1342 (1974); 10.1063/1.1663412 A Classical Magnetic Resonance Self-Oscillating Magnetometer Am. J. Phys. 41, 260 (1973); 10.1119/1.1987186 Response of SelfOscillating Rubidium Vapor Magnetometers to Rapid Field Changes Rev. Sci. Instrum. 40, 601 (1969); 10.1063/1.1684019

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 42. NUMBER 6 JUNE 1971

Self-Oscillating Magnetometer Utilizing Optically Pumped 4He*

ROBERT E. SLOCUM, P. CLAYTON CABINESS, JR., AND STEPHEN L. BLEVINS

Texas, Instruments Incorporated, Dallas, Texas 75222

(Received 11 January 1911; and in final form, 22 February 1971)

An optically pumped resonance magnetometer has been constructed which utilizes 4He gas as the resonant ele­ment in the self-oscillator mode of operation. This instrument is capable of monitoring anomalies and fluctuations in the magnetic field with a sensitivity of 0.1 ,..G over the range of the earth's field (0.25-0.75 G). Recordings of variations in the geomagnetic field are compared with recordings made simultaneously with a servo helium magne­tometer. The characteristics and advantages of the new system are discussed.

INTRODUCTION

OPTICALLY pumped resonance magnetometers are widely used to measure fluctuations and anomalies in

the geomagnetic field. Magnetic field information is ob­tained through the use of Zeeman transitions (~m= 1) to monitor changes in the Larmor frequency of an optically pumped sample. These instruments can be distinguished by (1) the method employed to monitor changes in the reso­nance frequency,I·2 and (2) the gas or vapor used as the resonant element. The servomethod of frequency tracking locks a resonance rf oscillator to the resonance frequency by means of a servoloop. A second method of frequency tracking is the self-oscillator technique which results in a more compact and simple system. In the self-oscillator approach the resonance frequency modulation of a light beam is used to construct an atomic oscillator.

The atomic species commonly used as resonant elements in optically pumped magnetometers are alkali atoms and helium (4He). Alkali vapors have been used in instruments of both the servo-3,4 and self-oscillatorS type. Helium has several recognized advantages over the alkali vapors in­cluding freedom from stringent temperature control of the sample. Up until the present time, however, helium magnetometers have utilized only the more complex servomode of operation.6 The present paper discusses a new self-oscillator helium magnetometer. The instrument combines the simplicity of the self-oscillator mode of fre­quency tracking with the advantages of the helium sample and achieves a sensitivity of 0.1 pG.

The Larmor frequency of the 23S 1 level of 4He which serves as the resonant element is relatively high ("" 2.8 Hz/ pG) with respect to the other optically pumped samples. This high Larmor frequency gave 4He an advantage over nuclear magnetometers7 with much lower Larmor fre­quencies in mobile applications where gyromagnetic effects can be introduced by platform motion. Because of the high Larmor frequency, helium self-oscillator operation was prohibited by the unmet demand for a large area, nonmag­netic near infrared photodetector with frequency response in the megahertz range. The development of the sensor described here was made possible by recent advances in the fabrication of large area (1 cm diam) silicon ir detectors with good responsivity to light of 1.08 p wavelength and

763

with good frequency response from 0.7 to 2.1 MHz (magnetic field from 0.25 to 0.75 G).

I. PRINCIPLES OF OPERATION

The optical pumping technique used to prepare the helium sample for resonance is basically that described by Colegrove and Franken8 and Schearer.9 The optical pumping apparatus is pictured schematically in Fig. 1. The atoms to be pumped are those which populate the 23S1

level, and this level is populated by electron excitation of ground state atoms in the absorption cell. The pumping light is composed of the three helium lines (Do, D1, and D2) with wavelengths grouped at 1.083 p. The light is obtained by rf excitation of a capillary helium lamp and is a colli­mated circularly polarized beam illuminating the sample. As this light is absorbed and emitted by the sample atoms, the optical pumping proceeds and results in a steady state nonequilibrium population distribution over the magnetic states of the 23S1 level. In this pumped condition the sample is prepared for resonance and is characterized by a net magnetization M=Me along the ambient magnetic field direction.

Resonance transitions between the Zeeman states of the pumped level are induced by subjecting the sample to an rf field of frequency P sufficiently close to the resonance fre­quency Po and with component H1 perpendicular to the static field Ho=H •. The rf drive field introduces Zeeman coherence, and the steady state behavior of the mag­netizathn M is well known from Bloch's equation.1° The

FIG. 1. Schematic diagram of single cell self-oscillator helium magnetometer.

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764 SLOCUM, CABINESS, JR., AND BLEVINS

0.04 0,06 0.06 O. t

FIG. 2. Relative frequency response of helium self-oscillator magnetometer to a 1 pG calibration signal measured through the analog output filters.

magnetization of the sample is no longer parallel Ho but now has a transverse component MJ. rotating around Ho at the frequency of H 1. The X -Z plane is defined as the plane containing the pumping light direction and the field vector Ho. The magnitude of the rotating component MJ. is (Mx2+My2)t, so that as Mr rotates around Ho at frequency II, the component M., is modulated at the same frequency.

Resonance can be observed optically because a com­ponent appears on the light beam modulated at frequency II

and with an amplitude proportional to 1M 11. This modula­tion results from a change in transmissivity of the sample which is synchronized with the drive field HI. The mag­netization equation predicts a 90° phase shift between the M., signal peak (the observed signal in our coordinate system) and HI. Optical modulation at the resonance fre­quency is the phenomenon employed to track the resonance frequency in the self-oscillator magnetometer. This effect was predicted by Dehmeltll and first observed by Bell and Bloom in sodium vaporI2 using the cross beam technique (separate beams are used to pump and detect resonance). Resonance modulation signals have been observed in the 4He first by Schearer9 using the cross beam technique and more recently by Afnas'yev et alP using a single beam to pump and detect. Phenomenological equations for the magnetization in an optically pumped sample are dis­cussed by Bell and Bloom12 and Cohen-Tannoudji.14

II. MAGNETOMETER AND RESULTS

In all resonance magnetometers field fluctuations are observed by monitoring variations in the center frequency of the resonance line. In the self-oscillator mode this is accomplished by constructing an atomic oscillator whose frequency is that of the resonance line center. This oscil­lator as shown in Fig. 1 employs the amplified output of the ir detector to drive the coils which produce the HI field

resonant with the sample. When the feedback loop gain is unity and the sum of all phase shifts is zero, spontaneous oscillation occurs at the resonance line center frequency. In practice a phase shifter is introduced in the feedback circuit in order to maintain the zero phase shift condition. The magnetometer output is an analog signal obtained by directing the oscillator output frequency into a frequency discriminator.

In our instrument the source of pumping radiation is a capillary lamp filled with 4He and excited by 4 W of 50 MHz power. The absorption cell is a Pyrex cylinder 36 mm in diameter and 46 mm long filled to a pressure of 0.75 Torr with spectroscopically pure 'He. The beam was passed through a circularly polarizing filter,15 directed through the sample, and monitored by an ir detector16 fabricated from high resistivity silicon. The active detector area is a circle 1 em in diameter. Devices of this type have a resistivity of 9000 12· em and a capacitance of approxi­mately 40 pF when operated with 135 V reverse bias. The detector-amplifier combination used in the instrument has a 3 dB high frequency rolloff point at 3.0 MHz. The detector-amplifier unit is very susceptible to rf pickup. The pickup was reduced by shielding the detector, cable, and wiring connectors with copper wire cloth.

The magnetometer was operated by adjusting the phase shifter in the feedback loop to zero and passing the analog output of the frequency discriminator through a set of output filters before recording on a strip recorder. The bandpass of the magnetometer including the output filters is shown in Fig. 2. The bandpass was determined by recording the magnetometer output for sinusoidal 1.0 p.G calibration signals which cover the frequency range of interest. The instrument was evaluated in the relatively quiet electromagnetic environment of the Texas Instru­ments Magnetic Test Facility at a remote location near Sherman, Texas. A recording of geomagnetic field varia­tions is shown in Fig. 3(a). A simultaneous recording of the output of a servo helium magnetometer with similar band­pass is shown in Fig. 3(b). The correlation between these

• II

··~fTL·

j I

I ... :0· .~-.

FIG. 3. Geomagnetic variation measured 28 September 1970, at Sherman, Texas with (a) single cell helium self-oscillator magnetom­eter, and (b) dual cell helium magnetometer.

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MAGNETOMETERS 765

two recordings indicates an equivalent level of performance of these two instruments.

III. CHARACTERISTICS

The helium self-oscillator magnetometer in the indicated operating bandpass (Fig. 2) is sensitive to field fluctuations as small as 0.1 J.lG as demonstrated by Fig. 3(a). The in­strument was subjected to a bias magnetic field and oper­ated over the range from 0.25 to 0.75 G. The precision of the instrument is determined by the light shifts14 intro­duced by the pumping beam and the phase shift in the oscillator feedback loop.l When the phase shift is adjusted to zero, the resonance frequency is expected to be identical to that found in the servo helium magnetometer. The light shift measured by Schearer17 was approximately 20 J.lG when the beam and field are parallel, but the shift for a beam orientation of 45° would be slightly less.1S

Two unique characteristics of the self-oscillator sensor are the variation in the resonance line for different values of drive field HI and dependence of signal amplitude on the pumping beam orientation (indicated by the angle 0) with respect to the static magnetic field. To study these effects the resonance signal was recorded by field sweeping the sample and monitoring the rf drive frequency modulation of the pumping beam. The field sweep was provided by a 93.4 cm diam Helmholtz coil aligned with the ambient field. The sensor was placed in the coil's region of homo­geneous field and, as the coil field swept through the helium resonance, the light modulation signal was fed to a Hewlett-Packard wave analyzer tuned to the rf drive frequency with a 3 kHz bandwidth. The analog output of the wave analyzer was recorded on the X-V plotter.

The family of resonance curves shown in Fig. 4 was recorded in this manner. As HI is increased, the signal amplitude is seen to increase to a maximum value and remain approximately constant. The amplitude of the line center increases with HI to the point of signal maximum and then decreases with increasing values of HI. This reso­nance signal behavior is discussed by Cohen-Tannondjil4

--=====" FIG. 4. Self-oscillator resonance curves measured at various rf sweep

amplitudes (mV).

FIG. 5. Orientation dependence of self-oscillator signal amplitude.

using a spin ! model. The rf drive coil for the sensor is wound in a Helmholtz configuration around the cylindrical absorption cell, and the coil constant is 37 J.lG/mV at the coil center. For magnetometer operation a value of HI was selected to achieve maximum signal but no line center reversal. The signal-to-noise ratio at the operating point was 184 in a 3 kHz bandpass. The width of the line at half­intensity is 1640 J.lG.

Both the optical pumping cycle and optical resonance detection depend on the direction of the pumping beam with respect to the magnetic field. The orientation depend­ence of the signal amplitude for a single helium cell in the servomode of operation has been reported.1s The orienta­tion dependence of the signal amplitude for the single cell self-oscillator sensor is shown in Fig. 5. Cylindrical sym­metry exists around the magnetic field direction for light absorption so the signal can be characterized by the angle 0 dependence.1 The signal amplitude values shown in Fig. 5 were obtained by recording the resonance curves (field sweep method) at 5° intervals over the range from 0 to 360°. The plane of rotation was perpendicular to the capillary lamp axis.

* This work was supported by the U. S. Naval Air Development Center under Contract No. N62269-70-C-0269.

1 A. L. Bloom, App!. Opt. 1, 61 (1962).

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766 SLOCUM, CABINESS, JR., AND BLEVINS

2 P. L. Bender, Quantum Electronics, edited by P. Grinet and N. Bloombergen (Columbia U. P., New York, 1964).

3 T. L. Skillman and P. L. Bender, J. Geophys. Res. 63, 513 (1958). • L. Malnar, Quantum Electronics (Columbia U. P., New York,

1964). • W. H. Farthing and W. C. Folz, Rev. Sci. Instrum. 38, 1023

(1067). 6 A. R. Keyser, J. A. Rice, and L. D. Schearer, J. Geophys. Res.

63, 513 (1958). 7 L. D. Schearer, F. D. Colegrove, and G. K. Walters, Rev. Sci.

Instrum.34, 1363 (1963). 8 F. D. Colegrove and P. A. Franken, Phys. Rev. 119,680 (1960). 9 L. D. Schearer, Advances in Quantum Electronics (Columbia U. P.

New York, 1961).

THE REVIEW OF SCIENTIFIC INSTRUMENTS

10 F. Bloch, Phys. Rev. 70,460 (1946). 11 H. G. Dehmelt, Phys. Rev. 105, 1559 (1957). 12 W. E. Bell and A. L. Bloom, Phys. Rev. 107, 1559 (1957). 13 V. F. Manas'yev, R. A. Zhitnikov, and P. P. Kuleshov,

Geomagtiz i Acronomiya 10, 183 (1970). 14 C. Cohen-Tannoudji, Ann. Phys. (Paris) 7, 423 (1962). 16 Polaroid type HR circular polarizing filter. 16 Infrared detectors of this type are the photoamperic silicon de­

tector manufactured by Texas Instruments Incorporated, Dallas, Texas and the ultrafast laser detector manufactured by the Harshaw Chemical Company, Cleveland, Ohio.

17 L. D. Schearer, Phys. Rev. 127, 512 (1962). 18 R. E. Slocum, Rev. Phys. App!. 5, 109 (1970).

VOLUME 42. NUMBER 6 JUNE 1971

A Cold Cathode Pulsed Gas Laser

R. K. LoYNES· AND J. c. W. TAYLORt

Department of Physics, University of Alberta, Edmonton 7, Alberta, Canada (Received 20 November 1970; and in final form, 26 February 1971)

Construction details are presented for a cold cathode pulsed gas laser which utilizes a plasma tube made from straight pieces of ordinary glass tubing. The laser features ease of construction with relatively low cost. Several gases may be used giving a wide spectrum of laser lines.

INTRODUCTION

I N this work construction details are given for a pulsed gas laser which produces peak light powers as high as

10 W. The system allows a number of gases to be used, giving a wide range of laser lines. In particular, the use of a cold cathode system makes it possible to operate the laser with oxygen gas which is normally undesirable in conven­tional thermionic cathode plasma tubes. We have found oxygen to be important owing to the simplicity of handling, low cost, and high laser light intensities obtainable at 5592 A.

Some further innovations in the laser are (a) the demountable electrode assembly which allows the laser tube to be constructed from straight pieces of ordinary

FIG. 1. The assembled laser head.

Pyrex glass tubing, and (b) provisions for a water cooled plasma tube built into the electrodes which allows for high pulse repetition rates without damage from heating.

THE LASER HEAD

The laser consists of a dustproof Plexiglas box containing the laser tube and mirror assemblies (Fig. 1). Connections to the gas leak, vacuum pump, cooling water, and high voltage are all made at couplings on the laser housing and are quickly detachable. The most novel feature of the laser head is the design of the two aluminum electrodes. These perform the multiple functions of (a) supporting the complete plasma tube assembly in the laser housing, (b) providing the inlets and outlets for the cooling water and vacuum system, and (c) holding the plasma tube, cooling jacket, and Brewster windows in place. A detailed diagram of one of these electrodes is shown in Fig. 2. The neoprene O-ring seals used for the vacuum and water

FIG. 2. Exploded diagram of electrode and central support system I-Brewster window; 2, 3, 4, 6, 7, and 9-machined aluminum elec­trode components; 5, 8, and 15-insulating rings; 10--5 mm o.d. Pyrex plasma tube; 11-18 mm o.d. Pyrex cooling tube; 12-Teflon spacer; 13, 14, and 16-machined aluminum components for central support; 17-vacuum connection; 18--cooling system connection.

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