DE2422194A1 - PROCEDURE FOR ELIMINATING SETTING ERRORS IN MAGNETOMETERS AND PERFORMING THE PROCEDURE SUITABLE MAGNETOMETER - Google Patents

Description

⁰ A ^- IRNTANW ¹ UT

"AIhN TANVYAL ¹ h 9/90 IQ /

DR. CLAUS REINiLANDER DIPL.-ING. KLAUS 3ERNHARiTT ^

D-8 Munich 60 · Orthstrasse 12 ■ Telephone 832024/5 Telex 5212744 · Telegram Interpatent

. May 3, 1974

V1 P377 D VARIAN Associates, Palo Alto, CaI., USA,

Method for eliminating setting errors in magnetometers, and for carrying out the method suitable magnetometer

Priority: May 17, 1973 ■ - U.S.A. - Ser. No. 361 081

summary

In an optically pumped, cross-coupled, self-oscillating Double cell magnetometers are orientation-dependent phase shifts, i. H. Adjustment errors, thereby reduced, that the self-oscillation frequency is phase modulated to a corresponding amplitude modulation in the Zeeman resonance in each of the two cells. This amplitude modulation of the Zeeman resonance is dependent on the corresponding inclination of the relevant, overlapping Zeeman resonance lines at the self-oscillating frequency. The two Zeeman resonance amplitude modulation signals are compared to derive a differential deviation signal which is used to determine the phase shift to adjust the self-oscillation frequency so that it is a

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Overlap frequency of the two Zeeman resonance lines corresponds to where both Zeeman lines have the same inclination so that setting errors are automatically minimized.

Background of the invention

The invention relates generally to optically pumped magnetometers and, more particularly, to an improved self-oscillating one Magnetometer with automatic phase correction.

Optically pumped, cross-coupled, self-oscillating double cell magnetometers are known. In such a system, the magnetometer itself vibrates at a frequency equal to corresponds to an overlap of the two Zeeman resonance lines of the two optically pumped cells. In case that the electrical Zeeman resonance signals around the closed Oscillating loop phase shifts have zero, the self-oscillation frequency corresponds to an overlap point of the two Zeeraan resonance lines, at which each line has the same tendency. If the phase shift around the closed electrical oscillation loop is not zero however, the self-oscillation frequency is closer to the center of one of the Zeeman resonance lines than that of the other, and this results in an error in the self-vibratory. frequency and thus an error for the measurement of the magnetic field.

Phase shifts can occur in the electrical oscillating loop due to minor optical misalignments in one of cells or misalignment of the RF coil. The misalignment results from the impossibility

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ability to have the optical and HF axes perfectly parallel make, as well as inhomogeneities in the RF field direction and imperfections in the optical components, which reduce the symmetry about the optical axis. A cross-coupled, self-oscillating, optically pumped one Dual cell magnetometer is described in U.S. Patent No. 3,256,500 ".

Despite the alignment problems, self-oscillating, optically pumped magnetometers are often compared to optically pumped ones Magnetometers with wobble are preferred because the former inherently have less noise and they are faster Respond to rapid field changes and aie quickly recover from an interruption in business.

Summary of the invention

The invention aims to make available an improved optically pumped, self-oscillating magnetometer, and in particular a device with which automatically all undesired electronically and optically induced phase shifts subtracted. to obtain a minimal adjustment error.

According to a feature of the invention, the self-oscillating frequency * frequency or phase modulated to derive outputs, the respective slopes of the Zeeman resonance lines correspond in each of the cells at the self-oscillating frequency. These outputs are compared with each other, so that a deviation signal is generated with which the phase of the self-

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oscillation frequency is controlled in such a way that at the self-oscillation frequency both overlapping Zeeinan resonance lines have the same inclination, so that the heading error is minimized.

According to another feature of the invention, the amplitude modulations of the two detected Zeeman resonance signals, which are caused by the phase modulation of the self-resonance frequency, compared with each other, to form a difference signal with the modulation frequency. This difference frequency becomes in phase with a sample the modulation frequency compared to a DC offset signal to form, which is used to control the phase position of the self-oscillating frequency, so that the Self-oscillation frequency is automatically controlled to a value of the same inclinations of the Zeeman resonance lines is equivalent to.

Further features and advantages of the invention emerge from the following description in conjunction with the drawing; show it:

Fig. 1 is a schematic block diagram of a self-oscillating, optical pumped magnetometers incorporating features of the invention;

2 graphically shows the Larmor signal amplitude (Zeeman resonance line amplitude) over the frequency of the two Zeeman resonance lines for the respective cells of the double cell magnetometer of Fig. 1; and

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Fig. 3 graphically the self-oscillation frequency, plotted over the phase shift caused by the electronic and optical misalignments in the magnetometer system of Fig. 1 is introduced.

In Fig. 1 is a cross-coupled, self-oscillating, optically pumped double cell magnetometer with features of the invention shown. The structure is basically the same as described in U.S. Patent No. 3,256,500.

In short, the magnetometer has an electrodeless discharge lamp T which is excited with RF from an excitation oscillator 2. The discharge lamp bulb 1 is filled with natural rubidium vapor which is mixed with an ignition medium such as krypton gas. The lamp is described in more detail in US Pat. No. 3,110,960. The optical pump light emerging from the lamp is sent in two directions through two absorption cells 3 and 4, respectively. Each of the two oppositely directed light beams successively passes through a collimator lens 5 and an interference filter 6, which _{suppresses the D 2} line of 7800 AU, but allows the D ₁ line to pass through at 7948 AU, so that the optical pumping in the absorption cells 3 and 4 is improved will. A circular polarizer 7 is provided in front of each of the absorption cells in order to effect a different sublevel absorption in the cells 3 and 4 in order to optically pump them.

The absorption cells 3 and 4 contain rubidium vapor, which is preferably isotopic with respect to either rubidium or rubidium 87, and mixed with a buffer gas, e.g. neon, to prevent disorienting wall

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reduce collisions so that long relaxation times and correspondingly narrow line widths are caused, as required for high sensitivity. In the case of Rubidium 85, the light flux is polarized and filtered with a precession frequency (Zeeman resonance frequency) of about 4.66 cycles per gamma of the measured magnetic field modulated, and in the case of Rubidium 87, the precession frequency is about 7.00 cycles per gamma (1 gamma = 10 ~ * ^ gauss). For example, in the average earth field of 0.5 Gauss the precession frequency of Rubidium 85 is about 233 kHz.

This intensity-modulated light beam is after passing through through the corresponding absorption cells 3 or 4 with a focus lens 8 focused on a corresponding photocell 9. The intensity modulation of the light impinging on the photodetector cell 9 is integrated with the photocells 9 alternating electrical signal of the same frequency converted. These alternating electrical signals are each in Amplifiers 11 and 12 are amplified and fed back in a cross-coupled relationship to the absorption cells 4 and 3, respectively, and Although via oppositely wound, coaxial RF coils 13 in the form of a magnetic Viechseifeld · «, this high-frequency Magnetic field maintains a forced precession of the rubidium atoms so that a self-sustaining vibration is maintained a frequency which is proportional to the strength of the measured unidirectional magnetic field. Between Amplifiers 11 and 12 and the corresponding RF coil 13 are .Blimer 14 provided to the amplitude of the cross-fed back electrical signal limit.

A sample of the self-oscillation frequency is taken from the output of the amplifier 11 and fed to a reading 20, for example a counter or the like. To obtain an output proportional to the intensity of the magnetic field.

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In FIG. 2, the envelopes 17 and 18 of the Zeeman or Larmor resonance line of the quantum systems (rubidium atoms) in cells 3 and 4 are shown. The magnetometer oscillates at a self-oscillation frequency corresponding to an overlap of the envelopes 17 and 18 of the two cells 3 and 4. For the special case in which the electronic and / or optical phase shift around the closed oscillation loop is zero, the self-oscillation frequency is at Oq, where both Lines 17 and 18 have the same absolute slope. Due to the great difficulty in obtaining a precise high-frequency and optical alignment of the two absorption cells 3 and 4, the phase shift around the loop is generally not zero, and therefore the self-oscillation frequency is at a point such as \> 'where the inclination of the two Envelopes 17 and 18 are not the same. Since the undesired phase shifts in the oscillation loop depends on the orientation of the cells 3 and 4 in the magnetic field to be measured, there is thus an undesirable adjustment error and it is desirable to eliminate this.

In Fig. 3 is graphically the relationship between ν '-wu (Setting error) and the phase shift around the oscillation loop. The setting error is as a curve 19 shown as a function of the phase shift.

In Fig. 2 it can be seen that one of the self-oscillation frequency . v "superimposed frequency (phase) modulation amplitude modulation signals in each of the defected Zeeman resonance line signals evokes. The amplitudes of the intensity modulation signals are proportional to the slope of the respective Zeeman resonance line envelopes at the self-oscillation frequency. According to the invention, therefore, the

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Sitätmodulationssignale compared to each other to form a difference signal, and the difference signal is used to adjust the phase shift around the closed .Schwingungsschleife so that all of the phase shift that occurs around the oscillation loop, ie that caused by the optical and electronic misalignments is canceled by an electronic phase shift introduced into the loop so that the self-oscillating frequency is corrected to a frequency corresponding to \) _Q so that undesirable adjustment errors from electronic and optical misalignment are substantially minimized.

The automatic phase correction part of the circuit is shown in FIG. 1 below the broken line 21. A voltage variable phase shifter 22 is in series with the closed Zeeman resonance oscillation loop by the phase shifter is connected between amplifier 12 and limiter 14. A low-frequency reference oscillator 23, whose frequency is less than each of the bandwidths for half power of the Zeeman resonance envelope, provides a modulation signal of, for example, 30 Hz into the phase shifter 22 to the phase (frequency) of the self-oscillating signal to modulate in the closed electrical loop.

This phase modulation causes a corresponding frequency modulation of the self-oscillation frequency, which in turn a corresponding amplitude modulation of each of the Zeeman resonance signals which is detected in the detectors 9, as shown by the signals 24 and 25 in FIG is. These amplitude-modulated signals 24 and 25 are from

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Output of the amplifier 11 and 12 removed and the input of the respective amplitude modulation detectors 26 and 27 are supplied to the respective modulation components 24 and 25 from the self-oscillation signal separate. The detected amplitude modulation signals 24 and 25 are fed to a summing amplifier 28, where they are summed up with 180 ° phase shift to produce a differential response at the modulation frequency of the Obtain reference oscillator 23.

The difference signal output of the summing amplifier 28 is fed to an input of a phase detector 29 in order to phase against the modulation signal from the reference oscillator

23 to be rectified, so that a direct current deviation signal is formed, the amplitude of which corresponds to the magnitude of the frequency difference between the self-oscillation frequency v "and the desired self-oscillation frequency ^ ₀ , and a sign (phase) either + or -, depending on the whether V ^{1 is} above or below the desired frequency zero.

The deviation signal is fed to an integrator 31 which integrates the deviation signal to form a rising voltage which is fed to the phase shifter 22 so that the phase of the self-oscillation signal is shifted in a direction determined by the polarity of the deviation voltage until the desired phase shift in the rest of the loop is subtracted. At this point in time, O ¹ equals ■ »Q , and the two intensity modulation signals

24 and 25 have equal amplitudes and their sum becomes zero, so that there is no further shift of the electronic phase in the closed Zeeman resonance loop.

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- ίο -

\ Ιθώώ. an initial phase shift or setting of the phase shifter 22 of 0 ° is initially assumed, the magnetometer only requires a few milliseconds to start oscillations at a frequency which is only defective by the remaining setting error plus the result of small electronic phase shifts. The part of the frequency error caused by electronic phase shifts and misalignment effects is corrected at a later point in time which is determined by the time constant of the correction loop. Since the correction loop need only react at a speed comparable to the speed of the magnetometer, the bandwidth of the loop can be limited to a fraction of a Hz in order to minimize the introduction of noise. In contrast to magnetometers with wobble, narrow-band correction does not reduce the response time of the magnetometer to rapid field changes.

An undesirable feature of the phase correction loop described is the frequency modulation that translates into the self-oscillating frequency which is read in reading 20. If a direct analog record as a reading 20 is used, a low pass filter can be used to block the modulation frequency so that the Reading does not respond to the modulation frequency. If a frequency counter is used, the low frequency Reference oscillator 23 can be replaced by signals from the counter; the signal from the counter would be a suitable one in such a case Be the harmonics of the reciprocal of the gate period used within the counter. In this way occurs an integer number of phase modulation cycles

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during the count on, so that the unwanted frequency modulation the self-oscillation frequency is averaged to zero in the selector 20. If a period measurement the Larmor frequency is made, the same averaging can be achieved by dividing a Larmor signal is used to replace the reference oscillator 23. For a typical period measurement the count is actually carried out for a large number of signal periods. The total count depends on the time interval between the first and last zero crossing of the signal. If an integer Number of frequency modulation cycles during this counting interval occurs, the modulation affects the relative time interval between the first and last Zero crossing not.

The Zeeman resonance vapor (quantum systems) does not need to be limited to rubidium vapor but can include other resonators such as Cs, K and other quantum system resonators which show magnetic field dependent quantum transitions.

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Claims (1)

- / JL- Claims . Method for eliminating setting errors in a self-oscillating, cross-coupled, optically pumped and optically monitored two-cell magnetometer, characterized in that a compensating phase shift is determined and inserted into the cross-coupling loop so that the overlap frequency of the oscillation occurs at the frequency that is essentially one equal inclination of the envelope of each of the two overlapping resonance lines corresponds. optically pumped magnetometer for swirl guidance of the Method according to claim 1 with two optical absorbers -Vdumina, each containing a collection of quantum systems contained in a unidirectional magnet- ' field can precess at a rate determined by the intensity of that field, facilities, with which optical radiation is directed onto the absorption volumes with such spectral characteristics, that it is absorbed differently with regard to the magnetic sublevels of the quantum systems, so that the intensity of the optical radiation received from each volume is intensity-modulated with a frequency is, which depends on the precession rate, devices with which the intensity modulation of the radiation received from the absorption volumes is converted into electrical alternating signals, devices for cross coupling of electrical alternating signals, which are derived from each absorption volume, to the other absorption volume in the form of a magnetic Alternating field, which is a forced precession of the quantum ... / A2 409849/02 9 5 systems causes, so that a self-sustaining oscillation of the electrical signals is effected at a frequency which is determined by the intensity of the unidirectional magnetic field and at a frequency which corresponds to an overlap of the envelopes of the resonance lines of the two quantum systems, wöbe: each of the Resonance lines corresponds to slightly different center frequencies of the precession rates in each of the volumes, characterized in that a phase control device is provided with which it is ensured that the overlap frequency oscillation occurs at a frequency which corresponds to an essentially equal inclination of the envelope of the overlapping resonance lines by introducing a compensating phase shift into the cross coupler to cancel phase shifts introduced by physical misalignments or unequal frequency-phase characteristics of the cross couplers. Magnetometer according to Claim 2, characterized in that the phase control has a device with which the phase of the self-oscillation frequency is modulated with a modulation frequency in order to effect a corresponding amplitude modulation of the two electrical alternating signals derived from the absorption volumes, comparison devices for comparing the amplitude modulation signals to derive a deviation signal proportional to the slope difference of the two resonance lines at the self-oscillation frequency, and a phase shifter for the phase of the self-oscillation signal responsive to the deviation signal, ... / A3 409 8 4 9/0295 to make the self-oscillating frequency appear at one frequency, the same inclinations of the two resonance line envelopes. 4. Magnetometer according to claim 3, characterized in that the comparison device has a device for comparing the amplitudes of the two modulation signals in order to derive a difference output at the modulation frequency, the amplitude of which is determined by the amplitude difference between the two modulation signals and the phase of which depends on which of the two resonant envelopes has the greatest slope at the self-oscillating frequency, and a comparator in which this difference output is compared with the modulation frequency which is used to modulate the phase of the self-oscillating frequency to derive the deviation signal. 5. Magnetometer according to claim 3 or 4, characterized in that the modulation frequency is smaller than the bandwidth at half power of the two overlapping resonance line envelopes. 6. Magnetometer according to claim 4 or 5, characterized in that an integrator is provided for the deviation signal, and a device with which the integrated deviation signal is fed to the phase modulator in order to shift the frequency of the self-oscillation frequency. 409849/029