DE102010020863A1 - Arrangement for noise reduction in optical magnetometer, has photodetector staying in connection with frequency generator, where output signal is supplied to photodetector, and alkali vapor cell subjected to magnetic alternating field - Google Patents

Abstract

The arrangement has an alkali vapor cell (3) attached to a branch parallel to an optical magnetometer (M1). The alkali vapor cell is exposed to magnetic field deviating from magnetic field detected by another alkali vapor cell (2a). Reinforcement signals of photodetectors (3a, 4a) are supplied directly to an electronic differentiator (5). An output signal is supplied to a third photodetector (6a), which stays in connection with a frequency generator (7a). The latter alkali vapor cell is subjected to defined adjustable magnetic alternating field (B1 power 1).

Description

In optical magnetometers, the vapor of alkali atoms is optically pumped with polarized light so that the spins of the valence electrons are aligned. An external magnetic field B ₀ causes a precession of the spins about the magnetic field direction with the Larmor frequency f _L or a splitting of the energy levels of the electron in Zeeman levels. By an additional alternating field B ₁ , the frequency of which is adapted to the Larmor frequency or the energy difference of the Zeeman levels, the magnetic field B ₀ to be measured can be determined, since Larmor frequency f _L and magnetic field B ₀ via the gyromagnetic factor γ , a material constant, are linked by f _L = γ · B ₀ . To measure the precession frequency, the photocurrent of a photoreceiver is evaluated on which the pumping light hits after passing through the alkali vapor, now partially modulated with the precession frequency.

the Measuring signal is superimposed on a number of noise contributions, which the possible magnetic field resolution of the optical Restrict magnetometer.

The Ultimate noise limitation of magnetic field measurement is due to the Shot noise of the photoreceptor given that of the number the incident photons, so the photocurrent is proportional. This shot noise limited resolution is often called the termed intrinsic resolution of the optical magnetometer and represents the theoretical or under ideal measuring conditions achievable value. In practical use, the magnetic field resolution but worsened by additional noise sources.

Two sources contribute significantly to the noise in practical use. These are the noise of the pumping light source (a gas discharge lamp or nowadays usually a laser tuned to the pump wavelength) and the noise of the magnetic field. The two sources introduce their noise into the measurement signal in various ways. The magnetic field and thus also its noise is mixed with the Lamor frequency and finds itself around this frequency again. The pump light source additionally introduces its noise into this frequency range. This leads to a noticeable deterioration of the signal-to-noise ratio compared to the ideal, purely shot-noise-limited value in the region of the Lamor frequency. After a demodulation of the signals, both noise contributions are found in the frequency range of the magnetic fields to be measured. The above problems are extensively studied and explained in the works S. Groeger, AS Pazgalev, A. Weis, Appl. Phys. B 80, 645-654, 2005 and Groeger, G. Bison, J.-L. Schenker, R. Wynands, A. Weis, Eur. Phys. J. D 38, 239-247, 2006 ,

In In all known realizations of optical magnetometers is the in Practical application achieved noise-limited magnetic field resolution significantly worse than the intrinsic, schrotrauschbegrenzte Value. This is due to noise of the light source and the magnetic field caused.

The influence of the pump light noise can be eliminated in principle, but requires a great effort in stabilizing the light source, which one would like to avoid. The magnetic field always contains fluctuations which are superimposed on the measurement signal via the frequency mixing at the Larmor frequency. This applies both to the natural earth field and artificially generated magnetic fields by means of coil systems. Corresponding EB Alexandrov, MV Balabas, AK Vershovski, AS Pazgalev, Technical Physics 49, 779-783, 2004 Only under magnetically extremely quiet measuring conditions could the actual resolution of optical magnetometers be brought to one, but still ten times worse than the intrinsic resolution.

Of the Invention is based on the object, the influence of noise the pump light source and, with appropriate circuitry, the magnetic field on the noise-limited magnetic field resolution of optical Magnetometers and the intrinsic, schrotrauschbegrenzten Approximate value.

The The object of the invention is characterized by the characterizing features of Claim 1 solved. Advantageous and further embodiments are the subject of the subordinate claims.

The essence of the invention is that when using a known optical magnetometer, with the usual per se assemblies there, the arrangement according to the invention changed to the effect that of the same light source, in addition to the usual alkali vapor cell of a magnetometer, a second alkali vapor cell with Light is applied, which is associated with a branch parallel to the magnetometer, said second alkali vapor cell is exposed to one of the first Alkalidampfzelle the magnetometer magnetic field deviating magnetic field, and which is also a photodetector and the amplified signals of the first photodetector and the second photodetector directly are then fed to an electronic difference former and only then its output signal is supplied to the magnetometer associated with this phase detector, which is in communication with a frequency generator, wel cher the alkali metal vapor cell of the magnetometer with a defined adjustable magnetic alternating field applied. In a special embodiment of the invention, it is further proposed that the second branch of the arrangement serving as the reference path, with the otherwise identical arrangement described above, also be designed as a magnetometer, which will be described in more detail in the following exemplary embodiments.

The Invention will be described below with reference to embodiments be explained in more detail. Show it:

1 the principle known construction of a Gradiometers, consisting of two optical magnetometers according to the known prior art;

2 a first basic arrangement of the invention for providing a noise-compensated optical magnetometer;

3 an arrangement consisting of two magnetometers incorporating the special solution according to 2 , here in further connection to a gradiometer;

4 a second basic arrangement of the invention to provide a noise-compensated optical magnetometer, here in a further interconnection to a gradiometer and

5 an exemplary representation of the position of the Larmor frequencies, the signals of the two in 4 correspond to alkaline cells and in 5 are included below in the same photo signal.

1 shows the usual structure for signal processing in a gradiometer, formed from two optical magnetometers, designated M ₁ , M ₂ (channel 1 and channel 2), according to the prior art. Light of a laser 1 passes through unspecified beam splitter and deflecting mirror in each case an alkali steam cell 2a and 2 B , These are each photodetectors 4a . 4b and phase detectors 6a . 6b subordinated, whose amplified signals only after an electronic subtractor 5 in the form of conventional operational amplifiers with subtraction function to obtain a Gradiometersignal G. With the phase detectors 6a . 6b stand each frequency generators 7a . 7b in conjunction with the respective alkali vapor cells 2a . 2 B with an alternating magnetic field B ₁ ⁽¹⁾ , B ₁ ^{(2) act} on the Lamorfrequenz. In the optically pumped magnetometer, however, noise mainly occurs, which is caused by wavelength fluctuations of the radiation source, which are converted into amplitude noise upon absorption in the alkali vapor cell. However, this noise can not be eliminated with an arrangement according to the prior art described above.

The According to the invention proposed below arrangements build on this famous gradiometer principle. Such a gradiometer is used two optical magnetometers in different places in the are arranged measuring magnetic field and subtracted their output signals become. In this way, same values of the magnetic field, the present at the locations of the two magnetometers, eliminated and only Differences detected. This is true only for the directly measured values of the magnetic field. The noise contributions from the pump light, by wavelength fluctuations of the radiation source caused, remain in the signal received. This is only in the absorption in the alkali vapor cell in amplitude noise which then no longer correlates with different cells is, so no longer by the subtraction of the magnetometer signals can be eliminated and the resolution deteriorates.

Of the inventive solution is the idea underlying, before the demodulation of the mixed with the Larmorfrequenz Signals to subtract the two photosignals from each other. On This way, much of the on the photodiode signals lying fluctuations of the photocurrent, the noise of the pump light come, eliminated. Then use both evaluation electronics this common difference signal.

In 2 is a first basic arrangement of the invention for providing a noise-compensated optical magnetometer, for ease of understanding with reference to only one magnetometer M ₁ , shown. Analogous to the prior art, the light passes, for example, a laser 1 into a first alkaline vapor cell 2a and in a subordinate photodetector 4a , At the same time according to the invention, a second Alkalidampfzelle 3 assigned in a branch parallel to the magnetometer, said second alkali vapor cell 3 one of the first alkali vapor cell 2a is exposed to beaufschlagendem of this deviating magnetic field. This additional alkali vapor cell 3 is also a photodetector 3a downstream and the amplified signals of the first photodetector 4a and the second photodetector 3a Immediately afterwards an electronic subtractor 5 supplied and only then its output on the magnetometer associated phase detector 6a fed to the frequency generator 7a which is the Alkalidampfzelle 2a with the defined adjustable alternating magnetic field B ₁ ⁽¹⁾ acted upon. On the two alkali vapor cells 2a and 3 otherwise, the measuring field B ₀ , which acts on the cell, also acts 3 , according to the invention, exclusively acts.

In a further embodiment of the invention is in 3 an arrangement consisting of two magnetometers M ₁ , M ₂ , including the special solution according to 2 and interconnection to a gradiometer G shown schematically. The in 3 Dashed line covered area I corresponds identically to the embodiment according to 2 and it is easy to see that the area II covered with another line shape is a mirror image of it. The additionally provided Alkalidampfzelle 3 , which is exposed exclusively to the measurement field B ₀ and no further magnetic field, serves via the photodetector 3a for reference signal formation, which is available in the example for two magnetometers M ₁ and M ₂ . It goes without saying within the scope of the invention to provide further magnetometer branches M _n , all of which are each provided with the reference branch (FIG. 3 ; 3a ), electronic subtractors 5a to 5n are paired. In order to stay in the example, reference is made here only to two magnetometers, so that the electronic subtractor 5a of the first magnetometer, the phase detector 6a is downstream and this with the frequency generator 7a which is the Alkalidampfzelle 2a acted upon by a coil with the alternating field B ₁ ⁽¹⁾ at the Lamor frequency. Identically, the interconnection of the second magnetometer M _{2 takes place} via the modules 6b . 7b and the loading of the alkali vapor cell 2 B with the alternating field B ₁ ⁽²⁾ at the Lamor frequency. How out 3 It can also be seen that the two magnetometer signals, as is usual with the use of other circuits, can be connected to a gradiometer G via another electronic difference former, here 8.

Another basic possibility for creating a noise-compensated optical magnetometer is in 4 shown schematically. Here, the requirement that two alkali vapor cells placed in relation to each other should be exposed to a magnetic field different from each other, is realized in that all branches are designed as magnetometers with all said assemblies, in which example the alkali vapor cell 2 B of the second path is acted upon with an additional, defined adjustable, static external magnetic field (.DELTA.B ₀ ) and the outputs of the two Alkalidampfzellen 2a . 2 B downstream photodetectors 4a . 4b immediately afterwards, analogously 2 , an electronic subtractor 5 are fed and only then its output to the magnetometers M ₁ , M ₂ (channel 1, channel 2) associated phase detectors 6a . 6b , which are fed via the respective frequency generators 7a . 7b the alkali vapor cells 2a . 2 B with the defined adjustable alternating magnetic field B ₁ ⁽¹⁾ , B ₁ ^{(2) act} on the Lamorfrequenz.

For the above example shows 5 the noise of the signals after both photoreceivers 4a . 4b as well as after the subtraction of the two signals by the electronic subtractor 5 (in the illustration from top to bottom). The two peaks in the photosignals are at the Larmor frequencies of the two channels, which result according to the relation f _L = γ · B ₀ from slightly different magnetic fields B ₀ at the locations of the two magnetometers. The underlying noise level is significantly increased in the individual signals and reflects the noise of the pump light. In the difference signal this is eliminated, so that the shot noise can be achieved as far as possible. This is reflected in the drastically reduced white noise of the magnetic field measurement.

In the usual difference formation with gradiometers 8th (see. 4 ), after demodulation by the phase detectors 6a and 6b In addition, the superimposed in the magnetometer signals direct low-frequency noise components are also largely eliminated. The latter applies analogously to the execution after 3 ,

In the new arrangement according to the invention according to 4 Both peaks (corresponding to the two Larmor frequencies of the two alkaline cells) are now contained in the same photo signal ( 5 below). For the electronics of the two magnetometers to work stably on their respective peak, they must be sufficiently far apart. In 5 For example, these are approximately 200 Hz. According to the relation f _L = γ * B ₀ , this corresponds to the requirement to provide means for adjusting a difference ΔB ₀ between the magnetic fields at the locations of the two alkaline cells.

To ensure the separation of two Larmor frequencies from a common photo signal, the Larmor frequencies must be far enough apart that two conditions are met: Firstly, the peaks of the common photo signal must be able to be resolved separately from the individual electronics. For example, with peak widths of 10 Hz, the minimum distance should be 20 Hz. On the other hand, it must be ensured that gradients occurring in the measuring field do not lead to a combination of the Larmor frequencies. If, for example, one expects gradients of a maximum of 30 nT, then according to the relation f _L = γ · B ₀ z. B. in cesium as alkali vapor with its gyromagnetic factor of 3.5 kHz / μT Larmorfrequenzen 100 Hz be moved. In addition, you must at least maintain this distance.

Below are some options to generate this magnetic field difference exemplified:

• - Near one of the two magnetometers (in 4 this is the cell 2 B ), a small amount of material is applied which either has its own magnetism or either displaces or focuses the external magnetic field. In the first case (for example use of a permanent magnet), a constant contribution to the external field is added, in the second case a part of the external magnetic field is directed in addition to or away from the alkali vapor cell. In each of these implementation variants, the resulting magnetic field at the cell will depend on how the external magnetic field B ₀ is oriented to the cell, since the individual magnetic fields add up vectorally and therefore the resultant changes with different position of the individual fields relative to one another. This solution is therefore preferably suitable for arrangements which have a fixed orientation relative to the earth's field or an artificially generated measuring field, that is to say for stationary measuring systems.
• - The additional field for one of the two cells ( 2a or 2 B in 4 ) can be generated by a current-carrying coil in the vicinity of this cell. The disadvantage here, however, is that then the noise of the current is impressed on the magnetometer cell. This disadvantage can be largely avoided if you provide both cells with coils that are traversed by the same stream, but are designed so slightly different that the required difference of the magnetic fields is realized. Likewise, a common field can be generated for both magnetometers, which has a slight gradient. It is also within the scope of the invention to apply one of the two coils additionally with a defined DC component. These arrangements also require a fixed orientation with respect to the external magnetic field, so they are suitable for stationary measuring systems.
• - A direction independent generation slightly different Fields can still be reached if one of the two alkali steam cells of a hollow body (not shown here, for example, a Ball), which is provided with a material, which slightly attenuates the magnetic field. Therefore a Mn-Zn ferrite powder has proved suitable. The damping factor can be set via the material selection and thickness.

The Advantages and the simple way of realization of solutions, according to the present task, become special clearly, if you look back to a comparison with known detailed solutions according to the prior art makes.

The subtraction of the photocurrents after the alkali vapor cells, as proposed in the present invention is not apparent from the prior art. Only solutions for reducing the noise of the radiation source alone are known, realized by two photoreceivers balanced to one another: "A Survey of Methods Using Balanced Photodetection", New Focus Appl. Grade 14, 2002 , In this solution alone the amplitude noise of the light is reduced. In the optically pumped magnetometer, however, noise mainly occurs, which is caused by wavelength fluctuations of the radiation source, which are converted into amplitude noise upon absorption in the alkali vapor cell. It is only by the inventive subtraction of the signals after the Alkalidampfzellen the desired reduction of the noise caused thereby without any further effort.

The methods for the targeted adjustment of the required different magnetic fields are so and for the intended use of the prior art can not be removed. The use of magnetic materials for guiding magnetic flux lines is in principle known (eg. SA Gudoshnikov, BY Liubimov, LV Matveets, AP Mikhailenko, YV Deryuzhkina, YS Sitnov, OV Snigirev, Physica C 368 (1-4), 66-69, 2002 ), but the targeted generation of locally different magnetic fields by these means is new. Likewise, there are arrangements of magnetic field sensors which use a common feedback of an external magnetic field (eg. RH Koch, JR Rozen, JZ Sun, WJ Gallagher, Appl. Phys. Lett. 63, 403-405, 1993 ), but always there is placed on the generation of equal fields at the locations of all magnetometers.

The arrangement according to the invention preferably uses at least two optical magnetometers which are pumped with the same source, namely a gas discharge lamp filled with the same material as the alkali vapor cells, or a laser whose wavelength is correspondingly adjusted. The signals from two downstream photoreceivers as described are subtracted and in turn evaluated separately in the subsequent electronics. These magnetometer signals can already be output separately. This gives signals with significantly less white noise than is the case with magnetometers without the inventive subtraction of the photodiode signals at the proposed location within the arrangement. If the output signals of the two magnetometers additionally subtracted, so formed a gradiometer, and the low-frequency noise of the magnetic field is considerably ver Ringert.

According to the prior art, various methods of phase synchronization are known, namely the M _x and M _z methods, in which an alternating field B _{1 is} used whose frequency is adapted to the Larmor frequency, the non-linear magneto-optical rotation (NMOR ) and the Bell Bloom method, in which the amplitude of the pump light is modulated with the Larmor frequency. To measure the precession frequency, the photocurrent of a photoreceiver is evaluated, to which the pumping light after passing through the alkali vapor, now modulated to a certain extent with the Larmor frequency impinges. The arrangement according to the invention for noise reduction can be used independently of the above methods for this modulation and demodulation. For simplicity and clarity, in the foregoing, without limiting the invention thereof, only the M _x method has been exemplified. Here is a common method for demodulating the magnetic field information from the frequency range around the Larmor frequency to the original frequencies, using a lock-in amplifier as the phase detector, and tuning the reference frequency generator to the Larmor frequency. Alternative other technical implementations of the basic idea according to the invention are obvious to the person skilled in the art.

1

light source

2a, 2 B

Alkali vapor cell

3

Alkali vapor cell in the reference path

3a

photodetector in the reference path

4a, 4b

photodetectors

5, 5a, 5b

electronic differentiator

6a, 6b

phase detectors

7a, 7b

frequency generators

8th

Another electronic subtractor

B ₁ ⁽¹⁾ , B ₁ ⁽²⁾

magnetic alternating fields

ΔB ₀

static outside magnetic field

M ₁ , M ₂

magnetometer

G

gradiometer

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited non-patent literature

• - S. Groeger, AS Pazgalev, A. Weis, Appl. Phys. B 80, 645-654, 2005 [0004]
• - S. Groeger, G. Bison, J. -L. Schenker, R. Wynands, A. Weis, Eur. Phys. J. D 38, 239-247, 2006 [0004]
• EB Alexandrov, MV Balabas, AK Vershovski, AS Pazgalev, Technical Physics 49, 779-783, 2004 [0006]
• - "A Survey of Methods Using Balanced Photodetection", New Focus Appl. Note 14, 2002 [0028]
• - SA Gudoshnikov, BY Liubimov, LV Matveets, AP Mikhailenko, YV Deryuzhkina, YS Sitnov, OV Snigirev, Physica C 368 (1-4), 66-69, 2002 [0029]
• RH Koch, JR Rozen, JZ Sun, WJ Gallagher, Appl. Phys. Lett. 63, 403-405, 1993 [0029]

Claims (8)

Arrangement for noise reduction in optical magnetometers, comprising at least one magnetometer, consisting of a light source ( 1 ), an alkaline vapor cell ( 2a ) with a downstream photodetector ( 4a ) and phase detector ( 6a ), the latter having a frequency generator ( 7a ), which contains the alkali vapor ( 2a ) with a defined adjustable alternating magnetic field (B ₁ ⁽¹⁾ ), characterized in that with light from the same light source ( 1 ), a second alkali vapor cell ( 3 ) is assigned in a branch parallel to the magnetometer, said second alkali vapor cell ( 3 ) one of the first alkaline vapor cell ( 2a ) is exposed to the magnetic field deviating magnetic field, which is also a photodetector ( 3a ) and the amplified signals of the first photodetector ( 4a ) and the second photodetector ( 3a ) immediately following an electronic subtractor ( 5 ) and only thereafter its output signal on the magnetometer associated phase detector ( 6a ) supplied with the frequency generator ( 7a ), which contains the alkali vapor cell belonging to the magnetometer branch ( 2a ) with the defined adjustable alternating magnetic field (B ₁ ⁽¹⁾ ) acted upon. Arrangement according to claim 1, characterized in that the branch in which the second Alkalidampfzelle is provided, is also designed as a magnetometer with all said assemblies, said Alkalidampfzelle ( 2 B ) of the second Magnetometerzweigs with an additional, defined adjustable, static external magnetic field (.DELTA.B ₀ ) is acted upon and the outputs of the two Alkalidampfzellen ( 2a . 2 B ) downstream photodetectors ( 4a . 4b ) immediately following an electronic subtractor ( 5 ) and only then its output to the magnetometers (channel 1, channel 2) associated phase detectors ( 6a . 6b ), via the respective frequency generators ( 7a . 7b ) the alkali vapor cells ( 2a . 2 B ) with the defined adjustable alternating magnetic field (B ₁ ⁽¹⁾ , B ₁ ⁽²⁾ ) act. Arrangement according to claim 2, characterized in that the static external magnetic field (ΔB ₀ ) is generated by placing in proximity to the second alkali vapor cell ( 2 B ) is a permanent magnet or material which focuses or displaces the magnetic field acting on the alkali vapor cell. Arrangement according to claim 2, characterized in that the static external magnetic field (ΔB ₀ ) is generated by the second alkali vapor cell ( 2 B ) is enveloped by a hollow body provided with a material such as a Mn-Zn ferrite powder which dampens the magnetic field applied to the alkali vapor cell. Arrangement according to claim 2, characterized in that the static external magnetic field (ΔB ₀ ) is generated by applying to the second alkali vapor cell ( 2 B ) acting coil is additionally traversed by a defined DC component, so that adds to the magnetic alternating field (B ₁ ⁽²⁾ ) generated by the coil, a defined adjustable static magnetic field (.DELTA.B ₀ ). Arrangement according to claim 1, characterized in that by the second Alkalidampfzelle ( 3 ) generated signal to several electronic subtractors ( 5 ), correspondingly amplified, is fed so that the respective difference formers ( 5a . 5b ) in each case a further magnetometer branch can be connected. Arrangement according to claim 2, characterized in that the through the second Alkalidampfzelle ( 2 B ) of the second branch, which is supplied with an additional, defined adjustable, static external magnetic field (.DELTA.B ₀ ) signal generated by a plurality of electronic difference formers ( 5 ) is supplied correspondingly reinforced, so that the respective difference formers ( 5a . 5b ) in each case a further magnetometer branch can be connected. Arrangement according to claim 1 or 2, characterized in that on the phase detectors ( 6a . 6b ) output signals of two optical magnetometers (M ₁ , M ₂ ), which are arranged at different locations in the magnetic field (B ₀ ) to be measured, by a further electronic difference former ( 8th ) are connected to a gradiometer (G).