Atomic magnetometer system

Abstract

An atomic magnetometer system is disclosed, comprising a variable magnetic field source (14) configured to provide an oscillating primary magnetic field to cause a sample (16) to generate a secondary magnetic field. The atomic magnetometer system also includes an atomic magnetometer for detecting the secondary magnetic field. The atomic magnetometer includes an atomic sample; a pump and detection subsystem configured to pump the atomic sample to generate polarization and detect atomic coherent precession within the atomic sample using a detection beam; and a detector configured to detect the detection beam to generate a detection signal. The system is configured to drive the variable magnetic field source (14) according to the detection signal, the detection signal having a frequency tuned to a radio frequency resonance. A method of operating an atomic magnetometer is also disclosed.

Description

Technical Field

[0001] The present invention relates to an atomic magnetometer system and method. Background Art

[0002] Radio frequency (RF) atomic magnetometers implemented as magnetic field sensors create new opportunities for non-destructive testing based on magnetic induction tomography (MIT) [1-8] (numbers in square brackets refer to the corresponding references at the end of the specification, which are incorporated into this application by reference). MIT relies on detecting the response of a sample to an oscillating RF magnetic field, the so-called primary field. The response of the material, the secondary field, is generated by eddy currents in the conductive sample and / or by magnetization in the case of magnetic permeability. In addition to having higher sensitivity, wider tuning range [9] and better spatial resolution

[10] than standard pick-up coils, atomic magnetometers can also perform semi-vector mapping of all components of the secondary field. Compared to other magnetic field sensors, such as giant magnetoresistance (GMR) magnetometers [10-12] or superconducting quantum interference devices (SQUIDs) [13, 14], atomic magnetometers have the advantage of operating in an unshielded environment [9, 15, 16], not requiring cryogenic temperatures and with few restrictions on miniaturization.

[0003] Tomographic imaging of defects in a sample using RF atomic magnetometers relies on observing changes in the amplitude and phase of the RF resonance as the sample moves under the RF primary field coil. Magnetically conductive samples present a challenge for atomic sensors because the local magnetization of the sample changes the RF resonance frequency. This problem has been partially addressed by active stabilization of the magnetic field in which the sensor operates, but the residual changes in the resonance frequency experienced can lead to reduced image acquisition rates [7]. Summary of the invention

[0004] Aspects of the present invention seek to provide an improved atomic magnetometer system and method.

[0005] According to one aspect of the present invention, there is provided an atomic magnetometer system, comprising:

[0006] a variable magnetic field source configured to provide an oscillating primary magnetic field to cause the sample to generate a secondary magnetic field;

[0007] An atomic magnetometer, used to detect the secondary magnetic field; the atomic magnetometer comprises:

[0008] Atomic samples;

[0009] a pump and probe subsystem configured to pump the atomic sample to generate polarization and to probe coherent precession of atoms within the atomic sample using a probe beam;

[0010] a detector configured to detect the probe beam to generate a detection signal;

[0011] The system is configured to drive a variable magnetic field source based on a detection signal having a frequency tuned to a radio frequency resonance.

[0012] Embodiments of the present invention can provide an atomic magnetometer system in which the secondary magnetic field is always tuned to radio frequency resonance. This can avoid the time-consuming resonant scanning required in some prior art systems. It should be noted that in embodiments of the present invention, the primary magnetic field is also tuned to radio frequency resonance. However, what the atomic magnetometer is to detect is the secondary magnetic field.

[0013] In an embodiment of the present invention, the radio frequency resonance is the frequency of the coherent precession of free atoms in the atomic sample.

[0014] In an embodiment of the invention, the atoms of the atomic sample together with the probe and detector act as a RF generator tuned to RF resonance. The secondary magnetic field can be automatically kept in resonance by driving a variable magnetic field source using a signal generated by detecting the coherent precession of atoms within the atomic sample.

[0015] Prior art systems typically use a separate RF generator to generate the RF signal to drive the variable magnetic field source.Using the atoms of the atomic sample together with the pump and detection subsystems and detectors as RF generators also means that embodiments of the present invention can avoid the need for a separate RF generator.

[0016] In some embodiments, the system is configured to drive the variable magnetic field source at a frequency corresponding to an oscillation frequency of the detection signal.

[0017] In some embodiments, a variable magnetic field source is coupled to the output of the detector so as to be driven according to the detection signal.

[0018] In some embodiments, the variable magnetic field source is configured to be placed adjacent to the sample.

[0019] In some embodiments, the system includes an instrument processor configured to receive the detection signal and determine its amplitude and phase. The instrument may be a processor.

[0020] In some embodiments, the system includes a voltage follower configured to drive the variable magnetic field source using a signal buffered from the detection signal.

[0021] A variable magnetic field source, particularly in the case of a low impedance coil, may be coupled to the output of the detector via a voltage follower to drive the variable magnetic field source with a signal buffered from the detection signal.

[0022] In some embodiments, the system includes an amplifier configured to drive the variable magnetic field source using a signal amplified from the detection signal.

[0023] The variable magnetic field source may be coupled to the output of the detector via an amplifier to drive the variable magnetic field source with a signal amplified from the detection signal.

[0024] In some embodiments, the system includes a phase shifter configured to drive the variable magnetic field source with a signal phase-shifted from the detection signal.

[0025] The variable magnetic field source may be coupled to the output of the detector via a phase shifter to drive the variable magnetic field source with a signal that is phase shifted by the detection signal.

[0026] In some embodiments, the system includes a bias magnetic field source configured to provide a bias magnetic field in a bias field direction to the atomic sample.

[0027] The bias magnetic field can define the quantization axis of the atomic sample.

[0028] In some embodiments, the variable magnetic field source is a radio frequency coil.

[0029] In some embodiments, the variable magnetic field source is driven according to a detection signal having a frequency tuned to radio frequency resonance by making the voltage signal dependent on the detection signal coupled across the radio frequency coil.

[0030] The voltage signal coupled across the radio frequency coil may be derived from the detection signal by amplifying and / or phase shifting it, but preferably has a frequency corresponding to the frequency of the detection signal.

[0031] In some embodiments, the output of the detector is coupled to a radio frequency coil, optionally via a voltage follower and / or an amplifier and / or a phase shifter.

[0032] In some embodiments, the atomic sample includes a collection of alkali metal atoms.

[0033] In some embodiments, the atomic sample includes alkali metal atomic vapor.

[0034] In some embodiments, the system includes a set of variable magnetic field sources configured to provide an oscillating primary magnetic field, wherein the system is configured to drive each variable magnetic field source based on detection signals having frequencies tuned to radio frequency resonance and phases shifted relative to each other.

[0035] Each variable magnetic field source is driven phase-shifted relative to each other allowing one of the set of variable magnetic field sources to be driven in phase with the detection signal without being driven via a phase shifter. Due to the phase shift, different members of the set of variable magnetic field sources can operate at different parts of the defect.

[0036] In some embodiments, the system includes at least one phase shifter configured to drive at least one of the set of variable magnetic field sources with a signal phase-shifted from the detection signal.

[0037] Where the signals driving the set of variable magnetic field sources are phase shifted from the detection signal, this may be achieved by a common phase shifter or by their own individual phase shifters.

[0038] The set of variable magnetic field sources can be driven in a similar manner as discussed for a single variable magnetic field source using signals amplified from the detection signal, respectively. Where the signals driving the set of variable magnetic field sources are amplified, they can be achieved through a common amplifier or through their own respective amplifiers.

[0039] In some embodiments, the atomic magnetometer is configured to reduce the effect of components of the primary and secondary magnetic fields on the atomic magnetometer in a main direction substantially orthogonal to the sample surface.

[0040] In an embodiment, the main direction is the direction from the variable magnetic field source to the atomic sample.

[0041] In an embodiment, the sample surface is the surface being investigated, typically the major surface of the sample.

[0042] Some embodiments comprise a compensation magnetic field source, which may comprise a coil arrangement for providing a compensation magnetic field at the atomic magnetometer, in particular at its atomic sample, including a component in a principal direction.

[0043] The compensation magnetic field source may be configured to provide a compensation magnetic field to reduce the effect of the components of the primary and secondary magnetic fields on the atomic magnetometer in the principal direction.

[0044] In some embodiments, the atomic magnetometer is in a self-compensating configuration.

[0045] In some embodiments, the atomic magnetometer has an insensitive axis arranged in a principal direction.

[0046] In some embodiments, the atomic magnetometer includes a bias magnetic field source configured to provide a bias magnetic field in a principal direction.

[0047] According to one aspect of the present invention, there is provided a method of operating an atomic magnetometer to detect a secondary magnetic field generated by a sample, comprising:

[0048] pumping the atomic sample to generate polarization and probing the coherent precession of atoms within the atomic sample with a probe beam;

[0049] detecting the probe beam to generate a detection signal;

[0050] The variable magnetic field source is driven according to the detection signal to provide an oscillating primary magnetic field to cause the sample to generate a secondary magnetic field having a frequency tuned to the radio frequency resonance.

[0051] In some embodiments, driving the variable magnetic field source according to the detection signal includes driving the variable magnetic field source with a signal buffered and / or amplified and / or phase-shifted from the detection signal.

[0052] In some embodiments, driving the variable magnetic field source according to the detection signal includes driving the variable magnetic field source at a frequency corresponding to the frequency of the detection signal.

[0053] In some embodiments, the method includes positioning the atomic sample in a bias magnetic field.

[0054] In some embodiments, the method includes determining an amplitude and a phase of a detection signal in order to detect the secondary magnetic field.

[0055] The method may include reducing the effects of components of the primary and secondary magnetic fields on the atomic magnetometer in a direction substantially orthogonal to the sample surface.

[0056] In an embodiment, the sample surface is the surface being investigated, typically the major surface of the sample.

[0057] The primary magnetic field may be substantially normal to the sample surface.

[0058] In some embodiments, reducing the effect of components of the primary and secondary magnetic fields on the atomic magnetometer in a direction substantially orthogonal to the sample surface includes:

[0059] A compensating magnetic field is provided at the atomic magnetometer (eg, at the atomic sample) including a component substantially normal to the sample surface.

[0060] In some embodiments, a compensation magnetic field is provided so that Bz'+bz=0, where Bz' is is a component of the secondary magnetic field, which is substantially orthogonal to the sample surface, bz is a component of the secondary magnetic field, which is substantially orthogonal to the sample surface, is the primary magnetic field, and To compensate for the magnetic field.

[0061] In some embodiments, reducing the effect of components of the primary and secondary magnetic fields on the atomic magnetometer in a direction substantially orthogonal to the sample surface includes:

[0062] The insensitive axis of the atomic magnetometer is aligned in a direction substantially normal to the sample surface.

[0063] In some embodiments, the method includes aligning the bias magnetic field in a direction substantially normal to the sample surface.

[0064] Radio frequency atomic magnetometers offer an attractive alternative to standard detection methods in non-destructive testing based on inductive measurements. Embodiments provide a magnetometer configured as a so-called spin maser that addresses two important challenges of this technology: the shift in the radio frequency resonance position caused by the magnetically permeable sample and the sensor bandwidth.

[0065] Embodiments provide a spin maser for nondestructive testing and inspection. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Embodiments of the present invention are described below by way of example only with reference to the accompanying drawings.

[0067] Figure 1 A standard alkali metal spin maser equivalent to that discussed in

[23] is shown.

[0068] Figure 2 is a diagram of a system according to an embodiment of the present invention.

[0069] Figure 3 for Figure 2 Schematic diagram of the system.

[0070] Figure 4 Shown is the statistical uncertainty (Allan variance) of the signal amplitude from the magnetometer in the standard configuration (red circles) and in spin maser mode (black diamonds).

[0071] Figure 5 The transient state of a self-oscillating magnetometer signal is shown.

[0072] Figure 6 Shown in 6mm thick carbon steel plate 64x64mm 2 Amplitude variations of the RF spectroscopy signal measured over an area of ​​a carbon steel plate containing grooves with a diameter of 24 mm and a depth of 2.4 mm, recorded in four measurement configurations: (a) the same self-compensating configuration as in [8], the self-compensating configuration in spin maser mode, (b) a single RF coil, (c) a single RF coil with an opposite phase to (b), and (c) two RF coils with opposite phases.

[0073] Figure 7 The dependence of the signal amplitude as a function of the driving phase is shown. DETAILED DESCRIPTION

[0074] Conventional non-destructive testing and inspection of ferromagnetic objects using magnetic induction tomography with atomic magnetometers is affected by the RF resonance shift caused by the object being tested. Spin maser operation as described in this application can solve this problem and can significantly reduce image acquisition time.

[0075] Embodiments provide an operating mode of a radio frequency atomic magnetometer, the so-called spin maser. The mode relies on spontaneous fluctuations of atomic spins being optically monitored and the signal from the optical detection being fed back to the atomic sample. The spin maser operating frequency automatically follows the current value of the bias field. The feedback loop may include buffering, amplification and phase shifting. Benefits include significantly reduced image acquisition time, image contrast and stealth operation in surveillance situations.

[0076] The embodiments of the present invention can be used in the energy field to detect corrosion under insulation, the transportation field to monitor reinforced concrete structures, nuclear waste container monitoring, object detection, and monitoring.

[0077] Embodiments provide a radio frequency atomic magnetometer operating in a spin maser mode for MIT measurement schemes. Positive feedback in a spin maser system not only provides a solution to the imperfect magnetic field stabilization problem, but also increases the sensor bandwidth, image acquisition rate and signal-to-noise ratio. Spin maser systems (also known as Zeeman masers) have been extensively studied theoretically and experimentally in the context of noble gas systems [17-20]. A typical spin maser system consists of two parts: a pumping bulb, in which nuclear spin polarization is generated, and a maser bulb, surrounded by a pickup coil, which detects the oscillation of spins in a static magnetic field and provides positive feedback. Spin polarization is achieved by direct optical pumping (e.g., using a discharge lamp)

[17] or by spin exchange pumping, which is to transfer polarization from directly optically pumped alkali metal atoms to noble gas atoms mediated by spin exchange collisions

[21] . The precession of nuclear spins around a static magnetic field induces currents in the coils surrounding the maser bubble, which in turn generates an oscillating magnetic field inside the bubble. This magnetic field reflects the spin evolution and provides positive feedback. In contrast to the nuclear spin maser configuration, the radio frequency atomic magnetometer monitors the spontaneous fluctuations in the polarized spin sample optically [22-24]. This is due to the fact that the oscillating magnetic field strength (greater than ~10 18 cm -3 ) compared to the precessing spins in low-density alkali metal vapor, the oscillating magnetic field strength is smaller (10 10-13 cm -3 )

[25] . The detected signal is fed to a radio frequency coil located near the atomic vapor. Figure 1 This separation of monitoring and feedback allows amplification and phase shift to be introduced into the feedback loop.

[0078] Embodiments of the present invention utilize radio frequency atomic magnetometers and can be used for material defect imaging.

[0079] A detailed example is described below, which is one implementation of the above-mentioned alkali metal spin microwave maser.

[0080] like Figure 2 As shown, an embodiment of the present invention includes a system 10, which includes a radio frequency atomic magnetometer 12 and a variable primary magnetic field source 14, and the variable primary magnetic field source 14 is configured to provide a primary magnetic field oscillating at a radio frequency frequency. The system can be called a tomographic magnetic induction imaging device, and can provide a spin maser in a magnetic induction measurement. In this embodiment, the primary magnetic field source 14 is a radio frequency coil; however, other variable magnetic field sources can be used in other embodiments. The radio frequency coil is configured to provide a primary magnetic field that is substantially orthogonal to the sample surface. In this embodiment, the surface is the surface being studied, which is the main surface of the sample.

[0081] In this embodiment, the system is configured to operate in a non-shielded environment using the same instrumentation as described in [6-8], which are incorporated into this application by reference.

[0082] In this embodiment, the RF coil 14 is a 1000 turn coil having a 0.02 mm diameter copper wire, a height of 10 mm, and inner and outer diameters of 2 mm and 4 mm.

[0083] The RF coil 14 is configured so that it can be placed adjacent to the sample 16 but completely to one side and in non-overlapping relationship therewith and is operable to generate an oscillating primary magnetic field to cause the sample to generate a secondary magnetic field indicative of the material response of the sample.

[0084] The samples should be electrically conductive (although not necessarily highly conductive) and / or should have a magnetic permeability so that they can be magnetized.

[0085] Atomic magnetometers are used to detect the secondary magnetic field.

[0086] It is worth noting that ferromagnetic targets produce two types of secondary fields:

[0087] a secondary magnetic field in the same direction as the applied primary field – the secondary magnetization, and

[0088] · Eddy current induced magnetic field in the opposite direction to the applied primary field - eddy current induced magnetic field.

[0089] In this specification, the z direction is a direction perpendicular to the sample surface, and the x and y directions are mutually perpendicular directions parallel to the sample surface.

[0090] In this embodiment, the atomic magnetometer 12 includes a detection unit 20. In this embodiment, the detection unit 20 is a magnetic field in a chamber containing cesium atomic vapor (whose atomic density n Cs =3.3x10 10 cm -3 ) at ambient temperature of 1cm 3 A paraffin-coated glass cell. Cesium vapor provides the atomic sample for the magnetometer.

[0091] The magnetometer includes a bias magnetic field source 24 ( Figure 2 ), configured to provide a bias magnetic field 26 at the detection unit 20 and thus to the atomic sample along a bias magnetic field direction. In the context of this magnetic field, the terms "bias" and "offset" are used interchangeably.

[0092] In this embodiment, the bias magnetic field is actively stabilized by three pairs of nested orthogonal square Helmholtz coils [3, 6-8, 35]. These provide the bias magnetic field source.

[0093] The magnetometer includes a pump laser ( Figure 2 The pump laser is configured to pump the atoms of the atomic sample in the detection unit 20 with a circularly polarized pump laser beam 28 propagating in the offset field direction. In this embodiment, the pump laser is a diode laser with a frequency stabilized at 6 2 S 1 / 2 F=3→6 2 P 3 / 2 F'=2 transition (D2 line, 852 nm).

[0094] The pump light beam 28 is configured to pump the atomic sample to generate a polarization or population imbalance or anisotropy along the bias magnetic field.

[0095] The atomic magnetometer consists of a detection laser ( Figure 2 ), which is configured as a detection unit 20 to detect the coherent precession of atoms in the atomic sample using a linearly polarized detection laser beam 32 orthogonal to the bias magnetic field 26 and phase-shift locked to the pump laser beam, in this embodiment, from 6 2 S 1 / 2 F=4→6 2 P 3 / 2 F'=5 transition blue shifted 800MHz.

[0096] The pump laser and the probe laser form a pump and probe subsystem.

[0097] The evolution of the total atomic spin is mapped onto the polarization state of a linearly polarized probe beam.

[0098] In this embodiment, the atomic magnetometer 12 is configured to reduce the influence of the component of the primary magnetic field and optionally also the component of the secondary magnetic field in a direction substantially orthogonal to the sample surface (referred to as the main direction or z) on the atomic magnetometer 12. In this embodiment, the main direction is the direction from the RF coil to the atomic sample along the RF coil axis. In this embodiment, this is achieved by the atomic magnetometer being in a self-compensating configuration, wherein the atomic magnetometer has an insensitive axis arranged in the main direction, wherein the axis of the RF coil and therefore the axis of the primary field is in the main direction and parallel to the bias field direction. Therefore, in this embodiment, the bias field direction is also in the main direction. For a bias magnetic field oriented along z It is preferably also the primary magnetic field Direction, Bz( The component along z) will not be present in the RF atomic magnetometer signal. Details are described in [8] and PCT / GB2019 / 051953 published as WO2020 / 016557, which is incorporated into this application by reference.

[0099] As disclosed in [8] and WO2020 / 016557, other means may be used to reduce the effect of the components of the primary and optional secondary magnetic fields on the atomic magnetometer in a direction substantially orthogonal to the sample surface. For example, some embodiments include a compensating magnetic field source, which may include a coil arrangement for providing a compensating magnetic field at the atomic magnetometer, in particular at its atomic sample, including a component in the main direction. In such an embodiment, a compensating magnetic field may be provided such that Bz'+bz=0, where Bz' is bz is the component of the secondary magnetic field along z and substantially orthogonal to the sample surface, is the primary magnetic field, and To compensate for the magnetic field.

[0100] Figure 8 The system is an example of a system comprising a compensation magnetic field source 41 for providing an oscillating compensation magnetic field, also called compensation magnetic field, at the atomic magnetometer, in particular at the detection unit 20, which comprises a component substantially orthogonal to the surface of the sample 16, reducing and preferably eliminating the influence of the magnetic field components of the primary and secondary fields on the atomic magnetometer in this direction. In particular, the compensation magnetic field compensates for the contribution of the primary field to the resultant field monitored by the vapor cell of the atomic magnetometer without changing the efficiency of the secondary field excitation.

[0101] from Figure 8 It can be seen that, in this embodiment, the compensation magnetic field source 41 is a compensation coil device including a first compensation coil 42 and a second compensation coil 44 .

[0102] The first compensation coil 42 has an axis substantially aligned with z (whose direction is normal to the surface of the sample 16 ) so as to provide a magnetic field substantially normal to the surface of the sample 16 at the atomic magnetometer, in particular at the detection unit 20 .

[0103] In this embodiment, the detection unit 20 is located between the RF coil 14 and the first compensation coil 42 , but this is not necessary in all embodiments.

[0104] The second compensation coil 44 has an axis substantially aligned with y (whose direction is parallel to the surface of the sample 16 and substantially orthogonal to the bias field direction) so as to provide a magnetic field at the atomic magnetometer, in particular at the detection unit 20, which is substantially parallel to the surface of the sample 16 and substantially orthogonal to the bias field direction.

[0105] In this embodiment, the detection unit 20 is located between the probe laser 30 and the second compensation coil 44, and the second compensation coil 44 is located between the detection unit 20 and the balance polarimeter 34, but this is not necessary in all embodiments.

[0106] The compensation coil device 41 is configured to provide a compensation magnetic field at the detection unit 20

[0107] In other words, a set of two RF coils oriented in the z and y directions ( Figure 8 ) produces an oscillating magnetic field, This compensates for the primary field seen by the atomic magnetometer. The coil is driven by the detection signal in a similar manner to that disclosed in this application to drive the RF coil 14 to generate This makes the field and Maintain a constant phase difference between them.

[0108] In other words, the primary field is compensated in the vapor unit by a set of two radio frequency coils oriented in the z and y directions.

[0109] However, for reasons explained below, the component of the secondary magnetic field can optionally also be compensated in the z-direction.

[0110] The amplitudes of the two components can be varied by changing the distance of the relevant coil from the steam unit.

[0111] The resultant magnetic field monitored by an RF atomic magnetometer includes components from the primary, secondary and compensation fields, in other words

[0112] This configuration allows the determination of the amplitude and phase (orientation) of the RF field in the yz plane. Amplitude of the RF spectroscopy signal and Phase Describes the change of the synthetic field, where bz +B' z and by+B'y are two orthogonal components of the RF signal.

[0113] As described in WO2020 / 016557 and [8], axis component, what is the condition for achieving maximum contrast when B′ z +b z,max =0.

[0114] Despite Figure 8 In the embodiment, has components in the y and z directions, but this is not necessary in every embodiment. For example, compensation can be performed only in the z direction. Figure 8 In the embodiment of FIG. 4 , this means that the second compensation coil 44 can be omitted.

[0115] It is not necessary in every embodiment to reduce the effect of the components of the primary and secondary magnetic fields on the atomic magnetometer 12 in a direction substantially normal to the sample surface.

[0116] return Figure 2 In an embodiment of the invention, the atomic magnetometer comprises a detector. In this embodiment, the detector comprises a polarimeter 34 configured to receive the probe laser beam after passing through the detection unit 20. The polarimeter 34 comprises a crystal polarizer oriented at 45° relative to the incident polarization and a commercial balanced photodetector.

[0117] The laser light transmitted through the cell 20 is analyzed by a polarimeter.

[0118] The detector is configured to output a detection signal via an output terminal in response to detection of the probe laser beam. The detection signal is typically a voltage or current signal representing the polarization and / or amplitude of the detected probe beam. In this embodiment, the output of the detector is the output of a balanced photodetector, providing a voltage signal representing the polarization of the probe beam.

[0119] In this embodiment, the system 10 includes a lock-in amplifier 36 (eg, Figure 3 ), configured to receive a detection signal from a detector. The lock-in amplifier is configured to record or determine the amplitude and phase of the detection signal (also called an atomic signal), the lock-in amplifier is referenced to the signal frequency, and provides a first output signal, for example, to a computer. The computer can use the amplitude and phase of the signal to detect the secondary magnetic field and thereby the material response of the sample, and in some cases perform material defect imaging.

[0120] In some embodiments, the computer may include a receiver to receive the first output signal from the lock-in amplifier 36 and determine therefrom a change in conductivity and / or permeability of the sample.

[0121] The RF coil 14 is coupled to an amplifier 40 (eg Figure 3 The amplifier 40 is configured to receive a detection signal from the detector and amplify it to drive the RF coil 14 according to the detection signal, the detection signal having a frequency tuned to the RF resonance, which is the frequency of the coherent precession of free atoms in the atomic sample. In this embodiment, the amplifier is configured to amplify the detection signal and apply the amplified signal as a voltage across the RF coil 14. Since the atomic sample is the signal source and acts as an RF generator together with the pump and detection subsystem and the detector, the frequency tuned to the RF resonance is automatically provided.

[0122] In some embodiments, in addition to or in lieu of the amplifier, the system may include a voltage follower configured to drive the variable magnetic field source with a signal buffered from the detection signal. This is particularly advantageous if the radio frequency coil is a low impedance coil.

[0123] It is not necessary to have an amplifier or voltage follower in all embodiments.In some embodiments, the detection signal may be applied directly to the radio frequency coil.

[0124] In some embodiments, the system may include a phase shifter configured to shift the phase of a signal driving the primary magnetic field source relative to the detection signal and thereby shift the phase of the primary and secondary magnetic fields.

[0125] Figure 2 The stages of spin maser operation in a magnetic induction tomography measurement according to this embodiment are shown. (1) Spontaneous fluctuations of atoms in an atomic sample produce small atomic spin precessions at the Larmor frequency. (2) A probe beam monitors the spin evolution, and the spin precessions are mapped onto the beam polarization. (3) A photodetector produces voltage oscillations following the polarization change. (4) The amplified and possibly phase-shifted signal from the photodetector produces a current in a primary RF field coil. (5) The primary and / or secondary fields provide positive feedback of the atomic spin precessions.

[0126] In operation, the bias magnetic field source 24 provides a bias magnetic field at the detection unit 20 .

[0127] The pump laser is operated to pump the atomic sample to produce a polarization or population imbalance or anisotropy along the bias magnetic field.

[0128] The probe laser is operated to detect the coherent precession of atoms within the atomic sample using the probe beam.

[0129] The detection beam is detected by the detector, and the detector outputs a detection signal.

[0130] The detection signal is amplified, and the amplified signal is applied to the RF coil to drive the RF coil 14 according to the detection signal to provide an oscillating primary magnetic field, thereby causing the sample to generate a secondary magnetic field having a frequency tuned to the RF resonance. As described elsewhere, in some embodiments, amplification may be omitted and / or phase shifting may be included.

[0131] Embodiments may be used to provide the following advantages:

[0132] Safe and non-destructive (i.e. non-ionizing radiation)

[0133] Detect corrosion on the inner wall of the pipeline.

[0134] Detect corrosion on the outer wall of the pipeline.

[0135] Ability to differentiate between corrosion and changes in pipe geometry from bends / T-joints / welds in the pipeline.

[0136] · Capable of scanning all insulation types.

[0137] ·low cost.

[0138] Provide improvements to current technology (resolution, switching scanning modes).

[0139] In some embodiments, the system may include a set of variable magnetic field sources configured to provide an oscillating primary magnetic field, wherein the system is configured to drive each variable magnetic field source according to a detection signal having a frequency tuned to radio frequency resonance and a phase relative to each other and phase-shifted from the detection signal, but not excluding being in phase with the detection signal. Each of the variable magnetic field sources effectively provides a component of each of the primary and secondary magnetic fields, and each of these components can be operated as described above (wherein the above reference is only for the primary or secondary magnetic field). The set of variable magnetic field sources can be a set of radio frequency coils, each configured as described above. At least one of the set of variable magnetic field sources is driven via a phase shifter as described above to ensure a relative phase difference between the set of variable magnetic field sources. In the case where a phase shifter, voltage follower and / or amplifier is used for more than one of the set of variable magnetic field sources, the phase shifter, voltage follower and / or amplifier can be shared by multiple, possibly all, of the set of variable magnetic field sources and / or multiple, possibly all of the set of variable magnetic field sources can use their own respective phase shifters, voltage followers and / or amplifiers.

[0140] Those skilled in the art will appreciate that the specific atomic magnetometer described above is not the only type of atomic magnetometer that can be used; for example, different detection units, different sizes, different powers, different laser frequencies, and different transitions can be used as appropriate. In particular, atoms other than Cs atoms can be used in solid, liquid, and / or vapor form, and the frequency and power can be adjusted accordingly. In addition, the manner of pumping, field generation, and detection of the probe beam can be changed. For example, the pump and detection subsystems can use one, two, three, or more lasers to perform pumping and detection, and in some embodiments the polarization of the beam can be changed. The detection beam detector can also be changed, but is preferably a photodetector capable of detecting the polarization and / or amplitude of the detection beam.

[0141] In some embodiments, the earth's magnetic field may be used as the bias field, and thus the bias magnetic field source may be omitted.

[0142] In some embodiments, the detection signal may be processed before being used to drive the variable magnetic field source. However, in all embodiments, the signal driving the variable magnetic field source and thus the oscillation of the primary and secondary magnetic fields should rely on the detection signal to have the same frequency.

[0143] Although in the above embodiment, the lock-in amplifier 36 is described as recording or determining the amplitude and phase of the atomic signal, any processor may be used as long as it is capable of demodulating the detection signal with reference to the frequency of the signal.

[0144] Although in the above embodiments, material response detection is used for material defect imaging, this is not required in every embodiment. In some embodiments, material response detection can be used for other purposes.

[0145] Example

[0146] The advantageous properties of the self-oscillating sensor are exhibited in both magnetically shielded and open environments. Demonstration of defect detection via magnetic induction tomography in a ferromagnetic carbon steel sample is shown. The discussed configuration can provide a simple and robust non-destructive material defect detection system based on atomic magnetometers.

[0147] The measurements described below were performed in two configurations: RF atomic magnetometers in a shielded environment [28–30] and in an open environment [6–8]. In the shielded environment, we explored the fundamental properties of spin masers. We compared the atomic responses to RF drive with and without a feedback loop. The implementation of spin masers in defect imaging of a carbon steel plate through MIT was demonstrated using an unshielded RF atomic magnetometer. So far, the role of spin masers in RF systems has been explored mainly in the context of geomagnetic measurements in the context of magnetometry [23, 26]. Feedback loops have also been investigated in self-oscillating DC atomic magnetometers [31–33]. The sensors were shown to deliver bandwidths (~5 kHz) that exceed those indicated by the linewidth of the spectral coherence profile. Previous demonstrations of spin masers in alkali metals also involved nonlinear coupling, i.e., a combination of nonlinear spin dynamics driven by a linearly polarized laser beam and indirect optical pumping

[34] . The case described here exploits the linear response of the atomic system.

[0148] Experimental setup. In the shielded environment, the ambient magnetic field is suppressed using a five-layer cylindrical shield with end caps made of a 2 mm thick high-permeability alloy (mu-metal) [28-30, 34]. A solenoid inside the shield generates a well-controlled bias magnetic field. In the open configuration, measurements are performed in a non-magnetic shielded environment, where the bias magnetic field is actively stabilized by three pairs of nested orthogonal square Helmholtz coils [3, 6-8, 35]. In both setups, cesium atomic vapor is contained in a paraffin-coated cell at ambient temperature (atomic density n Cs =0.33 10 11 cm -3 ). The atoms are optically pumped by a circularly polarized laser beam propagating in the direction of the offset field. Figure 1 The beam is provided by a diode laser, with a frequency stabilized at 6 2 S 1 / 2 F=3→6 2 P 3 / 2 F' = 2 transition (D2 line, 852nm. The evolution of the total atomic spin is mapped to the polarization state of a linearly polarized probe beam [29, 36, 37]. The probe beam is phase-locked to the pump beam, shifting from 6 2 S 1 / 2 F=4→6, 2 P 3 / 2 The F'=5 transition is blue-shifted by 800 MHz. The laser light transmitted through the cell is analyzed by a polarimeter consisting of a crystal polarizer oriented at 45° relative to the incident polarization and a commercial balanced photodetector. The resulting signal is measured by a lock-in amplifier or a 2MS / s data acquisition board referenced to the frequency of the driving RF field.

[0149] Shielding setup. The operation of the RF magnetometer in standard mode and in the spin maser configuration is compared by the Allan deviation of the signal amplitude. Our goal is to illustrate the advantage of the spin maser system being autonomously tuned to the RF resonance. To simulate the effects of a noisy environment, the stability of the current source Keithley 6220, which drives the offset field coil and defines the RF resonance frequency, is reduced. Figure 5 The time dependence of the statistical uncertainty of the amplitude of the RF magnetometer is shown in the standard configuration (red circles) and in the spin maser mode (black diamonds), where the RF field is generated by a tunable waveform generator. For the former, the drift of the offset magnetic field leads to deviations from a ~√τ dependence (blue dashed line) over an integration time of more than 10 milliseconds. The dependence of the amplitude statistical uncertainty on the integration time (~√τ, marked by the blue dashed line) shows a dominant "white noise" feature. The stability of the spin maser oscillation amplitude is limited by fluctuations in the power of the probe beam delivered to the system through a single-mode polarization-maintaining fiber.

[0150] One of the hallmarks of spin masers is the "start-up" oscillation, which is a transient periodic change in the amplitude of the maser oscillations at the start of maser operation

[19] . The amplitude change is comparable to that observed in any system with a high-gain feedback loop that approaches a stationary state after a sudden change in operating conditions. Two cases in alkali metal spin masers where transient dynamics can be observed are Figure 6 Shown: (a) at the beginning of spin maser operation in an offset field of 5.7 μT and (b) in the case of a significant frequency jump. The latter case is caused by the change of the coil Q factor with the operating frequency. In particular, Figure 6 The transient behavior of the self-oscillating magnetometer signal observed when the magnetic field is varied from 5kHz to 40kHz in terms of Larmor frequency is shown. The measurements were made using pump / probe beam powers of 500mW / 150mW. No transient amplitude oscillations are observed if the frequency variation is less than 1kHz, which is the largest excursion recorded in these ferromagnetic object experiments.

[0151] Spin masers in magnetic induction measurements. The alkali metal spin masers described above have been applied in a tomographic magnetic induction imaging setup. The measurements were performed in an unshielded environment using the same instrumentation already described in [6-8]. A radio frequency coil (1000 turns of 0.2 mm diameter copper wire, 10 mm height, 2 mm and 4 mm inner and outer diameters) generating the primary field has been placed about 3 mm above a 6 mm thick steel plate with grooves simulating structural defects. The radio frequency coil is connected to the output of a balanced photodetector that monitors the polarization of a probe beam in an atomic magnetometer located 200 mm above the plate. The measurements have been configured in a so-called self-compensating configuration [8]. In this configuration, the axis of the radio frequency primary field is parallel to the offset magnetic field direction and orthogonal to the surface of the plate. The amplitude of the atomic signal is recorded by a lock-in amplifier referenced to the signal frequency. The generation of the spin maser feedback effect in the induction experiment is more complex than reported in the previous section. The signal generated by the spontaneous precession of the atomic spins in the offset field is amplified and transmitted to the primary radio frequency field coil. This excites a response within the plate (secondary field), whose amplitude and direction are defined by its surface structure.

[0152] Since the atomic magnetometer is only sensitive to the RF field components that are orthogonal to the axis of the offset magnetic field, the atomic spin precession can only be driven by the secondary field components parallel to the slab surface. These components become non-zero only near the edges of the grooves. Therefore, there is no spin masering effect away from defects, inhomogeneities or edges, i.e. on the uniform surface of the slab. The condition for positive feedback in spin masering requires that the sum of all phase shifts in the feedback loop is zero

[23] , in other words, that there is phase matching between the atomic spin precession and the drive generated by the secondary field. The phase of the drive is defined by the direction of the secondary field. As shown in [8], when the primary field coil is scanned around the perimeter of the groove, the direction of the secondary field varies over the full range of possible values ​​(360°). This translates into eddies in the detected signal phase, i.e. the phase of the drive seen by the atoms. The spin masering effect will only be triggered in the region above the groove edge (i.e. in the direction of the secondary field), which guarantees phase matching conditions within the atomic sample. The choice of this region can be controlled by introducing a phase shift between the detected signal and the RF coil. Changes in the amplification in the feedback loop can be used to distinguish defects of different sizes and depths due to their effect on the secondary field strength. Figure 7 The board shown is 64x64mm 2 Images of an area, the plate containing grooves of 24 mm diameter and 2.4 mm deep, recorded in (a) standard and (b) spin maser self-compensation mode. The spin maser mode of operation ensures that the values ​​recorded in the measurement represent the amplitude of the RF spectrum resonance and therefore there is no need to monitor the entire spectral profile. This means that image acquisition time is significantly reduced. Figure 7The typical recording time for the image in (a) is 12 hours, defined by about 8 seconds of acquisition of the RF spectrum at a single location. Under the same measurement conditions, acquisition of the image in spin maser mode takes about 25 minutes. Figure 7 (bd). This time is limited only by the stepper motor's motion time. Due to the phase matching condition, only Figure 7 A portion of a groove feature is shown in (a). Figure 7 (c) shows an image of a groove recorded using the same RF coil and a 180° phase shift relative to that shown in (b). This suggests that in order to recover the full features of the groove, a set of coils, each with an associated phase shift, may be implemented. Thus, another embodiment of the present invention comprises a system as described above, but with two coils of opposite phase to record the groove. Figure 7 (d) Although the full features of the groove (i.e., the ring shape) are not recovered, the measurements demonstrate that a multi-coil system can be used to ensure spin masering over the entire circumference of the groove. The autonomous tuning of the spin maser system to the radio frequency resonance enables recording of Figure 7 The images shown in (bd) do not show degradation in amplitude and contrast even without active field stabilization. This greatly simplifies the instrumentation of the imaging system, as the fluxgate magnetometer and field stabilization electronics (e.g., PID unit) become obsolete.

[0153] Since the signal phase variation around the groove is monotonic, it can be obtained from Figure 7 The phase dependence of the spin maser signal is extracted in (b) and 7(c). As shown in [8], the phase of the secondary field varies monotonically around the groove circumference, which allows calibration of the horizontal axis. Figure 8 Shown along Figure 7 (b) Variation of the signal amplitude read around the periphery of the groove feature. The phase is arbitrarily set to 0° at the center of the image, representing a situation where the phase of the drive is perfectly matched to the atomic spin oscillations.

[0154] In summary, we have demonstrated alkali metal spin maser operation in both magnetically shielded and open environments. The self-regulating nature of the measurement facilitates reading the signal amplitude at the RF resonant frequency regardless of drift in the offset magnetic field. This can significantly increase the image capture rate. We also presented multi-RF coil system operation.

[0155] All details of the specific examples, in particular operating conditions, parameters and dimensions, can apply to any embodiment described in this application.

[0156] All optional and preferred features and modifications of the described embodiments and dependent claims can be used for all aspects of the invention taught in this application. In addition, the various features of the dependent claims and all optional and preferred features and modifications of the described embodiments can be combined and interchanged with each other.

[0157] The disclosures in UK Patent Application No. 1910213.6 from which this application claims priority and in the Abstract accompanying this application are incorporated herein by reference.

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Claims

1. An atomic magnetometer system, comprising: a variable magnetic field source configured to provide an oscillating primary magnetic field to cause the sample to generate a secondary magnetic field; an atomic magnetometer for detecting the secondary magnetic field; The atomic magnetometer comprises: Atomic samples; a pump and probe subsystem configured to pump the atomic sample to generate polarization and to probe coherent precession of atoms within the atomic sample using a probe beam; a detector configured to detect the probe beam to generate a detection signal; The system is configured to drive a variable magnetic field source based on a detection signal having a frequency tuned to radio frequency resonance; wherein the system is configured to drive the variable magnetic field source at a frequency corresponding to the oscillation frequency of the detection signal.

2. The system according to claim 1, wherein: A variable magnetic field source is coupled to the output of the detector so as to be driven according to the detection signal.

3. The system according to claim 1, wherein: The variable magnetic field source is configured to be positioned adjacent to the sample.

4. The system of claim 1, comprising an instrument processor configured to receive the detection signal and determine its amplitude and phase. 5 . The system of claim 1 , comprising a voltage follower configured to drive the variable magnetic field source using a signal buffered from the detection signal. 6 . The system of claim 1 , comprising an amplifier configured to drive the variable magnetic field source using a signal amplified from the detection signal.

7. The system of claim 1, comprising a phase shifter configured to drive the variable magnetic field source with a signal phase-shifted from the detection signal.

8. The system of claim 1, comprising a bias magnetic field source configured to provide a bias magnetic field in a bias field direction to the atomic sample.

9. The system according to claim 1, wherein: The variable magnetic field source is a radio frequency coil.

10. The system according to claim 9, wherein: The variable magnetic field source is driven according to a detection signal having a frequency tuned to radio frequency resonance by causing a voltage signal to be dependent on the detection signal coupled across the radio frequency coil.

11. The system according to claim 1, wherein: The atomic sample includes a collection of alkali metal atoms.

12. The system of claim 1, wherein: The atomic sample includes alkali metal atomic vapor.

13. The system of claim 1 , comprising a set of variable magnetic field sources configured to provide an oscillating primary magnetic field, wherein the system is configured to drive each variable magnetic field source based on a detection signal having a frequency tuned to a radio frequency resonance and a phase shifted relative to one another.

14. The system of claim 13, comprising at least one phase shifter configured to drive at least one of the set of variable magnetic field sources with a signal phase-shifted from the detection signal.

15. The system of claim 1, wherein: The atomic magnetometer is configured to reduce the influence of the components of the primary and secondary magnetic fields on the atomic magnetometer in a main direction substantially orthogonal to the sample surface.

16. The system of claim 15, wherein: The atomic magnetometer is in a self-compensating configuration.

17. A method of operating an atomic magnetometer to detect a secondary magnetic field generated by a sample, comprising: pumping the atomic sample to generate polarization and probing the coherent precession of atoms within the atomic sample with a probe beam; detecting the probe beam to generate a detection signal; driving a variable magnetic field source based on the detection signal to provide an oscillating primary magnetic field to cause the sample to generate a secondary magnetic field having a frequency tuned to the radio frequency resonance; Wherein, driving the variable magnetic field source according to the detection signal includes driving the variable magnetic field source at a frequency corresponding to the frequency of the detection signal.

18. The method according to claim 17, wherein: Driving the variable magnetic field source according to the detection signal includes driving the variable magnetic field source with a signal buffered and / or amplified and / or phase-shifted from the detection signal.

19. A method according to claim 17 or 18, comprising positioning the atomic sample in a bias magnetic field.

20. A method according to claim 17 or 18, comprising determining the amplitude and phase of the detection signal in order to detect the secondary magnetic field.

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