Atomic Magnetometer System

Abstract

An atomic magnetometer system is disclosed that includes a variable magnetic field source (14) configured to provide an oscillating primary magnetic field to generate a secondary magnetic field in a sample (16). 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 probe subsystem configured to pump the atomic sample to create polarized light and probe atomic coherence precession within the atomic sample with a probe beam, and a detector configured to detect the probe beam and generate a detection signal. The system is configured to drive the variable magnetic field source (14) in response to the detection signal at a frequency tuned to the rf resonance. A method of operating the atomic magnetometer is also disclosed.

Description

[Technical field]

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

[0002] The implementation of radio frequency (rf) atomic magnetometers as magnetic field sensors creates new opportunities for non-destructive testing based on magnetic induction tomography (MIT) [1-8] (numbers in square brackets refer to the respective references at the end of this description and are incorporated herein by reference). MIT relies on the detection of the response of a sample to an oscillating rf magnetism, the so-called primary magnetic field. The secondary magnetic field, which is the response of the material, is generated by eddy currents in the sample if it is conductive and / or by magnetization if it is permeable. Atomic magnetometers allow semi-vector mapping of all components of the secondary magnetic field, in addition to better sensitivity, a wide tuning range [9], and better spatial resolution

[10] than standard pickup coils. With respect to other magnetic field sensors, such as giant magnetoresistance (GMR) magnetometers [10-12] or superconducting quantum interference devices (SQUIDs) [13, 14], atomic magnetometers offer the advantage of operation in unshielded environments [9, 15, 16] without cryogenic temperatures and with few miniaturization limitations.

[0003] Tomographic imaging of defects in a sample using an rf atomic magnetometer relies on observing the change in amplitude and phase of the rf resonance as the sample moves beneath an rf primary magnetic field coil. Magnetically permeable samples pose a challenge for atomic sensors because the rf resonance frequency shifts with the local magnetization of the sample. This issue is partially addressed by active stabilization of the magnetic field in which the sensor operates, but residual changes in the resonance frequency that occur reduce the image acquisition rate [7]. Summary of the Invention [Problem to be solved by the invention]

[0004] Aspects of the present invention are directed to providing improved atomic magnetometer systems and methods. [Means for solving the problem]

[0005] According to one aspect of the present invention, there is provided an atomic magnetometer system comprising: a variable magnetic field source configured to provide an oscillating primary magnetic field to generate a secondary magnetic field in the sample; 1. An atomic magnetometer for detecting a secondary magnetic field, comprising: An atomic sample; a pump and probe subsystem configured to pump the atomic sample to create polarized light and to probe atomic coherence precession within the atomic sample with the probe beam; a detector configured to detect the probe beam and generate a detection signal; The system is configured to drive a variable magnetic field source in response to the detection signal at a frequency tuned to the rf resonance.

[0006] An embodiment of the present invention can provide an atomic magnetometer system in which the secondary magnetic field is always tuned to the rf resonance. This can avoid the time-consuming scan for resonance required in some prior art systems. Note that in an embodiment of the present invention, the primary magnetic field is also tuned to the rf resonance. However, it is the secondary magnetic field that the atomic magnetometer seeks to detect.

[0007] In an embodiment of the present invention, the rf resonance is the frequency of the free atomic coherence precession within the atomic sample.

[0008] In an embodiment of the invention, the atoms of the atomic sample, together with the probe and detector, act as an 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 resulting from detection of atomic coherence precession within the atomic sample.

[0009] Prior art systems typically use a separate rf generator to generate the rf signal to drive the variable magnetic field source. The use of the atoms of the atomic sample, together with the pump and probe subsystem and detector, as the rf generator also means that embodiments of the present invention can avoid the need for a separate rf generator.

[0010] In some embodiments, the system is configured to drive the variable magnetic field source at a frequency that corresponds to the oscillation frequency of the detection signal.

[0011] In some embodiments, the variable magnetic field source is coupled to the output of the detector such that it is driven in response to the detection signal.

[0012] In some embodiments, the variable magnetic field source is configured to be positioned adjacent to the sample.

[0013] In some embodiments, the system includes an instrument processor configured to receive the detection signal and determine its amplitude and phase. The instrument can be a processor.

[0014] In some embodiments, the system includes a voltage follower configured to drive the variable magnetic field source with a signal buffered from the detection signal.

[0015] Particularly for low impedance coils, the variable magnetic field source 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 detector signal.

[0016] In some embodiments, the system includes an amplifier configured to drive the variable magnetic field source with an amplified signal from the detection signal.

[0017] A variable magnetic field source can 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.

[0018] In some embodiments, the system includes a phase shifter configured to drive the variable magnetic field source with a signal that is phase shifted from the detection signal.

[0019] A variable magnetic field source can 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 from the detector signal.

[0020] In some embodiments, the system includes a bias magnetic field source configured to provide a bias magnetic field in a bias magnetic field direction to the atomic sample.

[0021] The bias magnetic field may define a quantization axis for the atomic sample.

[0022] In some embodiments, the variable magnetic field source is an rf coil.

[0023] In some embodiments, the variable magnetic field source is driven in response to the detection signal at a frequency tuned to the rf resonance by having a voltage signal dependent on the detection signal coupled across an rf coil.

[0024] The voltage signal coupled across the rf coil may be derived from the detection signal by its amplification and / or phase shift, but preferably has a frequency corresponding to the frequency of the detection signal.

[0025] In some embodiments, the output of the detector is coupled across an rf coil, optionally via a voltage follower and / or an amplifier and / or a phase shifter.

[0026] In some embodiments, the atomic sample comprises a collection of alkali metal atoms.

[0027] In some embodiments, the atomic sample comprises an alkali metal atomic vapor.

[0028] In some embodiments, the system includes a set of variable magnetic field sources configured to provide an oscillating primary magnetic field, the system being configured to drive each of the variable magnetic field sources in response to mutually phase-shifted detection signals at a frequency tuned to rf resonance.

[0029] Each of the variable magnetic field sources is driven with a phase shift relative to the others, allowing one of the sets to be driven in phase with the detection signal without being driven through a phase shifter. Because of the phase shift, different members of the set of variable magnetic field sources can act on different parts of the defect.

[0030] 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 that is phase shifted from the detection signal.

[0031] If the signals driving the set of variable magnetic field sources are phase shifted from the detection signals, this may be by means of a common phase shifter or by their own respective phase shifters.

[0032] Each of the sets of variable magnetic field sources can be driven with a signal amplified from the detection signal in a similar manner as discussed for the individual variable magnetic field sources. If the signals driving the sets of variable magnetic field sources are amplified, they can be by a common amplifier or by their own respective amplifiers.

[0033] In some embodiments, the atomic magnetometer is configured to reduce the influence on the atomic magnetometer of components of the primary and secondary magnetic fields in a primary direction substantially perpendicular to the surface of the sample.

[0034] In an embodiment, the primary direction is from the variable magnetic field source to the atomic sample.

[0035] In an embodiment, the surface of the sample is the surface under investigation, typically the major surface of the sample.

[0036] Some embodiments include a compensation magnetic field source, which may include a coil arrangement, for providing a compensation magnetic field to the atomic magnetometer, and in particular to the atomic sample thereof, that includes a component in a primary direction.

[0037] The compensation magnetic field source can be configured to provide a compensation magnetic field to reduce the influence of components of the primary and secondary magnetic fields in a primary direction on the atomic magnetometer.

[0038] In some embodiments, the atomic magnetometer is in a self-compensating configuration.

[0039] In some embodiments, the atomic magnetometer has an axis of insensitivity aligned in the primary direction.

[0040] In some embodiments, the atomic magnetometer includes a magnetic bias field source configured to provide a magnetic bias field in a primary direction.

[0041] According to one aspect of the invention there is provided a method of operating an atomic magnetometer to detect a secondary magnetic field produced by a sample, comprising: pumping the atomic sample to create polarized light and using a probe beam to probe atomic coherence precession in the atomic sample; detecting the probe beam to generate a detection signal; Driving a variable magnetic field source in response to the detection signal to provide an oscillating primary magnetic field to cause the sample to generate a secondary magnetic field at a frequency tuned to the rf resonance.

[0042] In some embodiments, driving the variable magnetic field source in response to the detection signal includes driving the variable magnetic field source with a signal that is buffered and / or amplified and / or phase shifted from the detection signal.

[0043] In some embodiments, driving the variable magnetic field source in response to the detection signal comprises driving the variable magnetic field source at a frequency corresponding to a frequency of the detection signal.

[0044] In some embodiments, the method includes placing the atomic sample in a bias magnetic field.

[0045] In some embodiments, the method includes determining an amplitude and a phase of a detection signal to detect the secondary magnetic field.

[0046] The method can include reducing the influence of components of the primary and secondary magnetic fields in a direction substantially perpendicular to a surface of the sample on the atomic magnetometer.

[0047] In an embodiment, the surface of the sample is the surface under investigation, typically the major surface of the sample.

[0048] The primary magnetic field may be substantially perpendicular to the surface of the sample.

[0049] In some embodiments, reducing the influence of components of the primary and secondary magnetic fields in a direction substantially perpendicular to the surface of the sample on the atomic magnetometer comprises: This involves providing a compensation magnetic field to an atomic magnetometer, e.g., an atomic sample, that includes a component substantially perpendicular to the surface of the sample.

[0050] In some embodiments, a compensation field is provided such that Bz′+bz=0, where Bz′ is

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[0051] In some embodiments, reducing the influence of components of the primary and secondary magnetic fields in a direction substantially perpendicular to the surface of the sample on the atomic magnetometer comprises: This includes aligning the axis of insensitivity of the atomic magnetometer substantially perpendicular to the surface of the sample.

[0052] In some embodiments, the method includes aligning the bias magnetic field in a direction substantially perpendicular to the surface of the sample.

[0053] High frequency atomic magnetometers offer an attractive alternative to standard detection methods for non-destructive testing based on induction measurements. Embodiments provide a so-called spin maser configured magnetometer, which addresses two key challenges of the technique: high frequency resonance position shifts induced by permeable samples and sensor bandwidth.

[0054] Embodiments provide a spin maser for non-destructive testing and detection.

[0055] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0056] [Figure 1] A diagram of a standard alkali-metal spin maser equivalent to the one described in

[23] .

[0057] [Diagram 2] FIG. 1 is a diagram of a system according to an embodiment of the present invention.

[0058] [Diagram 3] FIG. 3 is a schematic diagram of the system of FIG. 2.

[0059] [Figure 4] Diagram of the statistical uncertainty (Allan deviation) of the signal amplitude from the magnetometer in standard configuration (red circles) and in spin maser mode (black diamonds).

[0060] [Diagram 5] FIG. 1 is a diagram of a transient state of a self-excited vibrating magnetometer signal.

[0061] [Figure 6] Diagram of the variation in amplitude of the rf spectroscopy signal measured over a 64x64mm2 area of ​​a 6mm thick carbon steel plate containing a 24mm diameter indentation with a depth of 2.4mm, recorded in four measurement configurations: (a) a self-compensating configuration as in [8], (b) a single rf coil, (c) a single rf coil in anti-phase with (b), and (c) a spin maser mode with two rf coils in anti-phase.

[0062] [Figure 7] FIG. 13 is a diagram of the dependence of the signal amplitude as a function of the phase of the drive. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] Conventional non-destructive testing and detection of ferromagnetic objects by magnetic induction tomography using atomic magnetometers suffers from rf resonance shifts induced by the test object. The spin maser operation described herein can solve that problem and enable a significant reduction in image acquisition time.

[0064] The embodiment provides a mode of operation of a high frequency atomic magnetometer, a so-called spin maser. This mode relies on the spontaneous fluctuations of atomic spins being optically monitored and the signal from the optical detection being fed back to the atomic sample. The operating frequency of the spin maser automatically follows the current value of the bias magnetic field. The feedback loop can include buffering, amplification and phase shifting. Advantages include a significant reduction in image acquisition time, image contrast and stealth operation in case of surveillance.

[0065] Embodiments of the present invention may find application for detection of corrosion under insulation in the energy sector, monitoring of reinforced concrete structures in the transportation sector, monitoring of nuclear waste ships, object detection and surveillance.

[0066] The embodiment provides an implementation of an rf atomic magnetometer operating in spin maser mode for the MIT measurement scheme. The positive feedback of the spin maser system not only provides a solution to the problem of imperfect magnetic field stabilization, but also improves the sensor bandwidth, image acquisition rate, and signal-to-noise ratio. Extensive theoretical and experimental studies of spin maser systems, also called Zeeman masers, have been carried out in the context of noble gas systems [17-20]. A typical spin maser system consists of two parts: a pumping valve, where the nuclear spin polarization is generated, and a maser valve surrounded by a pickup coil that detects the oscillations of the spins in a static magnetic field and provides positive feedback. The spin polarization is achieved either by direct optical pumping, for example with a discharge lamp

[17] or by spin-exchange pumping, i.e. the transfer of the polarization from the directly optically pumped alkali metal atoms to the noble gas atoms mediated by spin-exchange collisions

[21] . The nuclear spin precession around the static magnetic field induces a current in the coil surrounding the maser valve, which consequently induces an oscillating magnetic field in the valve. This magnetic field reflects the evolution of the spins and provides positive feedback. In contrast to the nuclear spin maser configuration, the rf atomic magnetometer optically monitors the spontaneous fluctuations of a polarized spin sample [22-24]. This is because the spins of the noble gases (approximately 10 18 cm -3 Low density alkali metal vapor (10 10-13 cm -3 This is due to the small strength of the oscillating magnetic field generated by the precessing spins in the atomic vapor

[25] . The detected signal is sent to an rf coil located near the atomic vapor (Figure 1). This separation of monitoring and feedback allows the introduction of amplification and a phase shift in the feedback loop.

[0067] An embodiment of the present invention utilizes an rf atomic magnetometer and can be used for imaging material defects.

[0068] A detailed embodiment which is an implementation of the above-mentioned alkali metal spin maser is described below.

[0069] As seen in FIG. 2, an embodiment of the invention includes a system 10 including a radio frequency atomic magnetometer 12 and a variable primary magnetic field source 14 configured to provide a primary magnetic field oscillating at an rf frequency. This system is referred to as a tomographic magnetic induction imaging setup and can provide spin maser with magnetic induction measurements. In this embodiment, the primary magnetic field source 14 is an rf coil. However, other variable magnetic field sources can be used in other embodiments. The rf coil is configured to provide a primary magnetic field substantially orthogonal to a surface of the sample. In this embodiment, the surface is the surface under investigation, which is the primary surface of the sample.

[0070] In this embodiment, the system is configured to operate in an unshielded environment using the same equipment as described in [6-8], which is incorporated herein by reference.

[0071] In this embodiment, the rf coil 14 is a 1000 turn coil of copper wire with a diameter of 0.02 mm, with inner and outer diameters of 10 mm, 2 mm, and 4 mm in height.

[0072] The rf coil 14 is configured such that it can be positioned adjacent to, but entirely to one side of, the sample 16 in a non-overlapping relationship therewith, and is operable to generate an oscillating primary magnetic field and to cause the sample to generate a secondary magnetic field that is indicative of the material response of the sample.

[0073] The sample must be electrically conductive (but not necessarily highly conductive) and / or must have magnetic permeability so that it can be magnetized.

[0074] The atomic magnetometer is configured to detect a secondary magnetic field.

[0075] It is worth noting that a ferromagnetic target generates two types of secondary magnetic fields. A secondary magnetic field in the same direction as the applied primary magnetic field - secondary magnetization, · Eddy current induced magnetic field in the opposite direction to the applied primary magnetic field - Eddy current induced magnetic field.

[0076] In this description, the z-direction is perpendicular to the surface of the sample, and the x- and y-directions are mutually perpendicular directions parallel to the surface of the sample.

[0077] In this embodiment, the atomic magnetometer 12 includes a detection cell 20, which in this embodiment detects cesium atomic vapor (atomic density n Cs =3.3×10 10 cm -3 ) at ambient temperature 3 Paraffin-coated glass cell. Cesium atomic vapor provides the atomic sample for the magnetometer.

[0078] The magnetometer includes a bias magnetic field source 24 (not shown in FIG. 2) configured to provide a bias magnetic field 26 to the detection cell 20 and therefore to the atomic sample in the bias field direction. The terms "bias" and "offset" are used interchangeably in this magnetic field context.

[0079] In this embodiment, the offset magnetic field is actively stabilized by three pairs of nested, orthogonal square Helmholtz coils [3, 6-8, 35], which provide the bias magnetic field source.

[0080] The magnetometer includes a pump laser (not shown in FIG. 2) configured to pump atoms of the atomic sample in the detection cell 20 with a circularly polarized pump laser beam 28 propagating along the direction of the offset magnetic field. 2 S 1 / 2 F=3→6 2 P 3 / 2 F'=diode laser frequency stabilized to the 2 transition (D2 line, 852 nm).

[0081] The pump laser beam 28 is configured to pump the atomic sample to create an imbalance or anisotropy in the polarization or population along the bias magnetic field.

[0082] The atomic magnetometer includes a probe laser (not shown in FIG. 2) configured to probe the detection cell 20 to probe atomic coherence precession within the atomic sample using a linearly polarized probe laser beam 32 that is orthogonal to the bias magnetic field 26 and phase offset locked to the pump beam. 2 S 1 / 2 F=4→6 2 P 3 / 2 The F'=5 transition is shifted 800 MHz blue.

[0083] The pump laser and the probe laser form a pump and probe subsystem.

[0084] The evolution of the collective atomic spins is mapped onto the polarization state of a linearly polarized probe beam.

[0085] In this embodiment, the atomic magnetometer 12 is configured to reduce the effect on the atomic magnetometer 12 of the components of the primary magnetic field, and optionally the components of the secondary magnetic field, in a direction substantially perpendicular to the surface of the sample, referred to as the primary direction or z. In this embodiment, the primary direction is along the rf coil axis, from the rf coil to the atomic sample. In this embodiment, this is due to the atomic magnetometer being in a self-compensating configuration, where the atomic magnetometer has an axis of insensitivity arranged in the primary direction, where the axis of the rf coil, and therefore the axis of the primary magnetic field, is the primary direction, and is parallel to the bias magnetic field direction. Thus, in this embodiment, the direction of the bias magnetic field is also the primary direction. Bias magnetic field oriented along z

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[0086] As disclosed in [8] and WO 2020 / 016557, other methods of reducing the effect on the atomic magnetometer of the components of the primary and optionally secondary magnetic fields in a direction substantially perpendicular to the surface of the sample can be used. For example, some embodiments include a compensation field source, which may include a coil arrangement, for providing a compensation field to the atomic magnetometer, and in particular to the atomic sample thereof, including a component in the primary direction. In such embodiments, the compensation field may be provided such that Bz'+bz=0, where Bz' is

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[0087] 8 is an example of a system including a compensation field source 41 for providing an oscillation compensation field, also called a compensation field, to the atomic magnetometer, and in particular to the detection cell 20, with a component substantially perpendicular to the surface of the sample 16. The compensation field reduces, and preferably eliminates, the influence of the magnetic field components of the primary and secondary magnetic fields in that direction on the atomic magnetometer. In particular, the compensation field is

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[0088] As can be seen from FIG. 8, in this embodiment the compensation magnetic field source 41 is a compensation coil arrangement including a first compensation coil 42 and a second compensation coil 44 .

[0089] The first compensation coil 42 has an axis substantially aligned with z, a direction perpendicular to the surface of the sample 16, to provide a magnetic field to the atomic magnetometer, and in particular to the detection cell 20, that is substantially perpendicular to the surface of the sample 16.

[0090] In this embodiment, the detection cell 20 is located between the rf coil 14 and the first compensation coil 42, although this is not necessary in all embodiments.

[0091] The second compensation coil 44 has an axis substantially aligned with y, a direction parallel to the surface of the sample 16 and substantially perpendicular to the bias magnetic field direction, to provide a magnetic field to the atomic magnetometer, particularly to the detection cell 20, which is substantially parallel to the surface of the sample 16 and substantially perpendicular to the bias magnetic field direction.

[0092] In this embodiment, the detection cell 20 is positioned between the probe laser 30 and the second compensation coil 44, which is positioned between the detection cell 20 and the balanced polarimeter 34, although this is not necessary in all embodiments.

[0093] The compensation coil arrangement 41 generates a compensation magnetic field

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[0094] In other words, a set of two rf coils oriented along the z and y directions (Figure 8) generates an oscillating magnetic field

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[0095] In other words, the primary magnetic field is compensated in the vapor cell by a set of two rf coils oriented along the z and y directions.

[0096] However, for reasons explained below, the component of the secondary magnetic field in the z direction is also optionally compensated.

[0097]

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[0098] The resulting magnetic field monitored by an rf atomic magnetometer contains components from the primary, secondary and compensation fields, in other words

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[0099] With this configuration, the amplitude and phase (direction) of the rf magnetic field in the yz plane can be determined.

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[0100] As described in WO 2020 / 016557 and [8],

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[0101] In the embodiment of FIG.

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[0102] In all embodiments, it is not necessary to reduce the influence on the atomic magnetometer 12 of the components of the primary and secondary magnetic fields oriented substantially perpendicular to the surface of the sample.

[0103] Returning to the embodiment of Figure 2, the atomic magnetometer includes a detector. In this embodiment, the detector includes a polarimeter 34 configured to receive the probe laser beam after it passes through the detection cell 20. The polarimeter 34 includes a crystal polarizer oriented at 45° to the incident polarized light and a commercially available balanced photodetector.

[0104] The laser light transmitted through the cell 20 is analyzed by a polarimeter.

[0105] The detector is configured to output a detection signal via an output in response to detection of the probe laser beam. The detection signal is typically a voltage or current signal representative of the polarization and / or amplitude of the detected probe beam. The detector output, in this embodiment, is the output of a balanced photodetector providing a voltage signal representative of the polarization of the probe beam.

[0106] In this embodiment, the system 10 includes a lock-in amplifier 36 (shown in FIG. 3) configured to receive a detection signal from the detector. The lock-in amplifier is configured to record or determine the amplitude and phase of the detection signal, also known as the atomic signal, and the lock-in amplifier provides a first output signal, for example, to a computer, referenced to the signal frequency. The computer can use the amplitude and phase of the signal to detect the secondary magnetic field, thereby detecting a material response of the sample and potentially performing imaging of material defects.

[0107] In some embodiments, the computer may include a receiver for receiving the first output signal from the lock-in amplifier 36 and determining therefrom a change in the conductivity and / or permeability of the sample.

[0108] The rf coil 14 is connected to the output of the detector through an amplifier 40 (shown in FIG. 3). The amplifier 40 is configured to receive the detection signal from the detector, amplify it, and drive the rf coil 14 in response to the detection signal at a frequency tuned to the rf resonance. The rf resonance is the frequency of free atomic coherence precession within 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. The frequency tuning to the rf resonance is provided automatically as a result of the atomic sample being the source of the signal and acting as an rf generator together with the pump and probe subsystems and the detector.

[0109] In some embodiments, the system may include a voltage follower in addition to or instead of the amplifier, configured to drive the variable magnetic field source with a signal buffered from the detection signal, which is particularly advantageous when the rf coil is a low impedance coil.

[0110] In all embodiments it is not necessary to have an amplifier or voltage follower, in some embodiments the detection signal can be applied directly across the rf coil.

[0111] In some embodiments, the system may include a phase shifter configured to shift the phase of the signal driving the primary magnetic field source and thus shift the phase of the primary and secondary magnetic fields with respect to the detection signal.

[0112] Figure 2 shows the phases of spin maser operation in a magnetic induction tomography measurement according to this embodiment: (1) Spontaneous fluctuations of atoms in an atomic sample generate tiny atomic spins that precess at the Larmor frequency. (2) A probe beam monitors the evolution of the spins, and the spin precession is mapped to the polarization of the beam. (3) A photodetector generates a voltage oscillation following the change in polarization. (4) The amplified, possibly phase-shifted signal from the photodetector generates a current in the primary rf field coil. (5) The primary and / or secondary magnetic fields provide positive feedback to the atomic spin precession.

[0113] During operation, the bias magnetic field source 24 provides a bias magnetic field to the detection cell 20 .

[0114] The pump laser operates to pump the atomic sample to create an imbalance or anisotropy in the polarization or population along the bias magnetic field.

[0115] The probe laser operates to probe atomic coherence precession within the atomic sample with a probe beam.

[0116] The probe beam is detected by a detector, which outputs a detection signal.

[0117] The detection signal is amplified and the amplified signal is applied across the rf coil to drive the rf coil 14 in response to the detection signal to provide an oscillating primary magnetic field and cause the sample to generate a secondary magnetic field at a frequency tuned to the rf resonance. As noted elsewhere, in some embodiments it is possible to omit amplification and / or include a phase shift.

[0118] The embodiments can be used to provide the following advantages: · Safe and non-invasive (e.g., non-ionizing radiation) Detect corrosion on the inner walls of pipelines. Detect corrosion on the outer walls of pipelines. ·Ability to distinguish between corrosion and changes in pipeline geometry due to pipe bends / T-junctions / welds. -Ability to scan all insulation types. Low cost. Improve current technology (resolution, switching scan modes).

[0119] In some embodiments, the system may include a set of variable magnetic field sources configured to provide an oscillating primary magnetic field, the system being configured to drive each of the variable magnetic field sources in response to a detection signal that is phase-shifted from the detection signal relative to each other at a frequency tuned to the rf resonance, but not excluding that it may be in phase with the detection signal. Each of the variable magnetic field sources effectively provides a respective component of the primary magnetic field and the secondary magnetic field, each of which may be operated as described above (the above references are merely to the primary magnetic field or the secondary magnetic field). The set of variable magnetic field sources may be a set of rf coils, each configured as described above. To ensure a relative phase difference between the sets of variable magnetic field sources, at least one of the set of variable magnetic field sources is driven via a phase shifter as described above. If phase shifters, voltage followers and / or amplifiers are used for more than one of the sets of variable magnetic field sources, the phase shifters, voltage followers and / or amplifiers may be common to multiple, possibly all, sets of variable magnetic field sources, and / or multiple, possibly all, sets of variable magnetic field sources may use their own respective phase shifters, voltage followers and / or amplifiers.

[0120] Those skilled in the art will appreciate that the particular atomic magnetometers described above are not the only types of atomic magnetometers that can be used. For example, different detection cells, different dimensions, 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. Additionally, the means of pumping, magnetic field generation, and probe beam detection can be varied. For example, the pump and probe subsystem can use one, two, three, or more lasers to perform pumping and probing, and in some embodiments, the polarization of the beam can be varied. The probe beam detector can also be varied, but is preferably a photodetector that can detect the polarization and / or amplitude of the probe beam.

[0121] In some embodiments, the Earth's magnetic field can be used as the bias field, and thus the bias field source can be omitted.

[0122] In some embodiments, processing can be applied to the detection signal before it is used to drive the variable magnetic field source, but in all embodiments the signal that drives the variable magnetic field source, and therefore the oscillations of the primary and secondary magnetic fields, must depend on the detection signal to have the same frequency.

[0123] In the above embodiment, the lock-in amplifier 36 is described as recording or determining the amplitude and phase of the atomic signal, however any processor capable of demodulating the detected signal with reference to the frequency of the signal can be used.

[0124] In the above embodiments, material response detection is used to image material defects, but this is not necessary in all embodiments, in some embodiments material response detection can be used for other purposes.

[0125] example The advantageous properties of the self-excited vibration sensor are presented in both magnetically shielded and open environments. A demonstration of defect detection by magnetic induction tomography in a ferromagnetic carbon steel sample is presented. The described configuration can provide a simple and robust non-destructive material defect detection system based on an atomic magnetometer.

[0126] The measurements described below are performed in two configurations of rf atomic magnetometers: in a shielded environment [28-30] and in an open environment [6-8]. In the shielded environment, the fundamental properties of the spin maser are investigated. The atomic response to rf driving with and without a feedback loop is compared. The implementation of the spin maser in the imaging of defects in carbon steel plates via MIT with an unshielded rf atomic magnetometer is presented. So far, in the context of magnetometry, the action of spin masers in rf systems has been mainly investigated in the context of geomagnetic measurements [23, 26]. Feedback loops have also been studied in self-oscillating dc atomic magnetometers [31-33]. The sensor was demonstrated to provide a bandwidth exceeding that indicated by the linewidth of the spectral coherence profile (about 5 kHz). Previous demonstrations of alkali metal spin masers also involve nonlinear coupling, i.e. the 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.

[0127] Experimental setup. In the shielded environment, the ambient magnetic field is suppressed by using a five-layer cylindrical shield with end caps made of 2 mm thick mu metal [28-30, 34]. A solenoid inside the shield generates a well-controlled offset magnetic field. In the open configuration, measurements are performed in a magnetically unshielded environment, and the offset magnetic field is actively stabilized by three pairs of nested orthogonal square Helmholtz coils [3, 6-8, 35]. In both setups, the cesium atomic vapor is heated to 1000 K at ambient temperature (atomic density n Cs =0.3310 11 cm -3) in a paraffin-coated cell. The atoms are optically pumped by a circularly polarized laser beam propagating along the direction of the offset magnetic field (Figure 1). The beam is 2 S 1 / 2 F=3→6 2 P 3 / 2 The evolution of the collective atomic spins is provided by a diode laser frequency stabilized to the F'=2 transition (D2 line, 852 nm). The evolution of the collective atomic spins is mapped onto the polarization state of a linearly polarized probe beam [29, 36, 37]. The probe beam is phase-offset locked to the pump beam and 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° to the incident polarization and a commercially available balanced photodetector. The resulting signal is measured either by a lock-in amplifier referenced to the driving rf field frequency or by a 2 MS / s data acquisition board.

[0128] Shielding setup. The operation of the rf magnetometer in standard mode and in spin maser configuration is compared by the Allan deviation of the signal amplitude. Our goal is to illustrate the advantage of the autonomous tuning of the spin maser system on the rf resonance. To simulate the effects of a noisy environment, we reduced the stability of the current source (Keithley 6220) that drives the offset field coil and defines the rf resonance frequency. Figure 5 shows the time dependence of the statistical uncertainty of the amplitude of the rf magnetometer in the standard configuration (red circles). Here, the rf field is generated by a tunable waveform generator and in spin maser mode (black diamonds). In the former case, drifts in the offset field cause deviations from the approximately √τ dependence (blue dashed line) for integration times greater than 10 ms. The dependence of the statistical uncertainty of the amplitude on the integration time (approximately √τ, marked by the blue dashed line) shows a dominant "white noise" character. The stability of the spin maser oscillation amplitude is limited by the fluctuations of the probe beam power fed into the system via a single-mode polarization-maintaining fiber.

[0129] One of the characteristic properties of spin masers is the "start-up" oscillation, i.e., a transient periodic fluctuation of the maser oscillation amplitude at the beginning of the maser operation

[19] . The amplitude fluctuation is comparable to that observed in systems with high-gain feedback loops approaching a steady state after a sudden change in operating conditions. Two scenarios of an alkali-metal spin maser where transient dynamics can be observed are shown in Figure 6: (a) immediately after the start of the spin maser operation at an offset field of 5.7 μT and (b) in the case of a significant frequency jump. The latter case is caused by the variation of the coil's Q factor with the operating frequency. In particular, Figure 6 shows the transient behavior of the self-oscillating magnetometer signal observed when the magnetic field is changed from 5 kHz to 40 kHz with respect to the Larmor frequency. The measurements were performed with pump / probe beam powers of 500 mW / 150 mW. For frequency changes below 1 kHz, no transient amplitude oscillations are observed. This is the largest shift recorded in these experiments with ferromagnetic materials.

[0130] Spin maser in magnetic induction measurements. The above mentioned alkali metal spin maser is implemented in a tomographic magnetic induction imaging setup. The measurements are performed in an unshielded environment using the same equipment already described in [6-8]. The rf coil generating the primary magnetic field (1000 turns of copper wire with a diameter of 0.2 mm, height 10 mm, inner diameter 2 mm, outer diameter 4 mm) is placed about 3 mm above a 6 mm thick steel plate with an indentation mimicking a structural defect. The rf coil is connected to the output of a balanced photodetector that monitors the polarization of the probe beam with an atomic magnetometer 200 mm above the plate. The measurements are configured in the so-called self-compensating configuration [8]. In this configuration, the axis of the rf primary magnetic field is parallel to the offset magnetic field direction and perpendicular 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 action in the induction experiment is more complex than that reported in the previous part. The signal generated by the spontaneous precession of the atomic spins in the offset magnetic field is amplified and transferred to the primary rf field coil. This excites a response in the plate (secondary magnetic field) with an amplitude and direction defined by its surface structure. Since the atomic magnetometer is only sensitive to rf magnetic field components orthogonal to the axis of the offset magnetic field, the atomic spin precession can only be driven by secondary magnetic field components parallel to the plate surface. These components are nonzero only near the edges of the depression. As a result, there is no spin maser action away from defects, inhomogeneities, or edges, i.e., on the uniform surface of the plate. The condition for positive feedback in spin maser requires that the sum of all phase shifts in the feedback loop is zero

[23] , i.e., phase matching between the atomic spin precession and the drive generated by the secondary magnetic field. The phase of the drive is defined by the direction of the secondary magnetic field. As shown in [8], the change in the direction of the secondary magnetic field covers the full range of possible values ​​(360°) as the primary magnetic field coil scans around the depression. This translates into a vortex in the detected signal phase, i.e., the phase of the drive as seen by the atoms. The spin maser action is triggered only above the edge region of the cavity (i.e. with respect to the direction of the secondary magnetic field), which ensures a phase-matching condition within the atomic sample.The selection of this region can be controlled by introducing a phase shift between the detected signal and the rf coil. Changes in the amplification of the feedback loop affect the secondary magnetic field strength and can therefore be used to distinguish between defects of different sizes and depths. Figure 7 shows a 64 × 64 mm plate containing a 24 mm diameter indentation that is 2.4 mm deep, recorded in (a) standard and (b) spin maser self-compensated mode. 2 Figure 7(b) shows an image of the region of the dimples. In spin maser operation mode, it is guaranteed that the values ​​recorded in the measurement represent the amplitude of the rf spectrum resonances, thus eliminating the need to monitor the entire spectral profile. This leads to a significant reduction in image acquisition time. The typical recording time of the image in Figure 7(a), defined by the acquisition of the rf spectrum at a single location, of about 8 seconds, is 12 hours. It takes about 25 minutes to acquire an image in spin maser mode, with the same measurement conditions (Figure 7(b)). This time is limited by the movement time of the stepping motor. Due to the phase matching condition, only a part of the dimple features shown in Figure 7(a) are displayed in (b). Figure 7(c) shows an image of the dimples recorded with the same rf coil and a phase shift of 180° relative to the case shown in (b). This shows that a set of coils, each with an associated phase shift, can be implemented to recover the complete features of the dimples. Thus, a further embodiment of the invention includes a system as described above, but with two coils with opposite phases to record the image shown in Figure 7(d). Although the entire feature of the depression (i.e. the ring) is not recovered, the measurements prove that it is possible to ensure spin masing at the entire circumference of the depression using a multi-coil system. By autonomously tuning the spin maser system to the rf resonance, images such as those shown in Figure 7(b-d) can be recorded without any degradation in amplitude and contrast, even in the absence of active magnetic field stabilization. This significantly simplifies the instrumentation of the imaging system, as field stabilization electronics such as fluxgate magnetometers and PID units are eliminated.

[0131] The phase dependence of the spin maser signal can be extracted from Fig. 7(b) and (c) since the change in signal phase around the depression is monotonic. As shown in [8], the phase of the secondary magnetic field varies monotonically along the perimeter of the depression. This allows for the calibration of the horizontal axis. Figure 8 shows the change in the amplitude of the signal read along the perimeter of the depression feature in Fig. 7(b). The phase is arbitrarily set to 0° in the center of the image, representing the case where the phase of the drive and the atomic spin oscillations are perfectly aligned.

[0132] In conclusion, we have demonstrated the operation of an alkali metal spin maser in a magnetically shielded open environment. The self-adjusting property of the measurement lends itself to reading the signal amplitude at the rf resonance frequency, independent of the drift of the offset magnetic field. This allows for a significant increase in the image capture rate. We also demonstrated the operation of a multi-rf coil system.

[0133] All details of the specific examples, in particular the operating conditions, parameters and dimensions, may be applied to any embodiment described herein.

[0134] All optional preferred features and modifications of the described embodiments and dependent claims can be used in all aspects of the invention taught herein. Moreover, the individual features of the dependent claims, and all optional preferred features and modifications of the described embodiments, are combinable and interchangeable with each other.

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Claims

1. a variable magnetic field source configured to provide an oscillating primary magnetic field to generate a secondary magnetic field in the sample; an atomic magnetometer for detecting the secondary magnetic field, An atomic sample; a pump and probe subsystem configured to pump the atomic sample to create polarized light and to probe atomic coherence precession within the atomic sample with a probe beam; a detector configured to detect the probe beam and generate a detection signal; configured to drive the variable magnetic field source in response to the detection signal at a frequency tuned to rf resonance; the rf resonance is the frequency of the free atomic coherence precession within the atomic sample; Atomic magnetometer system.

2. The system described in claim 1, wherein the detection signal has an oscillation frequency and is configured to drive the variable magnetic field source at a frequency corresponding to the oscillation frequency of the detection signal.

3. 3. The system of claim 1, wherein the variable magnetic field source is coupled to an output of the detector so as to be driven in response to the detection signal.

4. The system of claim 1 , wherein the variable magnetic field source is configured to be positioned adjacent to a sample.

5. 5. The system of claim 1 , further comprising an instrument processor configured to receive the detection signal and determine its amplitude and phase.

6. 6. The system of claim 1 , further comprising a voltage follower configured to drive the variable magnetic field source with a signal buffered from the detection signal.

7. 7. The system of claim 1, further comprising an amplifier configured to drive the variable magnetic field source with an amplified signal from the detection signal.

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

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

10. 10. The system of claim 1, wherein the variable magnetic field source is an rf coil.

11. 11. The system of claim 10, wherein the variable magnetic field source is driven in response to the detection signal at a frequency tuned to rf resonance by having a voltage signal dependent on the detection signal coupled across the rf coil.

12. The system of claim 1 , wherein the atomic sample comprises a collection of alkali metal atoms.

13. 13. The system of claim 1, wherein the atomic sample comprises an alkali metal atomic vapor.

14. 14. The system of claim 1, comprising a set of variable magnetic field sources configured to provide the oscillating primary magnetic field, configured to drive each of the variable magnetic field sources in response to the detection signals that are phase shifted from one another at a frequency tuned to rf resonance.

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

16. 16. The system of claim 1 , wherein the atomic magnetometer is configured to reduce the influence of components of the oscillating primary magnetic field and the secondary magnetic field in a primary direction substantially perpendicular to a surface of the sample on the atomic magnetometer.

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

18. 1. A method of operating an atomic magnetometer to detect a secondary magnetic field produced by a sample, comprising: pumping an atomic sample to create polarized light and using a probe beam to probe atomic coherence precession within said atomic sample; detecting the probe beam to generate a detection signal; driving a variable magnetic field source in response to the detection signal to provide an oscillating primary magnetic field to cause the sample to generate the secondary magnetic field at a frequency tuned to rf resonance; the rf resonance is the frequency of the free atomic coherence precession within the atomic sample; method.

19. 20. The method of claim 18, wherein driving a variable magnetic field source in response to the detection signal comprises driving the variable magnetic field source with a signal that is buffered and / or amplified and / or phase shifted from the detection signal.

20. 20. The method of claim 18 or 19, wherein driving a variable magnetic field source in response to the detection signal comprises driving the variable magnetic field source at a frequency corresponding to a frequency of the detection signal.

21. 21. The method of any one of claims 18 to 20, comprising placing the atomic sample in a magnetic bias field.

22. 22. The method of any one of claims 18 to 21, comprising determining an amplitude and a phase of the detection signal for detecting the secondary magnetic field.

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