WO2021155428A1

WO2021155428A1 - Magnetic field sensor

Info

Publication number: WO2021155428A1
Application number: PCT/AU2021/050079
Authority: WO
Inventors: Xiaoke Yi; Liwei Li; Jianfu Wang
Original assignee: The University Of Sydney
Priority date: 2020-02-03
Filing date: 2021-02-03
Publication date: 2021-08-12

Abstract

A magnetic field sensor for magnetic field imaging, the magnetic field sensor comprising: a plurality of optical waveguide channels for receiving an optical signal; a plurality of cells arranged in an array, wherein the cells are in optical communication with the optical waveguide channels, wherein multiple cells in the plurality of cells are micro-resonator cells, where a micro-resonator cell comprises: at least one micro-resonator comprising a cell substrate, a cell optical waveguide on the cell substrate, and a magnetostrictive layer on the cell optical waveguide, wherein the micro-resonator cell is arranged to change an optical spectrum of the optical signal when the optical signal is transmitted along the optical waveguide channels, wherein the change in the optical spectrum is based on i) a strain exhibited by the magnetostrictive layer in the local magnetic field and ii) the effect of the strain on the cell optical waveguide.

Description

MAGNETIC FIELD SENSOR

Reference to Related Patent Application(s)

[0001] This application claims the benefit under Convention priority of the filing date of Australian Patent Application No. 2020900287, filed 03 February 2020, hereby incorporated by reference in its entirety as if fully set forth herein.

Technical Field

[0002] The present invention relates generally to a magnetic field sensor for magnetic field imaging.

Background

[0003] Magnetic field sensing and imaging is a powerful, important and non-invasive process and tool that has been the subject of intense research efforts due to the large number of technical applications in a wide range of areas, including, for example, healthcare, nuclear magnetic resonance imaging, magnetoencephalography, magnetic imaging, biochemical analysis, medical diagnosis, inertial measurement units, navigation systems, electrical power systems, aerospace, computer memories, defence, UVA sensing and so on. Many of these applications require highly sensitive magnetic field readings, in a compact size and with reduced susceptibility to electromagnetic interference.

[0004] Many conventional magnetic field sensors either suffer from limited sensitivity, or are difficult to expand to a high-density sensing array used for magnetic imaging. Magnetic field sensors based on magnetostriction have been built using microcantilevers, and piezoelectric elements, for which the sensitivities only reach the micro-Tesla range. Optical fibre magnetometers are based on the measurement of small changes in the optical path length from strain induced by magnetostrictive materials attached to the fibre, which only achieve sensitivities in the range of several hundred mTL/Hz. A microfabricated optical magnetometer based on epoxy bonding magnetostrictive material on to a silicon microtoroid has been demonstrated to achieve sensitivity in pico-Tesla range, however, it presents difficulty of scaling up the technique into a high-density sensor array, as the light source is evanescently coupled into the toroidal sensor via a tapered optical fiber that significantly limits the array size. [0005] Also in current systems, magnetometry cryogenically cooled superconducting quantum interference devices (SQUID) are used, which can measure magnetic fields with sensitivities in the femto-Tesla to pico-Tesla range. However, the SQUID system has a number of significant disadvantages, such as the need for a cryogenic cooling system to maintain low cryogenic temperatures, its large size and expense.

[0006] Therefore, there is a desire to achieve a compact, highly-sensitive, low-noise, cost- effective magnetic field sensor that can operate at room temperatures without the need for a cryogenic cooling system, as well as a desire for the sensor to be capable of mass production, and capable for expansion to a sensing configuration for high-density microarrays for magnetic imaging applications.

Summary

[0007] It is an object of the present invention to meet these desires or to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.

[0008] Disclosed are arrangements which seek to address the above problems using optical magnetometers that are intrinsically immune to electromagnetic interference, are light weight, have low power consumption and are generally made from materials that are magnetic insulators.

[0009] According to a first aspect of the present disclosure, there is provided a magnetic field sensor for magnetic field imaging, the magnetic field sensor comprising: a plurality of optical waveguide channels for receiving an optical signal; a plurality of cells arranged in an array, wherein the cells are in optical communication with the optical waveguide channels, wherein multiple cells in the plurality of cells are micro-resonator cells, where a micro-resonator cell comprises: at least one micro-resonator comprising a cell substrate, a cell optical waveguide on the cell substrate, and a magnetostrictive layer on the cell optical waveguide, wherein the micro resonator cell is arranged to change an optical spectrum of the optical signal when the optical signal is transmitted along the optical waveguide channels, wherein the change in the optical spectrum is based on i) a strain exhibited by the magnetostrictive layer in the local magnetic field and ii) the effect of the strain on the cell optical waveguide.

[0010] The array may comprise i) a plurality of rows of the plurality of cells, ii) a plurality of columns of the plurality of cells, or iii) a plurality of rows of the plurality of cells and a plurality of columns of the plurality of cells. [0011] The magnetostrictive layer may be formed from a material consisting of at least one of: Terfenol-D, nickel, iron, cobalt, Galfenol (iron and gallium), Ferrite (doped crystalline iron).

[0012] The cell substrate may be formed from a silicon dioxide layer that forms at least a portion of an SOI platform, the cell optical waveguide may comprise a silicon layer cantilevered above the silicon dioxide layer, and the magnetostrictive layer may be coated on the silicon layer.

[0013] The magnetostrictive layer may be coated on the silicon layer to partially cover the silicon layer.

[0014] The cell optical waveguide may be one of a microdisk resonator and a racetrack resonator.

[0015] The magnetic field sensor may further comprise at least one input optical coupler for receiving the optical signal; and a plurality of output optical couplers; wherein the plurality of optical waveguide channels is in optical communication with the input optical coupler and the output optical couplers.

[0016] The magnetic field sensor may further comprise a silicon substrate upon which the plurality of optical waveguide channels and the array of the plurality of cells are arranged.

[0017] The input optical coupler and the output optical couplers may be arranged upon the silicon dioxide layer, which is formed on the silicon substrate.

[0018] In the plurality of cells, at least one directional micro-resonator cell may be formed, wherein the directional micro-resonator cell may comprise a first micro-resonator that may comprise a first cell substrate, a first cell optical waveguide on the first cell substrate, a first cantilevered beam in optical communication with the first cell optical waveguide, and a first magnetostrictive layer on the first cantilevered beam, wherein the first cell optical waveguide may be in optical communication with a first optical waveguide channel that is in optical communication with the optical waveguide channel. The directional micro-resonator cell may comprise a second micro-resonator that may comprise a second cell substrate, a second cell optical waveguide on the second cell substrate, a second cantilevered beam, and a second magnetostrictive layer on the second cantilevered beam. The second cell optical waveguide may be in optical communication with a second optical waveguide channel that is in optical communication with the optical waveguide channel. The first cantilevered beam and the second cantilevered beam may be arranged perpendicular to each other. The directional micro- resonator cell may be arranged to change the optical spectrum of the optical signal on each of the first optical waveguide channel and the second optical waveguide channel when the optical signal is transmitted along the optical waveguide channel. The change in the optical spectrum may be for detecting the direction of the local magnetic field based on i) a strain exhibited by the first magnetostrictive layer and the second magnetostrictive layer in the local magnetic field and ii) the effect of the strain on the first cell optical waveguide and the second cell optical waveguide.

[0019] The first cell substrate and the second cell substrate may be formed from the cell substrate.

[0020] The magnetic field sensor may further comprise a plurality of photo detectors arranged in an array corresponding with the array of the plurality of cells, wherein each photo detector may be arranged to optically couple to each output of the optical waveguide channels, and wherein each photo detector may provide an output to a microprocessor.

[0021] A magnetic field sensor system may comprise the previously defined magnetic field sensor. The system may have a fixed wavelength optical source that may be arranged to couple the optical signal to the input optical coupler. The system may have a plurality of photo detectors where each photo detector in the plurality of photo detectors is arranged to couple to each output optical coupler of the output optical couplers. The system may have a plurality of transimpedance amplifiers arranged to amplify optical signals received by the photo detectors. The system may have a microcontroller arranged to i) control the fixed wavelength optical source, ii) develop a magnetic field image from the optical signals amplified by the transimpedance amplifiers.

[0022] The magnetic field sensor system may further comprise a frequency stabiliser comprising an add-drop microring.

[0023] A method of manufacturing the magnetic field sensor as described is also provided. [0024] Other aspects are also disclosed.

Brief Description of the Drawings

[0025] At least one embodiment of the present invention will now be described with reference to the drawings, in which: [0026] Figs. 1A-1C show a magnetic field sensor system according to the present disclosure;

[0027] Figs. 2A- 2B show a schematic of a microdisk resonator as a cross section and top view according to the present disclosure;

[0028] Fig. 2C shows a schematic of an all-pass microring according to the present disclosure;

[0029] Figs. 3A - 3B show different forms of a micro-resonator as a microdisk and a racetrack microdisk according to the present disclosure;

[0030] Fig. 4A shows a micro-resonator shape response of a cell according to the present disclosure;

[0031] Fig. 4B shows an optical spectrum response of a cell according to the present disclosure;

[0032] Fig. 5 shows a cell with two micro-resonators arranged to detect orientation of magnetic field according to the present disclosure;

[0033] Fig. 6 shows micro-resonators in a schematic for magnetic field direction decomposition according to the present disclosure;

[0034] Fig. 7 shows a magnetic field sensor system according to the present disclosure;

[0035] Fig. 8 shows a diagram indicating laser frequency drift according to the present disclosure;

[0036] Fig. 9 shows another magnetic field sensor system according to the present disclosure;

[0037] Figs. 10-10C show examples of micro-resonators according to the present disclosure; and

[0038] Fig. 10D shows an example of optical transmission spectrum for a micro-resonator structure according to the present disclosure.

Detailed Description including Best Mode

[0039] Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears.

[0040] Figures 1A - 1C show a schematic of an embodiment of a magnetic field sensor system in the form of a passive-only magnetic field pixel sensing probe, which may eliminate interference problems caused by the surrounding local environment.

[0041] The magnetic field sensor system 101 has at least one magnetic field sensor 103 and a control system 105.

[0042] This example magnetic field sensor 103 is arranged, positioned or formed on a substrate 107, such as a silicon substrate for a SOI (silicon on insulator) platform. Other suitable integration platforms could also be used, such as, for example, silicon nitride, silicon carbide, and lithium niobate.

[0043] In one example, arranged, positioned or formed on the substrate 107 may be at least one input optical coupler 109 for receiving and coupling an optical signal from an external optical source. For example, the input optical coupler may be a grating coupler for vertical coupling, a taper or inverse taper for butt-coupling.

[0044] In one example, also arranged, positioned or formed on the substrate 107 may be multiple output optical couplers 111 for outputting and coupling optical signals received from cells (as discussed below) in the magnetic field sensor 103. For example, the output optical couplers may be a grating coupler for vertical coupling, a taper or inverse taper for butt coupling.

[0045] Formed on the substrate 107 are multiple optical waveguide channels that are in optical communication with the input optical coupler 109 and the output optical couplers 111. The optical waveguide channels provide an optical medium to enable the optical signals to travel along the optical waveguide channels from the input optical coupler 109 to the output optical couplers 111 via the cells (as discussed below). In this example, the waveguide channels are arranged, positioned or formed in rows 113 and columns 115, where the number of rows and the number of columns are equal.

[0046] It will be understood that, as an alternative, the number of rows may be different to the number of columns. It will also be understood that, as an alternative, the waveguide channels may be arranged, positioned or formed only in rows 113 (e.g. in one column), only in columns 115 (e.g. in one row), or with any number of rows 113 and any number of columns 115.

[0047] Further, in this example, each of the rows and columns are arranged, positioned or formed as waveguides that are straight and parallel to other waveguides in the rows or columns. Also, in this example, each of the rows of optical waveguide channels are arranged, positioned or formed to be equally spaced apart. Further, in this example, each of the columns of optical waveguide channels are arranged, positioned or formed to be equally spaced apart.

[0048] It will be understood that, as an alternative, topographic arrangements of the rows and columns may be different to the above example, such that the waveguides of the rows and/or columns are curved, partially curved, and/or unevenly spaced apart.

[0049] As an alternative, it will be understood that the separate input optical and separate output optical couplers may not be required if, for example, the optical waveguide channels 115 are optically coupled directly with a connected silicon waveguide located on the control system 105. That is, the output of the control system 105 may be directly optically coupled to the input of the optical waveguide channels 115 via a silicon waveguide (in optical communication with a frequency stabiliser) formed on the control system 105. Further, the output of the optical waveguide channels 115 may be directly optically coupled to a silicon waveguide (in optical communication with high-speed photo detectors/diodes) formed on the control system 105.

[0050] It will be understood that the output structure may depend on the type of photodiode used. For example, free space transmission may occur between the silicon waveguide and the photodiode, and the optical coupler may be a vertical grating coupler (VGC). The VGC may also be made of silicon but include different thickness of “teeth”. The VGC could also guide the light from the silicon waveguide to air. If an integrated photodiode is used, the material responsible for the opto-electro effect may be directly coated on the surface of the silicon waveguide, which would then result in no requirement for the coupling between the silicon waveguide and the photodiode.

[0051] The multiple cells 117 are arranged, positioned or formed on the substrate 107 to be in optical communication with the optical waveguide channels. That is, in this example, multiple cells are arranged, positioned or formed along the rows and multiple cells are arranged along the columns. The multiples cells in this example form an array of cells, where the array has multiple rows and multiple columns. [0052] It will be understood that the cells 117 may be arranged, positioned or formed along only rows 113, along only columns 115, or along any number of rows 113 and/or any number of columns 115. Further, it will be understood that the term “array” in this document may mean multiple cells positioned or formed in a row (for example 20 cells positioned in a row forming a 20 * 1 array), multiple cells positioned or formed in a column (for example, 20 cells positioned in a column forming a 1 * 20 array), or multiple cells positioned in rows and columns (for example, 20 cells positioned or formed in each column and 20 cells positioned in each row forming a 20 * 20 array). It will be understood that any number of cells other than 20 (e.g. 10, 30, 50, 100 etc.) may be used in the rows and/or columns to form the array. It will also be understood that the number of cells positioned in the columns may be different to, or the same as, the number of cells positioned in the rows.

[0053] In the example shown in Fig. 1A, the cells 117 are arranged, positioned or formed to create a high-density photonic waveguide based two-dimensional micro-resonator sensor array.

[0054] In this example, the control system 105 includes a microcontroller (or microprocessor) 121, a fixed wavelength optical (e.g. laser) source 123, an optical source current driver 125, an optical source temperature controller 127, a laser frequency stabilizer unit to improve sensing stability 129, multiple high-speed photo detectors/diodes 131 with corresponding transimpedance amplifiers 133 that provide a transducer-array for optically detecting the light intensity from the optical signal received from the magnetic field sensor 103 via the output optical couplers 111.

[0055] Fig. 1B shows a detailed schematic of a micro-resonator cell 117 having a microresonator 119 arranged, positioned or formed to be in optical communication with the corresponding optical waveguide 115 associated with the cell 117, where the waveguide 115 is arranged, positioned or formed in the relevant row and/or column of the array of cells associated with the microresonator 119. There are multiple micro-resonator cells in the array of cells. In one example, all of the cells in the array are micro-resonator cells. That is, each cell in the array of cells is a micro-resonator cell.

[0056] Fig. 1C shows a cross-section schematic of the microresonator 119, which, in this example, has a cell substrate 135 formed from a silicon dioxide layer, a cell optical waveguide 137 formed from a silicon layer, where the silicon layer is formed so it is cantilevered above the silicon dioxide layer, and a magnetostrictive layer 139 formed, e.g. coated, on the cell optical waveguide (silicon layer) 137. The cell substrate 135 is formed on top of a silicon substrate/wafer (not shown). The cells are therefore formed as part of an SOI (Silicon on Insulator) platform.

[0057] Any suitable magnetostrictive material may be used, such as Terfenol-D for example. Terfenol-D is a unique magnetostrictive material that exhibits strains of the order of 2000x1 O ⁶ even at room temperature. This material is selectively coated on top of an optical integrated silicon-on-insulator (SOI) micro-resonator structure, which exhibits strains when the local magnetic field is applied to the microdisk and so changes the shape of the microdisk. The change in shape of the microdisk results in an optical frequency spectrum shift of the optical signal being transmitted on the adjacent optical waveguide channel 117.

[0058] Selectively coating refers to the selective region on which the coating is made. The suitable magnetostrictive material is only coated on a selective area of the waveguide which then leads to a significant change in the performance, such as tuning range and sensitivity. For example, the Terfenol-D material may be coated, for example, only on top of the microdisk, or only on top of the coupling region between the microdisk and the bus waveguide.

[0059] Other examples of magnetostrictive materials that may be used include materials that demonstrate magnetostrictive properties, such as nickel, iron, cobalt or several alloys that have been developed to maximise the magnetostrictive coefficient, such as Galfenol (iron and gallium), Terfenol-D (terbium, dysprosium and iron), or Ferrite (doped crystalline iron).

[0060] The magnetic field sensing array as described dramatically improves the signal-to-noise ratio and the measurement accuracy of the magnetic field sensor system as explained in more detail below.

[0061] Figs. 2A and 2B show an example of a microdisk structure as a cross-section and a top view respectively. The microdisk structure in this example is produced on a silicon-on-insulator (SOI) platform. It will be understood that other suitable manufacturing process may be used as an alternative.

[0062] It will be understood that other suitable manufacturing processes may be used other than the SOI platform. For example, another platform may be SiN (Silicon-Nitride), SiC (Silicon- Carbide), lithium niobate or polymer waveguides

[0063] In Fig. 2A a cross-section schematic of the microdisk is shown. On a silicon (Si) substrate 107 is formed a silicon dioxide (S1O2) layer 135. Formed from silicon (Si) on the silicon dioxide layer 135 is the microdisk 137. An etch process is used after the microdisk has been deposited (or formed) to form or undercut an etched region 136 of silicon dioxide to reveal the underlying substrate 107. This etch process causes the microdisk 137 to overhang, or cantilever above, the etched region 136. A Terfenol-D magnetostrictive layer 139 formed, e.g. coated, on the cell optical waveguide (silicon layer) 137. An optical waveguide 115 is also arranged, positioned or formed at the appropriate time on the silicon dioxide layer 135 in optical proximity to the cell optical waveguide 137.

[0064] An example process could be as follows using a SOI platform, which is a wafer formed from three layers (top to bottom) of 220nm Si (silicon), 2pm of S1O2 (silicon dioxide) on a Si (silicon) substrate. A first step of forming a microdisk using dry etching, a second step of depositing the magnetostrictive material, and a third step of forming the undercut region using wet etching based on HF (hydrogen fluoride) to undercut the S1O2 (silicon dioxide). It will be understood that at each step a corresponding photoresist coating may be required to protect areas not being etched.

[0065] In this example, the highly magnetostrictive material (Terfenol-D) 139 is coated on the partially under-cut microdisk structure at the centre of the microdisk 137 forming a round section of material. This arrangement provides high-sensitivity.

[0066] In more detail, the magnetic field sensor may be fabricated on a silicon-on-insulator (SOI) wafer using electron-beam lithography and reactive ion etching to firstly form the waveguide and sensing micro-resonators (such as microdisks) that provide a big contact area for magnetostrictive materials. To enable mass production, magnetostrictive material (such as Terfenol-D) will be coated onto each micro-resonator, acting as ultra-sensitive room temperature magnetometer cell with broad bandwidth. The outer silicon undercut can be achieved with a combination of wet chemical etching and dry etching, where the free-hanging structure will maximise the shape change of the micro-resonator, and make the system ultra sensitive where the Q-factor is compatible with microtoroid. Additionally, the free-hanging region can be altered depending on the needs of various applications. Consequently, the optical power from each micro-resonator sensing pixel cell is recorded simultaneously by a high-speed PDs array with transimpedance amplifiers (TIA).

[0067] The microdisk resonators described herein that are formed in cells as pixels can be cascaded to form a magnetic field sensing pixel array for magnetic field sensing with high sensitivity and accuracy. Assuming that every pixel in the array is identical in form, as the signals are summed coherently, while the noise is added together incoherently, the signal-to- noise ratio (SNR) dramatically improves by the square root of the number of array elements, as given by Eq. (1).

[0068] where M is the number of identical elements (e.g. there are M=64 elements in an 8x8 array), S and N are the signal and noise for each pixel respectively.

[0069] The intensity transmission spectrum of an all-pass ring resonator (Fig. 2C) can be obtained by:

[0070] where <f> = bå is the single-pass phase shift, with L the roundtrip length and /?the propagation constant a is the single-pass amplitude transmission, including both propagation loss in the ring and loss in the couplers. It relates to the power attenuation coefficient a [1/cm] as

2 a = exp(-a ) . r is the self-coupling coefficient. Similarly, k can be defined as the cross-coupling 9 9 coefficients, and so and Yr are the power splitting ratios of the coupler, and they are assumed to satisfy r² + k² = 1.

[0071] It will be understood that the roundtrip length mentioned above corresponds to the circumference of the ring waveguide in the example shown in Fig. 2C. However, it will be further understood that this may vary as the shape of the microdisk changes. The microring shown in Fig. 2C is an all-pass microring.

[0072] The micro-resonators can be produced in various forms, such as, for example, a round microdisk 301 A as shown in Fig. 3A or a racetrack shaped microdisk 301 B as shown in Fig. 3B. For the sake of brevity, the overhang feature is not shown in these images. The racetrack shape is effectively an oval with two straight opposing sides and two opposing curved (e.g. half circle) sides. The length of the two straight opposing sides are the same but may be of any suitable length. The angle of curvature of the two opposing curved sides are the same. [0073] When the magnetic field is applied to the microdisk, the field causes the magnetostrictive material dimensions to change, which results in deformation of the microdisk. A graph 401A showing an example of deformation in a microdisk 405 (corresponding to the microdisk 137 for example) is shown in Fig. 4A. The change in shape of the microdisk results in an optical frequency spectrum shift as shown in graph 401 B of Fig. 4B. This optical frequency spectrum shift is dependent on the magnetostrictive effect. This optical frequency spectrum shift can be detected using an optical edge filter, such as a single microdisk resonator, in combination with an optical laser. The optical edge filter maps the optical frequency change to optical power variations that can be detected using optical photodetectors or photodiode 131.

[0074] According to one example of use, the optical signal is generated from a fixed wavelength laser source 123, and launched to a laser frequency stabilizer block 129, which has an optical source temperature controller in the form of an add-drop microring to compensate the effects of disturbance in the ambient temperature. Any other suitable temperature controllers may also be used. An add-drop microring is formed from two parallel bus waveguides that are located opposite to and on either side of the ring in a symmetrical manner (as indicated in item 129 of Fig. 1A).

[0075] A feedback signal is provided from the drop-port power output of the laser frequency stabilizer for detection by a high-speed photo-detector, or photodiode 131, where the detected power (feedback signal) is used to compensate for laser frequency fluctuations. A micro controller 121 controls the laser current driver, laser temperature controller (TEC) and other monitoring circuits.

[0076] The optical signal output from the through-port of the laser frequency stabilizer circulates along the packaged sensing window that has the two-dimensional magnetic field sensing pixel array. The architecture of the pixels array includes a main bus waveguide, which evanescently couples the light into each waveguide row and then the columns to maximize the sensing performance.

[0077] The optical power is evenly coupled to every micro-pixel or cell 117, through precisely designing the coupling-coefficient of each coupler, which is varied by controlling the gap between the two bus waveguides of the directional coupler Lg while ensuring the coupler length Lc is unchanged. Alternatively, precise control of the coupling-coefficient can be achieved by varying the coupler length Lc while maintaining the gap between the two bus waveguides of the directional coupler Lg. [0078] As a further example, individual magnetic field cells can be adapted to form a “sub-array” structure in the form of a directional micro-resonator cell in which there are multiple (i.e. two or more) micro-resonators, where each micro-resonator is orientated in a different direction. The “sub-array” structure provides the capability of determining the direction or orientation of the local magnetic field. Fig. 5 shows an example of a directional micro-resonator cell that has two micro-resonators (119A & 119 B) , where each micro-resonator has a cantilevered beam with a magnetostrictive material coating. That is, in this particular directional micro-resonator cell structure, the microdisk does not have a magnetostrictive coating, only the cantilevered beams have the magnetostrictive coating (503A, 503B). Further, the coupling region between the beam and microdisk is undercut. The first micro-resonator is located along the X-axis plane and the second micro-resonator is located along the Y-axis plane such that the first and second resonators are substantially orthogonal, or substantially at right angles, to each other.

Therefore, the magnetostrictive coating (e.g. Terfenol-D) changes the position of the cantilevered beam, which affects the optical signal being transmitted on the optical waveguide channels (115A, 115B) in a relative manner when the cantilevered beam is under the influence of the magnetic field, and so shifts the optical spectrum of the signals on those channels in a relative manner. The direction or orientation of the magnetic field may be measured based on the response to different sub-vectors measured in relation to the different cantilevered beam directions. The dots ( ... ) in Fig.5 represent that there may be more micro-resonators positioned in between the two micro-resonators shown.

[0079] Therefore, in this example, one or more of the cells in the array of cells can have a directional micro-resonator cell as described above. Each of one or more of the directional micro-resonator cells has a first micro-resonator that has a first cell substrate, a first cell optical waveguide (507A) on the first cell substrate, a first cantilevered beam (505A) in optical communication with the first cell optical waveguide (507A), and a first magnetostrictive layer (503A) on the first cantilevered beam (505A). In this example, the cantilevered beam (505A) is a T-shape beam, where the “T” junction is arranged to be in close proximity and in optical communication with the first cell optical waveguide (507A).

[0080] The first cell optical waveguide (507A) is in optical communication with a first optical waveguide channel (115A) that is in optical communication with the optical waveguide channel (115). The distance between the first cell optical waveguide (507A) and the first optical waveguide channel (115A) is a fixed distance and does not change under the influence of the local magnetic field. [0081] A second micro-resonator has a second cell substrate, a second cell optical waveguide (507B) on the second cell substrate, a second cantilevered beam (505B), and a second magnetostrictive layer (503B) on the second cantilevered beam (505B). The second cell optical waveguide (507B) is in optical communication with a second optical waveguide channel (115B) that is in optical communication with the optical waveguide channel (115). The distance between the second cell optical waveguide (507B) and the second optical waveguide channel (115B) is a fixed distance and does not change under the influence of the local magnetic field.

[0082] The first cantilevered beam (505A) and the second cantilevered beam (505B) are arranged perpendicular to each other in this example directional micro-resonator cell. It will be understood that with the two micro-resonator embodiment, as an alternative the first cantilevered beam may be arranged relative to the second cantilevered beam at an angle that is offset from exactly 90 degrees and still produce a similar effect of detecting the direction of the local magnetic field.

[0083] The directional micro-resonator cell is arranged to change the optical spectrum of the optical signal on each of the first optical waveguide channel (115A) and the second optical waveguide channel (115B) when the optical signal is transmitted along the optical waveguide channel (115).

[0084] The relative change in the optical spectrum on each of the first and second optical waveguide channels (115A, 115B) enables detection of the direction of the local magnetic field based on i) a strain exhibited by the first magnetostrictive layer and the second magnetostrictive layer in the local magnetic field and ii) the effect of the strain on the first cell optical waveguide and the second cell optical waveguide.

[0085] The movement of the cantilevered beams modifies the effective refractive index of the associated cell optical waveguide. This introduces phase-shifting to the light traveling through the cell optical waveguide and then varies the resonance of the micro-resonator.

[0086] Although the phrase “first cell substrate” and “second cell substrate” has been used to describe the substrates in the first and second micro-resonators, it will be understood that these cell substrates may be part of the same substrate, or be part of different substrates.

[0087] It will be understood that the more micro-resonators provided at various directions, the more accurate the system may be for detecting the direction or orientation of the magnetic field. [0088] Further, the combination of the directional micro-resonator cells interspersed in an array having micro-resonator cells as described above with reference to Figs 1 to 4 provides a system that measures both intensity of a magnetic field as well as a directional component or orientation of the magnetic field. It will be understood that there may be multiple directional micro-resonator cells in the array at different positions, where the directional micro-resonator cells are in place of the micro-resonator cells.

[0089] Fig. 6 shows an example of a “sub-array” structure in the form of a directional micro resonator cell in which there are multiple (i.e. three or more) micro-resonators. That is, provided are a micro-resonator 1 (119A) and a micro-resonator 2 (119B) that measure the components of the magnetic field along XOZ and YOZ planes, with one more micro-resonators, such as micro resonator 3 (119C), provided in between to determine the magnetic field incident angle a on XOY plane. Therefore, with measured magnitudes Vxoz, Vyoz and Vxoy and angle a, the vertical angle Q to the z-axis in the space can be obtained, hence the direction or orientation of the magnetic field in the space can be determined. It will be understood that the cell optical waveguide of micro-resonator 3 is in optical communication with the second optical waveguide channel. It will be understood that the angle of Resonator 3 (and indeed other resonators) can be different in different cells.

[0090] Fig. 7 shows an alternative example of a magnetic field sensor system. This system is an all-in-one active photonic fully integrated magnetic field sensing pixel circuit. The Si-based photonic integrated circuit is produced using a complementary metal-oxide-semiconductor (CMOS) fabrication process, which holds significant advantages of a small footprint, low loss and great potential for seamless integration with microelectronics.

[0091] The magnetic field sensor system 701 has a laser source 703 (such as InP/Si hybrid (distributed feedback) DFB laser) with a frequency stabilization unit 704, a high-density photonic waveguide-based magnetic field sensing cell (pixel) array formed from columns 705 and rows 707, and an array of high-speed photodiodes 709 on one single chip.

[0092] Similar to the magnetic field sensor system 101 shown in Fig. 1A-1C, the optical power is evenly coupled into each waveguide row 707 and then the columns 705 via the bus waveguides that are directed towards each micro-resonator cell. Again, in this example, the high magnetostrictive material is coated at the centre of the microdisks and the outer region of silicon cell waveguide is undercut to maximise the sensing effect. [0093] An example of a packaged board design 701 shows that the photonic chip is located and packaged onto an interposer PCB board 711. Each electrical bond pad (not shown) on the photonics chip is wire-banded to the surface mounted electrical microcontroller terminal for system control and data-acquisition. A terminal 713 is provided for connecting the PCB board 711 to an external control system. Therefore, a new optical magnetic sensing pixel array structure is provided that has a simple and compact configuration to enable the realisation of high performance on-chip sensing with reduced size, light weight, low cost, low noise, low power consumption, and scalability to large-scale integration.

[0094] Embodiments described herein may optionally incorporate the feature of wavelength drifting insensitivity compensation. As shown in the graph 801 in Fig. 8 the working principle of a laser frequency drifting compensation unit is shown. The micro-resonator acts an edge filter shown as the solid line 803. For example, if the laser frequency/wavelength drifts within about 100pm shown as the strip region 805, the optical power of the light passing through the edge filter will also be changed as indicated by the power difference range 807. This power difference can be detected via an optical photodetector where the variation of the power reflects the wavelength change or drift. The signal from the optical photodetector can then be used to calibrate the laser drifting by controlling or recalibrating the frequency output by the laser.

[0095] Environmental disturbances, such as temperature drift, vibration and noise could introduce spurious changes in the measurements of magnetic field sensors and sensing systems as described herein. To mitigate these disturbances, a differential measurement approach can be used which includes one or more additional reference resonators to track the undesired drift caused by environmental disturbance.

[0096] Figure 9 shows an example of a magnetic field sensing cell (pixel) array 901 having reference resonators. In this example, the reference resonators (907A to 907H) are placed in each column of the array. The bottom row of the array includes reference resonators, which are the same or are similar in structure as the micro-resonator cells described with reference to Figures 1 to 3 above. However, a key difference is that the reference resonators do not have a magnetostrictive material coating on the cell optical waveguide. Therefore, the reference resonators are insensitive to the local magnetic field changes and can be used to detect variations induced by environmental disturbances. These measurements can then be used by the controller to negate any environmental differences from the measurements made by the micro-resonator cells. That is, the final magnetic field sensing measurement is the difference between the micro-resonator cells and the reference resonators, or the reference resonator signal is subtracted from the micro-resonator cells. [0097] The array 901 could form part of the magnetic field sensor system 701 as described above with reference to Figure 7. That is, it may include a laser source 909 (such as InP/Si hybrid (distributed feedback) DFB laser) with a frequency stabilization unit 911, a high-density photonic waveguide-based magnetic field sensing cell (pixel) array formed from columns 903 and rows 905, and an array of high-speed photodiodes 913 on one single chip.

[0098] Similar to the magnetic field sensor system 101 shown in Fig. 1A-1C, the optical power is evenly coupled into each waveguide row 905 and then the columns 903 via the bus waveguides that are directed towards each micro-resonator cell. Again, in this example, the high magnetostrictive material is coated at the centre of the micro-resonator cells 915 (but not the reference cells (907A to 907H)).

[0099] This system may be used in the packaged board design as described above with reference to Figure 7.

[00100] The measurement range of the micro-resonators based magnetic field sensing is restricted to one entire optical spectrum free spectral range (FSR), due to the periodic nature of the optical spectrum of micro-resonators. In order to enlarge the measurement range, arrangements using embedded structures can be employed to suppress unwanted adjacent modes, examples of which are provided in figures 10A to 10C. Using embedded structures in the resonator cavity can provide an optical filtering effect, filtering out unwanted components in frequency spectrum to reduce the FSR limitation. The embedded structures can be holes or gratings for example. Figure 10A shows an example of holes 1005 embedded in a microdisk of a micro-resonator 1010. The holes 1005 are non-uniform photonic crystal holes on the ring waveguide. Figure 10B shows an example of gratings 1015 in a microdisk of a micro-resonator 1020. The gratings 1015 are uniform tooth-shape Bragg gratings at an inner sidewall of the micro-resonator 1020. Figure 10C shows an example of gratings 1025 in a microdisk of a micro-resonator 1030. The gratings 1025 are uniform semi-circle Bragg gratings at an inner sidewall of the micro-resonator 1030.

[00101] Figure 10D shows a graph 1050. The graph 1050 shows a simulated optical transmission spectrum for the micro-resonator structures with an ultra-large measurement range. The graph 1050 shows that a difference between a dominant mode (notch 1055) and side-modes (for example notch 1060) is increased by over 25 dB via use of gratings such as the gratings shown in Figures 10B and 10C. The suppression ratio and dominant mode wavelength position can be flexibly altered via changing the parameters of the embedded structure such as the semi-circle radius or tooth size. Changing the parameters of the embedded structure can provide higher sensing performance in terms of resolution and range due to the increased FSR and enhanced extinction ratio.

[00102] It will be understood that alternative arrangements of reference resonator(s) to that shown in Figure 9 are envisaged. For example, the array may have one or more reference resonators in the array. Each of the one or more resonators may be positioned, placed or located at any of the pixels or cells of the array in any desired pattern or position. In Figure 9, a single reference resonator is positioned, placed or located in each column of the array to form a row of reference resonators. As an alternative, a single reference resonator may be positioned, placed or located in each row of the array to form a column of reference resonators. The reference resonator(s) may be positioned, placed or located in different rows and different columns throughout the array.

[00103] It will be understood that the microcontroller 121 shown in the control system 105 may be part of a computer system, upon which the various control arrangements described can be practiced to make the computer system application specific to control the magnetic field sensor systems and other components accordingly.

[00104] The computer system may include: a computer module; input devices such as a keyboard, a touch screen, a mouse pointer device, and output devices including a printer, a display device and loudspeakers. An external Modulator-Demodulator (Modem) transceiver device may be used by the computer module for communicating to and from a communications network via a connection. The communications network may be a wide-area network (WAN), such as the Internet, a cellular telecommunications network, or a private WAN. A wireless modem may also be used for wireless connection to the communications network.

[00105] The computer module typically includes at least one processor unit, and a memory unit. For example, the memory unit may have semiconductor random access memory (RAM) and semiconductor read only memory (ROM). The computer module also includes a number of input/output (I/O) interfaces for coupling to the input and output devices.

[00106] Storage devices are provided and typically include a hard disk drive (HDD). Other storage devices such as a floppy disk drive and a magnetic tape drive (not illustrated) may also be used. An optical disk drive is typically provided to act as a non-volatile source of data. Portable memory devices, such optical disks (e.g., CD-ROM, DVD, Blu-ray Disc™), USB-RAM, portable, external hard drives, and floppy disks, for example, may be used as appropriate sources of data to the system. [00107] The method of control of the magnetic field sensors described may be implemented using the computer system, wherein the control processes described, may be implemented as one or more software application programs executable within the computer system.

[00108] The software may be stored in any suitable computer readable medium. The software may be loaded into the computer system from the computer readable medium, and then executed by the computer system. A computer readable medium having such software or computer program recorded on the computer readable medium is a computer program product. The use of the computer program product in the computer system preferably effects an advantageous apparatus for controlling the magnetic field sensors.

[00109] In some instances, the application programs may be supplied to the user encoded on one or more CD-ROMs and read via the corresponding drive, or alternatively may be read by the user from the networks. Still further, the software can also be loaded into the computer system from other computer readable media. Computer readable storage media refers to any non-transitory tangible storage medium that provides recorded instructions and/or data to the computer system for execution and/or processing. Examples of such storage media include floppy disks, magnetic tape, CD-ROM, DVD, Blu-ray™ Disc, a hard disk drive, a ROM or integrated circuit, USB memory, a magneto-optical disk, or a computer readable card such as a PCMCIA card and the like, whether or not such devices are internal or external of the computer module. Examples of transitory or non-tangible computer readable transmission media that may also participate in the provision of software, application programs, instructions and/or data to the computer module include radio or infra-red transmission channels as well as a network connection to another computer or networked device, and the Internet or Intranets including e- mail transmissions and information recorded on Websites and the like.

[00110] Examples of benefits and advantages of the herein described disclosure include, but are not limited to, the use of the proposed magnetic field sensing array dramatically improves the signal-to-noise ratio, accuracy and sensitivity in the measurement.

[00111] The on-chip photonic magnetic sensing pixel array could be used on mobile sensing platforms such as UAV for remote sensing applications.

[00112] The proposed optical array nano-structure with simple, compact and reliable configurations that enable the realisation of on-chip sensing, which can be implemented to reduce size and cost with capability for massive production. [00113] The proposed laser frequency stabilizer in the system establishes a sensing system that is completely free of laser frequency drifting problem.

[00114] Each individual magnetic field pixel cell can also be extended to a subarray, which provides the capability of determining the direction of the magnetic fields.

[00115] The all-in-one active fully integrated photonic pixel array circuits provides key advantages of light weight, small footprint, low power consumption and scalability to large-scale integration.

[00116] The all-passive magnetic field sensing probe design eliminates the interference problems with the impact of surrounding environments.

[00117] The light power is easily coupled through rigid optical waveguide, where the main optical bus waveguide evanescently couples the light into each waveguide row and then the columns evenly, to maximise the performance and form a high- density sensing pixel array.

[00118] The herein described embodiments may be developed using a CMOS compatible fabrication platform, where the entire fabrication process is cost effective, reduced footprint, and may be reproduced with high yield for scalable applications, hence enabling the miniaturization of magnetic field sensors with the capability of large-scale integration.

[00119] In summary, described herein are examples of one or two-dimensional magnetic field pixels or cells arrays for magnetic sensing with high performance. In the system there is a laser frequency stabilizer block that is arranged to compensate for the frequency drifting problem. The optical signal(s) is easily coupled through rigid optical waveguides, where the main optical bus waveguide evanescently couples the light into each waveguide row and the waveguide columns evenly, to maximise the performance and form a high-density pixel array that dramatically reduces the system noise. Magnetostrictive material is coated on each pixel/cell sensor. Each cell has an optical integrated micro-resonator with an undercut/cantilever region. The sensor can be fabricated using CMOS compatible technologies, thus it is cost effective and can be reproduced with high yield for mass production. By extending individual magnetic field sensing pixel cells to a subarray, direction/orientation of the local magnetic fields can also be determined.

[00120] According to one example, interference problems associated with the surrounding environment may be reduced to improve sensing in harsh environments. According to another example, the optical source has a frequency drift compensation unit, magnetic field sensing pixels array, photodetectors on a single chip, which provide key advantages of light weight, small footprint, low power consumption and scalability for large-scale integration. Various embodiments may be used in a wide range of applications in different technology areas such as healthcare and remote sensing on unmanned aerial vehicles (UAV) and defence areas, for example.

Industrial Applicability

[00121] The arrangements described are applicable to the computer and data processing industries and particularly for the magnetic field sensing industries.

[00122] The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

[00123] In the context of this specification, the word “comprising” means “including principally but not necessarily solely” or “having” or “including”, and not “consisting only of”. Variations of the word "comprising", such as “comprise” and “comprises” have correspondingly varied meanings.

Claims

CLAIMS:

1. A magnetic field sensor for magnetic field imaging, the magnetic field sensor comprising: a plurality of optical waveguide channels for receiving an optical signal; a plurality of cells arranged in an array, wherein the cells are in optical communication with the optical waveguide channels, wherein multiple cells in the plurality of cells are micro-resonator cells, where a micro resonator cell comprises: at least one micro-resonator comprising a cell substrate, a cell optical waveguide on the cell substrate, and a magnetostrictive layer on the cell optical waveguide, wherein the micro-resonator cell is arranged to change an optical spectrum of the optical signal when the optical signal is transmitted along the optical waveguide channels, wherein the change in the optical spectrum is based on i) a strain exhibited by the magnetostrictive layer in the local magnetic field and ii) the effect of the strain on the cell optical waveguide.

2. The magnetic field sensor of claim 1, wherein the array comprises i) a plurality of rows of the plurality of cells, ii) a plurality of columns of the plurality of cells, or iii) a plurality of rows of the plurality of cells and a plurality of columns of the plurality of cells.

3. The magnetic field sensor of claim 1 or 2, wherein the magnetostrictive layer is formed from a material consisting of at least one of: Terfenol-D, nickel, iron, cobalt, Galfenol (iron and gallium), Ferrite (doped crystalline iron).

4. The magnetic field sensor of any one of claims 1 to 3, wherein the cell substrate is formed from a silicon dioxide layer that forms at least a portion of an SOI platform, the cell optical waveguide comprises a silicon layer cantilevered above the silicon dioxide layer, and the magnetostrictive layer is coated on the silicon layer.

5. The magnetic field sensor of claim 4, wherein the magnetostrictive layer is coated on the silicon layer to partially cover the silicon layer.

6. The magnetic field sensor of any one of claims 1 to 5, wherein the cell optical waveguide is one of a microdisk resonator and a racetrack resonator.

7. The magnetic field sensor of any one of claims 1 to 6 further comprising at least one input optical coupler for receiving the optical signal; and a plurality of output optical couplers; wherein the plurality of optical waveguide channels is in optical communication with the input optical coupler and the output optical couplers.

8. The magnetic field sensor of any one of claims 1 to 7 further comprising a silicon substrate upon which the plurality of optical waveguide channels and the array of the plurality of cells are arranged, wherein, for the magnetic field sensor of claim 7 when dependent upon claim 4, the input optical coupler and the output optical couplers are arranged upon the silicon dioxide layer, which is formed on the silicon substrate.

9. The magnetic field sensor of any one of the preceding claims, wherein, in the plurality of cells, at least one directional micro-resonator cell is formed, wherein the directional micro-resonator cell comprises: a first micro-resonator comprising a first cell substrate, a first cell optical waveguide on the first cell substrate, a first cantilevered beam in optical communication with the first cell optical waveguide, and a first magnetostrictive layer on the first cantilevered beam, wherein the first cell optical waveguide is in optical communication with a first optical waveguide channel that is in optical communication with the optical waveguide channel; a second micro-resonator comprising a second cell substrate, a second cell optical waveguide on the second cell substrate, a second cantilevered beam, and a second magnetostrictive layer on the second cantilevered beam, wherein the second cell optical waveguide is in optical communication with a second optical waveguide channel that is in optical communication with the optical waveguide channel; wherein the first cantilevered beam and the second cantilevered beam are arranged perpendicular to each other; wherein the directional micro-resonator cell is arranged to change the optical spectrum of the optical signal on each of the first optical waveguide channel and the second optical waveguide channel when the optical signal is transmitted along the optical waveguide channel, wherein the change in the optical spectrum is for detecting the direction of the local magnetic field based on i) a strain exhibited by the first magnetostrictive layer and the second magnetostrictive layer in the local magnetic field and ii) the effect of the strain on the first cell optical waveguide and the second cell optical waveguide.

10. The magnetic field sensor of claim 9, wherein the first cell substrate and the second cell substrate are formed from the cell substrate.

11. The magnetic field sensor of any one of claims 1 to 10 further comprising a plurality of photo detectors arranged in an array corresponding with the array of the plurality of cells, wherein each photo detector is arranged to optically couple to each output of the optical waveguide channels, and wherein each photo detector provides an output to a microprocessor.

12. A magnetic field sensor system comprising: the magnetic field sensor of any one of claims 1 to 11 ; a fixed wavelength optical source arranged to couple the optical signal to the input optical coupler; a plurality of photo detectors where each photo detector in the plurality of photo detectors is arranged to couple to each output optical coupler of the output optical couplers; a plurality of transimpedance amplifiers arranged to amplify optical signals received by the photo detectors, and a microcontroller arranged to i) control the fixed wavelength optical source, ii) develop a magnetic field image from the optical signals amplified by the transimpedance amplifiers.

13. The magnetic field sensor system of claim 1-12 further comprising a frequency stabiliser comprising an add-drop microring.

14. A method of manufacturing the magnetic field sensor of any one of claims 1 to 11.