WO2012038948A2 - Optical magnetometer sensor array - Google Patents

Abstract

The present invention is a large-scale, low cost, array of optical magnetometers. The invention utilizes known elements from planar optics and from optical magnetometry in order to achieve its goals. The basic principle of the invention is to use a single light source, which can be more powerful than the individual separate sources used with prior art optical magnetometers, and planar optical elements to direct the light to multiple sites of a sensor array. The invention could lead to monolithic fabrication of optical magnetometer sensor arrays.

Description

OPTICAL MAGNETOMETER SENSOR ARRAY

Field of the Invention

The present invention generally relates to the field of magnetic field sensing. Specifically, the present invention relates to magnetic field sensing using an optical magnetometer. More specifically the present invention relates to the realization of a large scale array of optical magnetic sensors.

Background of the Invention

Publications and other reference materials referred to herein, including reference cited therein, are incorporated herein by reference in their entirety and are numerically referenced in square brackets in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.

The following abbreviations are used herein:

μ-wave - microwave

μ\Υ - micro-Watts (lO ⁶ Watts)

rf— radio-frequency

SQUID - superconducting quantum interference device

μΤ, nT, pT, fT - micro-Tesla (ΙΟ ⁶ T), nano-Tesla (10 ⁹ T), pico-Tesla (10 ¹² T), femto-Tesla (10 ¹⁵ T)

Hz - Hertz = 1/second

NMR - nuclear magnetic resonance

MRI - magnetic resonance imaging

MEMS - micro-electro-mechanical systems

CPT - coherent population trapping

NMOR— nonlinear magneto-optical rotation SERF - spin exchange relaxation free

CCD - charged coupled device

CMOS - complementary metal-oxide-semiconductor

NIST - national institute of standards and technology

DC - direct current

AC - alternating current

ID, 2D, 3D - one-dimensional, two-dimensional, three-dimensional

Optical magnetometry relies primarily upon optical pumping and high resolution atomic spectroscopy, and has been developed in the second half of the 20^th century [1-6]. The field has matured rapidly in the last decade, and is now well-substantiated in the scientific literature [7-9]. Optical magnetometers were shown by several groups to be of comparable or even surpassing performance compared to other magnetic field sensing technologies [10-13], including perhaps the most widely used and highly sensitive SQUIDs (superconducting quantum interference devices) [14]. While SQUIDs are used quite extensively since the 1970s, and have demonstrated sensitivity on the order of 1 flYHz¹'² [14], they suffer from a high level of production and operation complexity, which includes for example the need for cryogenic cooling. This leads to high costs of such systems, and creates a motivation for finding competitive technologies which will be simple and less expensive. Optical magnetometers having sensitivities from the pT to the fT range and even below were demonstrated in shielded, moderately shielded, or even in the geophysical field, on the order of 50 μΤ [15,16].

These demonstrations have shown the potential use of the technology of optical magnetometry in commercial applications. Demonstrations of the capability of miniaturization of optical magnetometers, for example using MEMS techniques or methods from the world of micro-electronics fabrication [7] have also shown that optical magnetometers may be realized in a low-cost process, while still retaining a high level of performance.

Optical magnetometers are suitable for many applications. These include, among others, environmental applications, archaeology, NMR/MRI or other medical application, magnetic field measurements in space, mineral exploration, and more [9]. However to date very little actual commercialization of the field has been executed, especially outside of the research community.

Referring now to the drawings, Fig. 1 illustrates the main building blocks of a prior art optical magnetometer 100. A laser diode 10 is driven by a driving electronics assembly 60, typically in direct-current (DC) mode, or in a pulsed mode of some frequency and duty cycle. Optionally, the pulsed mode operation is obtained using other means for modulating the DC output radiation, such as using an optical chopper, electro-optical, acousto-optical or other means (not shown in the figure). It should be noted that the light source is not limited only to diode lasers, but could be any form of light source of adequate physical properties for the application, in particular a light source of well-defined power, wavelength, polarization, spectral line width, and stability required to interact with the atomic sample to be probed. The light from laser diode 10 (both in DC and pulsed modes) is also modulated by external modulation electronics 30 by adding either microwave [17] or an RF [7] signal, depending on the specific measurement technique used, to the laser diode 10 current using a bias-T to couple the radiation along with the DC driving current, or by using optical, electro- optical, acousto-optical or other modulation means directly on the laser light, and is sent to the vapor cell 4 through a first set of optical elements 2 (for example, a waveplate and or a polarizer to manipulate the light polarization, a lens to collimate the laser beam, etc.). Elements such as waveplates or polarizers are typically fabricated on a glass or polymer substrate, as thin films of a suitable material and thickness. The vapor cell 4 can be fabricated from a variety of materials such as glass, plastic, other types of polymer, silicon, and more, provided an optical access for light coming into and out of the cell is maintained. The cells typically contain a gas of alkali atoms, such as Li, Na, K, Rb, Cs, or a combination of them. According to the requirements of the application, the alkali gas is maintained at some working pressure and temperature. In addition, in order to reduce harmful relaxation or decoherence processes, vapor cells typically contain additional gasses, which are typically inert gasses such as nitrogen N2, Xe, Ar or combinations of these or other inert gasses. Moreover, in some cases the inner walls of the vapor cell 4 are coated with a material which serves to reduce the relaxation or decoherence of the alkali atoms caused by collisions with the cell walls. The most common material for wall coating is paraffin, although other materials, such as octadecyltrichlorosilane (OTS) or other materials may be suitable for this purpose as well.

In some cases the vapor cell is heated to ~100°C or a specified working temperature required for optimal performance of the sensor. This can be done externally, for example, by hot-air heating [22], by light-radiation heating [32] (both not shown in Fig. 1), or by thin-film heaters 6 [31] which are placed on one or more sides of the vapor cell.

In some techniques, the modulation is not done on the laser light by modulating the laser diode driver parameters, using the external modulation electronics 30, but rather the energy levels of the atoms in the vapor cell are modulated by using external coils on the vapor cell itself [26] to add an AC magnetic field. The external coils could also be fabricated as thin films elements on both sides of the vapor cell [31]. Herein, these will be denoted as the vapor cell coils 5, as illustrated in Fig. 1. In some measurement techniques there is an additional DC magnetic field applied to the vapor cell. This field could be applied by external coils 5 or by a providing an additional, separate set of external coils in case the measurement technique requires both a DC field and a modulating field on the vapor cell. It should be noted also that in principle a constant DC field could be realized using permanent magnets.

After going through the vapor cell 4, the light can again go through a second optical elements assembly 3, e.g. for separating polarization components, before reaching the photo-detector 1. The photo-detector can be, for example, a simple photodiode, or more than one photo-diode, e.g. a polarimeter composed of two photodiodes, each detecting the signal of one of two polarization components of the light, separated by optical elements assembly 3. The polarimeter signal is usually the (amplified) difference of the reading of the two photodiodes. The properties of the detector, e.g. it's rise- and fall-times, should be chosen adequately such that the bandwidth of the sensor is maximal or at least sufficient for the application used. The signal from the photo-detector 1 is sent to the signal electronics assembly 50 for amplification, analysis and/or control. In some cases [38] there is a closed feedback loop between the electronics assembly 50 and the driving electronics assembly 60, driving the laser diode and other components in the sensor.

For convenience, referring to Fig. 1 there is defined a part of optical magnetometer 100 that is referred to herein as the 'physics package' 25 which includes the first optical elements assembly 2, the vapor cell 4, the second optical elements assembly 3, and optionally also the vapor cell coils 5 and vapor cell heaters 6. In addition, there is defined the sensor head 20, comprising the physics package 25 and the photo-detector 1. The above configuration could be extremely miniaturized, while still retaining the advantages of the technology, namely an ultra-high sensitivity and a low level of operational and production complexity (see for example NIST [17]). Miniaturization could immediately lead to an improvement in the spatial resolution of the measurement, and to the possibility of fabricating large-scale arrays of sensors, which could be used in numerous applications requiring the detection and possibly tracking of minute sources of magnetic field, and/or forming three-dimensional maps of the magnetic fields at or near the sensor array. By use of fabrication techniques from the field of micro-electronics, the cost of realizing such large-scale sensor arrays could be greatly reduced. In addition, low power-consumption operation is realized [31].

There are several methods for performing optical magnetometry. These rely essentially on performing spectroscopy on an atomic sample which is optically manipulated to create polarization in the sample, and probing the change of state of polarization due to the presence of a magnetic field, by measuring the properties of light applied onto the atomic sample. The quantity to be measured in all methods is essentially always (although not necessarily directly) the Larmor frequency

which is proportional to the modulus of the magnetic field B. Here gF is the Lande factor of the hyperfine atomic energy level F, and μ is Bohr's magneton.

Among the methods for performing optical magnetometry are methods utilizing coherent population trapping (CPT) [17], nonlinear magneto-optical rotation (NMOR, either frequency- [7] or amplitude-modulated [18]), the Bell-Bloom scheme [19], and the Mx scheme [20]. One notable technique is the spin-exchange relaxation free (SERF) magnetometry [21], with which the highest sensitivity measurements were demonstrated. All of the aforementioned methods have been surveyed in detail in the literature, and will not be elaborated on. Apart from magnetic field measurements by a single detector, optical magnetometry was also applied for gradiometry. Here the magnetic field is measured in more than a single location, which may it be at different points of the same atomic sample [22], or in separate vapor cells [23]), and the differential signal is analyzed. In some cases, the light was gathered for detection by a photo-diode array or a CCDs chip [22, 24]. In a few cases, such as work done in the Weis group [25], arrays of more than two optical magnetometers were realized. Perhaps the most advanced demonstration of an array of optical magnetometers was done by Bison et al. [26], who constructed magnetic field maps resulting from the natural electrical signal from the human heart on the order of up to 100 pT right outside the body from measurements of a multilayered structure of magnetometers. The technique used in this work was the Mx method, which requires magnetic coils to be positioned around the vapor cell. The main component in the apparatus was an array of 19 optical magnetometers. The light was brought from a single laser source to each of the array sites by an optical fiber. The light was coupled to the fibers with the aid of a specially designed hologram. The highest sensitivity reported in this work was sub-pT/Hz^1/2 over a bandwidth from 0.1-lOOHz, when operated in gradiometer mode. It should be noted that this array suffered from a spatial resolution of several cm, due to the size of the individual sensors and surrounding components.

A number of technological aspects need to be considered when realizing an optical magentometer. A substantial effort in developing technological methods for realizing miniature, low cost, and low power magnetometers was done at NIST in the US [17]. This group has demonstrated several types of optical magnetometers, including ones with sensitivity on the order of 1-5 pT/Hz^1/2, in a miniaturized scheme. It should be noted that research was performed on such issues as miniature vapor cell fabrication [27], magnetic shielding [28], compact modulation electronics and integration [29], vapor cell heating methods [30,31], miniature vapor cell coils [30], etc. The group has also demonstrated a miniature all-optical single magnetometer [31], where both the input light and the output optical signal, were carried by optical fibers.

The state of the art in constructing magnetometer sensor arrays is typified by two US patent applications - US 2007/0167723 and US 2009/0149736. In these documents either a separate light source is used for each magnetometer in the array or a single light source is used in combination with a beam splitter or a plurality of fiber optic lines are used to direct light to some or all of the magnetometer in the array.

Fig. 2 illustrates the principle of prior art planar optical waveguides. The principle behind planar waveguides, as is schematically illustrated in the figure, is first to introduce light into a planar optical waveguide 70 (such as a piece of glass or polymer) in such a way that the light will start to move inside the glass with total internal reflection, meaning that the light stays inside the glass. The way to do that is, for example, by using a special input grating 71 written on the planar optical waveguide 70. The light may be coupled into the waveguide through more than one input grating 71. The light expands within the planar optical waveguide 70, while losing intensity as a function of distance, according to the geometrical constraints of the planar optical waveguide 70, and can be extracted from the planar optical waveguide 70 at any point on the planar optical waveguide 70 surface, for example, by use of a similar type of grating, now denoted as a top surface output grating 72.

Planar optics has been well established for several decades. Key features and principles are described, for example, in US patent 5,966,233 by Friesem et al., which is incorporated by reference for all purposes as if fully set forth herein, as well as in many other publications [32-34]. In addition to the basic principles, more advanced schemes have been developed, see for example [35-37]), for a variety of planar optical components. Among these is also the possibility to have 'planar optics' on curved surfaces, as is described, for example, in US patents 6,577,411 and 6,041,508 by David, which are incorporated by reference for all purposes as if fully set forth herein. Herein the term, "planar optical waveguide" is used in a generic sense to include waveguides that are truly planar but also waveguides comprised of curved surfaces as described in the above references patents.

State-of-the-art fabrication methods enable extreme miniaturization of optical magnetometers, leading to high spatial resolution while still retaining high sensitivity, low power consumption, and low-cost fabrication. However, scaling up to large arrays of sensors in a commercially attractive product poses a number of technological challenges:

First, constructing an array comprised of a large number of sensors requires bringing the laser light to many 'sites' of the array in a cheap and efficient manner. While this could be done with separate laser diodes, one per each site, this is not cost effective, and also requires many other components to be duplicated for each of the sites as well (for example, the electronics to modulate the laser diode current). Alternatively, a single source coupled to many optical fibers using specially designed optics may be used [26]. However, the level of complexity in using multiple fibers becomes too large in practice for large-scale, possibly three-dimensional (3D) arrays, and/or in a compact product. In addition, in order to optimize performance it is necessary to minimize the number of components carrying electrical current (and hence generating a magnetic field) in the vicinity of the vapor cell, where the measurement is performed.

Second, in order to have efficient current modulation to the laser diode, especially in the GHz range, good coupling of the modulation radiation into the laser diode is required. This means that either the modulation source needs to be very close to the diode (again, resulting in an unwanted magnetic disturbance near the vapor cell), or considerable effort is needed for efficient guiding and coupling of the modulation radiation over larger distances. This becomes complicated if there is an array (possibly in three dimensions) of sensors.

Third, as has been demonstrated by several groups [39,40], the carrier frequency of the modulation radiation of the laser diode 10 can be suppressed, resulting in a cleaner signal of the light interacting with the atoms in the vapor cell. This leads to improved sensitivity of the magnetic field measurement, as an unwanted noise component is eliminated. However, as could be seen from these works, carrier suppression requires a relatively large laser design, which is not possible in compact arrays and/or in arrays comprising large number of 'sites'.

Fourth is the issue of geometrical alignment and calibration. Even for a system incorporating a single sensor, proper analysis of the measured signal requires that the relative orientation of the sensor to that of the rest of the components in the system be very well known and also that the system can be calibrated to adjust for any errors in orientation. When moving from a single sensor to an array of multiple sensors, which may be at small or large distances from each other, the problem of geometrical/mechanical alignment of the sensors with respect to each other becomes much more difficult to solve.

It is a purpose of the present invention to provide an optical magnetometer sensor assembly that provides solutions to all of the above mentioned challenges. Further purposes and advantages of this invention will appear as the description proceeds.

Summary of the Invention

The present invention relates to the realization of large-scale, low cost optical magnetometers arrays. As described herein above with reference to Fig. 1, an optical magnetometer is generally comprised of a laser light source, optical elements for adjusting laser power, collimation and polarization, a vapor cell containing an atomic sample to be probed, further optical elements of a similar nature, and a photo-detector to collect the output light and convert it into an electrical signal to be processed. The laser light may be required to be modulated at a specific frequency, according to the selected measurement technique. Additionally, magnetic coils carrying direct current (DC) or alternating current (AC) are used to generate magnetic fields required by the measurement technique. Furthermore, the vapor cell may be required to be heated to some working temperature which is typically several tens of degrees above room temperature or higher.

The invention makes use of simple basic elements from the field of planar optics to realize a large scale array of optical magnetometers, meeting the challenges described herein above that have to date prevented the scaling up of the existing knowledge to create large arrays of sensors in a commercially attractive product. The basic building block of the invention is a planar optical waveguide, having means to couple light into and out of the waveguide. These means could for example be an input grating to couple light into the waveguide and output gratings at predetermined locations to couple the light back out. It should be noted that although in the following input and output gratings are used for the description of embodiments of the input devices and output devices of the present invention, these are only examples, and any other means, e.g. edge couplers, prism couplers, luminescent dye couplers, or V-groove based couplers, can be used to couple light into or out of the waveguide. While inside the waveguide, the light expands and travels with total internal reflection along the waveguide.

According to the invention an optical magnetometer array is realized by using a single, powerful, light source which, along with its associated driving electronics and modulation means, is located far from the sensor array. The light source is coupled to a planar optical waveguide having multiple output gratings. Optically coupled to each output grating is a sensor head comprised of all of the magnetometer elements, i.e. the physics package and, in some embodiments the photo-detector.

In one embodiment, some or all of the optical elements between the vapor cell and the photo-detector are also made of planar optical elements, such as a waveguide with input and output gratings.

In one embodiment, the photo-detector is a polarimeter, comprised of two photo-diodes, each of which samples a different light polarization component of the laser light coming from the vapor cell. The polarimeter signal is typically the amplified difference of the pair of photo-diodes.

The configuration of the invention provides a robust, monolithically fabricated structure, wherein light is brought from a single source to multiple array sites; thereby saving the need for multiple laser diodes, multiple laser modulation modules, and simplifying calibration and alignment issues. Some optical elements are also saved by proper grating design on the planar optical element. The level of magnetic disturbances, as well as sensor cross-talk, caused in part by the laser source and its surrounding electronics is greatly reduced, as they are located far from the sensor array. The embodiments of the invention described herein above may be one- or two-dimensional (ID or 2D, respectively) and may comprise a large number of array sites, in a very flexible spatial distribution, as needed according to the requirements of the application.

In one embodiment of the invention, magnetic shields are used either on each of the individual sensors in the array, or on the entire array, or both.

In one embodiment, the vapor cell used is large such that it serves as a joint vapor cell for more than a single array site. This enables further simplification in the fabrication of the array, as well as increased robustness and reduced calibration and alignment requirements.

In one embodiment, some or all the other optical and other components surrounding the vapor cell are large such that they serve as joint components for more than a single array site. This truly creates a layered device which could be monolithically fabricated.

In one embodiment, output gratings are located not only on one surface of the planar optical waveguide, but also on the opposite surface. Sensor heads are located by each output grating, thus creating a double-layer device, and realizing a 3D array which enjoys the benefits of the layered structure design. The distance between the vapor cells of the different layers, which is the distance between the locations of the physical measurements, is determined by the thickness of the various elements in the sensor head.

In one embodiment, the output light coming from the vapor cell is coupled to planar optical elements, such that it is carried to photo-detectors located at a remote location.

In one embodiment, the photo-detectors are part of a photo-detector array, for example such as a CCD or CMOS chip. In one embodiment, the distance between layers of the sensor array is enlarged by guiding the light through planar optical waveguides positioned perpendicularly to the individual array layers. The vertical waveguides are of a similar nature to the above described planar optical waveguides and contain input and output gratings as well.

In one embodiment magnetic markers, either passive or active, are placed at predetermined locations on or near the device, and serve as reference markers for the purpose of calibration of the array.

In further embodiments, and where the conditions equivalent to those of total internal reflection are maintained for the various optical waveguides in the system, the above embodiments are generalized to waveguides having a curved rather than strictly planar form.

The invention is an optical magnetometer sensor array comprising:

a. one or more planar waveguides;

b. one or more input devices for introducing light from a light source into the waveguide;

c. multiple output devices for extracting light from the waveguide;

d. an optical magnetometer sensor physics packages optically coupled to each of the output devices; and

e. one or more photo-detectors;

wherein the sensor physics package comprises a first optical elements assembly, a vapor cell, and a second optical elements assembly.

In embodiments of the invention the sensor physics package comprises one or more of vapor cell coils and vapor cell heaters.

In embodiments of the invention light from one light source is introduced into the waveguides through the input devices. In other embodiments light from two or more phase and coherence-locked light sources is introduced into the waveguides through the input devices.

In embodiments of the invention the light sources are located remotely from the input devices of the sensor array.

The input devices and the output devices of the sensor array can be gratings or can be selected from the group consisting of: edge couplers, prism couplers, luminescent dye couplers, and V-groove based couplers.

The photo-detector of the sensor array can be a photo diode or it can be selected from the group consisting of: a polarimeter, a CCD chip, and a CMOS chip.

The sensor array of the invention can be embodied in one-dimensional, two- dimensional, and three-dimensional embodiments.

In embodiments of the sensor array of the invention the output devices are located on both surfaces of the planar waveguides, thereby creating a double-layer device and realizing a three-dimensional array. In these embodiments the distance between the vapor cells on one surface of the planar waveguide and the vapor cells on the other surface of the waveguide is on the order of the distance from the source of the magnetic field being measured by the array to the vapor cells.

The sensor array of the invention can comprise magnetic shields on one or more of the individual sensor physics packages in the array, on the entire array, or on both or one or more of the individual sensor physics packages and the entire array. In embodiments of the invention the vapor cell of one or more of the sensor physics packages and/or one or more of the optical and/or the other components associated with the vapor cell of one or more of the sensor physics packages are large such that they serve as joint components for a plurality of array sites.

In embodiments of the invention the output light coming from the vapor cell of one or more of the sensor physics packages is coupled to planar optical elements, such that it is carried to a photo-detector located at a remote location.

In embodiments of the invention the distance between layers of the sensor array is enlarged by guiding the light through one or more planar optical waveguides positioned perpendicularly to the individual array layers.

In embodiments of the invention one or more passive and/or active magnetic markers is placed at one or more locations located near and/or on the array.

In embodiments of the invention one or more of the waveguides has a curved form.

In embodiments of the sensor array of the invention light from two or more light sources is introduced into the waveguides through the input devices. In these embodiments light from one or more of the sources is used for optical pumping and light from one or more of the other sources is used for probing the atoms.

All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of embodiments thereof, with reference to the appended drawings; wherein the following terms and reference numbers are used to identify the various components:

— Sensor, magnetometer 100

— Laser diode 10

— Physics package 25 (comprises: optical assemblies 2,3 and vapor cell 4 and optionally vapor cell coils 5 and/or heater 6 )

— Sensor head 20 (comprises physics package 25 and the detector 1)

— Photo-detector 1

— First optical elements assembly 2

— Second optical elements assembly 3

— Vapor cell 4

— Vapor cell coils 5

— Vapor cell heaters 6

— External modulation electronics 30

— Signal electronics assembly 50

— Driving electronics assembly 60

— Planar optical waveguide 70

— Input grating 71

— Top surface output gratings 72

— Optical fiber 300

— Fiber coupler 310

— First output planar optical waveguide assembly 90

— Input grating 91 of output planar optical waveguide assembly 90

— Output grating 92 of output planar optical waveguide assembly 90

— Bottom surface output grating 73

— Composite output planar optical assembly 110

— Detector array 111

— Perpendicularly positioned planar optical waveguide 80

— Input grating 81 of the perpendicularly positioned planar optical waveguide 80 — Output grating 82 of the perpendicularly positioned planar optical waveguide 80

Brief Description of the Drawings

— Fig. 1 schematically illustrates the main building blocks of a prior art optical magnetometer;

— Fig. 2 schematically illustrates the principle of a prior art planar optical waveguides;

— Fig. 3 schematically illustrates an array of sensors in accordance with the present invention;

— Fig. 4 schematically illustrates an embodiment of the present invention, wherein the light is coupled into the planar optical waveguide through multiple input gratings;

— Fig. 5 schematically illustrates an embodiment of the present invention, wherein a planar optical waveguide with specially patterned input and output gratings is used to separate the light coming from the vapor cell into its polarization components;

— Fig. 6 schematically illustrates an embodiment of the present invention, wherein the sensor array is two-dimensional;

— Fig. 7 schematically illustrates an embodiment of the present invention, in which a single large vapor cell is used, instead of a large number of small cells at each of the array sites;

— Fig. 8 schematically illustrates an embodiment of the present invention, in which the optical elements surrounding the vapor cell are each made of a single large component, rather than from many small components at each of the array sites;

— Fig. 9 schematically illustrates an embodiment of the present invention, in which the output coupling gratings on the planar optical waveguide are placed on both opposing faces of the waveguide, such that a two-layer array structure is realized; — Fig. 10 schematically illustrates an embodiment of the present invention, in which an assembly of planar optical waveguides or components is used to collect the light from each individual site in the sensor array, and direct it to a remote photo-detector;

— Fig. 11 schematically illustrates an embodiment of the present invention, in which an assembly of planar optical waveguides is used to collect the light from each individual site in the sensor array, and direct it to a suitable location in a remote detector array;

— Fig. 12 schematically illustrates an embodiment of the present invention, in which a perpendicularly positioned planar optical waveguide is used to guide the light between layers of sensor arrays across large distances; and

— Fig, 13 schematically illustrates the prior art concept of a curved planar optical waveguide.

Detailed Description of Embodiments of the Invention

The present invention is a large-scale, low cost, array of optical magnetometers. The invention presents a novel way of utilizing known elements from planar optics and from optical magnetometry in order to achieve its goals.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not intended by its inventors to be limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, dimensions, methods, and examples provided herein are illustrative only and are not intended to be limiting.

The basic building blocks of an optical magnetometer, as they are known in the art, have been described herein above and illustrated in Fig. 1. While the basic concept has been well-established, and the potential for miniaturization has been shown, a few issues arise from the prior art configuration that are presently limiting the capability of scaling the knowledge according to the present state of the art up to large arrays. The source of some of these issues comes from the design of a single sensor, while others arise only when scaling up to an array of sensors. Four of these issues were outlined herein above in the "background" section. It should be noted that these issues were not listed in any order of importance and they are not a complete or comprehensive list of the technical problems that are encountered in building arrays comprised of large numbers of optical sensors.

The present invention provides solutions to most of the above mentioned issues, in a low-cost way, which could also lead to monolithic fabrication. The basic principle of the invention is to use a single light source, which can be more powerful than the individual separate sources used with prior art optical magnetometers such as that shown in Fig. 1, and planar optical elements to direct the light to multiple sites of a sensor array. The light source may be a lamp, a laser (for example, an external cavity diode laser, a vertical cavity surface emitting laser, a distributed feedback laser, a solid state pumped diode laser, or a fiber laser), or any light source of adequate properties required for the light-matter interaction in the operation of the vapor cell for magnetometry. The planar optical elements are typically made of glass or polymer but may be made of other materials. They may be coated with some material such that the conditions for total internal reflection of the light traveling within the planar optical elements are maintained. Fig. 3 schematically illustrates an array of sensors in accordance with the present invention. While planar optics is a well-established field [33-38], in this invention the inventors suggest for the first time the use of planar optics for realizing a large scale array of optical magnetometers, as schematically illustrated in Fig. 3. In this figure the light source is not shown. Together with the driving electronics and modulating means, it is located far away from the sensor array. Herein the phrases "located far away", "remote source", etc. when related to the distance between the light source and the array have the meaning that the distance is large enough that magnetic disturbances caused by the electronics used to create and modulate the light and measured near the vapor cells of the array are negligible compared to the magnetic fields that are being measured at the sensor heads.

Light from the remote source is transferred via an optical fiber 300 and coupled into the planar optical waveguide 70 through the special input grating 71. It should be noted that the light coupled into the sensor array may be used as optical pumping light, as probing light, or as both, as is required by the specific measurement technique chosen to operate the array. In the case where the light used for optical pumping and for probing the atoms is of different properties (e.g. as in the case of AM-NMOR where the optical pumping light is amplitude-modulated while the probing light is not modulated), the light may come from separate sources.

The fiber may be polarization maintaining or non polarization maintaining, according to the requirements of the specific measurement technique used. An example of optical fibers suitable for use with the present invention is fibers having a silica core with a diameter for single-mode fibers on the order of 6 μπι in which the light propagates. It is noted that other means, e.g. simple free-space optical elements, can be used to transfer the light from the source to the planar waveguide. Also the input grating 71 used here and in the following description is an example of a means to couple light into the waveguide 70, although other means e.g. edge couplers, prism couplers, luminescent dye couplers, or groove based couplers could be used for this coupling. As illustrated here, further means for coupling the light from the fiber 300 to the input grating 71 are necessary. These include, for example, a fiber coupler and a mirror, denoted here generally as a fiber coupler 310. The fiber coupler is a mechanical structure which typically contains some optical component such as a lens, and has a length suitable to match the divergence of light coming out of the optical fiber to the desired spot size at the lens and/or at the point of coupling into the planar optical waveguide 70. The mirror could be, for example, a standard dielectric mirror or a silver or gold (broadband mirror) and could be totally reflecting or partially reflecting if there is a need for it. The light travels along the planar optical waveguide 70, and is coupled out again at predetermined locations through top surface output gratings 72. Again, the use of gratings 72 here and in the following description is brought as an example for a means to couple light out of the waveguide 70, while other means, such as those described with respect to gratings 71, for this coupling may be used. The relative intensity of the light which is coupled out from a particular output grating 72 is determined by the grating parameters; therefore, by engineering the output coupling strength of the various gratings, the distribution of power at the various sensor array sites, i.e. a uniform distribution in which the same power reaches all sensors or a non-uniform distribution that depends on the requirements of a particular application, can be set. The sensor at each site of the array, i.e. at each output grating 72, comprises of a sensor head 20 similar to that shown in Fig. 1. Note that in some of the figures illustrating the various embodiments of the invention, the optional vapor cell coils 5 and heaters 6 are not shown; however it is to be understood that they are present if the properties of the material in vapor cell 4 or of the particular measurement to be carried out requires their presence.

Fig. 4 schematically illustrates an embodiment of the present invention, wherein the light is coupled into the waveguide 70 through multiple input gratings 71. In this figure, two input gratings 71 are shown as an example (more than 2 are possible). The number and location of input gratings 71 on the planar optical waveguide 70 is set by the power requirements at the various array sites, and according to the characteristics of the light properties as it travels through the planar optical waveguide 70. Generally, more input gratings 71 are required as the array area is larger. In embodiments of the invention a single source of light is used to provide the input to all of the fibers 300 leading to the input gratings 71; however more than one source can be used, if suitable precautions employing methods well known in the art are taken to ensure the sources are phase- and coherence- locked, and substantially of the same optical properties as needed for the operation of an optical magnetometer, In the case that separate input gratings are used to introduced to couple optical pumping and probing light, as needed by the specific measurement method chosen for operating the sensor array the light components (for optical pumping and for probing) do not necessarily have to be phase- or coherence-locked with respect to each other. Only light components used for the same purpose in the operation of the sensor array are required to be phase- locked and coherence-locked when introduced to the array from different locations.

Fig. 5 illustrates an embodiment of the present invention, wherein a planar optical waveguide with specially patterned input and output gratings are used to separate the light coming from the vapor cell into its polarization components. The light exiting from vapor cell 4 is directed by the second optical assembly 3 onto a pair of photo-detectors used for polarimetry. The annotations "p-polarized light" and "s-polarized light" are for illustrative purposes only. The actual polarization components may differ. In cases in which the measurement technique requires separating the light coming from the vapor cell 4 into its polarization components, e.g. as in the case of NMOR, some or all of the elements in the second optical elements assembly 3 following the vapor cell 4 could also be replaced by a planar optics component, denoted as the first output planar optical waveguide 90, as is shown in Fig. 5. This waveguide includes its own special input 91 and output gratings 92.

Already from the schemes described in Fig 3 to Fig. 5, it can be seen that the current invention addresses some of the issues related to scaling up the prior art systems to large arrays that are noted herein above. First, the incoming light comes from a single source, which needs to be strong enough to provide sufficient light to all of the sensors that are in the array, also taking into account losses due to the coupling into, propagation within, and coupling out of the planar optical waveguides 70 and 90. Since the power required to operate a single highly sensitive optical magnetometer is typically on the order of 50 μ^Ν, the overall power required from the single source does not impose unrealistic requirements on the power of the single light source. The large number of laser diodes and other components that would be required according to the prior art is saved, by using a single source and a single set of electronics for modulating and stabilizing it. The scheme of the invention is also more easily realizable than using multiple optical fibers and holograms for light delivery to the array sites [26], certainly, as will be shown herein below, when scaled to 3D arrays and/or compact arrangement.

Secondly, some optical elements are saved, as collimated light coupled into a planar optical waveguide can also be coupled out collimated, at least on length scales relevant to the sizes of sensor heads 20. The same is true for maintaining light polarization properties in the planar optical components. Thirdly, the location of each site in the array is set by the location of the output gratings 72 and 92. These can easily be fabricated at almost any location in the waveguides 70 and 90, in a highly precise manner. This also allows very high precision and accuracy in the relative location of the individual sensors and their orientation, thus reducing the complexity of calibration and alignment needed for the system.

Since the light source 10 and its electronics 60 and 30 are set far away from the sensor array, the level of magnetic field disturbances at the measurement location, i.e. at the vapor cells 4, is suppressed. It should be noted that since, in the invention, the sensor array is truly all-optical, i.e. has the minimal amount of electronics near it, the level of cross-talk between sensors, which in any case is very small for this type of sensors, is further reduced. The goal of an all-optical sensor array can be achieved by using a measurement technique in which the light is modulated, rather than the magnetic field at the vapor cells.

In the amendments described herein above, magnetic shielding around the entire array or separately around each of the sensor heads can be added, as is known in the art [28]. Typically, magnetic shielding is realized by covering the vapor cells by up to 5 layers of a material with high permeability, such as μ-metal, arranged in a proper geometry. Shielding factors better than 10⁴ can be realized.

Fig. 6 schematically illustrates an embodiment of the present invention, wherein the sensor array is two-dimensional. Four input gratings 71 for coupling light into the planar optical waveguide 70 are illustrated, as well as nine output gratings 72, and their respective sensor heads 20. The light source, input coupling means, and light propagation path are not shown. In this figure, only a 3X3 array is shown, although generally the number of sites could be also smaller or larger than 9. The figure shows a generalization of the scheme described in Figs. 3 to 5 from ID to 2D. Since the location of the individual array sites is determined primarily by the location of the output gratings 72, the 2D array geometry is highly flexible, and could be of any reasonable size and number of sites.

Fig. 7 schematically illustrates an embodiment of the present invention, in which a single large vapor cell 4 is used, instead of a large number of small cells at each of the array sites. As was mentioned above, magnetic field measurements, as well as magnetic field gradient measurements, have also been performed within a single vapor cell [22]. For this application also, using a single, larger vapor cell, covering all of the array sites, is possible as is illustrated in Fig. 7. In this case, the properties of the internal composition of the vapor cell 4, e.g. types of gasses, partial pressures, temperature, are adjusted properly.

Figure 8 schematically illustrates an embodiment of the present invention, in which the optical elements surrounding the vapor cell are each made of a single large component, rather than from many small components at each of the array sites. Here, the 'layered' nature of the scheme is also used for all optical or other components 2,3,5,6 surrounding the vapor cell 4, such that the level of miniaturization is greatly increased, thereby potentially reducing the overall cost of the system, without compromising the performance.

In the embodiments illustrated in Fig. 7 and Fig. 8, some of the elements of any individual sensor head 20 are thus jointly shared with the other sensor heads 20 in the array. This truly creates a completely layered device, which could be monolithically fabricated using standard techniques. Fig. 9 schematically illustrates an embodiment of the present invention, in which the output coupling gratings on the planar optical waveguide 70 are placed on both opposing faces of the waveguide, such that a two-layer array structure is realized. This is another improvement of the basic scheme, taking advantage of the benefits of planar optics. The output-coupling gratings 72 are positioned not only on one (the top) surface of the planar optical waveguide 70, but also on its opposite side (these are shown as the bottom surface output gratings 73). The individual sensor heads 20 of each site are also duplicated on the 'bottom' side of the planar optical waveguide 70, thereby converting the 2D array into a double-layer structure, thus realizing a 3D device. This 3D device enjoys all of the advantages described above for the ID and 2D embodiments, including the monolithic fabrication enabling ease of alignment and calibration. It should be noted that according to the geometrical nature of the application, namely the required distance between the field source, the sensor array, and the distance between array sites, the desired 'thickness' of the structure (or, the distance between array layers, which is dependent upon one or more of the planar optical waveguide 70, the optical elements 2,3 surrounding the vapor cell 4, the vapor cell 4 itself, or additional buffer layers) should be properly chosen for obtaining best performance. As a rough rule of thumb, there is no use in having the two layers of the device very close to each other, while the source of the magnetic field being measured by the array, is located far away relative to the above thickness, otherwise the measurement resolution in the third dimension will not suffice. Hence, the typical layer-to-layer distance, i.e. distance between vapor cells 4, should be on the order of the magnetic field source-to-sensor distance.

Fig. 10 schematically illustrates an embodiment of the present invention, in which an assembly of planar optical waveguides or components is used to collect the light from each individual site in the sensor array, and direct it to a remote photo-detector. A further improvement of the scheme, for the purpose of a truly all-optical array, is shown, where a composite structure of output planar optical waveguides and elements 110 (as are known in the field of planar optics [32-37]), is used for extracting the signal from each sensor to a remote set of detectors.

Fig. 11 schematically illustrates an embodiment of the present invention, in which an assembly of planar optical waveguides is used to collect the light from each individual site in the sensor array, and direct it to a suitable location in a remote detector array. In this embodiment the signal is coupled out not from the top or bottom surfaces of the planar optical waveguides 70, 90 as has been described herein above, but rather from one of the side faces of the waveguides. In this manner, the output coupled light can be directed to a detector array 111, such as for example (but not limited to) to a CCD or CMOS chip.

As is the case in all previous figures, also in Fig. 10 and Fig. 11 only a small fraction of the sensor array (two sites) is shown, for illustrative purposes. In practice, a large-scale array will necessitate the composite output planar optical assembly 110 to be more complex than is shown in these figures. It should be noted, as was illustrated for the incoming light radiation into the device, that light could also be coupled out to optical fibers, using various optical components, such that the detector array 111 is positioned far away from the sensor array. This type of all-optical sensor array reduces even further the errors and instabilities due to stray magnetic fields arising from the detector electronics.

Fig. 12 schematically illustrates an embodiment of the present invention, in which a perpendicularly positioned planar optical waveguide 80 is used to guide the light between layers of sensor arrays across large distances. Similarly to what was shown in Fig. 11, when the signal is coupled out of one of the side faces of the planar optical waveguides 70, 90, the double layer scheme illustrated in Fig. 9 is enhanced to even larger 'thicknesses' between array layers, by guiding the light also through perpendicularly positioned planar optical waveguides 80, and their respective input and output gratings 81 and 82. This is illustrated in Fig. 12. It should be noted however, that the use of a perpendicularly positioned planar optical waveguide 80 results in an increased complexity of system alignment, calibration, and mechanical support difficulties, as the advantage of complete monolithic fabrication of the entire array is partially lost.

Another feature which could be added on the various embodiments of the invention is the use of magnetic markers, which are positioned at precisely known pre-determined locations on the device (for example, on the array boundaries). These markers serve as reference signals for the sensor array. Their number will vary according to the complexity of the array and the needs of the application in which the array is used. The markers can be passive, i.e. small permanent magnets serving as magnetic dipoles of known magnitude and orientation or active, i.e. they produce a known magnetic field only when excited (for example by an electrical current). The reference markers could be operated in the active case for system calibration, and then turned off before the actual measurements are performed. This allows for a 'cleaner' measurement of the target magnetic field source. The markers could also be activated during the regular operation of the sensor array.

Fig. 13 schematically illustrates the prior art concept of a curved planar optical waveguide. As far as the requirements of planar optical waveguides are maintained, namely keeping the conditions of total internal reflection or equivalent conditions to maintain the light traveling within the waveguide, the various waveguides 70, 90, 110 and their respective various input and output gratings 71, 72, 73, 91, and 92 need not necessarily be completely planar, but rather can have curvature, as described for example in US patents 6,577,411 and 6,041,508 by David. The principle is illustrated in Fig. 13, where a general curved 'planar' optical waveguide 75 is shown along with input grating 71 and output grating 72. This opens up the possibility of generalizing all of the above features of the invention to arrays of sensors distributed about curved structures.

An optical magnetometer array as have been described herein may be of use in many applications. Among these, but without loss of generality and for illustrative purposes only, one can include for example applications in the fields of medical instrumentation, imaging of magnetic materials (such as hard drives) or electrical current distribution in conductors, homeland- security systems, and more.

As a more specific example, in the field of medicine the magnetometer array could be used to detect and monitor the field distribution originating from one or more magnetic field sources. These could be natural sources, such as fields originating from electrical activity in the brain or the heart, or from artificial sources, such as from small permanent magnets implanted in the body, in tools used by medical staff in their procedures, or in instruments passing through the body, such as a 'camera-in-a-pill' type of device, ingested for the purpose of imaging the intestines. Additionally, the artificial sources may not be permanent magnets, but active components radiating magnetic fields which could be detected by the magnetometer array, which is placed in the vicinity of the field source to be measured.

The use of a magnetometer array for medical instrumentation may be in various fields of medicine. To give an example, the use of a magnetometer array in orthopedic diagnostic procedures, such as bone elongation can be considered. In this application, small magnets are implanted at suitable locations on a bone (such as, at its edges), and the magnetometer array, placed outside of the patient's body adjacent to the location of the bone, is used to map the field distribution and analyze the location of the magnets, and more specifically, the distance between the magnets. In a bone elongation procedure, the patient may be required to undergo a monitoring process, wherein this distance is measured repeatedly over a period of time. To date this is preformed with procedures which typically involve harmful radiation to be applied on the body. The use of a magnetometer array as described herein, which is passive, without harmful radiation, and with a high spatial resolution will reduce the dose of harmful radiation, increase accuracy, and may reduce the level of complexity of the procedure, enabling it to be moved from a hospital environment, to an outpatient facility.

Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, without exceeding the scope of the claims.

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Claims

Claims

1. An optical magnetometer sensor array comprising:

a. one or more planar waveguides;

b. one or more input devices for introducing light from a light source into said waveguide;

c. multiple output devices for extracting light from said waveguide; d. an optical magnetometer sensor physics package optically coupled to each of said output devices; and

e. one or more photo-detectors;

wherein said sensor physics package comprises a first optical elements assembly, a vapor cell, and a second optical elements assembly.

2. The sensor array of claim 1, wherein the sensor physics package comprises one or both of vapor cell coils and vapor cell heaters.

3. The sensor array of claim 1, wherein light from one light source is introduced into the waveguides through the input devices.

4. The sensor array of claim 1, wherein light from at least two light sources is introduced into the waveguides through the input devices, wherein said light sources are phase and coherence-locked light sources.

5. The sensor array of claim 1, wherein the light sources are located remotely from the input devices.

6. The sensor array of claim 1, wherein the input devices and the output devices are gratings.

7. The sensor array of claim 1, wherein the input devices and the output devices are selected from the group consisting of: edge couplers, prism couplers, luminescent dye couplers, and V-groove based couplers.

8. The sensor array of claim 1, wherein the photo-detector is a photo diode.

9. The sensor array of claim 1, wherein the photo-detector is one or more photo-detectors selected from the group consisting of: a polarimeter, a CCD chip, and a CMOS chip.

10. The sensor array of claim 1, wherein the array is one-dimensional.

11. The sensor array of claim 1, wherein the array is two-dimensional.

12. The sensor array of claim 1, wherein the array is three-dimensional.

13. The sensor array of claim 1, wherein the output devices are located on both surfaces of the planar waveguides.

14. The sensor array of claim 13, wherein the distance between the vapor cells on one surface of the planar waveguide and the vapor cells on the other surface of said waveguide is on the order of the distance from the source of the magnetic field being measured by said array to said vapor cells.

15. The sensor array of claim 1, comprising magnetic shields on one or more of the individual sensor physics packages in said array.

16. The sensor array of claim 1, comprising magnetic shields on the entire array.

17. The sensor array of claim 1, comprising magnetic shields on both one or more of the individual sensor physics packages and the entire array.

18. The sensor array of claim 1, wherein the vapor cell of one or more of the sensor physics packages is large such that it serves as a joint vapor cell for a plurality of array sites.

19. The sensor array of claim 1, wherein one or more of the optical components associated with the vapor cell of one or more of the sensor physics packages are large such that they serve as joint components for a plurality of array sites.

20. The sensor array of claim 2, wherein one or more of the vapor cell coils and vapor cell heaters associated with the vapor cell of one or more of the sensor physics packages are large such that they serve as joint components for a plurality of array sites.

21. The sensor array of claim 1, wherein the output light coming from the vapor cell of one or more of the sensor physics packages is coupled to planar optical elements, such that it is carried to a photo-detector located at a remote location.

22. The sensor array of claim 1, wherein the distance between layers of said sensor array is enlarged by guiding the light through one or more planar optical waveguides positioned perpendicularly to the individual array layers.

23. The sensor array of claim 1, comprising one or more passive magnetic markers placed at one or more predetermined locations on said array

24. The sensor array of claim 1, comprising one or more passive magnetic markers placed at one or more predetermined locations near said array.

25. The sensor array of claim 1, comprising one or more active magnetic markers, placed at one or more predetermined locations on said array.

26. The sensor array of claim 1, comprising one or more active magnetic markers placed at one or more predetermined locations near said array.

27. The sensor array of claim 1, wherein one or more of the waveguides has a curved form.

28. The sensor array of claim 1, wherein light from two or more light sources is introduced into the waveguides through the input devices, wherein light from one or more of said sources is used for optical pumping and light from one or more of the other sources is used for probing the atoms.