WO2012066527A1

WO2012066527A1 - Optical magnetometer sensor network

Info

Publication number: WO2012066527A1
Application number: PCT/IL2011/000566
Authority: WO
Inventors: David Tal; Gil Shalev; Avi Elmalem
Original assignee: Israel Aerospace Industries Ltd.
Priority date: 2010-11-18
Filing date: 2011-07-18
Publication date: 2012-05-24
Also published as: IL209428A; IL209428A0

Abstract

The present invention is a low cost network comprised of tens and even hundreds of optical magnetometers. The invention presents a novel way of realizing such a network that overcomes the problem of separation of signals coming from the various sensors in the network while using only a small number of optical fibers, very little power consumption along the line, and eliminating the strict requirements of alignment and calibration of each element on the network.

Description

OPTICAL MAGNETOMETER SENSOR NETWORK

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 a line or network of magnetic sensors. More specifically, the present invention relates to the realization of a large scale optical network of optical magnetic sensors.

Background of the Invention

Publications and other reference materials referred to herein, including references 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

μW - micro-Watts (10⁶ Watts)

μβ - micro-second (10⁶ seconds)

rf - radio-frequency

SQUID - superconducting quantum interference device

μΤ, nT, pT, fT - micro-Tesla (10⁶ T), nano-Tesla (lO ⁹ T), pico-Tesla (lO-" T), and femto-Tesla (10 ¹⁵ T) respectively

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

NIST - national institute of standards and technology

DC - direct current

Networks of sensors are widely used in many applications. These sensors could be of a large variety of technologies, depending on the needs of the application, and include, for example, optical sensors (in the visual range of the optical spectrum, the near infra-red range, or other spectral ranges), radar systems, magnetic sensors, acoustic or seismic sensors, and more. One particular application of interest for such a network of sensors is its use for the realization of a system for detection of sources of magnetic field, or magnetic field anomalies with respect to the Earth's field, at the vicinity of sensors in the network. Magnetic field sources include magnetic or electromagnetic generators, such as electronic equipment. Sources for field anomalies may include any ferromagnetic material, providing its magnetic dipole moment (to first order) is strong enough such that the field originating from it is discernible from the surrounding field in its environment.

A sensor network may have many configurations. These include, for example, a linear chain of sensors, a closed loop of sensors, or a complex grid or network of sensors.

The realization of a system for detection of sources of magnetic fields as described herein can be used for intrusion detection or tracking of magnetic field sources in the vicinity of the network. In general, the larger the secured area is (in the case of a closed loop or network), or the longer the secured line is (in the case of a linear chain of sensors), the higher the complexity and cost of the system, its deployment, and its maintenance. Various intrusion detection systems are known, typically classified by their type of sensors, or by their technical structure. Herein the focus is mainly on intrusion systems comprising magnetic field sensors.

In a magnetic sensor based intrusion detection system, the intrusion detection is realized by magnetic sensors which detect a variation in the magnetic field in the proximity of the sensor. This variation is used for determining whether an intrusion has occurred or not. Magnetic sensors of the passive type can particularly sense articles in the proximity of the sensor that are made of ferromagnetic materials, or more specifically, the magnetic sensor can sense a change in the magnetic field due to a change in the location or orientation of such articles, as they influence the properties of the magnetic field at the location of the sensor.

There are various advantages to the use of magnetic sensors, as follows: (a) magnetic sensors of various technologies are or can be made relatively cheaply and to be very reliable; (b) magnetic sensors, when applied to constant field or low-frequency field detection, can be used above or below the ground or underwater, or they can be installed and sense even from within concrete walls; (c) the magnetic sensors of the passive type do not radiate energy, and therefore are hard to be detected; and (d) magnetic sensors have relatively small mass and volume.

Generally, intrusion detection systems use a plurality of magnetic sensors, spaced apart from one another, for example, by 1-lOm. Typically, a number of magnetic sensors are arranged along a connecting line in a 'chain-like' structure (i.e. one after the other, sometimes with other supporting units in between) and they generally communicate with the neighboring sensors by means of said line, or wirelessly. They may also communicate with a control center directly or via a relay unit. More complex configurations, such as a closed loop or grid of sensors can be realized.

There are some intrusion detection systems in which variation measurements as obtained from two or more sensors enabling accurate determination regarding the exact location and direction of the intrusion with respect to each of the involved sensors. Such determination is generally based on the use of a detection algorithm that solves the 'physical problem' of determining the location of the intruding object (hereinafter also referred to as the 'target') and its magnetic moment. The detection algorithm may use numerical tools. However, in systems employing magnetic sensors measuring the field vector components (rather than the field magnitude directly), such determination of the exact location and direction of the intrusion, based on indications from a plurality of sensors is very dependent on the orientation of each sensor with respect to the others in its proximity, or with respect to a predefined coordinate system. In addition, sensors measuring the field vector components are typically comprised of sub-units, each measuring a single field vector component and arranged such that three such sub-units measure three orthogonal field components in order to construct a total field vector measurement. Such a configuration also has a disadvantage of the manufacturing and calibration of the internal sub-units in the sensor with respect with each other, and the actual level of orthogonality between the different measurement directions. Therefore, a direction-location determination requires very accurate leveling and calibration of the orientation of each of the sensors with respect to the predefined coordinate system at the time of installation and it also strictly requires continuous maintenance of all of the sensors at the calibrated orientation after the time of installation. This issue is even more problematic in a marine environment, where the sensor line or network is in constant motion within an aquatic medium. Scalar, or total field measuring, magnetometers directly give the absolute magnitude of the field, rather than its directional components. In principle, more scalar magnetometers are required to give the spatial information needed for analysis, than vector magnetometers (a single vector magnetometer gives three out of the six degrees of freedom which are the minimal requirement for solving the problem of target localization of a magnetic dipole moment according to a measured field magnitude. In principle, two vector magnetometers or six scalar magnetometers can provide the necessary information for solving this problem). However, since scalar magnetometers give directly the field magnitude, the level of complexity of alignment, orientation calibration and analysis is greatly reduced both at the single sensor level and certainly at the system level. This is especially emphasized in applications such as an underwater sensor network, where some or all magnetometers may be in continuous motion in the aquatic environment.

For example, it is common to install a network of magnetic sensors along a secured border or around a secured area. In some cases, the network of sensors is installed underground. In some cases, the system may be deployed underwater. In some cases a line of sensors may be mobile, for example being dragged in water by a manned or an unmanned ship. In both the ground-based, and the water-based applications, the procedure of installing the network is very cumbersome, expensive, and time consuming as it includes stages of careful alignment and calibration.

Another challenge in networks of magnetic sensors to-date is their electrical power consumption. This is due both to power consumption of each magnetometer unit distributed along the network, as well as losses due to the length of the network. For example, for a network having a power supply based on a nominal voltage of 60 V and for a single sensor power consumption of 800 mV (including for each sensor also a microcontroller, communications circuitry and a 90% efficiency supplier), the input voltage of the sensor unit can reach 45 V already at a total of 100 sensors spaced apart by 2 m from each other. Maintaining the voltage level is important, in order to avoid using high currents, the resulting unwanted magnetic fields and their disturbances, and large cross-section area conductors, which are expensive. This creates a severe limitation on the possible length of system segments and on the resulting system cost; thus, there is a need to reduce the power consumption both at the single sensor level, and also at the system level.

There is therefore a need in the realization of a network of magnetic sensors in which power consumption is reduced, deployment, installation, and maintenance are simplified, and the strict requirements of alignment and calibration of each of the network elements is eliminated.

Referring now to the drawings, Fig. 1 of the prior art schematically illustrates the concept of a detection system 1000 based on magnetic sensors. Such a system is described in co-pending Israeli Patent Application 208728 by the same applicant, the description of which, including publications referenced therein, is incorporated herein by reference. This patent application deals with an intrusion detection system comprising non- optical magnetic sensors, which are electrically fed by a standard electrical supply line. The application discloses a way to simplify the alignment and calibration of the system, by means of an accelerometer located near each magnetic sensor. The system is made up of several detection segments 100, comprising detection clusters 10, and segment coordinators 5. The segment coordinators 5 include coordinator units 3, and power supply units 4. They can be connected to a command, control, and communications (CCC) unit 6 for extracting data from the system. All components are generally connected from one or both sides of the line by power and communications cables 7. A detection cluster 10 includes several magnetic sensors 1, as well as a detection head 2, which may include a sensor 1 as well. Data from each of the sensors 1 is collected for preliminary (low level) analysis in the detection heads 2 of each cluster. Smart algorithms are used to classify the variations in magnetic field sensed in each cluster 10, and transfer an alert to the higher levels (segments 100) of the system. This scheme may have many variations and could be arranged in a flexible manner, according to the requirements of the specific applications used.

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-documented 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 the 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 fT/Hz^1/2 [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 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 manufacturing 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 applications, magnetic field measurements in space, mineral exploration, and more [9]. However to date very little actual commercialization of the technology has been executed, especially outside of the research community. Examples of commercialization are presented in a paper given by two engineers from Geometries, Inc. in April, 2010 [34]. In this paper, amongst other topics, the authors describe in very general terms a small-scale array comprised of nine sensors and mention the possibility of deploying the sensors in even greater numbers for use in detecting intruders.

Referring again to the drawings, Fig. 2 illustrates the main building blocks of a prior art optical magnetometer 2000. A laser diode 21 is driven by a driving electronics assembly 22, 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, an electro-optical or acousto- optical modulator, 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 21 (both in DC and pulsed modes) is also modulated by external modulation electronics 23 by adding either micro- wave [17] or an RF [7] signal, depending on the specific measurement technique used, to the laser diode 21 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 27 through a first set of optical elements 24 (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 27 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 27 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 [18], by light-radiation heating [19] (both not shown in Fig. 2), or by thin-film heaters 26 [20] 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 23, but rather the energy levels of the atoms in the vapor cell are modulated by using external coils on the vapor cell itself [21] to add an AC magnetic field. The external coils could also be fabricated as thin films elements on both sides of the vapor cell [20]. Herein, these will be denoted as the vapor cell coils 25, as illustrated in Fig. 2.

In some measurement techniques there is an additional DC magnetic field applied to the vapor cell. This field could be applied by external coils 25 or by 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 27, the light can again go through a second optical elements assembly 28, e.g. for separating polarization components, before reaching the photo-detector 29. 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 28. The polarimeter signal is usually the (amplified) difference of the reading of the two photodiodes. The properties of the detector, e.g. its 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 29 is sent to the signal electronics assembly 30 for amplification, analysis and/or control. In some cases [22] there is a closed feedback loop between the electronics assembly 30 and the driving electronics assembly 22, driving the laser diode and other components in the sensor.

For convenience, referring to Fig. 2 there is defined a part of optical magnetometer 2000 that is referred to herein as the "physics package" 20 which includes the first optical elements assembly 24, the vapor cell 27, the second optical elements assembly 28, and optionally also the vapor cell coils 25 and vapor cell heaters 26. In addition, there is defined the sensor head 200, comprising the physics package 20 and the photo-detector 29.

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 networks or 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 network/array. By use of fabrication techniques from the field of micro-electronics, the cost of realizing such large-scale sensor networks or arrays could be greatly reduced. In addition, low power-consumption operation is realized [20].

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 ω\ =γΒ, which is proportional to the modulus of the magnetic field B. Here γ is the gyromagnetic ratio.

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 [23]), the Bell-Bloom scheme [24], and the Mx scheme [25]. One notable technique is the spin-exchange relaxation free (SERF) magnetometry [26], 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 herein. Some of these measurement methods provide scalar measurements of the magnetic field, while some can also be used for measuring the vector components of the magnetic field.

Apart from magnetic field measurements by a single detector, optical magnetometry has also been applied for gradiometry. Here the magnetic field is measured at more than a single location, which may be at different points of the same atomic sample [18], or in separate vapor cells [27], and the differential signal is analyzed. In some cases, the light was gathered for detection by a photo-diode array or a CCDs chip [18, 28]. In a few cases, such as work done in the Weis group [21], 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. [29], who constructed magnetic field maps resulting from the natural electrical signal from the human heart on the order of up to 100 pT just 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 optical fibers. 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.

To-date, the majority of optical magnetometers are delicate, expensive, and large. A substantial effort in developing technological methods for realizing miniature, robust, 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. Research was performed on such issues as miniature vapor cell fabrication [30], magnetic shielding [31], compact modulation electronics and integration [32], vapor cell heating methods [19,20], miniature vapor cell coils [20], etc. The group has also demonstrated a miniature all-optical single magnetometer [19], 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. However, these two examples are oriented primarily for medical applications, rather than for a large-scale sensor line or network which could be used, for example, for intrusion detection or for magnetic anomaly detection in various environments such as in the marine environment.

Therefore, using scalar optical magnetometers in an all-optical sensor network has the advantages of: (a) ultra-high sensitivity; (b) elimination or reduction of the requirement of calibration and alignment of the system due to the ability to perform scalar measurements; (c) a wide dynamic range, allowing the sensitive device to operate in an unshielded environment, such as in the geophysical field; (d) extremely low power consumption along the network, reduced in practice only to several tens of iW of optical power per sensor; (e) a significant prospect of miniaturization.

It is a purpose of the present invention to provide an optical magnetometer sensor network that provides solutions to the problem of realizing a large- scale all-optical sensor network, comprising a small number of optical fibers for control and communications.

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 networks. As described herein above with reference to Fig. 2, 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.

According to the present invention, an optical magnetometer network is realized by using a single, powerful light source which, along with its associated driving electronics and modulation means, is located at one edge of the network. The light source is coupled to an optical fiber and is operated in a pulsed mode. The pulse of light travels within the optical fiber, and portions of it are split at various locations along the fibers using standard fiber splitters, and coupled into an optical magnetometer assembly. The light exiting the magnetometer assembly is coupled into similar fibers, which are then combined with a second optical fiber serving as an output bus. The signals from each of the magnetometers travel along the output optical fiber and are analyzed at its edge by electronics, following an optical- to-electronic conversion using a photo-detector.

Herein the term "line", when used in relation to arrangement of optical magnetometer sensors, and especially to arrangements of large numbers of optical magnetometer sensors, is used to designate a linear arrangement, wherein the sensors and optionally other components are arranged in a single line. A "network" of optical magnetometer sensors could be comprised of a single line (degenerate), a grid of lines (2D array), or even a 3D grid. A network can also have a closed topology (closed loop).

In a network as described herein the configuration is typically flexible and the light is brought to and from the location of each magnetometer in the network via optical fibers. This is as opposed to an "array", which typically has a more rigid structure. As an example, an array of optical magnetometer sensors is described in co-pending Israeli Patent Application IL 208258 by the same applicant, the description of which, including publications referenced therein, is incorporated herein by reference. In this application the light is brought to each site using planar optics.

The invention is an optical magnetometer sensor network. The network of the invention comprises:

a. an input box comprising at least one laser source, driving electronics to operate the lasers in pulse mode and modulation means to control the properties of the output pulses of the lasers;

b. at least one input fiber optically coupled to the at least one laser;

c. at least one output optical fiber;

d. a plurality of sensor segments distributed along the input fiber each of the sensor segments comprising: i. a fiber splitter, which splits off a small portion of the light pulse traveling in the input fiber;

ii. an optical magnetometer physics package, the input optical elements assembly of which are coupled to the fiber splitter;

iii. at least one fiber combiner distributed along the at least one output fiber wherein the

iv. fiber combiner is optically coupled to the output optical elements assembly of the physics package;

e. an output box optically coupled to an end of the at least one output optical fiber, the output box comprising electronic and optical components adapted to analyze the light traveling along the at least one output optical fiber.

Embodiments of the optical magnetometer sensor network of the invention comprise a mirror after the output optical elements assembly of the physics package in each sensor assembly. The mirror is adapted to reflect the light exiting the physics package back through the optical path, allowing the input fiber to also serve as the output fiber and components of the input box and the components of the output box to be located in a single box.

Embodiments of the optical magnetometer sensor network of the invention comprise an input monitor box optically coupled to the end of the input fiber that is not coupled to the input box. In these embodiments the input monitor box is adapted to probe changes in the properties of light traveling the entire length of the system via the input fiber and to be used in signal analysis.

In embodiments of the optical magnetometer sensor network of the invention the network is connected in a closed loop configuration. Embodiments of the optical magnetometer sensor network of the invention comprise an optical manipulation block associated with each sensor segment. The optical manipulation block adapted to perform manipulation on the light used in the optical magnetometer physics package, thereby allowing signals coming from different sensor segments along the network to be distinguished from each other. The optical manipulation block can be located at one of the following locations:

a. on the input fiber adjacent to and before the fiber splitter;

b. just before entering the physics package;

c. right after exiting the physics package; and

d. on the output fiber adjacent to and after the fiber combiner.

The properties of the optical pulses that are manipulated by the optical manipulation block are at least one of a group comprising: timing dynamics, light amplitude, polarization, spectral wavelength, and line-width.

In embodiments of the optical magnetometer sensor network of the invention manipulating the timing dynamics comprises delaying the light traveling along the input fiber for a predetermined time between adjacent sensor segments. The light can be delayed by being directed through a passive length of optical fiber of optical fiber; by an optical-to-electronic signal conversion, followed by an electronic delay circuit, and further followed by an electronic-to-optical conversion, or by use of an optical manipulation block composed of an assembly comprising a material which has a high degree of dispersion, which can be used to slow/store light, according to the dynamics applied on the light fields passing through it.

The optical manipulation block of the optical magnetometer sensor network of the invention may comprise magnetic shielding. Embodiments of the optical magnetometer sensor network of the invention comprise means for supplying electrical power to the consumers in each optical manipulation block. The means for supplying electrical power can comprise a power supply in the input box and a main power supply line from which are split segment power supply lines that bring electrical power to each optical manipulation block. The means for supplying electrical power can comprise a battery which feeds power to an electrical circuit in each optical manipulation block. In embodiments of the invention the means for supplying electrical power comprise at least one solar panel connected through segment power supply lines to a power unit in each optical manipulation block. In other embodiments the means for supplying electrical power comprise at least one control laser that is coupled to at least one control fiber, which runs along the input fiber of the sensor network, fiber splitters that direct a portion of the light from the control laser to a photo-voltaic cell in each of the optical manipulation blocks through segment control input fibers; wherein the photo-voltaic cells supply electrical power to an electrical circuit in the optical manipulation block.

In embodiments of the optical magnetometer sensor network of the invention the properties of the optical pulses are manipulated by means of optical switches. Embodiments of the optical magnetometer sensor network comprise at least one control laser emitting light with predetermined optical parameters, a main optical switch, which provides an optical signal via a separate segment optical switching component control signal input fibers to segment optical switching components, each located at each sensor segment of the optical magnetometer sensor network.

Other embodiments comprise at least one control laser unit that includes a wavelength scanning control laser that is coupled to at least one control laser fiber and a fiber splitter at each sensor segment. The fiber splitter splits a portion of the control laser light and directs the portion in the - in direction of the optical manipulation block through a fiber spectral filter. In these embodiments the optical manipulation block comprises an optical switch and each of the fiber spectral filters has a different central wavelength.

Embodiments of the optical magnetometer sensor network of the invention comprise at least one control laser unit that includes a control laser that emits light at a fixed wavelength coupled to at least one control laser fiber and a fiber splitter at each sensor segment that splits a portion of the control laser light towards the optical manipulation block. The optical manipulation block comprises a polarization controller, which is adapted to perform at least one of rotating the polarization of the light coming out of the magnetometer physics package by a fixed measure and modulating the polarization of the light in some time dependent fashion. A unique polarization manipulation is used for each of the sensor segments.

Embodiments of the optical magnetometer sensor network of the invention comprise at least one control laser unit that includes a control laser that emits light at a fixed wavelength coupled to at least one control laser fiber and a fiber splitter at each sensor segment that splits a portion of the control laser light towards the optical manipulation block. The optical manipulation block comprises a wavelength converter that receives light from the control laser and converts it to a different wavelength for each of the different sensor segments.

The optical magnetometer sensor network of the invention can comprise at least one optical sensor in addition to the optical magnetometers. The at least one additional sensor can be selected from the group consisting of: acoustic, seismic, pressure, chemical, or optical sensors. 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:

— Detection system 1000

— Detection segment 100

— Detection cluster 10

— Magnetic sensor 1

— Detection head 2

— Coordinator unit 3

— Power supply 4

— Segment coordinator 5

— Command, control, and communication unit 6

— Power and communication cable 7

— Optical magnetometer 2000

— Sensor head 200

— Physics package 20

— Laser 21

— Laser driving electronics 22

— External modulation electronics 23

— First optical elements assembly 24

— Vapor cell coils 25

— - Vapor cell heaters 26

— Vapor cell 27

— Second optical elements assembly 28

— Photo-detector 29

— Signal electronics assembly 30

— Input box 50

— Input fiber 51

— Sensor segment 60

— Fiber splitter 61

— Splitter output fiber 62

— Input optical elements assembly 63

— Vapor cell 64

— Output optical elements assembly 65

— Combiner input fiber 66

— Fiber combiner 67

— Mirror 68

— Output fiber 71

— Output box 70

— Second output fiber 72

— Combined input / output box 57 — Input monitor box 80

— Optical manipulation block 90

— Fiber delay line 91

— Optical-to-electrical signal converter 92

— Electronic delay circuit 93

— Electrical-to-optical signal converter 94

— Power unit 95

— Slow / stored light input optical elements assembly 96

— Slow / stored light vapor cell 97

— Slow / stored light output optical elements assembly 98

— Magnetic shielding 99

— Optical manipulation output fiber portion 101

— Main power supply line 300

— Segment power supply line 310

— Battery 320

— Electrical circuit 321

— Voltage supply line 322

— Solar panel assembly 330

— Control laser unit 340

— Control fiber 341

— Segment control input fiber 342

— Photo-voltaic cell 343

— Additional input optical elements assembly 401

— Additional Optical sensor 402

— Additional output optical elements assembly 403

— Additional output fiber 404

— Main optical switch 450

— Segment optical switching component control signal input fiber 451

— Control laser fiber 460

— Fiber spectral filter 461

— Segment optical switching component 463

— Polarization controller 470

— Wavelength converter 480

Brief Description of the Drawings

— Fig. 1 of the prior art schematically illustrates the concept of a detection system based on magnetic sensors;

— Fig. 2 of the prior art illustrates the main building blocks of an optical magnetometer;

— Fig. 3A to Fig. 3F schematically illustrate several basic schemes of the present invention; — Fig. 4 schematically illustrates the solution for the problem of distinguishing between signals coming from different sensors along the network;

— Fig. 5A to Fig. 5C schematically illustrate realizations of an optical manipulation block as an optical delay, according to the present invention;

— Fig. 6A to Fig. 6E schematically illustrate means for supplying electrical power to the consumers in the optical manipulation block;

— Fig. 7A and Fig. 7B schematically illustrate embodiments of an optical manipulation block based on optical switching;

— Fig. 8A and Fig. 8B schematically illustrate an embodiment of an optical manipulation block based on varying the polarization of the signal light coming from the vapor cell;

— Fig. 9A and Fig. 9B schematically illustrate an embodiment of an optical manipulation block that uses a wavelength converter; and

— Fig 10 schematically illustrates an embodiment of the invention in which the optical network contains additional optical sensors in addition to the magnetic sensor.

Detailed Description of Embodiments of the Invention

The present invention is a large-scale, low cost, network of optical magnetometers. Herein the term "large-scale" is used to describe networks comprised of tens and even hundreds of optical magnetometers. The invention presents a novel way of realizing such a network while overcoming the problem of separation of signals coming from the various sensors in the network while using only a small number of optical fibers, very little power consumption along the line, and eliminating the strict requirements of alignment and calibration of each element on the network.

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. 2. 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-scale, low- power and simple networks.

The present invention provides solutions to most of the above mentioned issues, in a low-cost way. 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. 2, optical fibers, and a means for optical manipulation which serves to enable distinguishing between signals coming from the various magnetometers distributed along the network.

Referring again to the drawings, Fig. 3A to Fig. 3F illustrate several basic schemes of the present invention. In Fig. 3A the most basic optical network with a single input fiber and single output fiber is illustrated. The input box 50 contains the laser source and optical means to manipulate the laser and couple it to the input fiber 51. The laser is operated in pulsed mode and light pulses having properties suitable for operating the sensors' physics packages 20 distributed along the line or network travel along the input fibers 51. It should be noted that, according to the operating technique used for magnetometry in this system, more than one type of optical field may be used. For example, the light used to optically pump the atoms in the vapor cells 64 may be of different wavelengths, different polarizations, etc. than that used for probing the changes in polarization of the atoms (when performing a measurement). Additionally, in some techniques such as in amplitude-modulated NMOR, the optical pumping light is amplitude modulated while the probing light is not. All of this manipulation is performed in the input box 50, which may contain more than one laser if required.

Along the input fiber 51 are distributed sensor segments 60. In this figure, two such segments are shown for illustrative purposes, although the number of segments is actually larger. The segments are spaced from each other by some distance x which can be on the order of 1-lOm, or any other distance. Each segment 60 comprises a fiber splitter 61 in which a small portion of the light pulse traveling in the input fiber 51 is split to the splitter output fiber 62. Typically, an optical magnetometer requires only a few tens of \iW for operation, although this number may vary according to the measurement technique and dynamics used. The light is directed from the splitter output fiber 62 to the physics package 20, containing an input optical elements assembly 63, followed by a vapor cell 64, and an output optical elements assembly 65. Finally, the light is coupled to a fiber combiner input fiber 66, followed by a fiber combiner 67, and to an output optical fiber 71. The light collected from each of the sensing segments 60 on the output fiber 71 travels to the output box 70, located at an edge of the line or network, where it is analyzed. The basic scheme shown in Fig. 3A has several variations. Fig. 3B illustrates a variation of the basic scheme, where two output fibers 71 and 72 are used, as is required in some measurement techniques, such as where the difference in two orthogonal light polarization component are analyzed, after being separated in the output optical elements assembly 65, and coupled to separate output fibers 71 and 72 via two similar fiber combiners 67. Again, in this scheme all of the analysis is performed in the output box 70, at the end of the line / network.

Another variation of the basic scheme is illustrated in Fig. 3C. Here, the input fiber 51 serves also as the output fiber. This is accomplished by adding a mirror 68 after the output optical elements assembly 65 at each segment 60. The signal is reflected back through the optical path, and analyzed at the edge of the line or network, where the input box is now a combined input / output box 57. Some measures need to be employed in this scheme to protect the laser sources from the radiation coming back in their direction from the sensor line, which can be harmful for their operation. In addition, some light properties need to be manipulated such that the light traveling back to the combined input / output box 57 will be directed inside the box for analysis. This may rule out some measurement techniques, for example if this manipulation is on the light polarization degree of freedom. This scheme has the advantage of even being more compact than the basic one, as it employs only a single fiber along the sensor line / network, fewer losses due to fiber splicing, and less components. It is thus also more robust than the basic scheme described in Fig. 3A above.

Fig. 3D illustrates another variation on the basic scheme, wherein the output box 70 is located on the same side of the array as the input box 50. This requires a slight change in the geometry of the fiber coupling out of the physics packages 20 such as to the direct the light to the desired direction, i.e. towards the beginning of the sensor line / network. This change of geometry, however, does not necessarily imply that different components must be used. This scheme serves to concentrate all electronics of both the input and output boxes 50, 70 at the same edge of the line / network, thus simplifying the system.

Fig. 3E shows a variation of the basic scheme, having an input monitor box 80 located at the end of the input fiber 51. Such a monitor box may serve for probing changes in the properties of light traveling the entire length of the system in the input fiber 51, and can be used in signal analysis.

Finally, Fig. 3F shows a scheme where the network is connected in a closed loop configuration.

It should be understood that the basic scheme is very flexible, and more complex configurations are possible, such as grids of sensors connected to a single input/output box 57, and to each other through various fiber splitters 61 and combiners 67.

However, a critical problem arises in the basic scheme described above. The pulse duration required for the operation of an optical magnetometer is related to the coherence time of the probed atoms. For the most sensitive operation, this coherence time is on the order of milliseconds. In some instances, the resulting requirement on the pulse duration could be as low as several tens of s depending upon the requirements of the specific application and the internal composition of the vapor cells used. This time scale is much longer than the time it takes an optical pulse to travel the distance between sensing segments 60, which is in the nano-second scale. This implies that the pulses accumulate on the output fiber 71 after probing each of the sensing segments 60 arrive to the output box 70 at essentially the same time, with a very high degree of overlap. There is therefore a stringent need for some kind of manipulation of the optical signals used in the system, in order that a single (or two) output fiber 71 may still be used, and such that the analysis at the output box 70 will remain simple.

In this invention there are disclosed several methods for overcoming this problem. Each has its advantages and short-comings, and the choice of the most suitable solution is highly dependent on the specific application amongst other factors.

One or more of the following properties of the optical pulses may be manipulated: timing dynamics, light amplitude, polarization, spectral wavelength or line-width. However, not all of these properties may be manipulated at any given part of the system, as there are clear requirements as to the light which is to be used for magnetometry. Moreover, it is preferable to perform manipulation on light before it is used for performing a measurement, rather than manipulate the light components which already contain information from the measurement. Generally, an optical manipulation block Mi is required for each i-th sensing segment 60.

Fig. 4 illustrates the solution for the problem of distinguishing between signals coming from different sensors along the network by introducing an optical manipulation block Mi 90 at each segment, according to the present invention. The four possible locations of the optical manipulation block 90, (i.e. on the input fiber 51 portion before it is split in the fiber splitter 61, just before entering the physics package 20, right after it, or on the output fiber 71), are shown, although only a single optical manipulation block is in principle needed at each segment 60. The optimal location of optical manipulation block 90 depends on the specific implementation. Some examples to illustrate the principle involved in choosing the location will be given herein below. In the following are presented several different examples of realizations of the optical manipulations that can be performed in the optical manipulation blocks 90. These include delaying the optical pulse for a predetermined time duration, switching solutions, polarization modulation, or wavelength conversion. These are described here as examples, and it should be understood that these suggestions should not limit other types of manipulations along the lines of the general description presented herein.

It may appear at first that the simplest manipulation is to delay the light traveling along the input fiber 51 for a predetermined time between adjacent sensing segments 60. Fig. 5A to Fig. 5C schematically illustrate realizations of the optical manipulation blocks 90 as an optical delay, according to the present invention.

First, Fig. 5A presents the simplest optical delay, based on a length of fiber. The light simply travels through a coiled length of fiber before being allowed to proceed forward. This type of manipulation block may be positioned either on the input fiber 51, or on the output fiber 71, the former being preferable. The fixed time delay is accumulated over the sensing segments 60 distributed in the system. This solution should not suffer from severe power losses, as it employs only simple fiber splices. It could also be used in the sensing segment itself, however in this case a different length of fiber is required at each segment in order to realize a different delay between each of the segments. This of course complicates the design considerably, and is less preferable. However this solution, of a simple passive length of fiber is impractical in many scenarios, since for a delay on the order of up to 1 ms, a fiber length of tens and even up to 200 km is needed. While technological advances in compact fiber spools are increasingly widening the delay range possible, this solution becomes very expensive, certainly in systems comprising many sensing segments, each of which needs such a component. Other realizations of an optical delay are possible. Fig. 5B illustrates an optical delay realization by an optical-to-electronic signal conversion, followed by an electronic delay circuit, and further followed by an electronic- to-optical conversion. Light of wavelength A_s enters the optical manipulation block 90 and is detected on an optical-to-electrical signal converter 92, such as a photodiode or an analog opto-coupler. The electrical (converted) signal is fed into an electronic delay circuit 93, and again converted to an optical signal through an electrical-to-optical converter 94. These components are well known in the field of electronics, and are commercially available. Generally, the output signal from this device is not of the same wavelength as that of the input signal, but rather at some wavelength λο. This implies that such a device cannot be located before the optical magnetometer, but only after it, as the magnetometer requires light having very specific properties for its operation. A device as is illustrated in this figure is not all- optical, and requires some electrical power for operation. Electrical feed solutions are described below and illustrated in Fig. 6.

Another realization for optical delay may be to use a technology similar to optical magnetometry, which also relies on ultra-fine laser spectroscopy of an atomic species, to generate slow light. Fig. 5C shows schematically a delay realization by using slowed and/or stored light. Here, the optical manipulation block 90 is composed of an assembly comprising a material which has a high degree of dispersion, which can be used to slow/store light, according to the dynamics applied on the light fields passing through it. Such an assembly has similar structure to that comprising the physics package 20 of an optical magnetometer. The light is coupled from the fiber to slow/stored light input optical elements assembly 96, followed by a slow/stored light vapor cell 97, and a slow / stored light output optical elements assembly 98, before being coupled back to the fiber on which this block is located. In light slowing and storing two light components are manipulated such that an optical pulse of the weaker of the two beams is slowed during the interaction with the atomic sample in the vapor cell, and can even be stored for a limited time. A notable example for light slowing and storage with minimal loss has been demonstrated by Novikova et. al [33]. In this work the dynamics of the light field was chosen to minimize signal loss during storage times of up to 320 μβ.

It should be noted that such an optical delay device is a novel idea, and is not yet commercially available. The location of this type of optical manipulation block will be determined according to its efficiency, namely how much light amplitude is lost through the device. Referring to Figs. 5 A to 5F, if negligible amplitude loss is achieved, then a series of identical (fixed) delay blocks could be positioned on the input- or output-fibers 51, and 71, with the former being preferable. If the amplitude loss is small but non- negligible, the block could be positioned on the fiber portions 62 and 66, and then the duration of the delay needs to be engineered to be different for each segment on the line / network. A clear advantage of this scheme lies in the device being truly all-optical, and thus it requires no electrical feed. The device can use the same optical field used for the optical magnetometry, although clearly careful engineering of the subsequently operating components needs to be done. It should be noted that typically light slowing and storing requires a precise light frequency, and environmental disturbances causing light shifts or broadenings are undesired. Therefore magnetic shielding 99 may be needed for some applications of this device. Very compact magnetic shielding has been demonstrated in the NIST group [31].

As was shown in Fig. 5B, in some realizations an electrical power feed is required for the optical manipulation block 90. As will be seen below, other types of realizations for the optical manipulation block 90 also require electrical power, and so we now pause to consider possible solutions for this issue. Fig. 6A to Fig. 6D illustrate means for supplying electrical power to the consumers in the optical manipulation block 90, where relevant, according to the present invention. Fig. 6A illustrates a power line along the network. Here, the input box 50 includes a power supply 4, and a main power supply line 300, from which segment power supply lines 310 are split. These bring electrical power to the optical manipulation block 90 at each segment. This solution is perhaps the most stable solution; however it suffers from the already known power feeding problem present in other systems. The all- optical nature of the optical magnetometer network is somewhat lost.

Fig. 6B illustrates a battery-based power supply. Here, the power unit 95 present in each optical manipulation block 90 comprises a battery 320 which feeds power to an electrical circuit 321 which serves to supply the operating voltage to consumers in the optical manipulation block 90, through voltage supply lines 322. The number of supply lines depends on the number of consumers of power in the unit. The electrical circuit 321 can be used to stabilize the voltage supplied by the battery 320, if needed. Limitations to this scheme are the finite capacity of batteries and their life time. The capacity issue poses a requirement that the consumers in the optical manipulation block 90 be capable to work at very low voltages. The lifetime of batteries, due to their internal mechanisms, is limited, but can today reach up to 10-20 years of operation.

Fig. 6C shows a solar panel-based power supply. A solar panel 330 is connected through segment power supply lines 310, to the power unit 4 in each optical manipulation block 90. The solar panel converts solar energy to electrical power, used to operate the consumers in the manipulation block. It could be understood, that according to the capabilities of the chosen solar panels and electrical circuits in this scheme, a single solar panel 330 may be used to supply power to more than a single optical manipulation block 90, thereby realizing a hybrid solution of those presented in Fig. 6A and Fig. 6C. Shortcomings of the solar panel scheme are in its limited power capabilities, its dependence on external conditions of sun light, and in it being visible - thereby a covert system employing it loses part of its advantage.

Fig. 6D illustrates a power supply scheme based on a photovoltaic cell 343 illuminated by a control laser unit 340 located at an end of the optical network, providing electrical power to the consumers in the optical manipulation block 90. The control laser 340 is coupled to a control fiber

341, which runs along the system's input fiber 51. Using fiber splitters 61, a portion of the light from the control laser 340 is directed to each of the optical manipulation blocks 90 through the segment control input fibers

342. As shown in Fig. 6E, the power unit of the optical manipulation block 90 in this case is composed of a photo-voltaic cell 343, which is optically fed from the control laser 340 through fibers 341 and 342, and supplies electrical power to the electrical circuit 321, which stabilizes the power and distributes it to the various consumers in the manipulation block 90, via voltage supply lines 322. In some instances this solution would be preferable, as this solution for supplying electrical power is done in an all- optical manner. The addition of an additional fiber along the system, and a control laser, should not impose a much higher level of complexity and cost.

In Figs. 5A to 5F realizations of an optical delay component to overcome the problem of distinguishing between overlapping signals coming from the various sensors along the line / network, in the analysis done in the output box have been illustrated. Optical manipulation could also be performed by varying the properties of the light, rather than its time dynamics.

Fig. 7A and Fig. 7B illustrate embodiments of the optical manipulation block 90 based on optical switching, according to the present invention. Fig. 7A illustrates use of a control laser 340 shining light with fixed optical parameters and a main optical switch 450 (Main OS), which provides an optical signal to a segment optical switching component 463 (OS) located at each segment 60 of the optical network via segment optical switching component control signal input fibers 451. The main OS 450 serves as a single switching unit at the edge of the line. Its power requirements are met by the infrastructure at the input box 50. The optical switch OS 463, controls whether the light coming to the magnetometer physics package 20 is allowed to pass the switch 463 or be blocked. This scheme is suitable for networks of limited number of sensors, since distributing a large amount of fibers 451 becomes cumbersome, even though solutions to similar problems have been found in the field of optical communications.

A variation on this embodiment is illustrated in Fig. 7B, where the control laser unit 340 includes a wavelength scanning control laser, coupled to a single control laser fiber 460. At each segment 60, a portion of the control laser light is split in an ordinary fiber splitter 61 towards the optical manipulation block comprising an optical switch 463. A fiber spectral filter 461, of different central wavelength (λί) is located between fiber splitter 61 and optical switch 463 at each segment 60 of the optical network. The spectral filter 461 is connected on one side to the fiber splitter 61 through the splitter's secondary output fiber 62, and on its other side to the OS 463 through the segment optical switching component control signal input fiber 451. The control laser 340 wavelength is scanned such that it covers the span of wavelengths defined by the various spectral filters 461 distributed in the system. These spectral filters may be made for example from free- space optical components such as gratings, or from fiber gratings. Light passes the filter only when its wavelength matches that of the spectral filter 461. Once passing the filter, this light feeds the OS 463 which serves to allow the light traveling to the magnetometer's physics package 20. The scanning rate of the control laser 463 wavelength is such that a suitable duration of time is introduced between each segment 60 hence allowing the signals coming from various segments 60 on the output fiber 71 can be distinguished. This embodiment has the advantage of being all-optical. Its complexity, and hence its cost, depend mainly on the properties of the spectral filters 461. The more these filters are spectrally narrow, the smaller the range of required wavelengths the scanning control laser 340, implying a simpler and less expensive laser.

Fig. 8A and Fig. 8B illustrate an embodiment of the optical manipulation block 90 based on varying the polarization of the signal light coming from the vapor cell 64 using a polarization controller (PC) 470 as shown in Fig. 8B, according to the present invention. In this embodiment light is directed from the control laser unit 340 (at a fixed wavelength) to the optical manipulation block 90, where it is coupled to serve as an operating signal to a polarization controller 470, which can either rotate the light coming out of the magnetometer physics package (of some known wavelength A_s and polarization ε) 20 by a fixed measure (2 angles define the light ellipticity), or modulate its polarization in some time dependent fashion. For each segment 60, a unique polarization manipulation is used, such that the various signals from the sensors along the line or network could be separated according to their polarization (ε', while still of the same wavelength A_s), and measured separately in the output box 70 at the end of the line / network. In this embodiment also, it should be noted that not all optical magnetometry measurement techniques may be favorable for use, especially those employing measurement on the light polarization degrees of freedom.

Fig. 9A and Fig. 9B illustrates an embodiment of the optical manipulation block 90 that uses a wavelength converter 480 (λ-C or λ-converter), according to the present invention. A wavelength converter is a component which receives light of a certain wavelength A_s and converts it to a different wavelength Ai for each of the different segments 60. Although in the field of optical communications wavelength converters are found which are all- optical, in general they require electrical power to operate. In the embodiments of this invention alternative types of power supply such as those described in relation to Figs. 6B to 6E can be employed.

An advantage of an optical network of sensors as have been illustrated herein is the possibility to use it not only for optical magnetometers. Fig 10 schematically illustrates an embodiment of the invention in which the optical network contains additional optical sensors in addition to the magnetic sensor. An additional optical sensor 402 may be coupled to the system in a similar fashion to that with which the optical magnetometer is. It may require additional input and output optical elements assemblies 401 and 403, respectively, as well as an additional output fiber 404. Such additional sensors may be acoustic, seismic, pressure, chemical, or other types of optical sensors. The inclusion of several types of sensors on the line / network enhances its capabilities, as a more complete solution to a variety of problems. An example for such an application is in a detection system for the marine environment. This could be realized by combining passive sonar (hydrophone) detectors and magnetometers on the same line. While hydrophones have a relatively large detection range, they rely on mechanical disturbances in the aquatic medium, which arise from the target. Hydrophones typically generate a detection signal which can give the range to the target, but not the orientation of it. Hydrophones can be fabricated using optical fibers which include fiber Bragg gratings as the mechanically sensitive element. Combining hydrophones with magnetometers will realize a system which can be used even if the target does not generate a mechanical disturbance in the water, can give the target's orientation, and can provide information whether the detected object comprises ferromagnetic material. 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.

Bibliographv

Claims

1. An optical magnetometer sensor network comprising:

a. an input box comprising at least one laser source, driving electronics to operate said at least one laser source in pulsed mode and modulation means to control the properties of the output pulses of said at least one laser source;

b. at least one input fiber optically coupled to said at least one laser; c. at least one output optical fiber;

d. a plurality of sensor segments distributed along said input fiber each of said sensor segments comprising:

i. a fiber splitter, which splits off a small portion of the light pulse traveling in said input fiber;

ii. an optical magnetometer physics package, the input optical elements assembly of which are coupled to said fiber splitter;

iii. at least one fiber combiner distributed along said at least one output fiber, said fiber combiner being optically coupled to the output optical elements assembly of said physics package; e. an output box optically coupled to an end of said at least one output optical fiber, said output box comprising electronic and optical components adapted to analyze the light traveling along said at least one output optical fiber.

2. The optical magnetometer sensor network of claim 1, comprising a mirror after the output optical elements assembly of said physics package in each sensor assembly; said mirror adapted to reflect the light exiting said physics package back through the optical path, allowing the input fiber to also serve as the output fiber and components of the input box and the components of the output box to be located in a single box.

3. The optical magnetometer sensor network of claim 1, comprising an input monitor box optically coupled to the end of the input fiber that is not coupled to the input box, wherein said input monitor box is adapted to probe changes in the properties of light traveling the entire length of the system via said input fiber and to be used in signal analysis.

4. The optical magnetometer sensor network of claim 1, wherein said network is connected in a closed loop configuration.

5. The optical magnetometer sensor network of claim 1, comprising an optical manipulation block associated with each sensor segment, said optical manipulation block adapted to perform manipulation on the light used in the optical magnetometer physics package, thereby allowing signals coming from different sensor segments along the network to be distinguished from each other.

6. The optical magnetometer sensor network of claim 5, wherein the optical manipulation block is located at one of the following locations:

a. on the input fiber adjacent to and before the fiber splitter;

b. just before entering the physics package;

c. right after exiting said physics package; and

d. on the output fiber adjacent to and after the fiber combiner.

7. The optical magnetometer sensor network of claim 5, wherein the properties of the optical pulses that are manipulated by the optical manipulation block are at least one of a group comprising: timing dynamics, light amplitude, polarization, spectral wavelength, and line- width.

8. The optical magnetometer sensor network of claim 7, wherein manipulating the timing dynamics comprises delaying the light traveling along the input fiber for a predetermined time between adjacent sensor segments.

9. The optical magnetometer sensor network of claim 8, wherein the light is delayed by being directed through a passive length of optical fiber of optical fiber.

10. The optical magnetometer sensor network of claim 8, wherein the light is delayed by an optical-to-electronic signal conversion, followed by an electronic delay circuit, and further followed by an electronic-to-optical conversion.

11. The optical magnetometer sensor network of claim 8, wherein the light is delayed by use of an optical manipulation block composed of an assembly comprising a material which has a high degree of dispersion, which can be used to slow/store light, according to the dynamics applied on the light fields passing through it.

12. The optical magnetometer sensor network of claim 11, wherein the optical manipulation block comprises magnetic shielding.

13. The optical magnetometer sensor network of claim 5, comprising means for supplying electrical power to the consumers in each optical manipulation block.

14. The optical magnetometer sensor network of claim 13, wherein the means for supplying electrical power comprise a power supply in the input box and a main power supply line from which are split segment power supply lines that bring electrical power to each optical manipulation block.

15. The optical magnetometer sensor network of claim 13, wherein the means for supplying electrical power comprise a battery which feeds power to an electrical circuit in each optical manipulation block.

16. The optical magnetometer sensor network of claim 13, wherein the means for supplying electrical power comprise at least one solar panel connected through segment power supply lines to a power unit in each optical manipulation block.

17. The optical magnetometer sensor network of claim 13, wherein the means for supplying electrical power comprise at least one control laser that is coupled to at least one control fiber, which runs along the input fiber of the sensor network, fiber splitters that direct a portion of the light from said control laser to a photo-voltaic cell in each of the optical manipulation blocks through segment control input fibers; wherein said photo-voltaic cells supply electrical power to an electrical circuit in said optical manipulation block.

18. The optical magnetometer sensor network of claim 5, wherein the properties of the optical pulses are manipulated by means of optical switches.

19. The optical magnetometer sensor network of claim 18, comprising at least one control laser emitting light with predetermined optical parameters, a main optical switch, which provides an optical signal via a separate segment optical switching component control signal input fibers to segment optical switching components, each located at each sensor segment of said optical magnetometer sensor network.

20. The optical magnetometer sensor network of claim 18, comprising at least one control laser unit that includes a wavelength scanning control laser, said control laser coupled to at least one control laser fiber; a fiber splitter at each sensor segment that splits a portion of the control laser light directing said portion in the direction of the optical manipulation block through a fiber spectral filter; wherein said optical manipulation block comprises an optical switch and each of said fiber spectral filters has a different central wavelength.

21. The optical magnetometer sensor network of claim 7, comprising at least one control laser unit that includes a control laser that emits light at a fixed wavelength coupled to at least one control laser fiber; a fiber splitter at each sensor segment that splits a portion of the control laser light towards the optical manipulation block; said optical manipulation block comprising a polarization controller; wherein said polarization controller is adapted to perform at least one of rotating the polarization of the light coming out of the magnetometer physics package by a fixed measure and modulating the polarization of said light in some time dependent fashion; wherein a unique polarization manipulation is used for each of said sensor segments.

22. The optical magnetometer sensor network of claim 7, comprising at least one control laser unit that includes a control laser that emits light at a fixed wavelength coupled to at least one control laser fiber; a fiber splitter at each sensor segment that splits a portion of the control laser light towards the optical manipulation block; said optical manipulation block comprising a wavelength converter that receives light from said control laser and converts it to a different wavelength for each of the different sensor segments.

23. The optical magnetometer sensor network of claim 1, comprising at least one optical sensor in addition to the optical magnetometers.

24. The optical magnetometer sensor network of claim 7, comprising at least one additional sensor selected from the group consisting of: acoustic, seismic, pressure, chemical, or optical sensors.