Distributed optical sensing system and method

Abstract

A distributed optical detection system comprising: a broadband light source; and a phase and amplitude receiver for measuring the phase and amplitude of a distributed backscatter signal from a sensing medium. Also disclosed is a method of quantitatively sensing changes in optical path length along a sensing medium in a distributed manner.

Description

[0001] This application is a divisional application of the Chinese national phase patent application with application number 201880047418.4, which was filed on January 16, 2020, after the international application PCT application with application number PCT / AU2018 / 050775, international application date July 26, 2018, entitled "Distributed Optical Sensing System and Method". Technical Field

[0002] This invention relates to systems and methods for quantitative and distributed measurement of changes in optical path length in an optically transparent medium, and more particularly to systems and methods for quantitative and distributed measurement of changes in optical phase along an optical path in an optically transparent medium.

[0003] The present invention was developed primarily for a method and system for quantitatively and distributedly sensing changes in the optical path length along an optical fiber caused by acoustic energy, elastic energy, seismic energy, vibrational energy, thermal energy, or kinetic energy, as described below with reference to this application. However, it will be understood that the invention is not limited to this specific field of use. Background Technology

[0004] Any discussion of the background art throughout the specification should not be construed as an admission that such background art is prior art, nor as an admission that such background art is well known in Australia or worldwide or is part of the general knowledge in the field.

[0005] All references cited in this specification (including any patents or patent applications) are incorporated herein by reference. No reference is acknowledged to constitute prior art. The discussion of the references states the claims of their authors, and the applicant reserves the right to challenge the accuracy and relevance of any cited references. It should be clearly understood that although this document cites numerous prior art publications, such citations do not imply an acceptance that any of these publications constitutes part of common general knowledge in the art, whether in Australia or in any other country.

[0006] When applying interference detection to perimeter or infrastructure security, it is often necessary to detect and locate a single point of interference (without quantitative measurement). US Patent No. 7,995,197 B2, granted to Sikora, discloses a continuous-wave incoherent light source for detecting singular interference on optical fibers, without teaching how to locate the interference point along the fiber. US Patent No. 8,670,662 B2, granted to Healey, extends Sikora's teachings, proposing a method for locating such singular interference using a frequency-modulated device. However, both Sikora and Healey detect interference by relying solely on detecting changes in the intensity of backscattered light using only an intensity detector that cannot directly measure phase. The magnitude of interference on an optical fiber has a highly nonlinear and non-unique relationship with the resulting intensity fluctuations. Therefore, it is practically impossible to accurately measure changes in optical path length (or other physical parameters such as strain). Furthermore, Sikora suggests using the unique spectral characteristics of multiple interferences to distinguish them. This requires prior knowledge of the interferences in order to locate multiple interferences. Therefore, the method taught by Sikora and Healey does not provide, for example, a distributed sensing device for measuring changes in the length of multiple optical paths within a sensing medium.

[0007] For example, as taught in U.S. Patent No. 6,459,486 B1 to Udd and European Patent No. EP1,496,723 B1 to Vakoc, Sagnac-based fiber optic sensing systems can operate using broadband or incoherent light sources. However, they rely on bidirectional or back-propagating optical paths to utilize the Sagnac effect and clearly cannot be extended to the case of distributed backscattering, where no access to both ends of the sensing fiber is provided.

[0008] Recently, other applications, such as seismic or acoustic recording, have required advanced system designs to enable quantitative, highly sensitive, and distributed measurements of fiber strain changes along optical fibers. Applications of this technology include: seismic profiling analysis of oil and gas fields; microseismic monitoring during hydraulic fracturing reservoir enhancement; rock and soil surface surveying; and fluid flow measurement. Technological advancements supporting these applications have led to increased cost and complexity of the systems used.

[0009] Distributed acoustic sensing (DAS, also known as distributed vibration sensing) is a method that uses Rayleigh backscattering from an optically transparent sensing medium (typically an optical fiber) to measure minute changes in the physical properties of the sensing medium along its entire length in a distributed manner. Therefore, as used herein, distributed sensing or distributed acoustic sensing (DAS) is defined as a technique that enables continuous, real-time measurement of the amplitude and / or phase of a distributed backscattered optical signal along the entire length of the sensing medium (i.e., at every point along the optical path).

[0010] Typically, using optical fibers (with or without cable sheath protection) as distributed sensors can replace many point sensors. As a result, it is likely the most cost-effective and weight- and space-efficient sensor system available, as it requires only one fiber capable of transmitting, receiving, and sensing signals from the same fiber, and a single monitor is sufficient to display localized changes in temperature, stress, vibration, and acoustic energy. Furthermore, optical fibers are well-suited for operation in harsh environments, at high temperatures, and are unaffected by damage or noise caused by electromagnetic interference. This significant lightweight advantage makes distributed sensors based on distributed light scattering in optical fibers the most versatile monitoring option, even compared to point fiber optic sensors, especially for monitoring long linear assets such as pipes, wells, railways, roads, conveyors, bridges, tunnels, buildings, and fences.

[0011] Known DAS systems operate by coupling highly coherent (narrowband) laser energy pulses into an optical fiber and analyzing Rayleigh backscattering caused by microscopic defects and inhomogeneities within the fiber. As the light pulse travels from the input end to the distal end of the fiber, these microscopic defects / inhomogeneities are reflected. This interaction causes a small amount of light to backscatter and return to the input end, where it is detected and analyzed. When acoustic waves interact with the materials constituting the fiber, elongation and minute changes in refractive index occur within the microstructure. These changes affect the backscattering characteristics, becoming detectable events. Using time-domain techniques, the location of these events can be precisely determined, providing fully distributed sensing with a resolution of 1 meter or less.

[0012] The difference between DAS and traditional distributed strain sensing is that it does not use nonlinear Brillouin backscattering for measurement. This enables highly sensitive, linear, and fast distributed measurements. For example, DAS can achieve measurement rates above 2 kHz and sub-nanometer strain sensitivity with spatial resolution below 10 m.

[0013] However, all previous DAS systems share the same drawback: the coherence length of the light source and the associated phase noise are the main factors limiting the system's signal-to-noise ratio, and thus its sensitivity. Therefore, for long-distance DAS, the light source used in the system must have a very narrow bandwidth (narrow linewidth), and thus a very high coherence length, to allow the forward and backward propagating light to interfere and generate an interference signal for analyzing the returned signal. In some DAS systems, the coherence length can reach tens of kilometers, enabling the analysis of Rayleigh backscattered signals at practical distances. Of course, requiring the light source optical signal to meet such a high coherence length parameter adds considerable complexity and cost to the entire DAS system.

[0014] Another drawback of existing DAS systems is the limitation on optical power, which can be transmitted to the sensing medium (e.g., optical fiber) before the nonlinear interaction between the forward-propagating light and the sensing medium itself. This can limit the optical power that can be transmitted over 10,000 or 100,000 meters, or introduce phase and intensity modulation that disrupts the Rayleigh backscattered signal. A typical nonlinear effect in optical fiber is the so-called modulation instability, which causes position-dependent signal attenuation in long-range DAS systems. This attenuation leads to complete masking of the interference signal at certain locations, and thus a loss of sensitivity at those locations. Another typical nonlinear effect that is particularly limiting in optical fiber is Brillouin scattering, which exhibits a significant effect, leading to a loss of the Rayleigh backpropagation signal even at input power levels as low as 100 mW. Since Rayleigh backscattering is a very weak phenomenon, this directly limits the system's sensitivity by limiting the amount of optical power available in the sensing fiber.

[0015] Current DAS methods are based on coherent optical time-domain reflectometry (c-OTDR) or coherent optical frequency-domain reflectometry (c-OFDR). Both methods utilize narrowband lasers with very high temporal coherence. Depending on the method, the coherence length can be greater than 10 m, but is typically much larger, often much greater than 100 km.

[0016] Typically, in c-OTDR and c-OFDR, the low coherence of the laser directly and negatively affects the phase noise of the measurement, and thus severely limits the sensitivity of the system. Furthermore, in many implementations, especially those involving heterodyne detection using a local oscillator, very high coherence is crucial for achieving long measurement ranges, as the coherence length of the laser directly determines the maximum length of the sensing fiber.

[0017] On the other hand, the high coherence of lasers typically leads to higher costs, greater complexity, and poorer system robustness during transport or operation in uncontrolled environments. Vibrations of the source or system can severely impair system performance. Furthermore, narrow-band / narrow-linewidth and therefore highly coherent laser sources exhibit lower nonlinear thresholds in optical fibers, such as stimulated Brillouin scattering, four-wave mixing, and modulation instabilities, which limit the optical power that can be emitted into the fiber. The coherence requirement of the source also makes it difficult to directly modulate the source without degrading coherence. Therefore, an external modulator is required, which increases the cost and complexity of the system.

[0018] Another limitation of using nearly monochromatic or coherent light sources for DAS is that Rayleigh backscattering from the fiber exhibits random amplitude variations along the fiber. At locations where the amplitude is relatively low or zero (known as signal fading), the measured phase is inaccurate or uncertain. This is a significant source of error in coherent optical time-domain reflectometry (c-OTDR) and coherent optical frequency-domain reflectometry (c-OFDR) systems, negatively impacting system sensitivity. To overcome this limitation, multiple highly coherent laser sources of different frequencies are used simultaneously in the system so that their signals can be combined in a way that mitigates attenuation, as taught by AH Hartog et al. in Geophysical Prospecting, Vol. 66, No. S1 (2017), “The use of multi-frequency acquisition to significantly improve the quality of fiber-optic distributed vibration sensing.” The use of multiple highly coherent light sources and multiple frequencies introduces significant cost and complexity into the DAS system.

[0019] Therefore, there is a need for a truly distributed sensing system and a method for querying and detecting changes in the length of multiple optical paths in a medium, with improved noise suppression, increased interference localization accuracy, improved vibration resistance, improved robustness, stability and reliability, and minimal system cost and complexity.

[0020] definition

[0021] The following definitions are provided as general definitions, and the scope of the invention should in no way be limited to these terms, but are set forth for a better understanding of the following description.

[0022] 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 pertains. It will be further understood that the terms used herein should be interpreted as having the same meaning as they have in the context of this specification and the relevant field, and will not be interpreted in an idealized or overly formal sense unless explicitly defined herein. Other terms are defined below for the purposes of this invention. Furthermore, unless there is doubt about the meaning of a particular term, all definitions defined and used herein should be understood as control over the general meaning of the term as defined in a dictionary, in reference to other documents, and / or as commonly used in the context of the term, in which case the general dictionary definition and / or common usage of the term shall prevail.

[0023] For the purposes of this invention, the following terms are defined as follows.

[0024] The articles “one” and “a” are used in this text to refer to one or more (i.e., at least one) grammatical objects of the article. For example, “an element” refers to one or more elements.

[0025] As used herein, the term "about" refers to a quantity that varies by up to 30%, preferably up to 20%, and more preferably up to 10% relative to a reference quantity. The use of the term "about" to limit a number indicates only that the number should not be interpreted as an exact value.

[0026] Throughout this specification, unless the context otherwise requires, the word “comprise” will be understood to imply inclusion of the stated steps or elements or groups of steps or elements, but does not exclude any other steps or elements or groups of steps or elements.

[0027] As used herein, any of the terms “including,” “which includes,” or “that includes” are also open-ended terms, meaning that at least the elements / features following the term are included, but other elements / features are not excluded. Therefore, “including” is synonymous with “comprising.”

[0028] In the claims, the overview above, and the description below, all transitional phrases (such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” etc.) should be understood as open-ended, meaning “including but not limited to.” Only the transitional phrases “consisting of” and “consisting essentially of” should be closed or semi-closed transitional phrases, respectively.

[0029] While any methods and materials similar to or equivalent to those described herein may be used in the practice or testing of the invention, preferred methods and materials are described. It will be understood that the methods, apparatus, and systems described herein can be implemented in various ways and for various purposes. The descriptions herein are merely illustrative.

[0030] As used herein in the specification and claims, the phrase “and / or” should be understood to mean “one or two” of the elements so combined, i.e., elements that appear together in some cases and separately in others. Multiple elements listed with “and / or” should be interpreted in the same way, i.e., “one or more” of the elements so combined. In addition to the elements explicitly identified by the “and / or” clause, other elements may optionally be present, whether related to or unrelated to those explicitly identified elements. Thus, as a non-limiting example, when used in conjunction with open-ended language such as “comprising,” a reference to “A and / or B” may refer only to A in one embodiment (optionally including elements other than B); only to B in another embodiment (optionally including elements other than A); to A and B in yet another embodiment (optionally including other elements); and so on.

[0031] As used herein in the specification and claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when the listed items are separated, “or” or “and / or” should be interpreted as inclusive, that is, including multiple or at least one of the listed elements, but also including more than one element, and (optionally) other items not listed. Terms that only explicitly indicate the opposite meaning, such as “only one of” or “exact one of”, or when used in the claims, “consisting of” will refer to including multiple or exactly one of the listed elements. In general, the term “or” as used herein should only be understood to indicate a unique alternative when following exclusive terms (such as “either,” “one of,” “only one of,” or “exactly one of”). When used in the claims, “consisting essentially of” should have the common meaning used in the field of patent law.

[0032] As used herein in the specification and claims, the phrase "at least one" in relation to a list of one or more elements should be understood to mean at least one element selected from any one or more elements in the list, but not necessarily including at least one element from every single element specifically listed in the list, and does not exclude any combination of elements in the list. This definition also allows for the optional presence of elements other than those explicitly identified in the list referred to by the phrase "at least one," whether related to or unrelated to those explicitly identified elements. Therefore, as a non-limiting example, “at least one of A and B” (or equivalently, “at least one of A or B”, or equivalently, “at least one of A and / or B”) may refer to at least one A in one embodiment, optionally including more than one A, with no B (and optionally including elements other than B); in another embodiment, refer to at least one B, optionally including more than one B, with no A (and optionally including elements other than A); in yet another embodiment, refer to at least one A, optionally including more than one A, and at least one B, optionally including more than one B (and optionally including other elements); and so on.

[0033] For the purposes of this specification, where method steps are described in sequence, such sequence does not necessarily mean that the steps should be performed in that order or in chronological order, unless there is no other logical way to interpret the sequence.

[0034] Furthermore, when the features or aspects of the invention are described in accordance with the Markush group, those skilled in the art will recognize that any individual element or subgroup of elements of the Markush group will also be described in the invention in accordance with the Markush group.

[0035] As used herein in the specification and claims, the phrase "broadband light source" relating to electromagnetic wave sources is used to indicate a light source that generates electromagnetic waves comprising more than one frequency. The frequencies in a broadband light source can be continuous, discrete, or both within a frequency range. The frequencies in a broadband light source can have random or indeterminate phase relationships (i.e., incoherent or partially coherent), such as amplified spontaneous emission (ASE) of an amplifier; SLED; multimode laser; or a combination of multiple independent laser outputs. A broadband light source can also have defined or fixed phase relationships (i.e., coherent) between frequencies, which can be obtained from: mode-locked lasers; amplitude-modulated lasers; polarization-modulated lasers; or frequency-modulated / phase-modulated lasers.

[0036] As used herein in the specification and claims, the phrase "phase and amplitude receiver" is used to describe a receiver system capable of accurately measuring and outputting the following two parameters (or "phase and amplitude measurement" is used to describe a receiving method): the phase difference between two electromagnetic wave inputs (e.g., optical inputs) having complete orthogonality, i.e., unambiguous over a range of 2π radians; and the amplitude of the interference between the two electromagnetic wave inputs (e.g., optical inputs).

[0037] As used herein in the specification and claims, the phrase "optical frequency" is used to describe frequencies ranging from 1 × 10⁻⁶. 13 Hz to 3×10 15 A frequency in the Hz range. A "light source" is an electromagnetic energy source at optical frequencies. Summary of the Invention

[0038] The purpose of this invention is to overcome or improve at least one or more disadvantages of the prior art, or to provide a useful alternative.

[0039] This invention relates to a method for quantitative and distributed measurement of changes in optical path length in an optically transparent medium, such as along an optical fiber. Multiple physical parameters can be sensed by the corresponding changes in optical path length they produce, and these changes can include: longitudinal strain, transverse strain, acoustic waves, seismic waves, vibration, motion, bending, torsion, temperature, optical delay, and chemical composition. Any other physical parameters having mechanisms that cause elongation and / or refractive index changes and / or deformation along the optical path can also be sensed. Changes in optical path length can also occur through the movement of scattering / reflecting particles in the sensing medium. Embodiments of the invention disclosed herein use an intensity-modulated (or pulsed) broadband light source, combined with precise phase and amplitude measurements of delay and distributed backscattering from an optically transparent sensing medium (e.g., an optical fiber).

[0040] According to a first aspect of the present invention, a distributed optical sensing system is provided. The system may include a broadband light source. The system may also include phase and amplitude receivers.

[0041] According to a specific arrangement of the first aspect, a distributed optical sensing system is provided, comprising: a broadband light source; and phase and amplitude receivers. The broadband light source may have a coherence time τ. coh And interchangeably have a coherence length l coh .

[0042] Coherence length l cohIt can be less than 1m, less than 50cm, less than 10cm, less than 9cm, less than 8cm, less than 7cm, less than 6cm, less than 5cm, less than 4cm, less than 3cm, less than 2cm, less than 1cm, less than 9mm, less than 8mm, less than 7mm, less than 6mm, less than 5mm, less than 4mm, less than 3mm, less than 2mm, less than 1mm, less than 0.9mm, less than 0.8mm, less than 0.7mm, less than 0.6mm, less than 0.5mm, less than 0.4mm, less than 0.3mm, less than 0.2mm, less than 0.1mm, or less than 0.05mm, less than 0.04mm, less than 0.03mm, less than 0.02mm, less than 0.01mm, less than 0.005mm, less than 0.004mm, less than 0.003mm, less than 0.002mm, or less than 0.001mm.

[0043] For example, in some arrangements, the coherence length l coh It can be approximately 0.001mm, 0.002mm, 0.003mm, 0.004mm, 0.005mm, 0.006mm, 0.007mm, 0.008mm, 0.009mm, 0.01mm, 0.015mm, 0.02mm, 0.03mm, 0.04mm, 0.05mm, 0.06mm, 0.07mm, 0.08mm, 0.09mm, 0.10mm, 0.11mm, 0.12mm, 0.13mm, 0.14mm, 0.15mm, or 0.16mm. , 0.17mm, 0.18mm, 0.19mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 0.12mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 1cm, 2cm, 3cm, 4cm, 5cm, 6cm, 7cm, 8cm, 9cm, 0.1m, 0.2mm, 0.3m, 0.4m, 0.5m, 0.6m, 0.7m, 0.8m, 0.9m, or 1m.

[0044] The bandwidth of broadband light sources can range from approximately 10MHz to 100MHz, 100MHz to 1GHz, 1GHz to 10GHz, 10GHz to 100GHz, 100GHz to 1THz, 1THz to 10THz, and 10THz to 100THz. That is, 10MHz to 100THz (i.e., 10 × 10⁻⁶). 6 Hz to 100×10 12 Hz).

[0045] For example, in some arrangements, the bandwidth of the broadband light source can be approximately 10MHz, 15THz, 20MHz, 25MHz, 30MHz, 35MHz, 40MHz, 45MHz, 50MHz, 60MHz, 70MHz, 80MHz, 90MHz, 100MHz, 200MHz, 300MHz, 400MHz, 500MHz, 600MHz, 700MHz, 800MHz, 900MHz, 10GHz, 15GHz, 20GHz, 25GHz, 30GHz, 35GHz, 40GHz, 45GHz, 50GHz, 60GHz, 70GHz, 80GHz, 90GHz, 10 ... GHz, 200GHz, 300GHz, 400GHz, 500GHz, 600GHz, 700GHz, 800GHz, 900GHz, 1THz, 1.5THz, 2THz, 2.5THz, 3THz, 3.5THz, 4THz, 4.5THz, 5THz, 5.5THz, 6THz, 6. 5THz, 7THz, 7.5THz, 8THz, 8.5THz, 9THz, 9.5THz, 10THz, 15THz, 20THz, 25THz, 30THz, 35THz, 40THz, 45THz, 50THz, 60THz, 70THz, 80THz, 90THz, or about 100THz.

[0046] Phase and amplitude receivers can be adapted to receive distributed backscattering of optical signals generated by a light source and directed to a medium.

[0047] Broadband light sources can be incoherent or partially coherent. Broadband light sources can also be coherent broadband light sources.

[0048] The system may also include at least one intentionally relative delay path (IRDP). The IRDP can be adapted to delay a portion of the light produced by the light source.

[0049] The light source can be a modulated light source. A modulated light source can be an intensity-modulated or pulsed light source.

[0050] The system may also include a forward optical path adapted to receive output light generated by a light source.

[0051] The system may also include a modulator adapted to modulate at least a portion of the light generated by the light source in the forward optical path. The modulator may be adapted to modulate the intensity of at least a portion of the light in the forward optical path. The modulator may be adapted to modulate the phase of at least a portion of the light in the forward optical path. The modulator may be adapted to modulate the polarization of at least a portion of the light in the forward optical path. The modulator may be adapted to modulate the frequency of at least a portion of the light in the forward optical path.

[0052] The forward optical path can also be adapted to transmit the modulated output light to an optically transparent medium, which is adapted to alter the optical signal in response to physical changes. The optically transparent medium can be adapted to interfere with the optical signal by modifying the path length of the optical signal propagating in the medium. The path length can be the length of the optical path of the optical signal propagating in the medium.

[0053] The forward optical path may include a beam splitter adapted to divide an optical signal into at least two parts. A modulator may be adapted to receive one or both parts of the separated optical signal to provide at least one modulated optical signal.

[0054] The forward optical path may include a first optical delay device having a first delay τ1. The forward optical path may be adapted to receive a first portion of the optical signal from the beam splitter. The forward optical path may also be adapted to subsequently transmit the delayed forward-propagating optical signal to an optically transparent medium.

[0055] The forward optical path may also include a direct optical transmission device adapted to receive a second portion of the intensity-modulated optical signal from the beam splitter and subsequently transmit the directly forward-propagating optical signal to the optically transparent medium, bypassing the first optical delay device. The delayed forward-propagating optical signal may be delayed in time by a first delay τ1 relative to the directly forward-propagating optical signal.

[0056] The system may further include a combining device for combining a direct forward propagation optical signal and a delayed forward propagation optical signal into a common forward optical path. The system may also include an optical guiding device adapted to receive backpropagation light from an optically transparent medium and guide the received backpropagation light to a return optical path. The return optical path may include a return beam splitter adapted to split the backpropagation light into at least two parts. The return optical path may also include a second optical delay device having a second delay τ2. The second optical delay may be adapted to receive a first part of the backpropagation light from the return beam splitter. The second optical delay may also be adapted to transmit the delayed return optical signal to a phase and amplitude receiver.

[0057] The system may also include a direct-return optical transmission device adapted to receive a second portion of the backpropagated light from the return beam splitter. The direct-return optical transmission device may also be adapted to subsequently bypass a second optical delay device to transmit the direct-return optical signal to a phase and amplitude receiver. The delayed-return optical signal may be delayed by a second delay τ2 relative to the direct-return optical signal.

[0058] The first optical delay device and the second optical delay device can be equipped with a delay τ. com Public delay device.

[0059] The system may include a first optical delay device. The system may include an optically transparent medium adapted to modify the path length of an optical signal in response to external interference. The system may include an optical intensity modulation device to receive optical output from a broadband light source to provide an intensity-modulated optical signal. The intensity-modulated optical signal may be divided into two parts, wherein a first part of the modulated optical signal may be directed to the first optical delay device to generate a delayed output signal, and subsequently to the optically transparent medium, and wherein a second part of the modulated optical signal (a direct output signal) may be directly directed to the optically transparent medium, thereby bypassing the first optical delay device and experiencing a delay smaller than that experienced by the first part of the modulated optical signal imposed thereon by the first optical delay device. The first part of the modified optical signal passing through the first optical delay device is delayed relative to the second part of the modified optical signal.

[0060] The system can be adapted to receive a return optical signal from an optically transparent medium, the return optical signal comprising a first backscattered optical signal generated by a first portion of an intensity-modulated optical signal and a second backscattered optical signal generated by a second portion of an intensity-modulated optical signal.

[0061] The system may include a second optical delay device.

[0062] The first backscattered optical signal can be divided into at least two parts: a first backscattered first part and a first backscattered second part. The first backscattered first part can be guided to a second optical delay device such that it is delayed in time relative to the first backscattered second part to generate a first delayed backscattered optical signal (generated by the first backscattered first part) and a first direct backscattered optical signal (generated by the first backscattered second part).

[0063] The second backscattered optical signal can be divided into at least two parts: a second backscattered first part and a second backscattered second part. The second backscattered first part can be guided to a second optical delay device such that it is delayed in time from the second backscattered second part to generate a second delayed backscattered optical signal (generated by the first backscattered second part) and a second direct backscattered optical signal (generated by the second backscattered second part).

[0064] The system may also include a phase and amplitude receiver adapted to detect interference signals between a first delayed backscattered optical signal and a second delayed backscattered optical signal to determine a phase change between the two signals, which indicates external interference causing the optical path difference in the optically transparent medium.

[0065] Transparent optical media can be optical fibers.

[0066] The first optical delay device can apply a time delay τ1 (relative to at least another portion of the output path) to the optical signal guided thereon. The second optical delay device can apply a time delay τ2 (relative to at least another portion of the return path) to the optical signal guided thereon.

[0067] The absolute difference between the time delays τ1 and τ2 from the first and second optical delay devices can be less than the coherence time τ of the broadband light source. coh That is, |τ1-τ2|<τ coh In the alternative arrangement, the absolute difference between the time delays τ1 and τ2 from the first and second optical delay devices can be less than the coherence time τ of the broadband light source. coh a times, that is, |τ1-τ2|<aτ coh Where |τ1-τ2| may be as high as the coherence time τ coh 100 times, that is, |τ1-τ2|<100τ coh .

[0068] Broadband light sources can have a coherence time τ coh The coherence time can satisfy the relationship |τ1-τ2|<aτ coh The multiplication factor a can be approximately 1. The multiplication factor a can be between approximately 1 and approximately 100. The multiplication factor a can be between about 1 and about 2, between about 1 and about 3, between about 1 and about 4, between about 1 and about 5, between about 1 and about 6, between about 1 and about 7, between about 1 and about 8, between about 1 and about 9, or between about 1 and about 10, or between about 1 and about 20, or between about 1 and about 30, or between about 1 and about 40, or between about 1 and about 50, or between about 1 and about 60, or between about 1 and about 70, or between about 1 and about 80, or between about 1 and about 90, or between about 1 and about 100, or between about 10 and about 20, or between about 20 and about 30, or between about 30 and about 40, or between about 40 and about 50, or between about 50 and about 60, or between about 60 and about 70, or between about 70 and about 80, or between about 80 and about 90, or between about 90 and about 100. The multiplication factor 'a' can be approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, or approximately 100.

[0069] The delays τ1 and / or τ2 can be greater than the round-trip time of the optical signal from the light source to the far end of the optically transparent medium and back. In an alternative arrangement, the delays τ1 and / or τ2 can be less than the round-trip time of the optical signal (i.e., from the light source to the far end of the optically transparent medium and back).

[0070] The forward optical path may include a beam splitter adapted to divide an optical signal into at least two parts. A modulator may be adapted to receive one or both parts of the separated optical signal to provide at least one modulated optical signal.

[0071] The forward optical path may include a first optical delay device having a first delay τ1 and adapted to receive a first portion of an optical signal from a beam splitter, and subsequently transmit the delayed forward propagation optical signal to an optically transparent medium. The forward optical path may also include a direct optical transmission device adapted to receive a second portion of an intensity-modulated optical signal from a beam splitter, and subsequently bypass the first optical delay device to transmit the direct forward propagation optical signal to the optically transparent medium; wherein the delayed forward propagation optical signal is time-delayed by the first delay τ1 relative to the direct forward propagation optical signal.

[0072] The system may further include an optical guiding device adapted to receive backpropagating light from an optically transparent medium and guide the received backpropagating light to a return optical path. The return optical path may include a return beam splitter adapted to split the backpropagating light into at least two parts. The return optical path may include a second optical delay device having a second delay τ2. The second optical delay device may be adapted to receive a first part of the backpropagating light from the return beam splitter and subsequently transmit a delayed return optical signal to an optical receiver. The return optical path may also include a direct return optical transmission device adapted to receive a second part of the backpropagating light from the return beam splitter and subsequently bypass the second optical delay device to transmit a direct return optical signal to the optical receiver. The delayed return optical signal may be delayed by a first delay τ1 relative to the direct return optical signal.

[0073] The first and second optical delay devices can be common. The first and second optical delay devices can be a common IRDP adapted to allow forward and reverse propagation of optical signals. Optical signals propagating in either direction via the common IRDP may experience delay. The delay imposed by the common IRDP on the forward-propagating optical signal may differ from the delay imposed by the common IRDP on the reverse-propagating optical signal.

[0074] Broadband light sources can have a coherence time τ coh And where |τ1-τ2|<aτcoh The multiplication factor a can be between 1 and 100. The delays of the first and second optical delay devices can respectively satisfy the relationship τ1>τ coh and τ2>τ coh .

[0075] Phase and amplitude receivers can be adapted to measure the phase difference of an optical signal incident upon them. The optical receiver can receive multiple signals, including: a direct + direct signal, comprising a backpropagation signal (signal 1) generated from a directly forward-propagating optical signal that has bypassed the second optical delay device; a direct + delayed signal, comprising a backpropagation signal (signal 2) generated from a directly output signal that has passed through the second optical delay device; a delayed + direct signal, comprising a backpropagation signal (signal 3) generated from a delayed forward-propagating optical signal that has bypassed the second optical delay device; and a delayed + delayed signal, comprising a backpropagation signal (signal 4) generated from a delayed output signal that has been transmitted through the second optical delay device.

[0076] Signals 2 and 3 can arrive at the optical receiver approximately simultaneously, allowing signal 2 to interfere with signal 3, thereby generating an interference signal adapted to provide a measure of the optical path difference between signal 2 and signal 3, which represents the difference in path length in the optically transparent medium caused by external interference.

[0077] The first optical delay device and the second optical delay device can be equipped with a delay τ. com A common delay device. The common delay device can be used as a first delay device to allow light to propagate through it in the forward propagation direction. The common delay device can also be used as a second delay device to allow light to propagate through it in the reverse propagation direction, where τ com >τ coh .

[0078] Phase and amplitude receivers can be adapted to measure the phase difference and interference amplitude of optical signals incident upon them. Phase and amplitude receivers can receive multiple signals, including:

[0079] ■ Direct + Delayed Signal, which includes a backpropagation signal (signal 2) generated from the direct output signal already transmitted through the second optical delay device; and

[0080] ■ Delayed + direct signal, which includes a reverse propagation signal (signal 3) generated by a delayed forward propagation optical signal that has bypassed the second optical delay device.

[0081] The received signals (signal 2 and signal 3) can arrive at the receiver approximately simultaneously, allowing signal 2 to interfere with signal 3, thereby producing an interference signal adapted to provide a measure of the optical path difference between signal 2 and signal 3, which can represent the difference in path length in the optically transparent medium caused by external interference.

[0082] Transparent optical media can be optical fibers.

[0083] The system may also include computing and analysis devices adapted to receive the output signals of phase and amplitude receivers. These computing and analysis devices may further be adapted to calculate distributed optical path length variations in the transparent optical medium. Finally, the computing and analysis devices may be adapted to infer physical changes in the transparent optical medium based on the measured changes in optical path length.

[0084] The system may also include multiple optically transparent media. The system may also include frequency selection devices for selecting multiple frequency bands within each of the direct and delayed forward propagating optical signals. Each selected frequency band may include forward propagating direct and delayed optical signals guided to the selected medium. The system may also include multiple receiving devices for receiving backward propagating optical signals from each selected optical medium. The system may also include frequency selection devices for selecting multiple frequency bands within each of the direct and delayed return propagating optical signals, each selected frequency band including return propagating direct and delayed optical signals guided to selected phase and amplitude receivers; multiple phase and amplitude receivers adapted to measure the amplitude phase difference of the received optical signals in the selected frequency bands, thereby determining the optical path length variation along each selected medium in a distributed manner to infer physical changes in the selected medium.

[0085] According to a second aspect of the present invention, a method for sensing changes in optical path length along a sensing medium in a distributed manner is provided. The method may include the step of providing a broadband light source for generating optical output. The light source may have a coherence time τ. cohThe method may further include the steps of: splitting the optical output into at least two parts and guiding each separate part to an independent forward optical path. The method may further include the steps of: providing a first optical delay device in the first of the optical paths, the delay device being adapted to delay at least a portion of the optical output on the first forward optical path relative to at least another independent forward optical path by a predetermined delay time τ1, thereby generating a delayed optical output signal and at least one direct optical output signal. The method may further include the steps of: providing a guiding device for guiding the output signal into a sensing medium. The method may further include the steps of: providing a receiving device for receiving a distributed backscattered signal from the sensing medium. The method may further include the steps of: splitting the backscattered signal into at least two independent return paths. The method may further include the steps of: providing a second optical delay device in the first of the return paths, the delay device being adapted to delay at least a portion of the backscattered signal received on the first return path relative to at least another return optical path by a predetermined delay time τ2, thereby generating a delayed backscattered signal and at least one direct backscattered signal. The method may further include the steps of: measuring the relative phase difference between the separated delayed and direct backscattered signals to determine the change in optical path length along the sensing medium.

[0086] According to a specific arrangement of a second aspect of the present invention, a method for sensing changes in the optical path length along a sensing medium in a distributed manner is provided, the method comprising the steps of: providing a broadband light source for generating optical output, the light source having a coherence time τ coh The optical output is divided into at least two parts, and each separate part is guided to an independent forward optical path; a first optical delay device is provided in the first of the optical paths, the delay device being adapted to delay at least a portion of the optical output on the first forward optical path relative to at least another independent forward optical path by a predetermined delay time τ1, thereby generating a delayed optical output signal and at least one direct optical output signal; a guiding device is provided for guiding the output signal into a sensing medium; a receiving device is provided for receiving a distributed backscattered signal from the sensing medium; the backscattered signal is divided into at least two independent return paths; a second optical delay device is provided in the first of the return paths, the delay device being adapted to delay at least a portion of the backscattered signal received on the first return path relative to at least another return optical path by a predetermined delay time τ2, thereby generating a delayed backscattered signal and at least one direct backscattered signal; and the relative phase difference between the separated delayed and direct backscattered signals is measured to determine the change in optical path length along the sensing medium.

[0087] The method may also include the following steps: inferring physical changes in the sensing medium based on the sensed changes in optical path length.

[0088] The light source can be the coherence time τ coh The delays of the first and second delay devices can satisfy the relationship |τ1-τ2|<aτ coh The multiplication factor a can be approximately 1. The multiplication factor a can be between approximately 1 and approximately 100.

[0089] Broadband light sources can be incoherent or partially coherent. Broadband light sources can also be broadband coherent. Light sources can be modulated or pulsed.

[0090] The bandwidth of broadband light sources can range from approximately 10MHz to 100MHz, 100MHz to 1GHz, 1GHz to 10GHz, 10GHz to 100GHz, 100GHz to 1THz, 1THz to 10THz, and 10THz to 100THz. That is, 10MHz to 100THz (i.e., 10 × 10⁻⁶). 6 Hz to 100×10 12 Hz).

[0091] The method may also include the following steps: modulating the optical output or delaying and / or directly outputting the signal before directing the output signal to the sensing medium.

[0092] The method may further include the step of providing a combining means for combining a delayed output signal and at least one direct output signal into a common forward optical path before directing the output signal into the sensing medium.

[0093] The first optical delay device and the second optical delay device can be common.

[0094] The method may further include the following steps: determining the location or physical parameter of the optical path length change using the travel time and modulation scheme of light in the sensing medium. The method may further include the following steps: quantitatively determining the optical path length change in a distributed manner using measured phase. The method may further include the following steps: inferring one or more physical parameters of the sensing medium based on the determined optical path length change.

[0095] The modulation process may include intensity modulation, amplitude modulation, frequency modulation, phase modulation, or polarization modulation of the optical signal. Modulation may be pulsed, thereby providing an intensity-modulated optical output.

[0096] The light source can be a modulated light source. The method may further include the step of modulating the optical output or delaying and directly outputting the signal before directing the output signal to the sensing medium. The modulation step may include amplitude modulation, frequency modulation, phase modulation, or polarization modulation of the optical signal. Modulation may be pulsed, thereby providing an intensity-modulated optical output.

[0097] The system may also include an optical combiner adapted to recombine a delayed optical signal and at least one direct output signal into a common forward optical path for guiding the output signal into the sensing medium.

[0098] The sensing medium can be an optically transparent medium of one or more predetermined wavelengths of light generated by a light source. The optically transparent medium can be an optical fiber or waveguide adapted to guide light having one or more predetermined wavelengths. The optically transparent medium can be water, seawater, fluids, glass, polymers, semiconductor materials, air, methane, compressed natural gas, liquefied natural gas, gases, or other suitable optically transparent materials.

[0099] The optically transparent medium may not be a guiding medium, and the method may further include the steps of: providing one or more collimators for substantially collimating the output signal and transmitting the collimated output signal into the sensing medium, the one or more collimators also being used to collect backscattered optical signals from the sensing medium.

[0100] The method may further include the step of providing a first frequency selection device. The first frequency selection device may be adapted to select multiple frequency bands within each of the direct and delayed forward-propagating optical output signals. Each selected frequency band may include forward-propagating direct and delayed optical signals. The first frequency selection device may be adapted to direct each pair of optical signals in each selected frequency band to a selected one of multiple sensing media. The first frequency selection device may be adapted to receive backward-propagating distributed backscattered optical signals from each selected optical medium. The first frequency selection device may be adapted to combine each of the received optical signals onto a common optical return path.

[0101] The method may further include the step of providing a second frequency selection device. The second frequency selection device may be adapted to receive an optical signal from a return path. The second frequency selection device may be adapted to select multiple frequency bands using the received optical signal. The second frequency selection device may be adapted to direct a signal within each of the selected multiple frequency bands to a selected one of multiple phase and amplitude receivers to measure the relative phase difference between the delay and direct backscattered optical signal in each selected frequency band, thereby determining the change in optical path length in each of the multiple sensing media and inferring physical changes in each sensing medium.

[0102] The first frequency selection device may include an optical frequency demultiplexer and a multiplexer. The second frequency selection device may include an optical demultiplexer.

[0103] Optical signals within multiple frequency bands selected by the frequency demultiplexer can each be directed to a unique phase and amplitude receiver. Alternatively, optical signals within multiple selected frequency bands selected by the frequency demultiplexer can each be directed to a common phase and amplitude receiver.

[0104] According to a third aspect of the invention, a method for sensing changes in the optical path length along a sensing medium in a distributed manner is provided. The method may include the step of providing a broadband light source. The method may further include the step of splitting light output from the light source into at least two output paths. The method may further include the step of delaying light in one output path relative to at least another output path by a delay time τ1. The method may further include the step of modulating optical light at any step before guiding light from different output paths into the sensing medium. The method may further include the step of guiding light from different output paths into the sensing medium. The method may further include the step of receiving distributed backscattering from the sensing medium. The method may further include the step of splitting the received backscattering into at least two return paths. The method may further include the step of delaying light in one return path relative to at least another return path by a delay time τ2. The method may further include the step of providing a phase and amplitude receiver for measuring the relative phase difference between the delayed and separated distributed backscattered light.

[0105] According to a specific arrangement of the third aspect, a method is provided for sensing changes in the optical path length along a sensing medium in a distributed manner, the method comprising: providing a broadband light source; splitting light output from the light source into at least two output paths; delaying light in one output path relative to at least another output path by a delay time τ1; modulating optical light at any step before guiding light from different output paths into the sensing medium; guiding light from different output paths into the sensing medium; receiving distributed backscattering from the sensing medium; splitting the received backscattering into at least two return paths; delaying light in one return path relative to at least another return path by a delay time τ2; and providing a phase and amplitude receiver for measuring the relative phase difference between the delayed and separated distributed backscattered light.

[0106] The method may further include the following steps: quantitatively determining the change in optical path length in a distributed manner using the measured phase. The method may further include the following steps: determining the location of the change in optical path length or one or more physical parameters causing the change in optical path length in the sensing medium using the travel time of light in the sensing medium. The method may further include the following steps: inferring one or more physical parameters based on the change in optical path length.

[0107] The light source can have a coherence time τ cohThe delays of the first and second delay devices can satisfy the relationship |τ1-τ2|<aτ coh The multiplication factor a can be approximately 1. The multiplication factor a can be between approximately 1 and approximately 100.

[0108] The first delay time and the second delay time can be similar within about 0.5% to about 2%. The first delay time and the second delay time can be similar within about 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or about 2%. The first delay time and the second delay time can differ by an optical delay length of less than 0.001m. The first delay time and the second delay time can differ by an optical delay length of approximately 0.001m to approximately 0.01m, approximately 0.001m to approximately 0.1m, approximately 0.001m to approximately 1m, approximately 0.001m to approximately 10m, approximately 0.001m to approximately 100m, approximately 0.001m to approximately 1000m, approximately 0.001m to approximately 1500m, or approximately 0.001m to approximately 2000m.

[0109] The first delay time and the second delay time can differ by an optical delay length of approximately 0.0005m, 0.001m, 0.005m, 0.01m, 0.05m, 0.1m, 0.5m, 1m, 5m, 10m, 50m, 100m, 500m, 1000m, 1500m, or approximately 2000m.

[0110] Broadband light sources can be incoherent or partially coherent. Broadband light sources can also be broadband coherent light sources.

[0111] The bandwidth of a broadband light source can range from approximately 10MHz to 100MHz, 100MHz to 1GHz, 1GHz to 10GHz, 10GHz to 100GHz, 100GHz to 1THz, 1THz to 10THz, and 10THz to 100THz. In other words, the bandwidth of a broadband light source can range from approximately 10MHz to 100THz (i.e., 10 × 10⁻⁶). 6 Hz to 100×10 12 Between Hz.

[0112] The intensity or amplitude of light generated by a light source can be modulated anywhere before the sensing medium. The frequency of light generated by a light source can be modulated anywhere before the sensing medium. The phase of light generated by a light source can be modulated anywhere before the sensing medium. The polarization of light generated by a light source can be modulated anywhere before the sensing medium.

[0113] The method may further include the step of providing a combining means for combining a delayed output signal and at least one direct output signal into a common forward optical path before directing the output signal into the sensing medium.

[0114] The first and second optical delay devices can be common. The intensity or amplitude of the light generated by the light source can be modulated anywhere before the sensing medium. The frequency of the light generated by the light source can be modulated anywhere before the sensing medium. The phase of the light generated by the light source can be modulated anywhere before the sensing medium. The polarization of the light generated by the light source can be modulated anywhere before the sensing medium.

[0115] The method may further include the step of providing a combining means for combining a delayed output signal and at least one direct output signal into a common forward optical path before directing the output signal into the sensing medium.

[0116] The first optical delay device and the second optical delay device can be common.

[0117] Phase and amplitude measurements can include the use of one or more of the following groups: phase and amplitude receivers; frequency shifts (e.g., acousto-optic shifts) and complex demodulation to recover the phase; 3x3 couplers or MxN couplers, where M≥2 or N≥3; swept-frequency optical signals to recover the phase using a Hilbert transform; phase modulators that receive the phase by time multiplexing with altered phase shifts; waveplate arrangements within a multiport interferometer; the use of a spectrometer or optical filters, with intensity or phase and amplitude measurements performed separately in different frequency bands; interferometric local oscillators; or any of the above with different polarizations (dual polarization). When the first (τ1) and second (τ2) delays are not equal (τ1≠τ2), it can be expected that the phase is not uniform across the entire spectrum; in this case, this fact can be used to perform phase and amplitude measurements, either separately in different frequency bands.

[0118] As is well known, interference using a reference light as a medium (called a local oscillator) can provide a phase difference between two signals without causing them to interfere with each other. This can be achieved in several ways, including:

[0119] ● Interfere the first signal with the local oscillator to measure the phase, and also interfere the second signal with the local oscillator to measure the phase, then subtract the two phase measurements; or

[0120] ● The first signal is interfered with by a local oscillator to measure the electronic interference with the first beat frequency signal, and the second signal is also interfered with by a local oscillator to measure the electronic interference with the second beat frequency signal. Then the first and second beat frequency signals are electronically mixed.

[0121] Phase measurements can include using a spectrometer or optical filters, and performing intensity or phase and amplitude measurements separately in different frequency bands. Electronic mixing of the receiver signal and the reference signal can be used to shift higher analog frequencies into lower frequency bands. Phase measurements can be performed continuously over time or intermittently. Phase measurements can be performed synchronously with amplitude measurements.

[0122] The physical parameters can be selected from the following groups: longitudinal strain; transverse strain; sound waves; seismic waves; vibration; motion; bending; torsion; temperature change; chemical composition change; or the motion of scattering / reflecting particles in the sensing medium.

[0123] The physical parameters can be selected from the following groups: longitudinal strain; transverse strain; sound waves; seismic waves; vibration; motion; bending; torsion; temperature change; optical delay change; chemical composition change; or the motion of scattering / reflecting particles in the sensing medium.

[0124] Methods for determining the location or physical parameters of the optical path length variation may include the step of using pulse modulation, wherein the measured phase is proportional to the optical path length variation with a delay time, and the sampling time is approximately linearly mapped to the location along the sensing medium. The method for determining the location or physical parameters of the optical path length variation may further include the step of performing a numerical deconvolution between the complex signal from the phase and amplitude receivers and the known or measured modulation. The method for determining the location or physical parameters of the optical path length variation may further include the step of performing a numerical cross-correlation between the complex signal from the phase and amplitude receivers and the known or measured modulation.

[0125] According to a fourth aspect of the present invention, a method for modulating the intensity of a broadband light source is provided. The method may include the steps of: providing an optical amplifier that generates a continuous amplified spontaneous emission (ASE) output when the input optical intensity is low. The method may further include the step of: modulating the intensity of the ASE output of the optical amplifier using an intensity modulator. The method may further include the step of: sending the modulated ASE to the input of the optical amplifier or a second optical amplifier for amplification. The method may further include the step of: using all or part of the amplified modulated light from the amplifier in the system. The method may further include the step of a modulation scheme wherein the modulation is maintained at a high intensity state for a period of time τ. P This time is longer than the time τ it takes for light to travel from the amplifier's output to its input. O Short, and then maintain a lower intensity than τ. O A long time.

[0126] According to a specific arrangement of the fourth aspect, a method for modulating the intensity of a broadband light source is provided, the method comprising the steps of: providing an optical amplifier that generates a continuous amplified spontaneous emission (ASE) output when the input optical intensity is low; modulating the intensity of the ASE output of the optical amplifier using an intensity modulator; sending the modulated ASE to the input of the optical amplifier or a second optical amplifier for amplification; using all or part of the amplified modulated light from the amplifier in the system; and a modulation scheme wherein the modulation is maintained at a high intensity state for a period of time τ. P This time is longer than the time τ it takes for light to travel from the amplifier's output to its input. O Short, and then maintain a lower intensity than τ. O A long time.

[0127] The bandwidth of broadband light sources can range from approximately 10MHz to 100MHz, 100MHz to 1GHz, 1GHz to 10GHz, 10GHz to 100GHz, 100GHz to 1THz, 1THz to 10THz, and 10THz to 100THz. That is, 10MHz to 100THz (i.e., 10 × 10⁻⁶). 6 Hz to 100×10 12 Hz).

[0128] The modulation scheme can be pulse modulation. The duration of the pulse can be longer than τ. O Short. The repetition period of the pulse can be shorter than τ. O long.

[0129] The methods of any of the second to fourth aspects can be applied to the system of the first aspect. The methods of the second to fourth aspects described above can be applied to one or more uses within the group comprising: distributed acoustic sensing over optical fibers or waveguides; surface vibration measurement; distributed acoustic LiDAR in air and atmosphere; wind speed measurement; distributed acoustic LiDAR in water; distributed acoustic LiDAR in pipe fluids; vertical seismic profile analysis in boreholes and wells; marine towed cables for seismic exploration; terrestrial seismic sensors for seismic exploration; permanent seismic monitoring arrays for repeating seismic imaging and inversion; passive seismic monitoring, such as seismic monitoring, microseismic monitoring, and induced seismic activity related to underground fluid injection or production; mine wall stability monitoring, such as microseismic monitoring and collapse monitoring; dam stability monitoring, such as seismic activity induced by dams and the stiffness of tailings dams; conveyor monitoring; rotating machinery monitoring. Measurement, such as compressors, fans, turbines, and generators; pipeline monitoring, such as leak detection and tampering; perimeter and security monitoring / surveillance, such as intrusion detection; infrastructure monitoring, such as strain and vibration control of bridges, tunnels, buildings, and wind turbines; vehicle structure monitoring, such as strain and vibration control of automobiles, aircraft, and ships; flow measurement, such as metering flow in pipelines; geotechnical surveying, such as surface wave inversion of near-surface shear wave velocity; airflow profile analysis, such as atmospheric profile analysis, wind tunnel profile analysis, and around aircraft; waterflow profile analysis, such as profile analysis around ocean currents, rivers, and maritime vehicles; medical devices, such as body strain sensors and blood flow measurement; telecommunications network monitoring, such as interference and faults; traffic and vehicle flow monitoring, such as roads, railways, and ships; audio recording; and fire monitoring, such as in tunnels and infrastructure. Attached Figure Description

[0130] Although any other forms may fall within the scope of this invention, one or more preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0131] Figure 1 A schematic block diagram of the DAS system and method disclosed herein is shown;

[0132] Figure 2 A schematic diagram of the arrival time of the distributed backscattered optical signal from the sensing medium is shown in the currently disclosed systems and methods;

[0133] Figure 3 A schematic block diagram of the DAS system and method under the public path configuration disclosed in this paper is shown;

[0134] Figure 4 A schematic diagram is shown, illustrating a specific arrangement of photonic components in the sensing system disclosed herein;

[0135] Figure 5A schematic diagram is shown, illustrating a further arrangement of photonic components in the sensing system with a common path configuration disclosed herein;

[0136] Figure 6 A schematic diagram is shown, illustrating a specific arrangement of photonic components in the sensing system disclosed herein;

[0137] Figure 7 A schematic diagram is shown, illustrating a further arrangement of photonic components in the sensing system with a common path configuration disclosed herein;

[0138] Figure 8 The photonic components of the sensing system disclosed in the common path implementation herein are shown, using all the output ports of a 3×3 coupler as phase and amplitude receivers.

[0139] Figure 9 The photonic components of a further arrangement of the sensing system disclosed herein in a common-path implementation with a polarization beamsplitter are shown;

[0140] Figure 10 An example of amplitude trajectory detection using a pulsed DAS system based on the systems and methods disclosed herein is shown;

[0141] Figure 11 An example is shown of the phase-to-position relationship measured along a sensing fiber optic cable at 430 m using a vibrating fiber optic stretcher for testing purposes according to the system and method disclosed herein;

[0142] Figure 12 An example is shown of the relationship between the calculated strain rate and the position along the sensing fiber relative to time at 430 m using a vibrating fiber optic tensioner for testing purposes, according to the system and method disclosed herein.

[0143] Figure 13 The photonic components of the sensing system disclosed herein are shown in a further arrangement, wherein amplified spontaneous emission from an optical amplifier is used as a light source;

[0144] Figure 14 A method for sensing distributed backscattered signals, as disclosed herein, is described;

[0145] Figure 15 The photonic components of the sensing system disclosed in the common path implementation herein are shown, using all the output ports of a 3×3 coupler as phase and amplitude receivers.

[0146] Figure 16 The methods for processing and analyzing amplitude and phase, as disclosed in this paper, are described;

[0147] Figure 17 A schematic block diagram of the DAS system and method disclosed herein is shown, which has two independent optical paths to the sensing medium.

[0148] Figure 18 The system layout is described, and Figures 18A to 18C A further arrangement of the system as described above is depicted, which, as disclosed herein, has a method for guiding the output signal into the sensing medium without recombination in a common outward path and for receiving the backscattered signal from the sensing medium; and

[0149] Figure 19 A further arrangement of the above system is described, which adds frequency-selective multiplexing / demultiplexing and frequency steering components for detecting path length variations on multiple sensing media. Detailed Implementation

[0150] In the following description, it should be noted that in different embodiments, similar or identical reference numerals denote the same or similar features.

[0151] This paper discloses a system and method for quantitative distributed acoustic sensing (DAS) systems, enabling quantitative distributed measurement of changes in the optical path length along a sensing medium, such as an optically transparent medium like an optical fiber. The DAS system disclosed herein includes a broadband light source and phase and amplitude measurements capable of accurately measuring the phase. The broadband light source can have a coherence time τ. coh And interchangeably have a coherence length l coh The coherence length is defined as the optical path length corresponding to propagation in the medium within a time equal to the coherence time. When used in the systems and methods disclosed herein, there is no lower limit in principle for the coherence length of the light source. The coherence time τ coh It is by increasing the coherence length l coh It is calculated by dividing by the speed of light in the medium, and by the relation. The approximation is given, where λ is the center wavelength of the light source, Δv and Δλ are the spectral linewidths of the light source in units of frequency and wavelength, respectively, and c is the speed of light in vacuum.

[0152] Multiple physical parameters can be sensed by the corresponding changes in optical path length they produce in the coupled medium. Examples of phenomena capable of causing changes in optical path length in the coupled medium can include: longitudinal strain, transverse strain, acoustic waves, seismic waves, vibration, motion, bending, torsion, temperature, optical delay, or chemical composition. Any other physical parameter with a mechanism that causes elongation and / or refractive index changes and / or deformation along the optical path can also be sensed. Changes in optical path length can also occur through the movement of scattering / reflecting particles in the sensing medium. The systems and methods disclosed herein utilize intensity-modulated broadband light sources combined with delay, phase, and amplitude measurements to perform accurate phase measurements of distributed backscattering from a sensing medium such as an optical fiber (referred to herein as the sensing fiber). Possible mechanisms by which light returns from the sensing optical path to the system include: Rayleigh backscattering; Mie backscattering; discrete reflections (intentional and unintentional reflections, such as those caused by malfunctions or connector connections); Bragg grating reflections; particle scattering in solids, such as dopants in crystals and glasses; particle scattering in liquids, such as cells or fissures in water; or particle scattering in gases, such as atmospheric aerosols in the air.

[0153] Possible optical paths for sensing media used in conjunction with currently disclosed systems and methods, or for implementing the optical delay devices disclosed herein, include: single-mode fiber; multimode fiber; multi-core fiber; polarization-maintaining fiber; photonic crystal; photonic bandgap fiber; fiber with a liquid or gas-filled core; planar waveguide, which may be made of any suitable material; or free-space propagation in a backscattering medium (gas, liquid, or solid) that may include air or water.

[0154] In the system disclosed herein, an optical output signal (OO) generated by a light source is split into, for example, two parts and then guided to an IRDP, wherein one part of the output signal is time-delayed relative to a second part (the direct output signal) of the output signal without delay (thus producing a delayed output signal). The forward-propagating delayed output signal (delayed OOS) and the forward-propagating direct output signal (direct OOS) are then guided to a sensing medium such that the delayed and direct output signals propagate through the sensing medium, and thus cause variations in the sensing medium due to external interference. The delayed and direct output signals may optionally be combined into a common optical path and / or common polarization before reaching the sensing medium. As those skilled in the art will appreciate, possible methods for splitting light into multiple paths to achieve a suitable IRDP with different delays may include: optical couplers (2x2, 3x3, or MxN); beam splitters; polarization beam splitters; switches (e.g., LCOS, holograms, MEMs, or electro-optics); acousto-optic modulators; optical filters; partial reflectors; or birefringence.

[0155] As they propagate through the sensing medium, the delayed and direct output signals are scattered in a distributed manner along the propagation direction of the output signal, and a portion of the scattered output signal propagates directly back to the forward propagation signal in the reverse propagation direction. The DAS system collects the backpropagated (or backscattered) light from the output signal to analyze any external interference that causes changes in the optical path length in the sensing medium.

[0156] Figure 1 A conceptual schematic block diagram of the DAS system according to the present invention is shown.

[0157] The output light OO 150 from light source 101 is optionally modulated by modulator 103. In an alternative arrangement, light source 101 is a pulsed light source. In a further arrangement, light source 101 may have modulator 103 associated with it. Preferably, the light source is modulated inside the laser or before separation, but alternatively, the light source may be modulated anywhere before entering sensing medium (such as sensing fiber) 160, i.e., at any point between light source 101 and sensing medium 160, in the forward propagation path 130. Modulator 103 may be adapted to modulate any one or more of the intensity, frequency, phase, or polarization of the light in the forward propagation path 130 of system 100 before sensing medium 160. Pulsed or coded modulation are examples of possible modulation schemes that may be used. As those skilled in the art will understand, possible alternative means for modulation may include, for example: electro-optic modulators; acousto-optic modulators; optical switches; direct light source modulation; and saturable absorbers. Possible modulation schemes may include: pulse; pseudo-random decoding; simplex code; Golay code; linear frequency chirp; or Barker code.

[0158] If the modulator is placed after the splitter 105 of the system 100, it can be adapted to operate on the delayed OOS 151 or the direct OOS 153 output section without operating on other output sections. In this case, the system 100 can operate in a manner similar to that in which the modulator is placed before the splitter 105.

[0159] If more than one modulator is used to act on the two output sections 151 and 153 respectively, then these modulators will preferably act on these sections before the delay and work together. If the modulators act on the output sections after the delay but before the combiner 111a, then the modulators will preferably use the same modulation mode with a delay equal to the optical delay.

[0160] If a modulator is used after combiner 111a and before sensing medium 160, the modulation will preferably repeat its modulation pattern after a period of time equal to the optical delay.

[0161] Possible broadband light sources may include: multimode lasers (e.g., Fabry-Perot lasers), single-mode lasers (e.g., DFB lasers); spontaneous emission or amplified spontaneous emission (ASE) lasers, such as EDFA and SOA; superluminescent diodes (SLEDs); supercontinuum sources; mode-locked lasers; amplitude-modulated sources; frequency-modulated sources; frequency-swept sources; phase-shift keying lasers; phase-modulated sources; natural light; fluorescence or phosphorescence; any filter sources listed above as will be understood by a person skilled in the art; or any combination of the above sources. In a particular arrangement, the coherence length of the light source is less than 100% of the system's spatial resolution.

[0162] In the following discussion, the modulator is described as an intensity modulator for illustrative purposes only. However, those skilled in the art will readily understand that intensity modulation can be readily replaced by modulation of the phase, polarization, or frequency of the light generated by the light source 101.

[0163] In a particular arrangement of system 100, the optical path between each element is provided by an optical fiber. In an alternative arrangement, the optical path between elements can be free space. Next, the light OO 150 from light source 101 is directed to beam splitter 105, where it is split into at least two parts: output light 152 and direct OOS 151. These are directed to a first intentionally relative delay path (IRDP) 106a, which includes a separate optical path (e.g., a separate optical fiber). The first IRDP 106a includes a first optical delay device 107a, which may be, for example, an optical fiber delay line of predetermined length, to apply a known delay time τ1 to the first part 152 of the separated output signal, thereby producing a delayed output signal delay OOS 153. In further discussion, the separated part 151 of the optical output 150 bypassing the first optical delay device 107a will be referred to as the direct output signal direct OOS 151. The delayed output signal 153 and the direct output signal 151 are then recombined in combiner 111a onto a common forward propagation optical path 171, and then directed to sensing medium 160, such as an optical fiber adapted to sense one or more parameters capable of sensing changes in the optical path length on sensing medium 160. In a particular arrangement, the difference delay applied to the separate signals 151 and 153 in IRDP 106a is preferably (though not necessarily) longer than the round-trip time of the light in sensing medium 160.

[0164] Distributed backscattering caused by forward-propagating light in sensing medium 160 and propagating backward along the same optical path in sensing medium 160 is collected by system 100 and initially split into a backscattering path 135 from the forward propagation path 130 in system 100 by backscattering receiving device 115. Backscattering receiving device 115 may be an optical circulator or the like, which a) receives forward-propagating light from the forward propagation path 130 and directs it to sensing medium 160, and b) receives backscattered light propagating backward from sensing medium 160 and directs it to the backscattering path 135. The backscattered optical signals 161 and 163 received by system 100 include a first backscattered return signal 163 caused by backscattering of delayed output signal 151 in sensing medium 160 and a second backscattered return signal 161 caused by backscattering of direct output signal 153 in sensing medium 160. Backscattered signals 161 and 163 each propagate along the backscattering path 135 and are each split by splitter 111b into at least two backscattered signal portions on separate optical paths. The separated backscattered signal portions are then directed to a second IRDP 106b. The second IRDP 106b includes a second optical delay device 107b, which may be, for example, a fiber delay line of predetermined length, to apply a known delay time τ2 to a first portion of each of the backscattered return signals 161 and 163.

[0165] The second optical delay device 107b is adapted to delay at least a portion of each of the received backscattered signals 161 and 163 on the return path by a predetermined delay time τ2 relative to the other return optical paths, thereby generating a delayed backscattered signal portion of each of the backscattered signals 161 and 163 and at least one direct backscattered signal portion of each of the backscattered signals 161 and 163, wherein the delays of the first (τ1) and second (τ2) delay devices satisfy the relationship |τ1-τ2|<aτ coh , where τ coh τ1 is the coherence time of light source 101, where the multiplication factor a can be between 1 and 100. In some cases, as described below, intentionally using τ1≠τ2 in the system design may be advantageous for phase and amplitude measurements. The direct and delayed portions 161 and 163 of the backscattered signal are then directed to phase and amplitude receivers 131, respectively.

[0166] According to the optical path of the system 100 described above, the backscattered signal from the sensing medium 160 received by the phase and amplitude receiver 131 includes multiple signals, including:

[0167] ■ Direct + direct signal, which includes a backpropagation signal 161a (signal 1) generated by the direct output signal 151 that has bypassed the second optical delay device 107b;

[0168] ■ Direct + Delayed Signal, which includes a reverse propagation signal 161b (signal 2) generated by the direct output signal 151 that has been transmitted through the second optical delay device 107b;

[0169] ■ Delayed + direct signal, which includes a reverse propagation signal 163a (signal 3) generated by the delayed forward propagation output signal 153 that has bypassed the second optical delay device; and

[0170] ■ Delay + Delay Signal, which includes a backpropagation signal 163b (signal 4) generated by the delayed output signal 153 that has been transmitted through the second optical delay device.

[0171] Signals 1 and 3 (161a and 163a) are collectively referred to as direct BS (referring to backscattered signals that bypass the second optical delayer 107b), and signals 2 and 4 (161b and 163b) are collectively referred to as delayed BS (referring to backscattered signals transmitted through the second optical delayer 107b).

[0172] In a particular arrangement, the direct BS and the delayed BS interfere with each other in the phase and amplitude receiver 131 to generate an interference signal for measuring the optical path length difference in the sensing medium 160. The signal 2161b of the delayed BS and the signal 3163a of the direct BS arrive at the receiver approximately simultaneously, allowing signal 2161b to interfere with signal 3163a, thereby generating an interference signal 170 at the output of receiver 131 adapted to provide a measure of the optical path difference between signal 2161b and signal 3163a, which represents the path length difference in the sensing medium 160 caused by external interference.

[0173] In a further arrangement, the direct BS and the delayed BS are each subjected to interference from a local oscillator as described above, in order to provide a measurement of the phase difference between the direct BS and the delayed BS without directly interfering with each other.

[0174] The detected signal is recorded and stored in memory 133, and analyzed by analysis processor 136 to calculate the change in effective path length in sensing medium 160 caused by external interference.

[0175] System 100 also includes optional amplifiers 113a and 113b, which are used for: 113a) optically amplifying the output forward-propagating optical signals 151 and 153 before they are transmitted into a sensing medium (e.g., a sensing fiber) 160; and 113b) optically amplifying the received backscattered signals 161 and 163.

[0176] In this way, broadband light traveling coherently throughout the system 100 and within the sensing medium 160 can travel nearly equal optical path lengths between the source and the phase and amplitude receivers (the so-called "white light" interference condition), independent of optical frequency and the location of backscattering along the sensing fiber. This condition generates an electronically measurable interference signal at the phase and amplitude receiver 131 between the broadband distributed backscatterings, which returns from the sensing medium 160 at different times. The relative phases of the delayed and non-delayed backscattered signals 163a and 161b, measured at the phase and amplitude receiver 131 (generated by the direct and delayed output signals, respectively), contain the key information needed to accurately determine the optical path length variation, which occurs in the sensing medium within the delay period caused by the first (forward propagation) IRDP and the first optical delay device 107a. As discussed in the signal model derivation below, the amplitude of the detected signal measured at the phase and amplitude receiver 131 can be used to estimate the quality of the phase information and can be used to improve the spatial resolution and sensitivity of distributed sensing by using a decoding scheme.

[0177] Possible phase and amplitude measurement methods may include: frequency shift (e.g., acousto-optic shift) of direct + delayed 161b and / or delayed + direct 163a signals (signals 2 and 3) and complex demodulation to recover the phase; 3x3 couplers or MxN couplers, where M≥2 or N≥3; frequency sweep and Hilbert transform for phase recovery; phase modulators receiving the phase via time multiplexing with varying phase shifts; waveplate arrangement within a multiport interferometer; using a spectrometer (e.g., a grating) or optical filter and detecting intensity or performing phase and amplitude measurements in different frequency bands; interferometric local oscillators; or any different polarization (dual polarization) version of the above, as understood by those skilled in the art. When the first (τ1) and second (τ2) delays are not equal (τ1≠τ2), it can be expected that the phase is non-uniform across the entire spectrum. In this case, this fact can be used for phase and amplitude measurements, either separately in different frequency bands. This may have advantages including improved ease of manufacture and cost, and compensation for dispersion in the sensing medium. Phase and amplitude measurements in different frequency bands can also be performed by using a frequency demultiplexer to guide the output signal to different sensing media and using the frequency demultiplexer to receive the returned backscattered signal, thereby enabling distributed sensing across multiple sensing media.

[0178] As is well known, interference using a reference light as a medium (called a local oscillator) can provide a phase difference between two signals (e.g., a direct BS and a delayed BS) without causing the two signals to interfere with each other. This can be achieved in several ways, including:

[0179] ● Interference with a first signal (e.g., direct BS) using a local oscillator to measure phase, and also interference with a second signal (e.g., delayed BS) using a local oscillator to measure phase, then digitally subtract the two measured phases; or

[0180] ● Interference with a local oscillator on a first signal (e.g., direct BS) to measure electronic interference to the first beat frequency signal, and interference with a second signal (e.g., delayed BS) on a second beat frequency signal to measure electronic interference to the second beat frequency signal, and then electronically mixing the first and second beat frequency signals.

[0181] As understood by those skilled in the art, possible arrangements of the first and second optical delay devices 107a and 107b may include delay optical devices such as: fiber delay lines; beam delay lines (e.g., free space); optical cavities; recirculation loops; or electromagnetically induced transparency.

[0182] Preferably, the separator and the phase and amplitude receiver may include a common device, such as Figure 8 , Figure 13 or Figure 15 As shown.

[0183] Now for reference Figure 14 A method for sensing distributed backscattered signals is described, comprising a method 1400 for quantitatively sensing changes in the optical path length along a sensing medium in a distributed manner. Method 1400 includes the step of providing a broadband light source 1401 to generate an optical output, the light source having a coherence time τ. coh Method 1400 further includes the steps of: dividing the optical output into at least two portions 1403 and directing each separated portion 1405 to an independent outward optical path. Separation can be achieved by spatial separation or by dividing into polarization components, but is not limited thereto. Independent polarization is considered an independent optical path. Method 1400 also includes providing a first optical delay device 1407 in a first optical path of the optical path. The first optical delay device is adapted to delay at least a portion of the optical output on the first forward optical path relative to another independent forward optical path by a predetermined delay time τ1, thereby generating a delayed output signal and at least one direct output signal. Method 1400 optionally includes providing an optical recombiner for recombining the delayed optical signal and at least one direct output signal 1409 into a common forward optical path and directing the output signal 1411 into a sensing medium, regardless of whether it is recombined. Method 1400 also includes providing a modulator 1420 for modulating the optical output generated by the light source. The modulation step 1420 can be performed anywhere between steps 1401 and 1411. The modulator can be adapted to modulate any one or more of the intensity, frequency, phase, or polarization of light in the forward propagation path 130 prior to the sensing medium 160.

[0184] Method 1400 further includes providing 1413 a receiving means for receiving a backscattered signal from a sensing medium. Method 1400 also includes splitting the backscattered signal into at least two independent return paths 1415. This separation can be achieved through spatial separation or splitting into polarization components, but is not limited thereto. Independent polarizations are considered independent optical paths. Method 1400 further includes providing 1417 a second optical delay device in a first return path. The second optical delay device is adapted to delay at least a portion of the backscattered signal received on the first return path relative to the other return optical path by a predetermined delay time τ2, thereby generating a delayed backscattered signal and at least one direct backscattered signal.

[0185] The delays of the first and second delay devices satisfy the relation |τ1-τ2|<aτ coh The multiplication factor 'a' can be between 1 and 100. The first and second delay devices can be common devices or common optical paths. Method 1400 also includes receiving the delayed and direct backscattered signals 1419 using a phase and amplitude receiver adapted to measure the relative phase difference between the separated delayed and direct backscattered signals. An analysis processor 136 can be used on the measured amplitude and phase to measure changes in the optical path length in the sensing medium in a distributed manner.

[0186] Now back Figure 2 This diagram illustrates a timeline depicting the arrival time of a distributed backscattered signal from a sensing medium to a phase and amplitude receiver 131 in currently disclosed systems and methods (such as system 100 with a modulated light source 101). In the case where the modulated light source is an intensity-modulated light source, the depth of intensity modulation is preferably greater than 50% up to 100% (e.g., from the output of a pulsed light source). For clarity, a pulsed light source is described in the diagram. Figure 2The arrival time is determined by the optical output (OO) pulse 200. Shortly after the generation of the optical output (OO) pulse 200, light traveling via the direct path (i.e., bypassing the first optical delay device 107a in the forward propagation direction and the second optical delay device 107b in the reverse propagation direction) before and after the sensing medium (optical fiber) 160 arrives at the phase and amplitude receiver 131. Due to distributed backscattering, it spreads over time. If the delay is longer than the round-trip time of the light in the sensing medium 160, the light arriving at the receiver 131 first travels via the direct path, then via the delayed path (i.e., signal 3163a), and arrives at the receiver without overlapping with signals 1161a and 4163b. Signal 3163a will coherently interfere with the light that first travels via the delayed path and then via the direct path, i.e., signal 2161b, which arrives almost synchronously with signal 3163a. ​​The relative phase of signals 2161b and 3163a is used in this sensing method. This is followed by light traveling via both delayed paths (i.e., signal 4163b). Repeat this process for subsequent pulses 205 and 206.

[0187] Figure 3 A conceptual schematic block diagram of a DAS system 200 with a further arrangement under a common path configuration according to the present invention is shown. Figure 3 In, with Figure 1 In contrast, common reference numerals are used to denote the same elements.

[0188] like Figure 3 As shown, the common path implementation uses a common optical path (e.g., a waveguide such as an optical fiber, although the polarization, orientation, or timing may differ) to achieve the delay before and after the sensing medium 160. Alternatively, the common path implementation can have exactly the same optical path for the delay before and after the sensing medium 160. Such an implementation greatly simplifies the manufacturing cost and complexity of the system because the system does not need to consider or compensate for delay variations between two or more individual IRDP or optical delay devices (physically, electronically, or in signal processing) relative to parameters such as temperature, pressure, or aging to ensure that the delay difference remains less than the coherence time of the light source according to the system and method disclosed herein. In the common path implementation (where the delay is static), when the sensing fiber is also static, the phase measured at the phase and amplitude receivers will be close to zero. Any change in the length of the optical path within the sensing medium 160 will result in a proportionally non-zero phase. In the absence of a common path implementation, and in the case of using different fibers of longer lengths before and after the sensing medium for the delay, care should be taken to ensure that within the coherence length of the light source (which can be as short as τ), coh The optical path length of the fiber is manufactured to be ~100 micrometers and will remain constant within the system's operating temperature range (e.g., 10°C to 50°C).

[0189] exist Figure 3 In the common path arrangement, the forward propagation IRDP with optical delay device 107a and the reverse propagation IRDP with optical delay device 107b are replaced by a single IRDP shared by the forward and reverse propagation paths with a single common optical delay device 207. System 200 further includes multiple optical circulators or couplers 206a and 206b to:

[0190] (a) Receiving light from light source 101 and guiding it through the IRDP in the forward propagation direction, wherein two separate portions of the output signal are combined in a hybrid combiner / splitter 211 to generate a backscattered signal before being emitted into sensing medium 160; and

[0191] (b) Backscattered light from sensing medium 160 (e.g., fiber optic sensing medium) is received, wherein each received backscattered signal is split into two parts by combiner / splitter 211, and each of the separated backscattered signals passes through a common path IRDP in the backscattering direction and is guided to phase and amplitude receiver 131 by circulators 206a and 206b. The detected signals are again recorded and stored in memory 133 and analyzed by analysis processor 136 to calculate the distributed path length variation in sensing medium 160 caused by external interference.

[0192] System 200 also includes an optional amplifier 213 for optical applications of the forward-propagating optical output signal and the backward-propagating received backscattered light signal from medium 160.

[0193] Figure 4 and Figure 5 The dual paths in the common system naming are shown in the further arrangements of systems 100 and 200, respectively (see example). Figure 1 ) and public paths (see, for example) Figure 3 Schematic layout of optical systems 400 and 500.

[0194] Figure 6 and Figure 7 The dual paths in the common system naming are shown in the further arrangements of systems 100 and 200, respectively (see example). Figure 1 ) and public paths (see, for example) Figure 3 Schematic layout of optical systems 600 and 700. Figure 6 and Figure 7This is achieved using a polarization-modifying Faraday mirror configuration in each interferometer arm arrangement. The polarization of the light returned by the Faraday mirror is rotated by 90° relative to the polarization of the input light. As those skilled in the art will understand, the Faraday mirror is thus used to compensate for any uncontrolled and / or random variations in the polarization state of light that has traveled a long distance through the fiber. In this way, a longer length of standard single-mode fiber (which is cheaper than polarization-maintaining fiber) can be used to delay the optical signal while ensuring a fixed relationship between the polarization input and output states.

[0195] Figure 8 The photonic components of a further arrangement 800 of the sensing system disclosed in the common-path implementation herein are shown, using all output ports of a 3×3 coupler 810 as phase and amplitude receivers. This arrangement is as understood by those skilled in the art. If we denote the three output signals detected from an ideal 3x3 coupler as I1, I2, and I3, then the real part of the complex interference signal can be determined by a linear combination (I1 + I2 - 2*I3), and the resultant can be obtained by a linear combination. Determine the imaginary part of the complex signal. The time gate or modulator 801 can be used to prevent the return signal from the sensing fiber from being guided back to the sensing fiber along with the output signal.

[0196] Figure 9 Photonic components of a further arrangement of the sensing system disclosed herein in a common-path embodiment with polarization beamsplitter 908 are shown. The polarization states of light traveling in different parts of the system are represented by vectors, and those skilled in the art will understand the effect of the various components on those polarization states. The purpose of this arrangement is to ensure, by means of polarization, that most of the light 901 entering the Intentional Relative Delay Path (IRDP) 902 is guided to the sensing fiber by the action of the polarization beamsplitter and Faraday mirrors. In this arrangement, the forward-propagating light 903 entering the sensing medium 160 is the fiber. Furthermore, this arrangement also ensures that most of the backscattered light 904, backscattered by the sensing fiber and entering the IRDP 902 by the same action of the polarization beamsplitter and Faraday mirrors 906 and 907, is guided to the phase and amplitude receiver 910. The light guided to the phase and amplitude receiver and its associated polarization state are labeled 905. In this arrangement, two orthogonal polarizations in a single fiber act as independent optical paths.

[0197] exist Figures 10 to 12 Experimental verification of the present invention is provided in the document. Figure 10The diagram illustrates the relationship between the amplitude of the signal from the phase and amplitude receivers and the timing of the detection of distributed backscattered radiation from a sensing medium comprising a telecommunications-grade optical fiber approximately 800 m in length. The first portion 1001 of the detected signal corresponds to the detected backscattered signal 161a (signal 1) generated by the direct output signal 151 that has bypassed the second optical delay device 107b. The last portion 1003 of the detected signal corresponds to the detected backscattered signal 163b (signal 4) generated by the delayed output signal 153 that has been transmitted through the second optical delay device. The central portion 1005 of the detected signal corresponds to: the detected backscattered signal 163a (signal 3) generated by the delayed forward propagation output signal 153 that has bypassed the second optical delay device, and the backscattered signal 161b (signal 2) generated by the direct output signal 151 that has been transmitted through the second optical delay device 107b, and is therefore a part of the detected signal, wherein interference between signals 2 and 3 occurs at the receiver and is used to analyze changes in the optical path length in the sensing medium.

[0198] In this example, the pulse length of the pulsed light output from the light source is 100 ns, the pulse repetition rate of the light source output is 20 kHz, and the delay from the IRDP used in the experimental system is 15 microseconds. The data in this example are obtained using... Figure 15 The common path setup shown is obtained. In this example, the coherence length of the light source is less than 0.05 mm, corresponding to a bandwidth of approximately 4 THz. A fiber stretcher with a single-frequency modulation is placed in the sensing fiber to provide a vibration signal that modifies the path length of the sensing fiber at that location, approximately 430 m from the input end of the sensing fiber.

[0199] Figure 11 It is a graph showing the relationship between the detected phase of the backscattered signal from the phase and amplitude receivers and the position along the sensing fiber. Figure 12 The calculated strain rate is shown as a function of position versus time. The strain ε is based on the phase gradient. Use formula The calculation is performed, where λ is the center wavelength of the light source, n is the refractive index of the optical fiber, and... It is the elastic optical coefficient, which is quantified in response to the change in fiber refractive index caused by mechanical strain due to changes in fiber length. The location 1011 of the external interference to the sensing fiber (fiber stretcher) can be easily observed in the detected signal at a distance of 430m along the fiber.

[0200] Figure 11The nonlinearity in the sensing fiber is also illustrated. This nonlinearity manifests in the detected signal as an upward tilt in the detected phase relative to distance, indicating the conventionally deterministic nature of nonlinear effects in the sensing medium in the systems and methods of the present invention. This effect is attributed to Kerr nonlinearity, where the refractive index of the fiber temporarily changes with the intensity of light in the fiber. The nonlinear effects in the detected signal can be easily corrected, as understood by those skilled in the art, if desired, by equalizing the power in the delay and direct path, or by removing the constant offset in the analysis. Figure 5 Experimental results with the arrangement in the middle and peak optical power exceeding 1W did not show any negative effects associated with modulation instability or nonlinear effects of stimulated Brillouin scattering, which are known to have adverse effects on coherent fiber optic sensing systems at lower peak power (peak optical power).

[0201] Figure 13 System 1300 has the same Figure 8 The system 800 is arranged similarly, utilizing a 3×3 coupler 1310, and can operate in two different modes. In the first operating mode, broadband amplified spontaneous emission (ASE) from amplifier 1301 serves as the system's light source. This ASE can be modulated by intensity modulator 1302 before entering IRDP 1305 and then directed to the sensing fiber. A second modulator / time gate 1312 acts as a time gate to allow this modulated ASE to enter the sensing fiber, but without any other unwanted light (e.g., preventing backscattered light from the sensing fiber from recirculating in the system). In the second operating mode, modulator 1302 is not required, and continuous ASE from amplifier 1301 enters the IRDP and is then modulated at intensity modulator 1312 before being directed to the sensing fiber. Intensity modulator 1312 can also be used in this operating mode to prevent unwanted light from entering the sensing fiber. In either operating mode, modulators 1302 and 1312 can be directly (respectively) combined in amplifiers 1301 and 1311 by direct modulation of the amplifier gain.

[0202] Figure 15 The common paths in the common system naming are shown in the further arrangements of systems 100, 200 and 300 respectively (see Figure 3 Schematic layout of optical system 1500. System 1500 is connected to... Figure 8 In a similar manner to System 800, all three optical signal outputs on the return path of the 3×3 coupler 1510 are used as phase and amplitude receivers, as will be understood by those skilled in the art.

[0203] exist Figure 16 In the middle, it is depicted that it will be Figure 1The analysis and processing method 1600 is implemented in the analysis processor 136. (Reference) Figure 1 System 100, method 1600 includes the following steps: reducing system noise 1601 by means such as filtering electronic signals; constructing complex signals 1602 using amplitude and phase measured by phase and amplitude receiver 131; applying deconvolution, cross-correlation, decoding, spike, dispersion compensation, polarization dispersion compensation, or nonlinear compensation processes / algorithms 1603 to the complex signals to compensate for known modulation or measurement modulation applied to output signals 151 and 153 or the propagation characteristics of the medium through which these signals pass; calculating the phase of the resulting complex signals 1604; taking the phase difference or phase gradient 1605 to calculate the change in optical path length or physical parameters of the sensing medium 160, such as strain (wherein the sensing medium 160 includes optical fiber), based on time and location; performing filtering or post-processing as required by a given application 1606; applying known automatic interpretation or classification methods 1607 as required by a given application; displaying 1608 and / or storing 1609 the obtained sensing data; and generating alarms for the user based on predefined criteria and as required by a given application 1610.

[0204] Figure 17 A conceptual schematic block diagram of a DAS system 1700 with a further arrangement under a common path configuration according to the present invention is shown. Figure 17 In, with Figure 1 and Figure 3 In contrast, common reference numerals are used to denote the same elements. System 1700 shows that delayed output signal 151 and direct output signal 153 are directed to sensing medium 160 without an intermediate step of recombining the output signals into a common forward optical path. Return path separator 108 receives backpropagation signals 161 and 163. Separator 108 directs a portion of both signals 161 and 163 to a common optical delay device 207 of the common IDRP to provide direct + delayed signal 2161b and delayed + delayed signal 4163b (collectively referred to as delayed BS). Separator 108 also directs a portion of both signals 161 and 163 to the direct path of the common IDRP to provide direct + direct signal 1161a and delayed + direct signal 3163a (collectively referred to as direct BS).

[0205] Figure 18 A conceptual block diagram 1800 shows the output signal being guided to the sensing medium, without the step of recombining the output signal onto a common forward optical path. Figure 18 In, with Figure 17 In contrast, common reference numerals are used to denote the same elements. Figures 18A to 18CThree example arrangements 1801, 1802, and 1803 for the photonic realization of system 1800 are shown respectively. Arrangement 1802 shows an example in which the two output paths and two return paths from the sensing medium may not be spatially separated, but are separated and independent due to orthogonal polarization rather than linear polarization states.

[0206] Figure 19 A further implementation scheme 1900 of the system disclosed above is described, wherein the forward propagation path 130 includes similar... Figure 1 The separator and delay device are shown (not shown). However, a frequency demultiplexer / multiplexer 1920 is provided before being directed to the sensing medium 160 to divide the forward propagation (direct and delayed) optical signals into multiple frequency bands and direct each pair of forward propagation signals in each frequency band to a selected one of the multiple sensing media 160.

[0207] The frequency multiplexer / demultiplexer (Mux / Demux) 1920 is also adapted to receive the backscattered signal propagated from each sensing medium 160 and direct the signal pairs from each sensing medium 160 to a similar frequency range. Figure 1 On return path 135 of the return path shown. System 1900 also includes a frequency demultiplexer 1930 to direct selected frequency bands from the backpropagating signal to a corresponding plurality of phase and amplitude receivers 131, which are adapted to measure the amplitude phase difference of the received optical signal in the selected frequency band, thereby determining the optical path length variation along each selected medium 160 in a distributed manner to infer physical changes in the selected medium 160. Figure 3 As shown, forward path 130 and return path 135 can share a common IRDP. In some embodiments, it may be advantageous to direct the backpropagation signal from multiple frequency bands to a single phase and amplitude receiver 131, in which case the number of sensing media is greater than the number of phase and amplitude receivers. In other embodiments, it may be advantageous to split the backpropagation signal from a single sensing medium into multiple frequency bands and direct the light in each band to a separate phase and amplitude receiver, in which case the number of sensing media is less than the number of phase and amplitude receivers.

[0208] Derivation of signal model

[0209] Using the coupled-mode equation to couple between forward and reverse propagation modes in an optical medium (specifically, optical fiber), Froggatt and Moore (M. Froggatt and J. Moore, “High-spatial-resolution distributed strain measurement in optical fibet with Rayleigh scatter”, Appl. Opt., Vol. 37, No. 10, pp. 1735-1740, 1998) derived the following expression:

[0210]

[0211] in

[0212] R(β) is the complex amplitude of the Rayleigh (non-frequency shifted) backscattered wave;

[0213] ε co It is the dielectric constant of the fiber core;

[0214] ε(z)-ε co It is the random variation of the dielectric constant of the fiber core;

[0215] We assume that outside the sensing fiber, (z)-ε co ≡0;

[0216] β is the propagation constant in the waveguide; and

[0217] E0(β) is the complex amplitude of the excitation field at z = 0.

[0218] The results show that the complex amplitude of the backscattered field caused by random dielectric constant fluctuations is the spatial Fourier transform of the dielectric constant fluctuations evaluated at twice the specific frequency of the excitation field.

[0219] Then the following replacements can be made:

[0220] z = v p τ / 2,

[0221] Where τ is the bidirectional (phase velocity) travel time; and

[0222] v p =c / n eff It is the phase velocity;

[0223] S(ω)=R(β(ω)) is the signal in the frequency domain;

[0224] E(ω)=E0(β(ω)) is the emission field in the frequency domain; and

[0225] β=ω / v p It is the propagation constant.

[0226] Given:

[0227]

[0228] Now we define:

[0229]

[0230] And use the Fourier transform formula:

[0231]

[0232] get:

[0233] S(ω)=E(ω)(-iω)Y(ω).

[0234] Alternatively, the signal model can be rewritten using simple notation as follows:

[0235] S(ω)=E(ω)G(ω)

[0236] s(τ)=e(τ)*g(τ) (2)

[0237] in

[0238] * indicates convolution.

[0239] G(ω)=(-iω)Y(ω) has been replaced

[0240] In the time domain, g(τ) can be interpreted as the impulse response function of the optical fiber.

[0241] s(τ) is a signal in the time domain

[0242] e(τ) is the emission field in the time domain.

[0243] g(τ), s(τ), and e(τ) are related to G(ω), S(ω), and E(ω) through Fourier transform.

[0244] Using identities:

[0245]

[0246] We found that:

[0247]

[0248] To determine the interference between two backscattered fields on the same optical fiber, we consider Analysis of detected signals The geometry shown. In this geometry, the phase and amplitude receivers have two backscattered signal fields:

[0249]

[0250]

[0251] Where T is the delay Then, the impulse response function of the optical fiber. It represents the perturbation applied to the sensed g(t).

[0252] Consider the interference term:

[0253]

[0254]

[0255] By using time, frequency, or polarization multiplexing, terms 1, 2, and 4 can be forced to equal zero, leaving only the third term:

[0256]

[0257] Alternative:

[0258] ■e T (t) = e(tT);

[0259] ■ That is, after time T, g(t) has undergone a very small deformation, which is equivalent to a position-dependent displacement / expansion;

[0260]

[0261] We get

[0262]

[0263] Then, replace t # =t′+ρ(t′) yields:

[0264]

[0265]

[0266] because

[0267] and

[0268]

[0269] therefore:

[0270]

[0271] Substitute t′=t # :

[0272]

[0273] When using sampling time t j When sampling at time Δt:

[0274]

[0275]

[0276] Assume the following stochastic model of the impulse response function is:

[0277]

[0278] The detected intensity is given by the following equation:

[0279]

[0280] We obtained:

[0281]

[0282] or:

[0283]

[0284] Or, equivalently, represented by convolution notation:

[0285]

[0286] Therefore, in order to determine the change in optical path length ρ(t) j Intensity modulation I(t) was applied. j -ΔT) and recorded data d(t) j deconvolution between )

[0287]

[0288] therefore:

[0289]

[0290] Alternatively:

[0291]

[0292] Cross-correlation can form an approximate deconvolution, especially if And the mutual correlation is written as:

[0293]

[0294] If ΔT is greater than the coherence length, then:

[0295]

[0296]

[0297] Or, equivalently, represented by convolution notation:

[0298]

[0299] if To represent different sensing media (e.g., optical fiber), then

[0300]

[0301]

[0302] Or, equivalently, represented by convolution notation:

[0303]

[0304] Field of application

[0305] The above derivation shows that the change (t) in the optical path length induced in the sensing medium (i.e., optical fiber) is given by the following expression:

[0306]

[0307] in:

[0308] ■t is the sampling time;

[0309] ■z = vt / 2 is the position along the optical fiber;

[0310] ■v is the speed of light in an optical fiber;

[0311] ■d(t) is a complex signal from the phase and amplitude receivers;

[0312] ■I(t) is the modulated laser intensity;

[0313] ■ Indicates Fourier transform;

[0314] ■ Indicates the inverse Fourier transform;

[0315] ■ω0 is the center frequency of the light source; and

[0316] ■ΔT is the difference between the delay before and after sensing the optical fiber.

[0317] Therefore, the change in optical path length can be determined by deconvolving the complex signals from the phase and amplitude receivers with the modulated laser intensity. If Then cross-correlation can be used instead of deconvolution.

[0318] When the intensity modulation is pulse modulation, the change in optical path length can be directly determined from the phase of the complex signal, such as when measured at the phase and amplitude receivers:

[0319]

[0320] Advantages

[0321] Possible applications of the systems and methods disclosed herein may include: distributed acoustic sensing over fiber optics or waveguides; surface vibration measurement; distributed acoustic LiDAR in air and atmosphere; wind speed measurement; distributed acoustic LiDAR in water; distributed acoustic LiDAR in pipe fluids; vertical seismic profiling analysis in boreholes and wells; marine towed cables for seismic exploration; terrestrial seismic sensors for seismic exploration; permanent seismic monitoring arrays for repeating seismic imaging and inversion; passive seismic monitoring, such as seismic monitoring, microseismic monitoring, and induced seismic activity related to underground fluid injection or production; mine wall stability monitoring, such as microseismic monitoring and collapse monitoring; dam stability monitoring, such as seismic activity induced by dams and the stiffness of tailings dams; and pipeline monitoring. Applications include: leak detection and tampering; perimeter and security monitoring / surveillance, such as intrusion detection; infrastructure monitoring, such as strain and vibration control of bridges, tunnels, buildings, and wind turbines; vehicle structure monitoring, such as strain and vibration control of automobiles, aircraft, and ships; flow measurement, such as metering flow in pipelines; geotechnical surveying, such as surface wave inversion of near-surface shear wave velocity; airflow profile analysis, such as atmospheric profile analysis, wind tunnel profile analysis, and around aircraft; waterflow profile analysis, such as profile analysis around ocean currents, rivers, and maritime vehicles; medical devices, such as body strain sensors and blood flow measurement; telecommunications network monitoring, such as interference and faults; traffic and vehicle flow monitoring, such as roads, railways, and ships; audio recording; and fire monitoring, such as in tunnels and infrastructure.

[0322] ​

[0323] As those skilled in the art will understand from the disclosure herein, the systems and methods disclosed herein overcome the inherent limitations of existing distributed acoustic sensing systems, including the advantages set forth below.

[0324] Direct phase and amplitude measurements allow for the accurate determination of the rate and amplitude of changes in optical path length within the sensing fiber with very high sensitivity. This also allows for a wider range of applications, such as machine condition monitoring, which is quantitative and extends beyond basic interference detection for security alarms. As proof of sensitivity and fidelity, the system disclosed herein has demonstrated its ability to acoustically record normal human voices and play back the recorded audio with fidelity comparable to microphone recordings.

[0325] Direct phase and amplitude measurements provide the ability to clearly distinguish the amplitude and phase changes of interfering light, which (in practice) overcomes the error in measuring changes in optical path length, which can be caused by attenuation effects, joints, connectors, and nonlinear effects in optical fibers (including Kerr nonlinearity).

[0326] Phase measurements performed in a fully orthogonal (2π range) manner, along with the subsequent expansion, eliminate any π-range ambiguity that could lead to errors in the notation and interpretation of physical quantities, such as ambiguity between compression and stretching. Furthermore, it extends the dynamic range of the system disclosed herein by a factor of two.

[0327] Direct phase and amplitude measurements overcome the problem that the system disclosed herein is prone to drifting into a completely insensitive state during practical operation. This insensitivity occurs near a specific phase value at which small phase changes do not produce measurable changes in interference intensity. A 3×3 coupler can only be used to generate a 120° relative phase offset between the optical fields at its output ports, thereby using the phase offset to select the higher slope region of the coupler's transfer function to improve sensitivity when the amplitude of the interference is very small. However, this solution is not reliable in practice because the offset requirement drifts with factors such as time, position along the fiber sensing medium, system parameters, and environmental conditions.

[0328] Direct phase and amplitude measurements allow the use of decoding and / or modulation schemes with the systems disclosed herein to improve system performance, including improved sensitivity and extended measurement range.

[0329] Direct phase and amplitude measurements avoid light source intensity noise from interfering with phase measurements, thus preventing interference signals that would otherwise limit the system's signal-to-noise ratio discrimination.

[0330] Intentional intensity modulation of the light source enables precise distributed sensing in sensing media, such as sensing optical fibers. The systems and methods disclosed herein are not limited to detecting and locating individual disturbances in the sensing medium, but also allow for the classification of disturbances based on location, time, or frequency.

[0331] The systems and methods disclosed in this paper also overcome the inherent limitations of existing c-OTDR and c-OFDR systems, such as significantly reducing system cost, complexity, and robustness by using broadband light sources without introducing phase noise, reducing sensitivity, or limiting measurement range.

[0332] The system and method disclosed herein intrinsically and simultaneously average the distributed sensing signal at all optical frequencies present in a broadband light source. This is a key advantage compared to existing narrowband / coherent c-OTDR and c-OFDR systems because it significantly improves sensitivity, accuracy, and linearity by eliminating problems associated with the relationship between amplitude attenuation and position along the sensing fiber. The amplitude attenuation phenomenon in narrowband / coherent c-OTDR and c-OFDR is the basis of the stochastic nature of Rayleigh backscattering, which produces random amplitude and phase at every location along the fiber. This phenomenon is analogous to spots on a rough surface when illuminated with coherent light. At locations where the backscattered amplitude is exactly close to zero, the corresponding phase at these locations cannot be accurately determined or may be undefined when the amplitude is exactly equal to zero. Furthermore, the near-zero amplitude "appears" as a sign change in the signal, which introduces highly nonlinear errors in any estimated phase change. These phase errors are a large and fundamental source of error in c-OTDR and c-OFDR DAS systems, thus necessitating the use of multiple narrowband, highly coherent laser sources in the system to overcome the limitations of amplitude attenuation; this is achieved by leveraging the fact that each different laser wavelength produces a different but random amplitude and phase realization (i.e., different speckle patterns). Using amplitude-weighted averaging of the phase in digital signal processing can reduce phase errors in the system because the phase measured near zero amplitude is largely ignored. The system and method disclosed in this paper utilize a sufficiently wide light source to ensure that no backscattering amplitude can randomly appear near zero along the sensing fiber. This is achievable because the phase at each location at each optical frequency is not random (although the amplitude is random), but rather proportional to the change in optical path length (the desired measurement) and the delay difference. Therefore, each frequency of the light source makes a constructive contribution to the amplitude and phase at each location. Random amplitude patterns are then washed away, analogous to eliminating speckles on a rough surface when using a sufficiently wide light source.

[0333] The use of broadband light sources increases the optical power available for sensing because the power threshold for unwanted nonlinear effects is higher compared to coherent light sources. Nonlinear effects with improved thresholds include stimulated Brillouin scattering, four-wave mixing, and modulation instability. In fact, since the effect of nonlinear mechanisms on the backscattered signal in low-coherence states is deterministic and can therefore be corrected in the analysis of coherently detected signals, the systems and methods disclosed herein can be used even when strong nonlinear effects exist within the sensing medium.

[0334] The systems and methods disclosed in this paper exhibit excellent stability and robustness in uncontrolled environments and in the presence of vibration and noise sources. This can be critical for reliable performance in outdoor applications with large machinery, such as seismic monitoring at oil drilling platforms and mining sites.

[0335] Typically, phase noise in the light source is undesirable in c-OTDR and c-OFDR systems because it increases overall noise and thus reduces the system's signal-to-noise ratio (SNR). This encourages the use of narrower-bandwidth and higher-coherence laser sources. The system and method disclosed herein teach the opposite. That is, simulations of the system and method disclosed herein show that, contrary to conventional wisdom, increasing the light source bandwidth (linewidth) and encouraging greater incoherence (less coherence, i.e., shorter coherence time and length) in the light source will actually reduce noise and improve the SNR. This is because incoherent interference between backscattered light returning from different segments of the sensing medium (intervals greater than the coherence length) generates background electronic (jitter) noise dispersed across the entire bandwidth of the light source. Since the electronic bandwidth of this noise can be greater than 1 THz, it can be filtered and removed from the signal very effectively.

[0336] Pulsed light sources have advantages over continuous wave sources, which can "mask" the system in practical applications due to strong reflections in the sensing medium. This is common in fiber optic links involving one or more connectors or devices, which can render existing distributed acoustic sensing systems unusable in many practical situations.

[0337] When the direct + direct and delayed + delayed backscattered signals arrive at different times than the direct + delayed and delayed + direct signals, the pulse of the light source improves the signal-to-noise ratio discrimination of the system disclosed herein.

[0338] Unlike Sagnac-type systems that require access to both ends of the sensing fiber, the system disclosed herein can achieve distributed sensing even with the limitation of only accessing one end of the sensing fiber. This offers advantages in borehole and well applications or other access-restricted applications.

[0339] The polarization management method proposed in this paper avoids the main errors caused by the polarization stability of the laser or the polarization mode dispersion in optical components or sensing media.

[0340] The system disclosed herein allows for implementation using fiber optic components, bulk optical components, micro-optical components, and / or planar waveguide techniques, enabling greater versatility in the implementation of systems for a variety of sensing applications.

[0341] Because broadband light sources have multiple optical wavelengths that can be used for independent sensing, multiple sensing fibers (or available sensing media) can be connected to the system via wavelength division multiplexing.

[0342] explain

[0343] in accordance with

[0344] As described in this article, “based on” can also mean “according to” and is not necessarily limited to the integers specified therein.

[0345] Implementation plan / arrangement

[0346] Throughout this specification, references to “an embodiment,” “an embodiment,” “an arrangement,” or “an arrangement” refer to a specific feature, structure, or characteristic described in connection with that embodiment / arrangement that is included in at least one embodiment / arrangement of the invention. Therefore, the phrases “in one embodiment / arrangement” or “in an embodiment / arrangement” appearing throughout this specification do not necessarily refer to the same embodiment / arrangement, but may refer to the same embodiment / arrangement. Furthermore, in one or more embodiments / arrangements, specific features, structures, or characteristics may be combined in any suitable manner, as will be apparent to those skilled in the art from this disclosure.

[0347] Similarly, it should be understood that in the above description of exemplary embodiments / arrangements of the invention, various features of the invention are sometimes combined in a single embodiment / arrangement, illustration, or description thereof to streamline the disclosure and aid in understanding one or more aspects of the invention. However, this approach should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the appended claims, aspects of the invention lie in fewer than all features of a single foregoingly disclosed embodiment / arrangement. Therefore, the claims are hereby expressly incorporated into the detailed description, wherein each claim independently serves as a separate embodiment / arrangement of the invention.

[0348] Furthermore, while some embodiments / arrangements described herein include certain features but not others included in other embodiments / arrangements, combinations of features from different embodiments / arrangements are intended to be within the scope of the invention and form different embodiments / arrangements, as understood by those skilled in the art. For example, in the appended claims, any claimed embodiment / arrangement may be used in any combination.

[0349] Specific details

[0350] Numerous specific details are set forth in the description provided herein. However, it should be understood that embodiments of the invention can be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description.

[0351] the term

[0352] In describing the preferred embodiments of the invention as shown in the accompanying drawings, specific terminology will be used for clarity. However, the invention is not intended to be limited to the specific terminology chosen so far, and it should be understood that each specific term includes all technical equivalents that operate in a similar manner to achieve similar technical purposes. Terms such as “forward,” “reverse,” “radial,” “circumferential,” “upward,” “downward,” etc., are used as convenient terms to provide a reference point and should not be construed as limiting terms.

[0353] Different instances of an object

[0354] As used herein, unless otherwise specified, the ordinal adjectives “first,” “second,” “third,” etc., used to describe common objects merely indicate that different instances of similar objects are being referenced, and are not intended to imply that objects described in this way must be in a given order in time, space, hierarchy, or any other way.

[0355] Scope of Invention

[0356] Therefore, although the preferred arrangement considered to be in the present invention has been described, those skilled in the art will recognize that other further modifications can be made thereto without departing from the spirit of the invention, and all such changes and modifications falling within the scope of the invention are intended to be claimed. Functions can be added or removed from the block diagrams, and operations can be interchanged between functional blocks. Steps can be added or removed from the methods described within the scope of the invention.

[0357] Although the invention has been described with reference to specific examples, those skilled in the art will understand that the invention can be implemented in many other forms.

[0358] Industrial applicability

[0359] It is evident from the above that the described arrangement is applicable to the mobile device industry, particularly to methods and systems for distributing digital media via mobile devices.

[0360] It will be understood that the methods / apparatus / devices / systems described / shown above at least substantially provide an improved system and method for quantitative distributed measurement of changes in optical path length in optically transparent media.

[0361] The systems and methods described herein and / or illustrated in the accompanying drawings are presented by way of example only and do not limit the scope of the invention. Unless otherwise specifically stated, various aspects and components of the systems and methods described herein may be modified or may have been replaced, and thus known equivalents or still unknown alternatives may be developed in the future or may be found to be acceptable alternatives in the future. Due to the wide range of potential applications and because the systems and methods of the invention described herein are intended to be adaptable to many such variations, the systems and methods described herein can also be modified for a variety of applications while remaining within the scope and spirit of the claimed invention.

Claims

1. An optical sensing system, comprising: Sensing medium; A light source, used to produce optical output, is called OO; A forward propagation unit is configured to guide a first portion of the OO, referred to as direct OOS, to the sensing medium, and to guide another portion of the OO, referred to as delayed OOS, to the sensing medium, which is delayed by a first delay time τ1 relative to the direct OOS. A device for modulating the intensity of light from the light source or light in the forward propagation unit; The backpropagation unit is configured to receive a distributed backscattered signal from the sensing medium and delay a first part of the backscattered signal, referred to as the delayed BS, relative to another part of the backscattered signal, referred to as the direct BS, by a second delay time τ2. A device for quantitatively measuring the difference in optical phase between delayed BS and direct BS in a time-varying manner; and An analysis processor is configured to receive the measured optical phase and provide quantitative distributed optical sensing in the sensing medium; The system is capable of continuously and in real time measuring the amplitude and / or phase of the distributed backscattered light signal along the entire length of the sensing medium.

2. The system of claim 1 further includes an optical receiver configured to receive and interfere with a delayed BS having a direct BS.

3. The system of claim 2 further includes a phase modulator and / or a frequency shifter configured to act on direct OOS, delayed OOS, direct BS, and / or delayed BS to provide an optical phase measurement with fully orthogonal determination of unambiguity over a 2π radian range.

4. The system of claim 1 further includes an optical receiver configured to receive and interfere with a common optical local oscillator having a delayed base (BS) and a direct base (BS) to quantitatively measure the difference in optical phase between the local oscillator and the backscattered signal.

5. The system according to any one of claims 1 to 3, wherein, The light source is a multi-wavelength light source.

6. The system according to any one of claims 1 to 3, wherein, The light source is either partially coherent or incoherent.

7. The system according to any one of claims 1 to 3, wherein, The light source is a low-coherence light source.

8. The system according to any one of claims 1 to 3, wherein, The light source is selected from an SLED, a DFB laser, an FP laser, an optical frequency comb, or a group of multiple such light sources.

9. The system according to any one of claims 1 to 4, wherein, The first delay time and the second delay time are similar and within 1%.

10. The system according to any one of claims 1 to 4, further comprising an optical amplifier and a filter, the optical amplifier and the filter being configured to at least partially separate the signal light from the spontaneous emission amplified by the optical amplifier.

11. The system according to any one of claims 1 to 4, further comprising a modulator configured to act as a time gating device to prevent light from entering the sensing medium at an undesirable time.

12. The system according to any one of claims 1 to 4, further comprising means for combining direct OOS and delayed OOS into the same optical path in the forward propagation unit.

13. The system according to any one of claims 1 to 4, wherein, The forward propagation unit guides the direct OOS and delayed OOS to the sensing medium in separate optical paths or spatial modes.

14. The system according to any one of claims 1 to 4, wherein, The backpropagation unit receives direct BS and delayed BS from the sensing medium in a separate optical path or spatial mode.

15. The system according to any one of claims 1 to 4, further comprising a plurality of sensing media, an optical multiplexing device for guiding light from the forward propagation unit to each of the sensing media, and an optical multiplexing device for receiving light from each of the sensing media into the reverse propagation unit.

16. A method for performing optical sensing in a sensing medium, comprising the following steps: A light source is provided, which is used to generate light output, referred to as OO; The first part of the OO is referred to as direct OOS and guided to the sensing medium; The other part of the OO is called delayed OOS, which is delayed by a first delay time τ1 relative to the direct OOS; Direct the delayed OOS to the sensing medium; The intensity of at least a portion of the light from the light source is modulated before the light from the light source is directed to the sensing medium; Receive distributed backscattered signals from the sensing medium; The first part of the backscattered signal is called the delayed BS, and the other part of the backscattered signal is called the direct BS, with a second delay time τ2. The difference in optical phase between delayed BS and direct BS is quantitatively measured in a time-varying manner; as well as Using the measured optical phase and providing quantitative distributed optical sensing in the sensing medium; The method is capable of continuously and in real time measuring the amplitude and / or phase of the distributed backscattered light signal along the entire length of the sensing medium.

17. The method of claim 16, further comprising the steps of interfering with direct BS and delaying BS.

18. The method of claim 17, further comprising modulating the phase of the direct OOS, delayed OOS, direct BS and / or delayed BS and / or shifting the frequency of the direct OOS, delayed OOS, direct BS and / or delayed BS to provide an optical phase measurement with fully orthogonal determination of unambiguity over a 2π radian range.

19. The method of claim 16, further comprising the step of interfering with a conventional optical local oscillator using delayed BS and direct BS to quantitatively measure the difference in optical phase between the local oscillator and the backscattered signal.

20. The method according to any one of claims 16 to 19, wherein the first delay time and the second delay time are similar to each other within 1%.

21. The method according to any one of claims 16 to 19, further comprising modulation or time gating to prevent light from entering the sensing medium at an undesirable time.

22. The method of any one of claims 16 to 19, further comprising combining the direct OOS and the delayed OOS onto the same optical path before directing the direct OOS and the delayed OOS into the sensing medium.

23. The method according to any one of claims 16 to 19, wherein direct OOS and delayed OOS are directed into the sensing medium in separate optical paths or spatial modes.

24. The method according to any one of claims 16 to 19, wherein the direct BS and the delayed BS are received from the sensing medium in separate optical paths or spatial modes.

25. The method according to any one of claims 16 to 19, further comprising multiplexing light to a plurality of said sensing media; and receiving light from each of said sensing media.

26. The method according to any one of claims 16 to 19, wherein the distributed backscattering is at least partially attributed to Rayleigh backscattering in the optical fiber.

27. The method according to any one of claims 16 to 19, wherein the method of performing distributed optical sensing in the sensing medium involves numerical deconvolution and / or numerical cross-correlation between the sensing signal and a known or measured modulation.

Images