Single reflection enhancement pattern distributed sensor with maximum signal level

Abstract

A fiber-optic sensor and a distributed fiber optic sensing system. The fiber-optic sensor includes an optical fiber and gratings formed within the optical fiber. One or more of the gratings are configured to generate a refractive index perturbation within the optical fiber so that upon exposure of the fiber-optic sensor to a light, a power ratio of a backscattered light signal that corresponds to a total backscattered light signal from a substantial entirety of the optical fiber to a backscattered light signal that corresponds to a spatial resolution length is below a value that would adversely impact the performance of a detector or related distributed fiber optic sensing system component.

Description

SINGLE REFLECTION ENHANCEMENT PATTERN DISTRIBUTED SENSOR WITH MAXIMUM SIGNAL LEVELCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patent application serial number 63 / 702,703, filed on October 3, 2024, by Paul S. Westbrook, and having the title SINGLE REFLECTION ENHANCEMENT PATTERN DISTRIBUTED SENSOR WITH MAXIMUM SIGNAL LEVEL, which is incorporated herein by reference in its entirety.BACKGROUNDFIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to an optical communications and more particularly to distributed fiber-optic sensor (DFOS) systems.DESCRIPTION OF RELATED ART

[0003] A traditional DFOS couples an optical signal, a length of optical fiber, and a detector such that a signal that has been transmitted from the source and reflected or otherwise backscattered from the optical fiber is received into the detector for analysis of any perturbations to the reflected signals relative to those originally emanating from the source. While system performance may be improved by increasing the amount of reflected backscattering, there reaches a point where too much can saturate the detector, thereby degrading the performance of the system. Thus, a heretofore unaddressed need exists to overcome these deficiencies and inadequacies.SUMMARY

[0004] The present disclosure provides systems and methods for maximizing the effective power of an optical signal that is reflected (or backscattered) from a single length (or portion) of an optical fiber that is used in a DFOS system while keeping an aggregate reflected signal from the entirety of the optical fiber or other effects arising from high back scatter such as optical cross talk of multipath interference (MPI) below a maximum allowed by the system’s signal detector.

[0005] Briefly described, in architecture, one embodiment is a fiber-optic sensor. The fiber-optic sensor includes an optical fiber that has an axial length and a spatial resolution length. The fiber-optic sensor further includes gratings formed within the optical fiber and extending alongat least a portion of the axial length to define a grating length. One or more of the gratings are configured to generate a refractive index perturbation within the optical fiber. In this way, when the optical fiber is exposed to a light selected from the group consisting of pulsed infrared, visible radiation, ultraviolet radiation, a femtosecond pulse, a picosecond pulse, a nanosecond pulse, and continuous wave radiation, a power ratio of a backscattered light signal that corresponds to a total backscattered light signal from a substantial entirety of the optical fiber to a backscattered light signal that corresponds to the spatial resolution length is less than 1,000,000.

[0006] Briefly described, in another architecture, one embodiment is a DFOS system that includes an optical signal source, an optical fiber, gratings formed within the optical fiber, and a detector. The optical fiber, which is signally (that is to say, optically) coupled to the optical signal source, has an axial length and a spatial resolution length. The gratings extend along at least a portion of the axial length of the optical fiber. One or more of the gratings is configured to generate a refractive index perturbation within the optical fiber. The detector is signally coupled with the optical fiber such that backscattered light received by the detector from the optical fiber defines a power ratio wherein a total backscattered light signal from a substantial entirety of the optical fiber to a backscattered light signal that corresponds to the spatial resolution length is less than 1,000,000.

[0007] Other systems, devices, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

[0009] FIG. 1 depicts a block diagram of an embodiment of the DFOS system discussed herein;

[0010] FIG. 2 A depicts a graph showing a first embodiment of nominally-equivalent gratings formed along the length of an optical fiber that may be used as part of a DFOS system;

[0011] FIG. 2B depicts a graph showing the first embodiment of FIG. 2A with some incidental uneven grating attributes;

[0012] FIG. 3A depicts a graph showing a second embodiment of nominally-equivalent gratings formed along the length of an optical fiber that may be used as part of a DFOS system; and

[0013] FIG. 3B depicts a graph showing the second embodiment of FIG. 3A with some incidental uneven grating attributes.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] While enhanced backscattering within a DFOS system can improve the signal-to-noise ratio, if such backscattering becomes too large, the system performance may degrade due to one or more of detector saturation, backscatter cross talk, MPI, or the like.

[0015] Such problems may be remedied by the various aspects of the present disclosure where a maximum back reflected power permitted by the detector remains below a certain value while still maintaining the highest possible signal returns from each of the individual sensor spatial resolution lengths within the length of the fiber, cable, or related strand that makes up the DFOS. As such, a ratio of the total (or aggregate) back reflected power over the entire sensor length to the back reflected power from one spatial resolution length would remain below a maximum threshold (such as 1,000, 10,000, 100,000, or 1,000,000) permitted by the detector for a given series of nominally similar exposures to optical source-provided light. Particular calculations for the 10,000 and 1,000,000 thresholds will be discussed in more detail as follows. Moreover, by keeping the backscattered signal from each individual pattern or grating within the DFOS nominally the same, ease of manufacturing is promoted as complicated grating fabrication methods that vary grating parameters along the length of a fiber sensor may be avoided.

[0016] Having provided a broad technical solution to a technical problem, reference is now made in detail to the description of the embodiments as illustrated in the drawings. While several embodiments are described in connection with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents.

[0017] As a threshold matter, the combination of a backscattered signal with an input or related reference signal may employ known techniques, such as time domain analysis (for example, Optical Time Domain Reflectometry (OTDR)), both time domain and spectral domain analysis (for example, Time Domain Multiplexing (TDM) and Wavelength Division Multiplexing (WDM)), as well as other approaches such as optical frequency domain reflectometry (OFDR) or swept wavelength interferometry (SWI). For any of these sensor systems, the ratio of total back reflected power to back reflected power from a spatial resolution length, a single grating or related element length may be determined.

[0018] By way of example using OFDR, an approach to providing an enhanced reflection pattern and associated signal level for each sensor within a DFOS while maintaining a maximum total signal level is described next. It will be appreciated that an analogous derivation may be carried out for OTDR, TDM, WDM, SWI, or other related DFOS configurations, and that all such variants are deemed to be within the scope of the present disclosure. It will further be appreciated that by using OFDR, the frequency of the incoming light may be scanned after which a large number of backscattered signals may be subjected to a Fourier transform in order to determine with precision the location on the optical fiber 30 from which the backscattered signal emanated.

[0019] Referring first to FIG. 1, a DFOS system 10 is shown. The system 10 includes a controller 20, an optical fiber sensor made up of an optical fiber 30 with one or more nominally equivalent gratings, an optical signal source 40, and a detector 50. In the present context (and absent the context indicating to the contrary), the term “nominally equivalent” is meant to represent “equally-spaced” such that both terms are understood to represent the same thing. For example, in situations where the terms would not represent identical quantities is when unintended manufacturing variations are present. The gratings (which will be discussed in more detail as follows) are formed within the optical fiber 30 and extend along its axial length. In one form, thereis a single core in the fiber, while in others more than one; in the case of the latter, there may exist multiple, separated core regions. In one form, the optical fiber 30 may include one or more cores, each of which guides light at the wavelength used by the DFOS system 10. The cores may guide a single mode or they may guide more than one mode at the wavelength used by the DFOS system 10. These cores are surrounded by a cladding region and the cladding region may be surrounded by one or more protective coatings. In one form, the optical fiber 30 may be fabricated from silica glass and doped silica glass such as Ge-doped silica glass, or silica doped with any combination of other dopants such as B, Al, F, Sn, P, Ti, Pb, or any rare earth element such as Yb, Er, Tm, or any other element that can be introduced into the silica matrix. Other materials may also be used for one or more of the cores and cladding, including polymers, soft glasses or the like. In one form, the glass or core regions may also contain hollow regions or air-filled regions with no solid material. In one form, the coating may be made up of polymers such as acrylates or silicones, or the coating may be made of metals, extruded materials, UV or thermally cured materials, carbon layers, or chemically or biologically active materials. Additional pre- or post-signal processing equipment 60 may also be included; such equipment may include one or more of a coupler, signal processing circuitry, converters (such as analog-to-digital or digital-to-analog), amplifiers, repeaters, signal shapers or the like. In one form, the remaining components may be made to operate autonomously or semi-autonomously. In one form, the remaining components may be made to cooperate either directly or indirectly with the controller 20. It will be appreciated that one or more of the components that make up the DFOS system 10 to perform the associated processes described herein may be implemented in hardware, software, firmware or a combination thereof. In one form, they are implemented in software or firmware that is stored in a memory and that is executed by the controller 20 or other suitable instruction execution system. If implemented in hardware, as in an alternative embodiment, they can be implemented with any or a combination of the following technologies, which are all well known in the art: one or more discrete logic circuits having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), or the like.

[0020] In situations where operation of the DFOS system 10 is automated, a program that comprises an ordered listing of executable instructions for implementing logical functions may be used. In one form, such a program can be embodied in any computer-readable medium for use byor in connection with an instruction execution system, apparatus, or device, such as the controller 20 or other computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (electronic), a read-only memory (ROM) (electronic), an erasable programmable read-only memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured via, for instance, optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory.

[0021] The optical signal source 40 may be used to emit light in the form of pulsed infrared, visible or ultraviolet (UV) radiation, femtosecond, picosecond, or nanosecond pulses, as well as continuous wave (CW) radiation of any wavelength such as 1550nm, either tunable or fixed in wavelength. As such, input light may be in the form of a pulse or CW such as — in one form — a wavelength tuned CW input from an OFDR. It will be appreciated that all of these variants are within the scope of the present DFOS system 10. In one form, the light from the optical signal source 40 enters one end of the optical fiber 30 such that at least some of the backscattered light traverses the fiber 30 back to the one end to be received by the optically-coupled detector 50 that records, collects or otherwise register the backscattered signal. In one form, the optical signal source 40 and the detector 50 may be separate components, while in another as part of a single, integrated whole. Likewise, one or more of the controller 20, optical signal source 40, detector 50, and pre- or post-signal processing equipment 60 may be formed of separate, discrete components or as part of an integrated whole, such as placed in a common housing, on common circuitry or the like. For example, in one form the optical signal source 40 and the detector 50may form an interrogator from which source light is emitted and reflected or backscattered light is received and then analyzed such as to quantify a measured parameter of interest. In one particular form, such an interrogator is configured to perform OFDR.

[0022] As will be discussed elsewhere, a given fiber optic sensor made with optical fiber 30 may have a particular spatial resolution length Lres that is determined by this interrogator or detector 50. In other words, for an OFDR-based approach, a particular length of optical fiber 30 may be understood by the detector 50 to be sectioned into numerous lengths by scanning over a range; the amount of reflected signal is thus related to the particular spatial resolution length Lres. By comparing the reflectivity from the spatial resolution length Lres to reflectivity from the entire length of optical fiber 30, and keeping within a threshold value as determined by the detector 50, a more accurate measurement may be taken. In this regards, if a measurement of 1mm is taken over an optical fiber that is 3m in length, then by the use of the fiber optic sensor discussed herein, this understanding will provide an enhanced signal for the 1mm portion that corresponds to the spatial resolution length Lres while keeping an overall backscattered signal from the remainder of the optical fiber 30 low.

[0023] Referring next to FIGS. 2A through 3B, plots of the wavelength of the gratings 300 in the form of Bragg gratings versus position along an axial length Lsensor of the optical fiber 30 are shown for two different enhancement patterns and their corresponding changes in refractive index perturbations. Within the present context, the gratings 300 allow the optical fiber 30 to acquire sensor-based features through partial reflection of the input light where the gratings 300 are a property of the sensor that was fabricated with actinic radiation (such as UV, visible, IR, pulsed or continuous). In one form, each grating 300 may be tuned to define a particular Bragg wavelength in order to reduce the impact of more randomized reflections such as those associated with Rayleigh scattering or the like.

[0024] Referring with particularity to FIGS. 3A and 3B, within the optical fiber 30, a series of periodic gratings (or patterns) 300 are formed along an axial dimension (or transmission direction) Z of the optical fiber 30 that coincides with the axial length Lsensor of the optical fiber 30 to provide a corresponding series of substantially identical chirps (or chirped perturbations) that modify the refractive index which in turn causes at least a portion of a signal received from the optical signalsource 40 to be backscattered or otherwise reflected. In other words, the same periodic perturbations are repeatedly used over the axial length Lsensor of the optical fiber 30 that makes up the sensor. The spatial resolution length Lres is also shown in FIGS. 2A. through 3B. This is the length that defines the spatial resolution of the sensor. The reflected signal originating from this length should be made as large as possible to improve the sensing accuracy over this length. The present disclosure depicts how the gratings 300 may be designed to maximize this backscattered power reflected from a single spatial resolution length Lres while maintaining the total back reflected power from the entire axial length Lsensor of the optical fiber 30 that makes up the sensor below a value that is deleterious to sensor performance and which can be the result of detector saturation, MPI, nonlinearities, or other defects arising from an excess of back reflected power. It will be understood that if the DFOS system 10 functions to perform OFDR, then the reflected signals from each wavelength as the wavelength is scanned arise from the entire sensor and not necessarily a given spatial resolution length Lres. Relatedly, in OFDR the amount of light that is reflected from the spatial resolution length Lies is then determined by a Fourier transform of the reflectivity spectrum obtained by the OFDR wavelength (or equivalently frequency) scan. Therefore the light reflected from the spatial resolution length Lres may also be understood to be the amount of light observed in the Fourier processing in the OFDR that results in the spatial sensing information.

[0025] Referring with particularity to FIG. 2A (and applicable to FIGS. 2B, 3A and 3B as well), and which will be discussed in more detail as follows, this spatial resolution length Lres is the shortest of three length indicators along the optical fiber 30, the others being successively longer in the form of an effective length Leff over which the periodic perturbations will reflect a given wavelength of received light, and a step length (also referred to as grating length) 2LMC (or LMC in FIG. 3) that corresponds to an enhancement pattern period, and the pattern has an amplitude from minimum to maximum grating period of AA. It will be appreciated that the relative lengths of these indicators should not be inferred from their present representation in the drawings insofar as their depictions therein are not necessarily to scale. As will be discussed with more particularity through an example, the spatial resolution length Lies is generally much smaller than the effective length Leff, that in turn is less than the step length LMC where the lengths are 1mm, 6mm and 20mm, respectively. The step length (or equivalently the grating length) LMC depicted in FIGS. 3A and 3B is shown as both the pitch of the periodic array and also has the chirp rate AA / LMC,while the embodiment of FIGS. 2A and 2B has a chirp rate that is AA / LMC. This has the practical effect of making the following arguments the same for the gratings of both figures. In one form, the beams formed by the optical signal source 40 define predetermined, relatively short fixed lengths along the axial dimension Z. By way of non-limiting example, the grating 300 length is two centimeters (2cm, or 20 millimeters (mm)), although other suitable lengths may be used. Two different forms of the optical fiber 30 with the periodic gratings 300 are shown and will be described next.

[0026] Referring with particularity to FIG. 3A, a change in grating period (resulting in a total change AA) rises steadily for each grating 300 that produces a refractive index perturbation, while the step length LMC that forms an enhancement pattern period defines the spacing corresponding to each period. Moreover, there are small or no axial gaps between the exposures along the axial dimension Z. For instance, each of the periodic gratings 300 may be fabricated by a single actinic pulse projected through a simple, uniform phase mask or related diffractive optical element (none of which are shown) that has periodic grooves sized to define the desired chirped periodic grating 300. In one form, such actinic radiation may be pulsed UV of various lengths, such as 248nm, 193 nm, and 157nm, while in another visible, or IR radiation with pulse duration in the nanosecond, picosecond or femtosecond range, while in yet another CW radiation of any wavelength is used. It will be appreciated that while minute exposure and related formation variations and tolerances may cause the individual ones of the chirped periodic gratings 300 to vary slightly (which in turn may cause random variations in exposure conditions), each will nonetheless produce gratings 300 that all have substantially the step length LMC. This in turn means that each grating 300 of FIG. 3 A has a chirp rate C equal to:AA C = — LMC where AA is the change in period over each step length LMC. For a Bragg grating, this may be related to the optical bandwidth of the reflection through the following:AA = 2neffAA where neff is the effective index of the guided mode in the optical fiber core or cores, and AA is the range of grating period in the chirped grating for a given neffand C. From this, a total back reflected power Rtotai and a back reflected power Rres over one spatial resolution length for thissensor may be compared. In particular, the reflected power Rres over one spatial resolution lengthLres isRres (KLres) , where K is the coupling constant of the enhancement pattern associated with a particular grating300. The relation Rres= (i<Lres)2holds ifLres « Leff.It also requires that the reflected power Rres«l . This is a requirement of many sensors that rely on single scatter along a length and in turn requires only a small single back reflection at each location rather than a large number of back reflections or cross talk or MPI along the optical fiber 30. Moreover, K may be related to the index perturbation amplitude An and mode overlap q throughThe total back reflection from the sensor may be computed by considering the effective length of the chirped gratings Letr. This is a length scale over which the chirped grating appears to be approximately a uniform period grating. It can be used to obtain the total back reflection from a chirped grating when light of a single wavelength is incident. By equating Fourier transform and chirp limited power reflection bandwidths, the following approximate relationship for the effective length Leff is obtained:where the factor of 1.5 is an approximate factor that has been determined empirically. From this, the total back reflection is obtained by summing the randomly phased back reflections from all of the chirped perturbations. It will be understood that the step length LMC is meant to be much larger than the spatial resolution length Lres. In other words, the detector 50 measures the total back reflected power Rtotai of the backscattered signal from the equally-spaced gratings 300 such that a ratio of this total back reflected power Rtotai to that from a single spatial resolution length Lresis below a maximum one of the aforementioned thresholds. As previously noted, enhancing the reflected light while avoiding saturation of the detector 50 is beneficial. This arises from the fact that excess backscatter contains both wanted and unwanted signals. By keeping the ratio below the threshold, physical limitations of the detector 50 related to maximum back reflected power,ambiguous signal readings such as those associated with detector saturation, backscatter cross talk, MPI, etc. are avoided. This in turn permits the DFOS system 10 to unambiguously quantify a sensed parameter corresponding to the perturbed light. In one form, such a parameter may be a physical item of interest, such as strain, temperature, fiber shape or other measurable parameter that may exhibit itself in the optical fiber 30 that in turn may correspond to an article of manufacture to which the optical fiber 30 is signally cooperative, such as through physical connection, securing or the like. From this, the total back reflected power Rtotai (also referred to as total back reflection or total reflectivity) is obtained by summing the randomly phased backscatter from all of the chirped perturbations that correspond to the gratings 300. For the two design examples shown in FIGS. 2A through 3B, the value of the total back reflected power Rtotai is desired when these two designs have equal sensor length. As depicted in FIGS. 3 A and 3B, the period is LMC, while for FIGS. 2A and 2B, the period has both an up and down chirped pattern and is shown to have length 2LMC. For the example depicted in the FIGS. 3A and 3B, if there are Npchirped grating patterns of length LMC, then the total reflection and total length of the sensor are related to LMC and Leff through:and^sensor NpL^c where RMC is the total power backscattered from all of the gratings 300 over the entire axial length of the optical fiber 30. For the example depicted in FIGS. 2A and 2B, which has an up and a down chirped portion in one period, the pattern of one period is taken as 2L C for simplicity and withoutN„ loss of generality. A sensor of equivalent length to that in FIGS. 3A and 3B would require periods. Moreover, the value of Leir would be the same for both designs since both designs show a change of AA over a length of L C and the computation of Leir is the independent of the sign of the chirp. In this case, the design of FIGS. 2A and 2B would have the same value of total reflection and sensor length:RMC = Np(KLeff)2^sensor NpL^cIn order to satisfy the limit Rmax (also referred to herein as a threshold or threshold limit) of such power back reflection, then:Thus, by having the gratings 300 be equally-spaced (i.e. nominally equivalent rather than changed along the length Lsensor of the optical fiber 30 that makes up the sensor, whether linearly or otherwise) refractive index perturbations in the form of the periodic gratings 300, varying chirp, and other time-related artefacts and their associated reflected wavelength changes are avoided, which significantly simplifies the manufacturing process of the entire optical fiber 30.

[0027] The improved results from the foregoing may be compared to those approaches that utilize one or more of time domain and frequency domain approaches, such as the aforementioned WDM / TDM, OFDR or SWI. Such methods may use step chirped grating arrays, that have each grating with a fixed period Lstep dm-p period, typically approximately equal to the sensor length or a multiple of the sensor length, and step length Lstep chirp. In such case, the period changes at each step. The pattern of step chirped gratings has a total variation of AA as in FIGS. 2 A through 3B. Furthermore, the sequence of steps may repeat many times. For example, comparison to a WDM / TDM step chirp approach may be achieved by equating the length Lsensor of the optical fiber 30 and the total variation AA and setting the total reflected powers as equalwhere the step chirp corresponds to a known discontinuous (that is to say, instantaneous) variation in the periodicity of a particular grating along its length such that step-like changes in a reflected wavelength of the received light is produced. In such case, frequency gratings change in a stepped manner. The total reflection from the step chirped pattern may be computed from the reflection of a single step. Because a single step has a uniform period, the reflection of this step is simplyThis is equated to the total reflection from the step chirp design and the designs of FIGS. 2A through 3B.Since the value of K is the same for both sensors, this leads toSuch an optical fiber 30 with axial length Lsensor could therefore have a length equal toWhen this condition is true, the ratio of the total back reflection to the reflection from the resolution length would be lower for the designs of FIGS 2A through 3B because the spatial resolution lengths Lres are the same and the coupling constant K is the same.

[0028] For example, comparing the two designs, it can be seen for what sensor length there is improved performance. IfLstepchirp 100mmLMC= 20mmAXchirp=lnmandA = 1550nm then for neff= 1.45, Leff~6 mm. Then for sensor lengths:the ratio of total reflected power to reflected from the resolution length will be improved from the stepped chirp design.Taking a sensor of this length and considering a resolution length of 1mm, the improved sensors would have a ratio of total power to power from the spatial resolution length Lres being less than:Moreover, if the spatial resolution length Lres = 0.1mm, then the ratio of total power to power from the spatial resolution length Lres would be less than m2= io6. \ 0 1 / In the foregoing example, the length of the optical fiber 30 that makes up the sensor was chosen as 5.5m or less insofar as such length is a useful length of fiber in various sensing applications, where more particular useful lengths of 2m to 4m are common.

[0029] Referring with particularity to FIG. 3 A, unlike the refractive index perturbation of FIG. 2A that first rises and then falls within each step length 2LMC that is associated with acorresponding grating 300, the period AA associated with the refractive index perturbation of FIG. 3 A steadily increases or rises over period A A for each grating 300. The enhancement pattern period is — as with that of the embodiment depicted in FIG. 2A — less than the axial length Lsensor of optical fiber 30. A calculation similar to that discussed previously in conjunction with FIG. 2A yields a similar total back reflection. It will be appreciated that although both FIGS. 2A and 3A depict a linear chirp for each grating 300, the chirp may also be of different form, including quadratic, sinusoidal, random or even nonlinear.

[0030] Referring with particularity to FIGS. 2B and 3B, the grating period versus fiber length indicates that while every grating 300 is nominally the same such as the enhancement pattern of FIG. 2A that has a refractive index perturbation that steadily rises is nominally the same, as well as the enhancement pattern of FIG. 2B that has a refractive index perturbation that first rises then falls within a single enhancement pattern, FIGS. 2B and 3B both show that some gaps or other unintended variations may be present. Such variations may in one form be caused by limitations in a given manufacturing process. In any event, even the presence of these variations does not detract from the ability of the optical fiber 30 to deliver consistent, substantially equal reflected returns for the total sensor length or the resolution length.

[0031] When the optical fiber 30 is used as part of the DFOS system 10, it will be understood that if the optical signal source 40 and detector 50 use many wavelengths, the results as disclosed herein will be valid for all of these wavelengths. Moreover, in one form, all of the gratings 300 are nominally equivalent, having been formed by a fabrication process that use nominally the same exposure of actinic radiation at equally spaced locations along the optical fiber 30. Furthermore, the gratings 300 have very little or no space between them, making them approximately continuous with only very small gaps of remaining unexposed, grating-less portions of the optical fiber 30. In one form, gap lengths are at or below the spatial resolution length Lres. In addition, the spatial resolution length Lres is less than the length of one of the grating 300 patterns.

[0032] Any process descriptions or blocks in flow charts should be understood as representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps in the process, and alternate implementations are included within the scope of the preferred embodiment of the presentdisclosure in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present disclosure.

[0033] One or more of the following claims may utilize the term "wherein" as a transitional phrase. For the purposes of defining features discussed herein, this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the acts or structure and should be interpreted in like manner as the more commonly used open-ended preamble term "comprising" and its variants that do not preclude the possibility of additional acts or structures.

[0034] Terms such as “preferably”, “generally” and “typically” are not utilized to limit the scope of the claims or to imply that certain features are critical, essential, or even important to the disclosed structures or acts. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the disclosed subject matter. Likewise, it is noted that the terms “substantially” and “approximately” and their variants are utilized to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement or other representation. As such, use of these terms represents the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

[0035] The use of the prepositional phrase "at least one of' is deemed to be an open-ended expression that has both conjunctive and disjunctive attributes. For example, a claim that states "at least one of A, B and C" (where A, B and C are definite or indefinite articles that are the referents of the prepositional phrase) means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. By way of example, if a claim recites that data is being acquired from at least one of a first sensor, a second sensor and a third sensor, and if such data is being acquired from the first sensor alone, the second sensor alone, the third sensor alone or any combination of the first, second and third sensors, then such data acquisition satisfies the claim.

[0036] The singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. The modifier “about” used in connection with a quantity is inclusive ofthe stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of 9% to 11%, and “about 1” may mean from 0.9 to 1.1. Other meanings of “about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from 0.5 to 1.4.

[0037] For the recitation of any numeric ranges herein, each intervening number therebetween with the same degree of precision is explicitly contemplated. For example, for the range of 6 to 9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0 to 7.0, the number 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9 and 7.0 are explicitly contemplated.

[0038] Within the present disclosure, the following claims are not intended to be interpreted based on 35 USC 112(f) unless and until such claim limitations expressly use the phrase "means for" or “steps for” followed by a statement of function void of further structure. Moreover, the corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims that follow are intended to include any structure, material or act for performing the function in combination with other claimed elements as specifically claimed.

[0039] The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims that follow are intended to include any structure, material or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Aspects disclosed herein were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

[0040] Having described the subject matter herein disclosed in detail and by reference tospecific embodiments, it is noted that the various details disclosed should not be taken to imply that these details relate to elements that are essential components of the various described embodiments, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Further, it will be apparent that modifications and variations are possible without departing from the scope of the present disclosure, including, but not limited to, embodiments defined in the appended claims. More specifically, although some aspects disclosed herein may be identified as preferred or particularly advantageous, it is contemplated that the invention is not necessarily limited to these aspects.

[0041] Although exemplary embodiments have been shown and described, it will be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the disclosure as described may be made. All such changes, modifications, and alterations should therefore be seen as being within the scope of the disclosure.

Claims

What is claimed is:

1. A fiber-optic sensor comprising: an optical fiber defining an axial length and a spatial resolution length; and gratings formed within the optical fiber and extending along at least a portion of the axial length to define a grating length, wherein at least one of the gratings is configured to generate a refractive index perturbation within the optical fiber such that in response to exposure from an input optical signal, a power ratio of a backscattered light signal that corresponds to a total backscattered light signal from a substantial entirety of the optical fiber to a backscattered light signal that corresponds to the spatial resolution length is less than 1,000,000.

2. The fiber-optic sensor of claim 1, wherein the power ratio is less than 100,000.

3. The fiber-optic sensor of claim 2, wherein the power ratio is less than 10,000.

4. The fiber-optic sensor of claim 1, wherein the gratings are all nominally equivalent.

5. The fiber-optic sensor of claim 1, wherein the spatial resolution length is less than the grating length.

6. The fiber-optic sensor of claim 1, wherein a power corresponding to the backscattered light signal from the spatial resolution length is proportional the square of the spatial resolution length.

7. The fiber-optic sensor of claim 1, wherein the optical fiber comprises a core surrounded by a cladding region and a protective coating.

8. The fiber-optic sensor of claim 1, wherein the optical fiber comprises a plurality of cores surrounded by a cladding region and a protective coating.

9. The fiber-optic sensor of claim 1, wherein the optical fiber is made from a material selected from the group consisting of silica glass and doped silica glass.

10. The fiber-optic sensor of claim 9, wherein at least a portion of the doped silica glass comprises dopants comprising at least one of boron, aluminum, fluorine, tin, phosphorous, titanium, lead, germanium or a rare earth element.

11. A distributed fiber optic sensing system comprising: an optical signal source; an optical fiber defining an axial length and a spatial resolution length, the optical fiber signally coupled to the optical signal source; and gratings formed within the optical fiber and extending along at least a portion of the axial length, wherein at least one of the gratings is configured to generate a refractive index perturbation within the optical fiber; and a detector coupled with the optical fiber such that backscattered light received by the detector from the optical fiber defines a power ratio wherein a total backscattered light signal from a substantial entirety of the optical fiber to a backscattered light signal that corresponds to the spatial resolution length is less than 1,000,000.

12. The distributed fiber optic sensing system of claim 11, wherein the power ratio is less than 10,000.

13. The distributed fiber optic sensing system of claim 11, wherein the optical signal source and the detector form an interrogator.

14. The distributed fiber optic sensing system of claim 11, further comprising a controller that is cooperative with at least one of the optical signal source, optical fiber, and detector .

15. The distributed fiberoptic sensing system of claim 11, wherein the controller, optical signal source, optical fiber, and detector are configured to perform optical frequency domain reflectometry.

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