Fiber optic sensing apparatus including fiber gratings and method for sensing parameters involving different parameter modalities

Aspects of the present invention relate generally to fiber optics sensing, and, more particularly, to a sensing apparatus and method based on fiber optics and fiber gratings, which may be adapted to sense multiple parameters, as may involve different parameter modalities.

Certain types of industrial assets, such as may be used for the transportation of a commodity through a pipeline, may extend over relatively long distances (e.g., hundreds of kilometers or more). For example, since the asset may be subject to variable physical conditions (e.g., temperature, mechanical stress, pressure) or may develop malfunctions (e.g., ruptures) anywhere along its length, it is desirable to provide the ability to reliably and cost-effectively monitor/sense various parameters along the length of the asset. Other industrial assets which may require monitoring of variable physical conditions may be deployed at multiple sites over a relatively large geographical area, such as wind energy farms, solar farms, etc.

Certain optical sensing systems have attempted to measure various parameters at multiple locations. However, such sensing systems generally involve costly and burdensome hardware, such as may entail a number of optical fibers and concomitant connectors as well as different types of sensors to sense the parameters of interest. It is believed that no optical sensing system has been developed which in a cost effective manner and in an integrated fashion is able to monitor multiple parameters, which may involve different parameter modalities.

Generally, aspects of the present invention in one example embodiment may provide an apparatus including an optical fiber. At least two sites in the fiber may comprise a respective fiber grating arranged to have a respective optical response in a wavelength spectrum comprising a distinguishing feature indicative of a value of a respective local parameter at a respective grating site. The two fiber gratings may be further arranged to form, in combination with a respective portion of the optical fiber which extends between said at least two sites, respective optical backscatter portions that when combined with one another are effective to sense an optical change in the fiber portion between the sites indicative of a value of a distributed parameter. This may be a parameter modality different from a parameter modality of the respective local parameters at the respective grating sites.

Further aspects of the present invention, in another example embodiment may provide an optical-based method for sensing parameters. The method includes providing an optical fiber. The method further includes arranging at least two sites in the fiber comprising a respective fiber grating arranged to have a respective optical response in a wavelength spectrum comprising a distinguishing feature indicative of a value of a respective local parameter at a respective grating site; and configuring the two fiber gratings to form, in combination with a respective portion of the optical fiber which extends between said at least two sites, respective optical backscatter portions that when combined with one another are effective to sense an optical change in the fiber portion between the sites indicative of a value of a distributed parameter. This may be a parameter modality different from a parameter modality of the respective local parameters at the respective grating sites.

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a schematic representation of one example embodiment of an optical sensing apparatus depicting an optical fiber embodying aspects of the present invention and one example embodiment of an optical interrogator.

FIG. 2 is a simplified conceptual schematic illustrating certain sensing features of an optical sensing apparatus embodying aspects of the present invention, which may allow sensing distributed parameters.

FIG. 3 is a schematic representation of an optical fiber embodying aspects of the present invention, as may be affixed onto an example asset (e.g., a pipeline) for monitoring parameters of interest in connection with the asset.

FIGS. 4 and 5 are plots representing example optical responses, such as may indicate reflectivity of light as a function of wavelength in connection with an example fiber grating site.

FIG. 6 is a schematic of another example embodiment of an optical interrogator.

FIG. 7 is a schematic of an example embodiment of an optical fiber including groupings of fiber gratings, as may be arranged along the length of the fiber.

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments of the present invention. However, those skilled in the art will understand that embodiments of the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, to avoid pedantic and unnecessary description well known methods, procedures, and components have not been described in detail.

Furthermore, various operations may be described as multiple discrete steps performed in a manner that is helpful for understanding embodiments of the present invention. However, the order of description should not be construed as to imply that these operations need be performed in the order they are presented, nor that they are even order dependent. Moreover, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may. Lastly, the terms “comprising”, “including”, “having”, and the like, as used in the present application, are intended to be synonymous unless otherwise indicated.

FIG. 1 shows a schematic representation of one example embodiment of an optical-based sensing apparatus 10 embodying aspects of the present invention. Apparatus 10 includes an optical fiber 12 , which may be housed in a jacket 13 ( FIG. 3 ), and which may be affixed onto an asset 20 ( FIG. 3 ) for monitoring parameters of interest in connection with the asset. One example of an asset 20 may be a pipeline. It will be appreciated, however, that a broad range of categories of assets in a variety of diverse applications may benefit from aspects of the present invention, such as assets in the realm of civil engineering (e.g., building structures, bridge structures, tunnel structures, water control structures, etc.); assets in the realm of energy production (e.g., wind energy farms, solar power farms, nuclear power plants; assets in the realm of transportation (air and marine vessels, trains, etc.); assets in the realm of intrusion sensing. The foregoing should be construed in an example sense and not in a limiting sense being that any application which may involve monitoring of parameters over many locations may benefit from aspects of the present invention.

In one example embodiment, optical fiber 12 may include at least two sites having a respective fiber grating ( 14 1 , 14 2 . . . 14 n ), such as a fiber Bragg grating (FBG), as may be arranged to have a respective optical response in a wavelength spectrum with a distinguishing feature, which, for example, may be indicative of a value of a respective local parameter at a respective grating site. Examples of local parameters which could be measured at the grating site due to local effects—depending on how the grating site is packaged—may include strain, temperature, acceleration, pressure, chemistry. For example, bonding the grating to the cable jacket and structure would enable the measurement of strain. Conversely, loosely routing the grating in the cable without bonding would enable the measurement of temperature. Additional packages can be manufactured by those skilled in the art to transfer the effects of acceleration, pressure, and chemistry into strain for measuring these parameters locally at the grating site.

The two fiber gratings, e.g., 14 1 , 14 2 , may be further arranged to form, in combination with a respective portion of the optical fiber which extends between the grating sites, respective optical backscatter portions that when combined with one another are effective to sense an optical change in the fiber portion between the sites, which may indicate a value of a distributed parameter, such as dynamic strain, dynamic pressure, an acoustic signal, etc. It will be appreciated that this is a parameter modality (e.g., distributed over the fiber portion between the sites and which could involve up to tens of KHz variation) different from the parameter modality of the respective local parameters sensed at the respective grating sites, which may involve relatively slowly varying parameters. As used herein, a local parameter refers to a parameter for measurement of a local effect at the grating site; and a distributed parameter refers to a parameter for measurement of an effect in the fiber portion between the sites.

In one example embodiment, the fiber gratings may be adapted to induce a relatively higher backreflection, (e.g., inducing enhanced reflectivity at off-Bragg wavelengths) and this may be effective for sensing of dynamic parameters, (e.g., acoustic sensing). Example techniques to induce the relatively higher backreflection may be as follows:

Write a narrow-band grating response (e.g., standard peaked response, (in one example embodiment approximately ≦1 nm)) and a chirped grating response (e.g., broadened spectral response) at a given grating site. (See Tables 1 and 2, which respectively list example steps, as may be implemented in two example embodiments involving a phase masking technique) Write a standard grating response, apply a structure that appropriately stresses the fiber over a length to increase backscatter: one example may be a splice protector with a corrugated strength member; another example may be recoating with a suitable coating (e.g., metal coating, etc) to achieve the increased backscatter

FIG. 2 is a simplified schematic to visually appreciate the foregoing concepts. For example, wavefronts 15 may represent an example of a dynamic stimulus (e.g., an acoustic signal) impinging on fiber gratings 14 1 , 14 2 and the fiber portion 17 located between fiber gratings 14 1 , 14 2 . In this schematic, arrows 19 represent respective optical backscatter portions that when combined may be effective to sense an optical change in the fiber portion between the grating sites, and the sensed optical change may, for example, be processed to indicate the value of the dynamic distributed parameter.

FIGS. 4 and 5 are plots representing example optical responses, such as may indicate reflectivity of light as a function of wavelength in connection with an example grating site. A desired optical response may be appropriately engineered in the fiber gratings embodying aspects of the present invention to effectively sense with the same optical hardware different parameter modalities. By way of example, a fiber grating could be engineered so that its optical response in a first wavelength range (e.g., conceptually represented by wavelengths to the left of line 21 in FIG. 4 ) provides the distinguishing feature (e.g., a lobe 22 ), which may be processed to indicate the value of a respective local parameter at the respective grating site (e.g., sensing of slowly varying parameters). The same fiber grating could also be engineered to exhibit a relatively broad, low-level response (e.g., as may be effected by a broad-band chirped fiber Bragg grating) in a second wavelength range (e.g., conceptually represented by wavelengths to the right of dashed line 21 ) effective to form the respective optical backscatter portions that when combined may be effective to sense an optical change in the fiber portion between the sites (e.g., dynamic distributed sensing). It will be appreciated that the distinguishing feature in the wavelength spectrum need not be a lobe since other distinguishing features could be engineered in a given fiber grating, such as a band edge 24 , a notch 26 , a pattern 27 , as may conceptually illustrated in FIG. 5 . For example, spectral shifts (conceptually represented by arrows 29 ) of edge 24 may be tracked by way of suitable signal processing to determine changes in the value of a parameter at a given grating site.

Returning to FIG. 1 , apparatus 10 may further include an optical interrogator 30 including an optical source 32 . In one example embodiment, optical source 32 may comprise a narrow-band swept-wavelength light source, such as where the optical frequency of the light source may be swept over a wavelength range sufficiently wide to, for example stimulate an appropriate optical response, as discussed in the examples of FIGS. 4 and 5 . In one example case, presuming low-reflectivity FBGs with lobes are used (e.g., example Bragg response from approximately 1530 nm to approximately 1545 nm), then one example wavelength range for light source 32 may be from approximately 1530 nm to approximately 1560 nm. In this example, line 21 ( FIG. 4 ) would be disposed approximately at 1545 nm. It will be appreciated that aspects of the present invention are not limited to any particular type of FBGs and/or wavelength range. Thus the foregoing description is just and example and should not be construed in a limiting sense.

Optical source 32 may be optically coupled through a circulator 34 to one end of fiber 12 to apply at least one optical pulse, (conceptually represented by a pulse 36 ) which when reflected at a respective grating site conveys the feature, which may be indicative of the value of the respective local parameter at the respective grating site, and which when passing through the respective fiber portion which extends between the two grating sites, conveys the respective optical backscatter portions. In one example embodiment, light source 32 may be triggered on (e.g., to transmit a pulse of light) at a trigger time (t on ) and may be triggered off at time t off =t on +2 dn/c, where d represents the distance between gratings, n represents the index of refraction of the fiber and c represents the speed of light. The idea being that the width of the pulse of light is not so wide as to simultaneously encompass two adjacent grating sites for measuring the distinguising features at the grating sites.

In one example embodiment, optical source 32 may be controlled to function in two modes: a first mode where the optical frequency of the light source is swept over a first wavelength range to measure the distinguishing features at the grating sites; and a second mode where it is tuned to a second wavelength range to measure the response in the fiber between the respective gratings sites.

In one example embodiment, optical interrogator 30 may include a wavelength demultiplexer 38 , as may provide a first detection channel 40 (e.g., to process the wavelengths to the left of line 21 in FIG. 4 ) configured to monitor the feature indicative of the value of a respective local parameter at a respective grating site, and may further provide a second detection channel 42 (e.g., to process the wavelengths to the right of line 21 ) configured to combine the optical backscatter portions to sense the optical change in the respective fiber portion (e.g., fiber portion 17 ) which extends between two grating sites (e.g., grating sites 14 1 , 14 2 ) indicative of the value of a distributed parameter. It will be appreciated that the respective optical backscatter portions comprise wavelengths which at least partially overlap. In one example embodiment, second detection channel 42 may include an interferometer 46 (e.g., unbalanced interferometer) arranged to combine the optical backscatter portions, and a photodetector 48 (e.g., photodiode) coupled to receive the combined optical backscatter portions from interferometer 46 .

In one example embodiment, first detection channel 40 may include a time division modulator 44 (labeled T.D.M.) arranged to relate a respective reflected pulse to a respective grating site. In one example embodiment, modulator 44 may be conceptualized as a shutter, which is open at time t open =t on +2 Dn/c, and which is closed at time t closed =t on +2 (D+d)n/c, where D represents a distance to the grating site being interrogated, (e.g., grating site 14 2 ). First detection channel 40 may further include an spectrometer 45 , as may be arranged to detect changes in the feature indicative of the value of a respective local parameter at a respective grating site. It will be appreciated by one skilled in the art that the functionality provided by demodulator 44 may be optionally implemented by way of spectrometer 45 . For example, spectrometer 45 could include a temporal gating functionality to process the light pulses from a desired fiber grating site.

It will be appreciated by those skilled in the art that spectrometer 45 can be implemented in a variety of ways, such as by way of dispersive optics to distribute the light by wavelength onto a photodiode array; or by way of a tunable filter, as may allow predefined wavelengths to illuminate a photodiode at different times. It is also feasible to integrate the two detection channels into a single detection channel. For example, one or more photodiodes may be arranged in spectrometer 45 to combine the optical backscatter portions used to measure the dynamic distributed parameter while using further photodiodes or filter settings in the spectrometer to measure the local parameter of the grating at the grating site. This example embodiment would eliminate in FIG. 1 the following elements: wavelength demultiplexer 38 , interferometer 46 , and photodetector 48 . This example would eliminate a separate second detection channel, as illustrated in FIG. 1

FIG. 6 is a schematic of an example embodiment of an optical interrogator 60 where an optical source 61 is made up of a source 62 of broad-band light over a first wavelength range, and further includes a source 64 of narrow-band light over a second wavelength range. In one example embodiment, the second wavelength range may have longer wavelengths relative to the first wavelength range. For example, the first wavelength range provided by the broad-band light source 62 may correspond to wavelengths to the left of line 21 in FIG. 4 while the second wavelength range provided by narrow-band light source 64 may correspond to wavelengths to the right of dashed line 21 . It will be appreciated that narrow-band light source 64 is conducive to pulse self-interference on the photodetector (e.g., conducive to sensing of dynamic parameters) and thus this example embodiment may provide the flexibility to optionally eliminate interferometer 46 (this optionally is schematically represented by the dashed line for the block diagram which represents interferometer 46 ) in the second interrogator channel 42 .

In accordance with further aspects of the present invention, FIG. 7 is a schematic of a fiber 80 including a plurality of fiber gratings ( 82 , 84 , 86 ) distributed along a length of the fiber in respective groups, such as example groups labeled G1 and G2. In one example embodiment, the fiber gratings which constitute a respective group may have respective non-overlapping wavelength optical responses relative to one another. For example, fiber gratings 82 may exhibit their respective distinguishing features over a first wavelength range (e.g., λ1), fiber gratings 84 may exhibit their distinguishing features over a second wavelength range (e.g., λ2) and fiber gratings 86 may exhibit their distinguishing features over a third wavelength range (e.g., λ3). This grouping arrangement may provide the following example advantages. For example, in an arrangement where the fiber includes a total of 100 gratings having their optical responses essentially over a common wavelength range (λc), then light in such an arrangement would be attenuated 100 times the attenuation of one grating over the common wavelength range. Compare the foregoing example with another example arrangement where the fiber also includes 100 gratings but in this case their optical responses are distributed over three different non-overlapping wavelength ranges (e.g., λ1, λ2, λ3), then light in such an arrangement in each wavelength range would be attenuated ⅓ rd lower when compared to the light attenuation of the foregoing example. Additionally, such non-overlapping optical responses from a respective group may be simultaneously processed in the time domain to concurrently extract the respective distinguishing features indicative of respective values of one or more respective local parameters at the respective grating sites. For example, presuming the distance to a given fiber grating group (e.g., group G2) is distance D, further presuming the distance of the fiber group is distance d s (e.g., intragroup distance), then the modulator which provides a shutter functionality may be configured to open at time t open =t on +2 Dn/c, and to close at time t closed =t on +2 (D+d s )n/c. The idea being that the optical responses of the constituent fiber gratings (e.g, fiber gratings 82 , 84 , 86 ) of a respective group being interrogated (e.g., group G2) could be simultaneously processed to contemporaneously obtain the sensing information from the constituent grating sites of the group being interrogated.

In operation, aspects of the present invention allow cost-effective and reliable monitoring of assets, which may involve relatively long-distances and/or large geographical areas. One example may be a pipeline, which may be subject to varying levels of stress or may develop leaks. For example, to monitor the pipeline for excessive stress, measurement of strain along the pipe may be desirable; whereas to monitor leaks, acoustic and temperature measurements would be more suitable. In operation, an apparatus comprising a single fiber and a common optical interrogator may allow cost-effective and reliable monitoring of parameters, which may comprise different parameter modalities, such as relatively slowly-varying parameters as well as relatively rapidly-changing parameters. It will be appreciated that this versatility to measure different modalities of measurands leads to an integrated determination about the health of the asset, such as substantially reducing the probability of false positives or negatives.

While various embodiments of the present invention have been shown and described herein, it is noted that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.