Patent Publication Number: US-2023152131-A1

Title: Techniques and apparatus for improved spatial resolution for locating anomolies in optical fiber

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 17/389,742, filed Jul. 30, 2021, entitled Techniques and Apparatus for Improved Spatial Resolution for Locating Anomalies in Optical Fiber, which claims priority to U.S. provisional patent application Ser. No. 63/129,295, filed Dec. 22, 2020, entitled Techniques and Apparatus for Improved Spatial Resolution for Locating Anomalies in Optical Fiber, and further claims priority to U.S. provisional patent application Ser. No. 63/059,633, filed Jul. 31, 2020, entitled Techniques and Apparatus for Improved Spatial Resolution for Locating Anomalies in Optical Fiber, each of which application is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to the field of optical communication networks and relates more particularly to techniques for distributed measuring moving anomalies in optical fibers. As used herein an anomaly may refer to any change in physical parameters induced in the fiber, including strain, temperature, and so forth. 
     BACKGROUND 
     Optical fibers are employed ubiquitously for applications such as communications in land and sea based technology. For example, optical fibers having lengths as great as 100 km or more are commonly employed in undersea fiber optic cables. These undersea fiber optic cables are commonly employed for transmitting data across expanses of ocean between terrestrial landing sites which are often located in different countries and on different continents. 
     Techniques including backscattering techniques, such as Brillouin Optical Time Domain Reflectometry have been adapted for analyzing defects or anomalies in optical fibers, either intrinsic or induced by environmental physical parameters around the optical fibers, where defects may be located at any position along many kilometers of an optical fiber. This technique may be used to determine the location of strain or temperature differences in an optical fiber. This technique is non-destructive and therefore allows for measurement of the optical fiber at any suitable location, including at the factory, during installation, or in-situ after installation of an optical cable. 
     Notably, a backscattering measurement may be performed as a distributed measurement at multiple wavelengths, to allow the acquisition of sufficient distributed spectral property information. During backscattering measurement such as when manufacturing and deploying cable, a relative motion may take place between the optical fiber used as a sensor and the location and distribution of fiber anomalies or physical parameters. This relative motion may accordingly skew the spatial profile of spectral properties from the backscattering measurements, as required to determine the profile of physical parameter within the fiber, leading to degraded spatial resolution and therefore limited accuracy. 
     It is with respect to these and other considerations that the present improvements may be useful. 
     BRIEF SUMMARY 
     A method of measuring an anomaly in an optical fiber is provided according to one embodiment. The method may include launching a plurality of probe pulses from a probe source into the optical fiber; recording a Brillouin back-scattering spectrum from a plurality of reflection signals generated in the optical fiber, responsive to the plurality of probe pulses; determining a relative motion between the probe source and the anomaly during the recording the Brillouin back-scattering spectrum; and dynamically adjusting the Brillouin scattering spectrum according to the relative motion. 
     In another embodiment, a method of measuring an anomaly in an optical fiber, may include measuring a relative motion between a probe source and the anomaly; synchronizing a start of an acquisition of a Brillouin gain spectrum (BGS) and an anomaly motion detection, wherein the BGS comprises a plurality of backscatter traces, acquired at a plurality of instances; and after completing of the acquisition of the BGS, correcting the BGS based on a position of the anomaly at a time when a given BGS trace of the plurality of BGS traces is acquired. 
     In a further embodiment, an apparatus is provided, including a probe source; a pulse modulator to receive first portion of a probe beam from laser, over an optical fiber, and to output a plurality of probe pulses to a fiber under test; and a heterodyne receiver arranged to receive second portion of the probe beam from the probe source, and arranged to receive a Brillouin back-scattered portion of the probe beam from an anomaly of a fiber under test. The apparatus may further include a motion sensor or a position sensor, arranged to detect a relative motion or position, with respect to the fiber under test; and a digital processor, coupled to the motion sensor or to the position sensor, for determining a relative motion of the optical fiber/probe source with respect to the anomaly while measuring the Brillouin back-scattered portion of probe beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a schematic diagram illustrating a conventional measurement arrangement for testing an optical fiber, according to the prior art; 
         FIG.  1 B  is a schematic diagram illustrating another conventional measurement arrangement for testing an optical fiber, according to the prior art; 
         FIG.  2    illustrates a schematic showing components of a measurement arrangement according to the present embodiments that incorporates components of the arrangements of  FIG.  1 A  or  FIG.  1 B  during operation; 
         FIG.  3 A  illustrates a reference Brillouin gain spectrum resulting from measurement of a moving anomaly in an optical fiber; 
         FIG.  3 B  illustrates a Brillouin gain spectrum resulting from measurement of the moving anomaly of  FIG.  3 A  after correction for movement during the measurement or after the measurement, in accordance with embodiments of the disclosure; 
         FIG.  4 A  illustrates a Brillouin gain spectrum resulting from measurement of a moving anomaly in an optical fiber, where Brillouin back-scattering intensity is plotted as a function of frequency and distance; 
         FIG.  4 B  illustrates an adjusted Brillouin gain spectrum resulting from correction of the Brillouin gain spectrum of  FIG.  4 A  to account for movement of the anomaly during measurement, in accordance with embodiments of the disclosure; 
         FIG.  5    presents an exemplary process flow; 
         FIG.  6    presents an additional exemplary process flow; 
         FIG.  7    presents a further exemplary process flow; and 
         FIG.  8    presents another exemplary process flow. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of a measurement arrangement and techniques for testing an optical fiber, will now be described more fully with reference to the accompanying drawings. The measurement arrangement and techniques may be especially suitable for testing and measurement of optical fibers when deployed in circumstances where relative movement of a given anomaly in the optical fiber with respect to the optical fiber takes place during measurement. As used herein an anomaly may refer to any change in physical parameters induced in a fiber, including strain, temperature difference, and so forth. For example, it may be useful to measure a strain/temperature profile induced in an optical fiber, then environmental temperature/strain profile, where relative motion takes place between a sensing fiber of a measurement apparatus and the environmental temperature/strain profile during measurement. This circumstance may obtain when an optical fiber, such as a fiber under test, is being measured while deployed underseas, where at least a portion of the measurement apparatus is located on a ship or other vessel. Another circumstance is when a fiber is under test while being cabled, where the fiber is in motion relative to the cabling apparatus. 
     Referring to  FIG.  1 A , there is shown a schematic diagram illustrating an example of a measurement arrangement according to the prior art for testing an optical fiber, in accordance with the present disclosure. In this embodiment, a measurement apparatus  101  is depicted with respect to a fiber under test (FUT), shown as optical fiber  110 . The optical fiber  110  may be arranged in any suitable form and location, for example, in an undersea fiber optic cable, or alternatively in a terrestrial setting. The measurement apparatus  101  may include a probe source  102 , to probe the optical fiber  110 , such as a probe laser, arranged to generate a probe beam  104  at a suitable wavelength for probing the optical fiber  110 . The measurement apparatus  101  may further include a pulse modulator to receive first portion of a probe beam  114  from the probe source  102 , over a sensing fiber  103 , and to output a plurality of probe pulses  108  to a fiber under test, shown as optical fiber  110 . 
     As shown in  FIG.  1 A , the probe pulses  108  are generated at a frequency υ 0 , and are characterized by a pulse width. The probe pulses, when conducted to the fiber under test, optical fiber  110 , may encounter an anomaly  112 , such as a temperature/strain change, where the anomaly may generate Brillouin scattering of the probe pulses  108 . 
     By way of background, when light enters an optical fiber photons may be scattered back toward the optical source, as well as forward. Brillouin scattered light is shifted in frequency from the original frequency of the probe beam toward lower frequency or higher frequency. More particularly, Brillouin scattering may be generated by inelastic scattering of light in a physical medium by acoustical phonons with an accompanying Brillouin Frequency Shift (BFS). Both temperature and strain affect the medium density, then acoustic velocity ν a  and cause changes in the frequency of the Brillouin frequency shift ν BFS . 
     Techniques including Brillouin Optical Time Delay Reflection (BOTDR) and Brillouin Optical Time Delay Analysis (BOTDA) harness the measurement of Brillouin scattered light to measure anomalies, such as localized strain changes or localized temperature changes in an optical fiber, where the localized or distributed strain will affect the Brillouin frequency shift. Generally, for Brillouin scattering, the change in BFS frequency can be represented as Δν BFS =C T ·ΔT+C ε ·Δε (1) where C T  is approximately in the range of 0.75 MHz/C and C ε  is in the range of 500 MHz/1% strain, where equation (1) forms the foundation of temperature/strain measurement based on detection of Brillouin Scattering, for both BOTDR and BOTDA. 
     In accordance with embodiments of the disclosure, the measurement apparatus  101  may be used to perform Brillouin Optical Time Delay Reflection as detailed below. For purposes of illustration, as shown in  FIG.  1 A , when a packet of 10 3  photons are directed to the anomaly  112 , generally one photon or less on average is scattered as Brillouin scattered light, meaning that the Brillouin scattering yield is less than or equal to 0.1 percent of the initial photons directed to the anomaly. 
     As depicted in  FIG.  1 A , the measurement apparatus  101  further includes a heterodyne receiver  114  that is arranged to receive a second portion of the probe beam  104  from the probe source (at frequency Do), and that is further arranged to receive a Brillouin scattered portion  115  of the probe beam, scattered from the anomaly  112  of the fiber under test, optical fiber  110 . As shown in  FIG.  1 A , this Brillouin scattered portion  115  is received at a frequency υ 0 +/−υ B , where υ B  is the Brillouin frequency shift generated by the anomaly  112 . 
     As further shown in  FIG.  1 A , the measurement apparatus  101  further includes a digital processor  116 , coupled to the heterodyne receiver  114 , and arranged to detect a position of the probe source with respect to the anomaly  112  while measuring the Brillouin scattered portion  115  of probe beam  104 . In accordance with various embodiments of the disclosure, the probe source  102  and heterodyne receiver  114 , as well as digital processor  116  may be collocated with one another, where the distance between probe source  102  and anomaly  112  may be approximately the same as the distance between heterodyne receiver  114  and anomaly  112 . Thus, the relative position of the heterodyne receiver  114  and anomaly  112  may be taken to be the relative distance between probe source  102  and anomaly  112 . 
     According to embodiments of the disclosure, the apparatus  101  may be employed to generate a Brillouin gain spectrum (BGS) based on probing on the anomaly  112  in the fiber under test, optical fiber  110 . 
     A Brillouin gain spectrum may generally comprise a plurality of backscatter traces, acquired at different instances. A given pulse of the plurality of pulses is launched at a frequency ν 0 , as shown in  FIG.  1 A , for example, where a given backscatter trace along the optical fiber is detected by the heterodyne receiver  114  at a frequency ν B , offset from the frequency ν 0 . Notably, the plurality of backscatter traces may span a predetermined frequency range that is characteristic of the Brillouin backscattering shift for a given fiber system, depending on the properties of the anomaly, as shown above for equation (1). 
     More particularly, the measurement apparatus  101  may acquire a BGS along an optical fiber in the following manner: For each launched pulse at ν 0 , the back-scattered signal trace along the fiber at ν B  is detected by the heterodyne receiver  114 , while multiple probe pulses  108  are launched by the pulse modulator  106  to improve the signal-to-noise ratio of the BGS. The heterodyne receiver  114  may step the frequency through a full coverage of a predetermined frequency range to acquire the entire BGS. Notably, as discussed below with respect to  FIGS.  4 A and  4 B , a BGS may be presented as a three-dimensional graph plotting BGS intensity as a function of distance along a fiber on one axis and Brillouin frequency shift along an orthogonal axis. Generally in a BGS, the spatial resolution is determined by the pulse width of the probe pulse, such as probe pulse  108 , while the total time for data acquisition of the entire BGS includes the scan duration for every frequency step. 
     Notably, in measurement scenarios where there is relative motion between a sensing fiber and an anomaly such as a steady temperature and strain profile in the fiber under test, assuming that the relative motion, V, &lt;&lt;Speed of light in fiber for simplicity, the spatial resolution will be determined not only by the pulse width of a launched pulse but also by V. The spatial resolution of a BGS may then be equal to the sum of the limitation due to limited pulse width and the spatial distance of the relative motion during the duration between the start and end of frequency sweeping. 
     In one example of generating an adjusted BGS, the first trace of a BGS may be collected for a first frequency step of −Δν, where the adjustment is as follows: I 11 (−Δν, x)→I′ 11 (−Δν, x)=I 11 (−Δν, x). In this example, the intensity at −Δν, x is mapped to the same coordinates in a graph where distance (x) and frequency −Δν are parameters plotted in an “X-Y” plane and BGS intensity I is plotted along a Z-axis of a Cartesian graph. 
     The N th  trace of the BGS may be collected for the same first frequency of −Δν, where the adjustment is as follows −Δν: I 1N (−Δν, x)→I′ 1N (−Δν, x)=I 1N (−Δν, x−d 1N ), where d 1N  the distance moved away along the direction of the pulse launch since the 1 st  trace scan at the 1 st  frequency step. 
     The N th  trace of the BGS may be collected for a different frequency of −Δν+m×δν: I mN (−Δν+m×δν, x as follows→I′ mN (−Δν+m×δν, x)=I mN (−Δν+m×δν, x−d mN ), where d mN  is the distance moved away along the direction of pulse launch since the 1 st  trace scan at the 1 st  frequency step, and (−Δν, Δν) and is δν the frequency range and step, respectively. 
     In the above manner, an initially unadjusted BGS may be adjusted and reconstructed as an adjusted BGS to account for movement of the anomaly with respect to the fiber as sensor during collection of the BGS spectrum.  FIG.  2    illustrates a schematic showing components of a measurement arrangement according to the present embodiments that incorporates components of the arrangements of  FIG.  1 A  or  FIG.  1 B  for incorporating motion correction techniques during operation. As shown therein, the anomaly  112  may be represented as a moving profile of local temperature/strain changes in the fiber under test. Said differently, the location or spatial distribution of the anomaly  112  may change in the optical fiber  104  under test during collection of a BGS, resulting in a moving profile as shown. The measurement apparatus  101  may in turn collect this information (position d) in real time at the digital processor  116 , to account for the change in relative position of the anomaly  112  with respect to the optical fiber  104  under test, thus facilitating the ability to generate an adjusted BGS. 
     Turning now to  FIG.  1 B  there is shown a schematic diagram illustrating another conventional measurement arrangement  150 , according to the prior art, for testing an optical fiber, in accordance with the present disclosure. In this example, the measurement apparatus  151  shares similar components with measurement apparatus  101 , with like components labeled the same. As such, operation of these components will not be discussed in detail. The measurement apparatus  151  differs from measurement apparatus  101  in that a pump source  118  is provided, coupled to an opposite end of the optical fiber  110  as the probe source  102 . The pump source  118  may be a tunable CW laser, for example. As such, the measurement apparatus  151  may be suitable to perform Brillouin Optical Time Delay Analysis (BOTDA) for the fiber under test, where the scattering from the acoustic wave stimulated by the probe pulses  108  at anomaly  112  is amplified by counter-propagated CW pump light launched from the other end of the fiber under test, e.g., generating Stimulated Brillouin Scattering (SBS). In such implementations, the BOTDA measurement approach generates a much better signal to noise ratio than BOTDR, then longer reach, and better frequency and spatial resolution. During collection of a BGS using the measurement apparatus  151 , the relative motion of the anomaly  112  with respect to the fiber  104  under test may be recorded and taken into account to generate an adjusted BGS, generally as discussed above with respect to  FIG.  1 A . Notably, the arrangement of  FIG.  1 B  requires access to both ends of a fiber under test. 
       FIG.  3 A  illustrates a reference Brillouin gain spectrum resulting from measurement of a moving anomaly in an optical fiber.  FIG.  3 B  illustrates a Brillouin gain spectrum resulting from measurement of the moving anomaly of  FIG.  3 A  after correction for movement during the measurement, in accordance with embodiments of the disclosure. These spectra are simulated spectra to illustrate operation of the present embodiments. In  FIG.  3 A , BGS intensity is plotted on the Z-axis, while Frequency and distance are plotted on mutually orthogonal axes in the “X-Y” plane. The values of frequency and position are arbitrary. As shown, the BGS is generated by a plurality of traces represented by the vertical planes, where the intensity increases to a peak that is centered about zero frequency. Notably, the peak in BGS intensity at each successive frequency step is shifted along the “position” axis with respect to the prior frequency step. 
     In  FIG.  3 B , a BGS is shown that is derived from the BGS of  FIG.  3 A , where the relative movement of the anomaly during BGS acquisition is accounted for, such as described above. In this example, the BGS peak caused by the anomaly does not shift in position with the different frequency steps. 
       FIG.  4 A  illustrates a Brillouin gain spectrum resulting from measurement of a moving anomaly in an optical fiber, where Brillouin scattering intensity is plotted as a function of frequency and distance.  FIG.  4 B  illustrates an adjusted Brillouin gain spectrum resulting from correction of the Brillouin gain spectrum of  FIG.  4 A  to account for movement of the anomaly during measurement, in accordance with embodiments of the disclosure. These spectra are experimentally measured. For better visibility the BGS is reversed. In  FIG.  4 A  there are actually two spectral peaks that are discernable. In  FIG.  4 B , a single well defined peak is observed, where the distance is shifted 100 m or so than is apparent in  FIG.  4 A . Thus the position of the anomaly is more accurately and precisely determined. 
       FIG.  5    presents an exemplary process flow  500 , in accordance with embodiments of the disclosure. At block  502  a plurality of probe pulses are launched from a probe source, such as a laser source. The plurality of probe pulses may be output by a pulse modulator, coupled to the laser source. The plurality of probe pulses may be output over a given time interval and may be directed to an anomaly of an optical fiber that represents a fiber under test. 
     At block  504  a Brillouin gain spectrum is recorded from a plurality of reflection signals generated in the optical fiber, responsive to the plurality of probe pulses. In some examples, a group of reflection signals may be collected by a heterodyne receiver for each frequency step of a series of frequency steps centered around a characteristic frequency, representing the Brillouin frequency shift generated by an anomaly in the fiber under test. In various embodiments, the anomaly may be characterized as a local change in strain/temperature in the optical fiber, where a change in Brillouin frequency Δν BFS  is generated according to Δν BFS =C T ·ΔT+C ε ·Δε, where C T  represents the frequency shift per degree Celsius change in temperature, and C ε  is the change in frequency per % strain in the optical fiber. In some non-limiting embodiments, the value of C T  is in the range of 0.75 MHz/C and the value of C ε ˜500 MHz/1% strain. 
     At block  506 , the relative motion between the anomaly in the fiber under test and the probe source or heterodyne receiver may be determined during the recording of the Brillouin gain spectrum. 
     At block  508 , the Brillouin scattering spectrum dynamically adjusting according to the relative motion between probe source and anomaly. In one example of generating an adjusted BGS, from a first trace to a last trace of a BGS may be collected where the N th  trace of the BGS may be collected for a different frequency of −Δν+m×δν: I mN (−Δν+m×δν, x as follows→I′ mN (−Δν+m×δν, x)=I mN (−Δν+m×δν, x−d mN ), where d mN  is the distance moved away along the direction of pulse launch since the 1 st  trace scan at the 1 st  frequency step, and (−Δν, Δν) and is δν the frequency range and step, respectively. 
       FIG.  6    presents another exemplary process flow  600 . At block  602  a relative motion is measured between a probe source an anomaly in an optical fiber under test. The probe source may be a laser source that launches a series of probe pulses that are directed to the anomaly. The anomaly may be a local temperature/strain profile in the optical fiber under test. At block  604 , a start of acquisition of a Brillouin gain spectrum (BGS) is synchronized with the detection of motion of the temperature/strain profile. The BGS may constitute a plurality of backscatter traces, that acquired at a plurality of instances. 
     At block  602 , after completing the of acquisition of the BGS, the BGS is corrected based on a position of the anomaly at a time when a given BGS trace of the BGS is acquired. 
       FIG.  7    presents a further exemplary process flow, shown as flow  700 . At block  702  one or more pulses is launched at each wavelength into sensing fiber with a pulse rate no larger than 1/round trip time. At block  704  synchronized real time acquisitions of back-scattering light power and relative motion/position between sensing fiber and spectral profile of physical parameters, such as BGS, under test are performed. At block  706  the operation is performed to convert real time acquisition of each back-scattering power data point or each trace from time-space to distance-space with instant correction. At block  708  the operation is performed to align spatially distributed traces for the same wavelength based on relative position readings before averaging or other statistical analysis. At block  710 , the operation is performed to align spatially distributed traces from step above for all wavelengths based on relative position readings to construct the actual spatially distributed spectral profile such as BGS under test with the skewing from relative motion corrected. 
     After block  704 , the flow may proceed to block  712  to step to the next wavelength. After block  708 , the flow may proceed to block  714  to step to the next wavelength. After block  704 , the flow may proceed to block  716  where the operation is performed to convert real time acquired back-scattering power data points from time-space to distance-space and make proper data alignments. For example, the data set for each trace may be converted with relative motion correction, the operation may be performed to align and then statistically process spatially distributed traces for each wavelength, and the operation may be performed to align processed spatially distributed traces for all wavelengths to construct spatially distributed spectral profile with correction based on relative motion/position. 
       FIG.  8    presents another exemplary process flow, shown as process flow  800 . At block  802  one or more pulses is launched at each wavelength into sensing fiber with a pulse rate no larger than 1/round trip time. At block  804  each pulse triggers a sequence of real time acquisitions of back-scattering light power data points and one acquisition of relative motion/position between sensing fiber and profile of physical parameters under test. At block  806  real time acquisition of each trace is converted from time-space to distance-space without correction of motion skewing between data points. At block  808  align spatially distributed traces are aligned based on the relative position readings at each pulse before averaging or other statistical analysis is performed. At block  810  spatially distributed traces are aligned from the step above for all wavelengths based on relative position readings to construct the actual spatially distributed spectral profile such as BGS under test with skews between traces and/or between wavelengths from relative motion corrected. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     While the present disclosure makes reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claim(s). 
     For example, the aforementioned techniques may be applied to any fiber optical distributed sensing of spectral profile of physical parameters, such as BGS with BOTDR and Raman spectrum with Raman-OTDR. Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.