Sensing in passive optical networks

Abstract

This disclosure teaches distributed sensing (DXS), such as distributed acoustic sensing (DAS), distributed vibration sensing (DVS), or other distributed sensing that employs optical time domain reflectometry (OTDR) or optical frequency domain reflectometry (OFDR). In particular, the DXS is performed in a passive optical network (PON) using an enhanced back-scattering optical fiber (ESF) that is configured for DXS. The ESF is located in an optical signal communication path between an optical line terminal (OLT) (generally at a central office (CO)) and an optical network unit (ONU) in the PON. The ESF comprises an in-band wavelength range that exhibits an in-band back-scattering that is higher than native scattering. The ESF further comprises an out-of-band wavelength range that exhibits an out-of-band back-scattering that is lower than the in-band back-scattering.

Description

SENSING IN PASSIVE OPTICAL NETWORKSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of United States provisional patent application serial number 63 / 536,800, filed 2023-September-06, having the title "Sensing in Passive Optical Networks," with Mark A. Boxer as first-named inventor, which is incorporated herein by reference in its entirety.BACKGROUNDFIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to optics and, more particularly, to fiber optics.DESCRIPTION OF RELATED ART

[0003] Passive optical networks (PONs), which are known in the art, are systems that bring fiber-optic signals most (if not all) of the way to an end user. Depending on where the PON terminates, PONs are also known as fiber-to-the-curb, fiber-to-the-building, fiber-to- the-home, or generally fiber-to-the-X (FTTX), with X designating the location where the fiber terminates. Typically, a PON has an optical line terminal (OLT) at a central office (or center office) (CO), several optical network units (ONUs) located at or near the end users, and one or more optical signal communication paths between the OLT and each of the ONUs.

[0004] Because optical networks, such as PONs, provide fast communications for large amounts of data, there are ongoing developments with reference to PONs.SUMMARY

[0005] The present disclosure provides systems and methods for distributed sensing, such as distributed acoustic sensing (DAS), distributed vibration sensing (DVS), or other distributed sensing (DXS) that employs optical time domain reflectometry (OTDR) or optical frequency domain reflectometry (OFDR). In particular, the DXS is performed in a passive optical network (PON).

[0006] Briefly described, in architecture, one embodiment of the system comprises an enhanced back- scattering optical fiber (or, simply, enhanced-scattering fiber (ESF)) that is configured for DXS. The ESF is located in an optical signal communication path between an optical line terminal (OLT) (generally at a central office (CO)) and an optical network unit (ONU) in the PON. The ESF comprises an in-band wavelength range that exhibits an in- band back- scattering (BS) that is higher than native scattering (as defined below).

[0007] ESF further comprises an out-of-band wavelength range that exhibits an out- of-band BS that is lower than the in-band BS.

[0008] 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

[0009] 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.

[0010] FIG. 1 is a diagram showing an embodiment of a passive optical network (PON) with an enhanced back-scattering optical fiber (or, simply, enhanced-scattering fiber (ESF)) configured for distributed sensing (DXS).

[0011] FIG. 2 is a diagram showing another embodiment of a PON with an ESF for DXS.

[0012] FIG. 3 is a diagram showing an embodiment of a PON with ESFs that are each coupled to its respective optical network unit (ONU).

[0013] FIG. 4 is a diagram showing another embodiment of a PON with ESFs that are each coupled to its respective ONU.

[0014] FIG. 5 is a diagram showing an embodiment of a PON with spatially modulated ESFs that are each coupled to its respective ONU.

[0015] FIG. 6 is a diagram showing an embodiment of a PON with ESFs that have different in-band wavelength ranges that exhibit higher-than-native back-scattering (BS) within those in-band wavelength ranges.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] Passive optical networks (PONs), which are known in the art, are systems that bring fiber-optic signals most (if not all) of the way to an end user. Depending on where the PON terminates, a PON is also known as fiber-to-the-curb, fiber-to-the-building, fiber-to-the- home, or generally fiber-to-the-X (FTTX), with X designating the location where the fiber terminates. Typically, a PON has an optical line terminal (OLT) at a central office (CO), several optical network units (ONUs) located at or near the end users, and one or more optical signal communication paths (or, simply, pathways) between the OLT and the ONUs.

[0017] To implement distributed sensing (such as distributed acoustic sensing (DAS) or distributed vibration sensing (DVS), generically designated as DXS) in PONs using optical time-domain reflectometry (OTDR), there needs to be a detectable back-reflected (BR) signal (or, interchangeably designated herein as a back- scattered (BS) signal). Often, however, large signal losses that occur at one or more optical splitters (or, simply, splitters) within the pathways, thereby attenuating any BS signals to the point that they become unobservable for purposes of DXS using OTDR. In other words, the signal-to-noise-ratio (SNR) of the BS signal becomes too low for reliable DXS. Additionally, because multiple pathways exist after a splitter, even when BS signals are detectable, there is no reliable way to detect which pathway affected the BS signal.

[0018] The disclosed systems and processes address both the SNR of BS signals and determining which pathway affected the BS signals. The low SNR of BS signals is overcome by introducing enhanced back- scattering optical fibers (or, simply, enhanced- scattering fibers (ESFs)) after the splitter. Different signal-distinguishing approaches (e.g., spatially distinguishable pattern of BS enhancements, different wavelength ranges for the BS signal, etc.) are introduced to determine which of the pathways affected the BR signal.

[0019] As those having skill in the art will appreciate, the difficulties of introducing the ESFs to a PON or applying different signal-distinguishing approaches to ESFs in a PON arise from a two-fold issue, namely: signal attenuation through a passive splitter; andmultiple indistinguishable optical signal communication paths downstream of the attenuated signal at the splitter. Neither of these issues presents a readily available solution in conventional PONs.

[0020] 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.

[0021] FIG. 1 is a diagram showing an embodiment of a passive optical network (PON) 100 with enhanced back-scattering optical fiber (or, simply, enhanced- scattering fiber (ESFs)) configured for DXS. The PON 100 comprises a central office (or center office) (CO) with an optical line terminal (OLT) 105 at one end and optical network units (ONUs) 110 at the other end. The CO / OLT 105 is optically coupled to the ONUs 110 through various optical components 115 that establish one or more optical signal communication paths (or, simply, pathways) between the CO / OLT 105 and the various ONUs 110.

[0022] The CO / OLT 105 comprises a PON OLT transmitter (Tx) 120 for transmitting optical signals and, also, a PON OLT receiver (Rx) 125 for receiving optical signals. Additionally, the CO / OLT 105 comprises a wavelength-division multiplexer (WDM) 130 that multiplexes outgoing signals for down-stream (DS) transmission 145 and de-multiplexes incoming up-stream (US) signals 150. Additionally, to perform DXS in the PON 100, the CO / OLT 105 also comprises a DXS interrogator 135 (shown in FIG. 1 as a DAS interrogator), which transmits a DXS signal, receives a BR DXS signal, and determines the location in the PON 100 that affected the DXS signal. For some embodiments the interrogator 135 is a conventional interrogator that is capable of DXS, while for other embodiments the interrogator 135 is a customized device that is specifically configured for DXS in a particular PON. Thus, a sensing channel 190 exists within the feeder fiber 155 (between the CO / OLT 105 and the splitter 160). The optical components 115 comprise a feeder transmission fiber 155 (typically a standard single-mode fiber (SSMF or SMF)) that is optically coupled between the WDM 130 and a splitter 160 (shown in FIG. 1 as a 1-to-n (or Ixn) splitter).

[0023] To illustrate DXS within the PON 100, the embodiment of FIG. 1 shows afirst ESF 165a (ESF1) optically coupled to a first ONU 140a (ONUl), a second ESF 165b (ESF2) optically coupled to a second ONU 140b (0NU2), and a single-mode fiber 175 (SMF) optically coupled to a third ONU 140c (ONUn). In other words, ONUl 140a and ONU2 140b are configured for both telecommunication (telecom) operation as well as DXS, while ONUn 140c is only configured for telecom operation. Insofar as conventional telecom operations are well known in the art, only a truncated discussion of the SMF 175 and ONUn 140c is provided herein, with greater details being discussed with reference to the ESFl-to-ONUl pathway and the ESF2-to-ONU2 pathway. Although an embodiment of a PON 100 with three (3) ONUs (namely, ONUl 140a, 0NU2 140b, and ONUn 140c) is shown for illustrative purposes, it should be appreciated that the disclosed configuration is extendable to PONs with additional distribution fibers (or drop fibers (DFs)) and additional ONUs.

[0024] As shown in FIG. 1, by introducing the ESF1 165a and the ESF2 165b after the splitter 160 and in their respective DFs to ONUl 140a and ONU2 140b, at least a portion of each pathway to ONU 1 140a and ONU2 140b exhibits enhanced BS. The enhanced BS increases the SNR of the BS signal, thereby overcoming SNR issues that ordinarily arise after the splitter 160 in conventional PONs.

[0025] Focusing on ESF1 165a and ONUl 140a, one embodiment of a DXS system comprises the ESF1 165a with an ESF input end and an ESF output end. The ESF input end is configured to optically couple to the splitter 160 (or any first location in an optical signal communication path located in the PON 100), thereby forming a portion of one segment (or first segment) of the optical signal communication path between the splitter 160 and the ONUl 140a. The ESF output end is configured to optically couple to the ONUl 140a (or any second location in the optical signal communication path located in the PON 100), thereby forming another segment (or second segment) of the optical signal communication path between the splitter 160 and the ONUl 140a.

[0026] It is possible that the bare fiber can have other contributions to BS from, for example, core cladding defects, dopant concentration fluctuations, Mie scattering, or other BS from fiber imperfections (which are, in conjunction with Rayleigh scattering, defined herein as "native scattering" (as compared to enhanced scattering that occurs in ESFs)). While the additional scattering that contributes to native scattering is often less than theRayleigh scattering, the additional scattering may also be similar to or stronger than the BS from Rayleigh scattering. In general, BS due to native scattering can include scattering from any and all imperfections in the optical fiber waveguide. While the imperfections giving rise to Rayleigh scattering are typically the largest contribution to BS, it is possible that BS from other imperfections can also contribute to total BS.10027] With this in mind, the ESF1 165a comprises an enhanced BS segment between the ESF input end and the ESF output end. The enhanced BS segment has a wavelength range (denoted as an in-band wavelength range) that exhibits a BS that is higher than native scattering (for simplicity, this higher-than-native scattering in the in-band wavelength range is designated herein as an in-band BS). The enhanced BS segment also has a wavelength range (denoted as an out-of-band wavelength range) that exhibits a BS that is lower than the in-band BS (designated herein as the out-of-band BS). Generally, the out-of- band BS is comparable to (or substantially the same as) Rayleigh scattering (or the back scattering level of bare telecom fibers used in the PON, such as native scattering).

[0028] As one having ordinary skill in the art will appreciate, the ESF2-to-ONU2 pathway comprises a similarly functioning DXS system. Additionally, one having ordinary skill in the art will appreciate that the ESFs are capable of carrying a telecom signal, which can be carried in a telecom wavelength range that either overlaps partially with the in-band wavelength range or, alternatively, resides solely within the out-of-band wavelength range. It should also be appreciated that the telecom signal can be carried in a separate fiber (e.g., SSMF) that is arranged in parallel with the ESF, thereby having a separate dedicated fiber (e.g., SSMF) for telecom signals and a separate dedicated ESF for DXS applications. With proper configuration, the telecom wavelength range can also reside fully within the in-band wavelength range.

[0029] The enhanced BS segment is characterized by an enhanced BS parameter, Re, which represents a ratio of BS in the ESF1 165a or ESF2 165b (which is higher than Rayleigh scattering or native scattering) to BS of a SSMF (which is on the order of Rayleigh scattering or native scattering). Thus, for example, Re= 10 decibels (dB) exhibits a ten-fold increase in scattering in the ESFs than for SSMF. Stated differently, if a SSMF exhibits - 43dB per meter (dB / m) of BS, then the ESF having Re= lOdB would exhibit -33dB / m. This ten-fold increase in BS can mitigate for some of the signal loss experienced at a splitter. Byway of example, if a 1x8 splitter causes an 18dB loss (a 9dB loss as the signal traverses the splitter in one direction and another 9dB loss as the signal returns and traverses the splitter again), then an ESF with Re= 18dB compensates for the loss at the 1x8 splitter. Similarly, if a 1x16 splitter causes a 24dB loss, then an ESF with Re= 24dB compensates for the loss at the 1x16 splitter. Consequently, the use of such ESFs permits DXS in OTDR (or optical frequency-domain reflectometry (OFDR)) as though there existed little-to-no splitter attenuation. One embodiment of an ESF that is introduced to the PON for DXS is the high backscattering waveguide that is disclosed in U.S. Patent Number 9,766,396 (by Kremp, Westbrook, and Geisler), issued on 2017-September-19, which is incorporated by reference herein as if set forth expressly in its entirety.

[0030] FIG. 2 is a diagram showing another embodiment of a PON 200 with ESFs for distributed sensing. Similar to the PON 100 of FIG. 1 , the PON 200 of FIG. 2 comprises a CO / OLT 205, a PON OLT Tx 220, a PON OLT Rx 225, a WDM 230, a DXS interrogator 235, ONUs 240a, 240b, 240c (collectively, 240), a feeder transmission fiber (e.g., a SMF) 255, a SMDF 275, and ESFs 265a, 265b.Briefly, unlike FIG. 1, the PON 200 of FIG. 2 comprises distributed and cascaded splitters. However, unlike the PON 100 of FIG. 1 (which shows a single (Ixn) splitter 160), the PON 200 of FIG. 2 comprises two (2) splitters, namely, a first (1-to-n or Ixn) splitter 260 and a second (1-to-m or Ixm) splitter 270 that is serially connected to the first splitter 260. In operation, the primary difference between the PON 200 of FIG. 2 and the PON 100 of FIG. 1 is that the distributed and cascaded splitters 260, 270 produced different degrees of attenuation after their respective splits. Thus, one having ordinary skill in the art can determine an appropriate Rebased on the specific attenuation for a particular after-split pathway.

[0031] As one can appreciate from FIGS. 1 and 2, the ESFs provide a mechanism by which BS is sufficiently enhanced to permit DXS in PONs. However, the ESFs, standing alone, do not distinguish between different post-splitter pathways. In other words, although the ESFs allow for detection of enhanced BS signals, the DXS still needs to determine which DF pathway reflected a particular enhanced BS signal. FIGS. 3 through 6 provide systems and methods for distinguishing BS signals from different DF pathways after a splitter.

[0032] FIG. 3 is a diagram showing an embodiment of a PON 300 with ESFs that are each coupled to its respective optical network unit ONU. The following components inFIG. 3 are comparable to corresponding components in FIGS. 1 and 2: a CO / OLT 305, ONUs 310, 312, 314, 316, 318, 355, a splitter 340, and a feeder transmission fiber 350. Consequently, only a truncated discussion of those components is provided, with greater emphasis being placed on the remaining components of FIG. 3.

[0033] With this in mind, FIG. 3 comprises a tap 345 that is placed between the splitter 340 and the CO / OLT 305. A pre-splitter ESF 360 (ESF0) extends from the tap 345 to a pre-splitter ONU 355. Note that such a tap may also be installed after the splitter. For purposes of illustration, there are five (5) ONUs 310, 312, 314, 316, 318 shown after the splitter 340, with two (2) pathways to ONUs 316, 318 comprising SSMFs 336, 338 for standard telecom operations. The remaining three (3) pathways to ONUs 310, 312, 314 include ESFs 320, 322, 324 that are each appended to a different length of SSMF fiber 330, 332, 334, thereby creating a different total overall length (L) of DF between the splitter 340 and the ONUs 310, 312, 314. The different L effectively creates a different detectable signal delay that is proportional to L for each post-splitter pathway. An example of the different detectable signals is shown graphically in FIG. 3, with OTDR (or OFDR) signal power 370 plotted as a function of distance 372 (labeled as a z-axis). By way of example, the graph of FIG. 3 represents an interrogator trace (or a reading at the interrogator) of a round-trip signal (from CO / OLT to ONUs). For reference, a noise level 384 is shown (slightly above the distance axis 372).

[0034] In the embodiment of FIG. 3, the pathway between the splitter 340 and 0NU1 310 comprises a first (shorter) length of SSMF 330 (SSMF1) in conjunction with the ESF1 320. The pathway between the splitter 340 and 0NU2 314 comprises a second (medium) length of SSMF 332 (SSMF2) in conjunction with the ESF2 322. The pathway between the splitter 340 and 0NU3 316 comprises a third (longer) length of SSMF 334 (SSMF3) in conjunction with the ESF3 324. By making the combined length of SSMF3+ESF3 (e.g., 3L) longer than the combined length of SSMF2+ESF2 (e.g., 2L), which in turn is made longer than the combined length of SSMF1+ESF1 (e.g., IL), a returning BS signal can be associated with its respective post-splitter pathway based on the round-trip time for the signal from the CO / OLT 305 to its respective ONU 310, 312, 314. Examples of round-trip-time-based signals are shown in the graph of FIG. 3.

[0035] As shown in the graph of FIG. 3, a measured BS signal 374, 378 that isattributable to the feeder fiber 350 is above the noise level 384 (or noise floor). The enhanced BS signal 376 produced by the ESFO 360 is higher than the feeder-fiber signal 374, 378. The graph of FIG. 3 also shows signal attenuation 380 due to losses at the splitter 340. After the splitter 340, the BS signals 386, 388, 390 from each of the different pathways to 0NU1 310, 0NU2 312, and 0NU3 314, respectively, are above the noise level 384 (approximately at the level of Rayleigh scattering or native scattering) and, further, above the combined, unenhanced signals 382 from all of the pathways (or DFs).

[0036] Significantly, each of the signals from their respective pathways is distinguishable from the others based on the round-trip time, which is directly proportional to the total length (L) of the DF. Thus, if an ESF is optically coupled to the splitter 340 by a shorter (first) segment of SSMF and to the ONU by a longer (second) segment of SSMF, then the round-trip characteristics will be different than an ESF that is optically coupled to the splitter 340 by a longer (first) segment of SSMF and to the ONU by a shorter (second) segment of SSMF. This is because each of the different optical signal communication paths (or pathways) will exhibit an enhanced BS at a different location (and, consequently, at a different return time).

[0037] Specifically, in the example of FIG. 3, the BS signal 386 from ESF1 320 will appear toward the back end of L (or some time proportional to L), while the BS signal 388 from ESF2 322 will appear toward the back end of 2L (or some time proportional to 2L), while the BS signal 390 from ESF3 324 will appear toward the back end of 3L (or some time proportional to 3L). Regardless of how many different pathways exist after the splitter 340, introducing a different L between the splitter 340 and each ONU permits each pathway to be discernible from the properties of the round-trip signal.

[0038] This concept may be generalized. The delay lines may have other lengths and do not have to be multiples of a given length. Importantly, to distinguish the different ESFs, their optical distance from the interrogator should be different, such that they may be observed in the OTDR or OFDR sensor traces at distinct (meaning, non-overlapping) or at least partially distinct (meaning, partially non-overlapping) time (or space) intervals. More generally, the ESFs may be located along each optical link in such a way that the ESFs have different optical path lengths to the interrogator. While this may require the addition of extra delay lines in some PONs, it may require only a few or possibly no extra delay lines (forexample, when the optical path lengths are already sufficiently different). In another preferred embodiment, the delay lines should not increase the length of the optical link from the OLT to the ONUs beyond the reach of the PON US and DS transponders.

[0039] In addition to differentiating the pathways based on delay line lengths L, the different pathways can be distinguished through distinct patterns along a portion of their lengths (as shown in FIG. 4) or by applying orthogonal patterns of modulations across their entire lengths (as shown in FIG. 5) or by providing different in-band wavelength ranges for each pathway (as shown in FIG. 6). To the extent that the following components are already discussed in detail with reference to FIGS. 1, 2, and 3, only a truncated discussion of the following components is provided: CO / OLT 405, 505, 605, feeder fiber (e.g., SSMF) 445, 545, 645, splitter 440, 540, 640, ONUs 410, 510, 610, 412, 512, 612, 414, 514, 614, 416, 516, 616, 418, 518, 618, and transmission fibers 534, 436, 536, 636, 438, 538, 638. Rather, the detailed descriptions will focus more on the ESFs 420, 520, 620, 422, 522, 622, 424, 624.

[0040] With this in mind, as shown in FIG. 4, each ESF 420, 422, 424 comprises a distinct pattern 486, 488, 490. Thus, if an interrogator trace exhibits a first pattern 486 (e.g., shown without any code or pattern applied), then one can determine that the BS signal is associated with the optical signal communication path to ONU1 410. Similarly, if the interrogator trace exhibits a second pattern 488, then the BS signal is associated with the pathway to 0NU2412. Likewise, if the interrogator trace exhibits a third pattern 490, then the BS signal is associated with 0NU3 414. For completeness, the graph in FIG. 4 plots the signal power 470 as a function of position 472 (with L representing the length of the total pathway), with the noise level 484 shown for reference. As one can appreciate, when a particular pattern associated with its respective pathway is known, the pattern that arises in the interrogator trace provides the location associated with that trace.

[0041] In FIG. 5, ESF1 520 comprises a spatial modulation that is orthogonal to a spatial modulation that is applied to ESF2 522. The spatial modulation could be periodic or aperiodic. The graph in FIG. 5 shows separate interrogator traces for ESF1 520 and ESF2 522, plotting the signal power 550, 570 as a function of position 552, 572, with the feeder fiber trace 554, 574 and the splitter loss 556, 576 shown for reference. Specifically, the first spatial modulation 558 (Al) has a higher-frequency repeating rate (meaning, shorter on-off periods), while the second spatial modulation 578 (A2) has a lower-frequency repeating rate(meaning, longer on-off periods). Because the spatial modulations are orthogonal and have different modulation frequencies, even when both signals are present in the same interrogator trace, the signals can be distinguished based on known techniques (e.g., use of Fourier Transforms or other known deconvolution techniques).

[0042] FIG. 6 is a diagram showing an embodiment of a PON with ESFs that have different in-band wavelength ranges. For example, the ESF1 620 comprises a first in-band wavelength range 654, while the ESF2 622 comprises a second in-band wavelength range 656, while the ESF3 624 comprises a third in-band wavelength range 658, with none of the in-band wavelength ranges 654, 656, 658 overlapping. For such an embodiment, a telecom signal can be transmitted at a center wavelength 670 (and a telecom wavelength range) that resides below all of the in-band wavelength ranges 654, 656, 658 that contribute to the enhanced BS. Alternatively, the telecom signal can be transmitted at a center wavelength 680 (and a telecom wavelength range) that resides above all of the in-band wavelength ranges 654, 656, 658. Although not shown in FIG. 6, it is also possible to transmit the telecom signal between the in-band wavelength ranges, presuming that sufficient bandwidth exists between those in-band wavelength ranges.

[0043] By separating the different pathways with different in-band wavelength ranges (meaning, different enhanced BS wavelengths), the location of the enhanced BS can be determined from the wavelength properties of the BS signal. Additionally, by interrogating the enhanced BS at a wavelength range that resides outside of the telecom wavelength range, both the telecom signal (using the PON OLT Tx and PON OLT Rx) and the DXS signal (using the DXS interrogator) can be deployed simultaneously in the PON with little-to-no interference between the two signals. Consequently, some embodiments of the interrogator are specifically configured to compensate for high splitter loss, while other embodiments of the interrogator are configured to employ differentiation techniques that permit isolation or differentiation of each DS pathway after the splitter. For clarity (and as shown in FIGS. 1 and 2), the DS direction is from the CO / OLT to the ONUs, while the US direction is from the ONUs to the CO / OLT.

[0044] Those having skill in the art will appreciate that ESFs have slightly higher attenuation than SSMFs. However, the slight attenuation can be compensated with higher power levels (meaning, a proper calculation of the power budget for signal transmissions cancompensate for any differences between ESFs and SSMFs). Also, PONs typically employ different wavelength ranges for US traffic as compared to DS traffic. Additionally, DXS applications often use yet another (different) range of wavelengths. Because signal attenuation and signal integrity are dependent on wavelengths, finding appropriate wavelength ranges that can accommodate all three (US, DS, and DXS) for a single ESF is neither intuitive nor trivial, thereby requiring judicious application of power and other signal parameters. Furthermore, for those embodiments that have a different L for each pathway, special fixturing or storage may be necessary for excess cable length.

[0045] As shown in FIGS. 1 through 6, the disclosed systems and processes address both the SNR of BS signals (FIGS. 1 and 2) and determining which pathway affected the BS signals (FIGS. 3, 4, 5, and 6). The low SNR of BS signals is overcome by introducing enhanced back- scattering optical fibers (or, simply, enhanced-scattering fibers (ESFs)) after the splitter (as shown in FIGS. 1 and 2). Different signal-distinguishing approaches arc introduced to determine which of the pathways affected the BR signal (as shown in FIGS. 3 through 6). Consequently, the disclosed systems and processes permit DXS in a PON with little-to-no adverse effects on a telecom signals within the same PON.

[0046] Any process descriptions or blocks in flow charts should be understood as representing steps in a process, and alternate implementations are included within the scope of the preferred embodiment of the present disclosure in which the process 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 ail of the present disclosure.

[0047] 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 within the scope of the disclosure.

Claims

What is claimed is:

1. A system comprising: an enhanced scattering optical fiber (ESF) configured for distributed sensing, the ESF comprising: an ESF input end configured to optically couple to a first location in an optical signal communication path, the first location being in a passive optical network (PON); an ESF output end configured to optically couple to a second location in the optical signal communication path, the second location being different from the first location, the second location being in the PON ; and an enhanced back-scattering segment between the ESF input end and the ESF output end, the enhanced back-scattering segment comprising: an in-band wavelength range exhibiting an in-band back-scattering that is higher than native scattering; and an out-of-band wavelength range exhibiting an out-of-band back- scattering that is lower than the in-band back-scattering.

2. The system of claim 1, wherein the optical signal communication path is located between an optical splitter and an optical network unit (ONU), wherein the system further comprises: a first segment of the optical signal communication path between the optical splitter and the ESF input end; and a second segment of the optical signal communication path between the ESF output end and the ONU.

3. The system of claim 2, wherein the first segment of the optical signal communication path is longer than the second segment of the optical signal communication path.

4. The system of claim 2, wherein the first segment of the optical signal communication path is shorter than the second segment of the optical signal communication path.

5. The system of claim 1, wherein the optical signal communication path is located between an optical tap and an optical network unit (ONU), wherein the system further comprises: a first segment of the optical signal communication path between the optical tap and the ESF input end; and a second segment of the optical signal communication path between the ESF output end and the ONU.

6. The system of claim 1, wherein the ESF further comprises: a telecommunication signal wavelength range for carrying an optical telecommunication signal, the telecommunication signal wavelength range at least partially overlapping with the in-band wavelength range.

7. The system of claim 1, wherein the ESF further comprises: a telecommunication signal wavelength range for carrying an optical telecommunication signal, the telecommunication signal wavelength range being located within the out-of-band wavelength range.

8. The system of claim 1, wherein the out-of-band back- scattering consists essentially of native scattering.

9. The system of claim 1, further comprising: a standard single-mode fiber (SSMF) located in the optical signal communication path, the SSMF and the ESF being arranged in parallel to each other.

10. A system comprising: a first enhanced scattering optical fiber (ESF1) configured for distributed sensing, the ESF1 comprising: an ESF1 length; an ESF1 input end configured to optically couple to a first location in a first optical signal communication path, the first location being in a passive optical network(PON); an ESF1 output end configured to optically couple to a second location in the first optical signal communication path, the second location being different from the first location, the second location being in the PON; and a first enhanced back-scattering segment between the ESF1 input end and the ESF1 output end, the first enhanced back-scattering segment comprising: a first in-band wavelength range exhibiting a first in-band back- scattering that is higher than native scattering; and a first out-of-band wavelength range exhibiting a first out-of-band back-scattering that is lower than the first in-band back-scattering; and a second enhanced scattering optical fiber (ESF2) configured for distributed sensing, the ESF2 comprising: an ESF2 length; an ESF2 input end configured to optically couple to a third location in a second optical signal communication path, the third location being different from the first location, the third location further being different from the second location, the third location being in the PON; an ESF2 output end configured to optically couple to a fourth location in the second optical signal communication path, the fourth location being different from the first location, the fourth location being different from the second location, the fourth location being different from the third location, the fourth location being in the PON ; and a second enhanced back- scattering segment between the ESF2 input end and the ESF2 output end, the second enhanced back- scattering segment comprising: a second in-band wavelength range exhibiting a second in-band back- scattering that is higher than native scattering; and a second out-of-band wavelength range exhibiting a second out-of- band back- scattering that is lower than the second in-band back-scattering, the second out-of- band back- scattering further being lower than the first in-band back-scattering, the first out- of-band back-scattering further being lower than the second in-band back-scattering.11 . The system of claim 10, wherein:the first optical signal communication path is located between an optical splitter and a first optical network unit (0NU1); and the second optical signal communication path is located between the optical splitter and a second optical network unit (0NU2).

12. The system of claim 11, wherein: the ESF1 further comprises: a first segment in the first optical signal communication path, the first segment comprising a first length, the first segment optically coupling the optical splitter to the ESF1 input end; and a second segment in the first optical signal communication path, the second segment comprising a second length, the second segment optically coupling the ESF1 output end to the ONU 1 ; and the ESF2 further comprises: a third segment in the second optical signal communication path, the third segment comprising a third length, the third length being longer than a combination of the first length and the ESF1 length, the third segment optically coupling the optical tap to the ESF2 input end; and a fourth segment in the second optical signal communication path, the fourth segment comprising a fourth length, the fourth segment optically coupling the ESF2 output end to the 0NU2.

13. The system of claim 10, wherein: the first in-band wavelength range at least partially overlaps with the second in-band wavelength range; the first enhanced back-scattering segment comprises a first spatial modulation pattern; and the second enhanced back-scattering segment comprises a second spatial modulation pattern, the second spatial modulation pattern being distinguishable from the first spatial modulation pattern.

14. The system of claim 10, wherein there is no overlap between the first in-band wavelength range and the second in-band wavelength range.

15. The system of claim 10, wherein: the ESF1 further comprises a telecommunication signal wavelength range for carrying an optical telecommunication signal; and the ESF2 further comprises the telecommunication signal wavelength range.

16. The system of claim 15, wherein: the telecommunication signal wavelength range at least partially overlaps with the first in-band wavelength range; and the telecommunication signal wavelength range at least partially overlaps with the second in-band wavelength range.

17. The system of claim 15, wherein: the telecommunication signal wavelength range at least partially overlaps with the first in-band wavelength range; and the telecommunication signal wavelength range is located within the second out-of- band wavelength range.

18. The system of claim 15, wherein: the telecommunication signal wavelength range is located within the first out-of-band wavelength range; and the telecommunication signal wavelength range is located within the second out-of- band wavelength range.

19. The system of claim 10, wherein: the first out-of-band back- scattering consists essentially of native scattering; and the second out-of-band back-scattering consists essentially of native scattering.

20. The system of claim 10, wherein:the first enhanced back-scattering segment comprises a first periodic spatial modulation pattern; and the second enhanced back-scattering segment comprises a second periodic spatial modulation pattern, the second periodic spatial modulation pattern being distinguishable from the first periodic spatial modulation pattern.

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