Vibration Sensing over Passive Optical Networks (PON)

Abstract

Aspects of the present disclosure describe systems and methods that advantageously enable vibration-induced optical phase measurements at centralized optical line terminals (OLTs) in PON architectures. In sharp contrast to existing distributed fiber sensing systems and methods, the optical phase measurements of the present disclosure do not rely on backscattering mechanisms and maintain sufficient optical signal-to-noise ratio (OSNR) even after round-trip splitting losses in the PON.

Description

[Technical Field]

[0001] The present disclosure relates generally to optical communication systems, methods and structures. More particularly, , Pa A method and structure for providing vibration sensing over a passive optical network (PON) is described. [Background technology]

[0002] As those skilled in the art can readily appreciate, centralized radio access networks (C-RANs) are an essential component of 5G mobile fronthaul access systems. To provide urban and suburban 5G coverage, centralized baseband units (BBUs) are connected to clusters of remote radioheads (RRHs) using fiber-based passive optical networks (PONs) deployed every few square miles.

[0003] Providing sensing capabilities over C-RAN has the potential to offer a wide range of applications, including network health monitoring for carriers, as well as new business opportunities such as traffic / building identification and earthquake detection. Fiber sensing in PON architectures has the potential to offer significant sensing advantages over other fiber networks due to its wider coverage area.

[0004] However, current standard PON architectures include passive splitters (typically 32 or 64 splits) between the optical line terminal (OLT) and the end-user optical network unit (ONU), making fiber sensing nearly impossible due to the round-trip loss imposed on the sensing signal due to the passive split (30-40 dB). Furthermore, due to the characteristics of the PON architecture, when the interrogator is located at the OLT, all sensing signals from the drop fiber after the passive split are combined, making it difficult or impossible to sense each of the multiple paths individually.

[0005] Previously, applicants have disclosed techniques for distributed fiber sensing (DFS) over PONs using reflective optical gain elements and time-domain multiplexing (TDM) switch sensing control, which generally solves both the splitting loss and multipath interference problems. However, the high cost of typical DFS interrogators and the additional reflective semiconductor optical amplifiers (R-SOAs) required for each ONU may make them unsuitable for large-scale deployment. Summary of the Invention

[0006] An advancement in the art is provided by aspects of the present disclosure that are directed to systems and methods that advantageously enable vibration-induced optical phase measurements at centralized optical line terminals (OLTs) in PON architectures. In contrast to distributed fiber sensing, optical phase measurements do not rely on backscattering mechanisms and maintain sufficient optical signal to noise ratio (OSNR) even after round-trip splitting losses in PONs.

[0007] According to one aspect of the present disclosure, the system and method employ TDM switching with optical reflectors from different optical network units (ONUs) to identify vibration sources in different drop fiber paths. The sensing mechanism is advantageously integrated into the upstream (US) data channel, where the optical carrier is shared and generated at the optical line terminal (OLT). Given the topology of the PON architecture and its short-reach applications, the inventive system and method according to aspects of the present disclosure provides vibration source identification based on fiber segments rather than specific spatial resolutions like DFS. Importantly, the inventive system and method according to the present disclosure can identify vibration sources not only from the feeder fiber before the splitter but also from individual drop fiber paths after the splitter.

[0008] In a first aspect, the system and method according to the present disclosure measures vibration-induced phase changes using a forward optical signal instead of a backscattered signal in a DFS. As a result, the scheme of the present invention produces a much higher OSNR even after round-trip splitting losses in a PON, and therefore does not require advanced amplification schemes.

[0009] From another perspective, to perform separate phase measurements of different drop fiber paths in a centralized OLT, the system and method of the present invention employs low-cost reflectors and switches at each ONU node. A TDM scheme is applied to the switches, and only one reflected optical signal is measured at each instance. As an additional advantage, the TDM synchronous clock can be provided from the centralized interrogator or can be obtained from existing PON data communications that use a shared clock.

[0010] Finally, yet another important aspect of the disclosed system and method is its integration with existing PON communication channels. Note that the system and method of the present invention allows for the acquisition of optical phase information from payload-bearing channels in a slightly modified coherent receiver DSP. Thus, vibration-induced phase measurements can be performed at the OLT if the same carrier is used by all ONUs providing US communications. Both TDM and frequency-division multiplexing (FDM) can be advantageously used for US data communications, since the phase recovery algorithms of the present application are applicable to both schemes.

[0011] A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram illustrating TDM switching control of vibration detection and reflected optical signals via an optical phase interferometric PON architecture according to an embodiment of the present disclosure.

[0013] [Figure 2] FIG. 2 is a schematic diagram illustrating a TDM switching mechanism for reflected signal conditioning (ONU to OLT) according to an embodiment of the present disclosure.

[0014] [Figure 3(A)] FIG. 3(A) is a pair of plots showing simultaneous recovery and vibration-induced phase signals in individual PON drop fiber paths, according to an embodiment of the present disclosure. [Figure 3(B)] FIG. 3(B) is a pair of plots showing simultaneous recovery and frequency spectra corresponding to the vibration-induced phase signals shown in FIG. 3(A), according to an embodiment of the present disclosure.

[0015] [Figure 4]FIG. 4 is a schematic diagram of an example configuration for a forward phase recovery sensing system overlay with 5G C-RAN using WDM, according to an embodiment of the present disclosure.

[0016] [Figure 5(A)] FIG. 5(A) illustrates forward phase recovery over a PON upstream (US) channel using a centralized laser source and a coherent receiver, according to an embodiment of the present disclosure. [Figure 5(B)] FIG. 5(B) is a diagram illustrating joint phase recovery over FDM subbands according to an embodiment of the present disclosure.

[0017] [Figure 6] FIG. 6 is a schematic diagram of an example physical cabling configuration of a PON system in a 5G fronthaul, according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0018] The exemplary embodiments are more fully described in the drawings and detailed description, however, embodiments in accordance with the present disclosure may be embodied in many different forms and are not limited to the specific or exemplary embodiments set forth in the drawings and detailed description.

[0019] The following merely illustrates the principles of the present disclosure, and it should thus be understood that those skilled in the art will be able to devise various arrangements which embody the principles of the present disclosure, even though not explicitly described or shown herein, and which are within the spirit and scope of the present disclosure.

[0020] Furthermore, all examples and conditional language provided herein are meant to be for educational purposes only to aid in understanding the principles of the present disclosure and concepts presented by the inventors to further the present technology, and should not be construed as being limited to the specifically listed examples and conditions.

[0021] Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Furthermore, such equivalents are intended to include both currently known equivalents and equivalents developed in the future, i.e., elements developed that perform the same function, regardless of structure.

[0022] Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.

[0023] Unless otherwise specified, the drawings herein, including the figures, are not drawn to scale.

[0024] For additional background, we begin by noting that Passive Optical Networks (PONs) are an optical fiber communications technology for providing broadband network access to end customers. The architecture implements a point-to-multipoint topology in which a single optical fiber serves multiple endpoints using unpowered (passive) fiber optic splitters to divide the fiber bandwidth between the endpoints. Passive optical networks are often referred to as the last mile between Internet Service Providers (ISPs) and their customers.

[0025] A passive optical network typically includes an optical line terminal (OLT) located at a service provider's central office (hub) and a number of optical network units (ONUs) or optical network terminals (ONTs) located closer to end users. PONs reduce the amount of fiber and central office equipment required compared to point-to-point architectures. A passive optical network is a type of optical fiber access network.

[0026] In most cases, the downstream signal is broadcast to all facilities sharing multiple fibers. Encryption can be used to prevent eavesdropping. The upstream signal is combined using a multiple-access protocol, usually time-division multiple access (TDMA).

[0027] 1 illustrates an architectural configuration according to an embodiment of the present disclosure. As can be seen, the exemplary configuration includes a PON system including a feeder fiber interconnecting an OLT to a 1:N splitter, followed by N drop fibers to individual ONUs located at remote nodes.

[0028] For data communications over such PONs, downstream (DS) and upstream (US) signals are designed to withstand a one-way splitting loss of 3 × log2N dB. For typical 32- to 64-split PONs, this one-way loss ranges from 15 to 18 dB. Sensing systems operating over such PONs must detect every fiber path in the PON, necessitating the installation of an interrogation unit at the centralized optical line terminal (OLT). Most distributed fiber sensing (DFS) systems rely on weak backscattering effects within the fiber, and their detection schemes typically cannot overcome the additional 30 to 36 dB round-trip loss imposed by 1:N splitters, resulting in significantly degraded performance.

[0029] In the innovative configuration presented in this disclosure, an optical phase interferometry system is used in a PON. Instead of relying on fiber backscattering, the optical signal is reflected back to the interrogator using a reflector at the ONU. A low phase noise laser is used in the phase interferometer and is located at the OLT. A coherent receiver is used to detect the reflected optical signal from the ONU after central amplification by a local oscillator (LO) supplied by the same laser source. Because the phase interrogation in this disclosure uses a continuous optical signal (rather than pulsed) and most of the signal power is reflected, a high optical signal-to-noise ratio (OSNR) can be achieved without optical amplification at the ONU.

[0030] Furthermore, with the innovative configuration of the present invention, the phase interference system provides segment-based vibration sensing with a PON architecture, and detects vibrations in the feeder fiber segment (S0) and each drop fiber segment (S1-S N ) can be used to identify the vibration source. To identify the vibration source, the present invention employs a TDM switching scheme for the reflected optical signals at each ONU, so that only one reflected signal is measured at a time by the centralized interrogator. Phase measurements at each drop fiber branch are performed sequentially by sequentially closing TDM-controlled optical switches. Another way to provide vibration source identification is by using a communication channel carrying a payload, as will be described in more detail later.

[0031] As reported in several previously published papers, ultra-stable optical lasers with very low phase noise can be used in conventional linear fiber networks to detect vibrations along the fiber route. For systems and methods according to aspects of the present disclosure, we present details of the modifications required to perform optical phase interferometry in PON networks and the main differences compared to previous disclosures.

[0032] According to one aspect of the present disclosure, a sensing laser and coherent receiver are placed in the optical line terminal (OLT) to detect optical phase changes caused by the PON network. Using the same laser as the LO for the coherent receiver results in a self-phase-beating configuration, as previously reported, effectively suppressing the inherent low-frequency phase noise of the laser. Advantageously, the remaining laser phase noise can be further suppressed by a phase-locked loop that tracks the low-frequency drift of the laser, and the remaining phase changes can be attributed to physical vibrations in the PON.

[0033] One of the many differences between the system and method of the present disclosure and the prior art is that the optical signal is reflected from the remote ONU along the same fiber path and returned to the OLT for detection. Because the PON architecture distributes a continuous interrogated signal to all ONUs simultaneously, the system and method of the present disclosure uses TDM switching control for each individual drop fiber path.

[0034] Such an exemplary configuration provides segment-based vibration detection, with only one ONU at a time reflecting an interrogated optical signal back to the centralized interrogator, as illustratively shown in FIG.

[0035] Using TDM switching, vibrations affecting individual drop fibers can be detected using different time slots within the sensing frame. One or more phase samples can be taken within a frame for each optical path. Since most of the vibrations to be detected are low frequency, it is advantageous to employ low-cost optical switches for TDM with millisecond-level switching speeds. For ONUs that do not require sensing along the drop fiber path, the fiber can be terminated as shown by ONU3 in Figure 1.

[0036] Because most PON data communication systems use TDM for upstream signaling, synchronization of TDM switching for phase interferometry can be achieved by utilizing / borrowing timing information from the data communication equipment. An individual time synchronization system for the TDM switches can also be provided by a phase interrogator. In this system, a clock is centrally distributed to each ONU, and a token is given to each ONU to close its optical switch during an assigned time window within each sensing frame. Implemented in this way, the length of the sensing frame, T, determines the sampling rate (1 / T) of the phase interrogation scheme.

[0037] To evaluate our system and method, we performed a simple experiment in an experimental PON setup with a 1:32 split to demonstrate TDM switching control in reflective phase interferometry. One feeder fiber and three drop fibers were connected before and after the splitter, each approximately 1 km long. Each of the three drop fibers was connected to an optical switch and reflector as shown in Figure 1, and the TDM control sampling frame rate (1 / T) was set to 20 kHz (10 μs window each). Two piezoelectric (PZT) fiber stretchers were installed in the two drop fiber paths to emulate fiber vibration modulated with a sinusoidal strain signal. Simultaneous phase interferometry measurements on the three paths using the centralized TDM sampling method are shown in Figures 3(A) and 3(B). As can be seen, three individual time-domain waveforms were recorded (Figure 3(A)), showing the phase measured on a stationary fiber path (black) and two paths with PZTs (blue and red). By analyzing the frequency content in Figure 3(B), the differences in the modulated frequencies of the PZT for the two individual paths can be further identified. The plotted results clearly demonstrate that the present invention can detect various vibrations and identify the fiber paths that generated them.

[0038] In addition to identifying vibration sources from different drop fiber paths, the present invention also provides a method for identifying vibrations in feeder fibers. Because all phase interference paths in a PON architecture necessarily pass through the feeder fiber, vibrations on the feeder fiber are present in all recorded phase measurements. Therefore, a simple signal correlation between all recorded waveforms can identify vibrations in the feeder fiber if the correlation is high. Because most PON systems installed for 5G fronthaul networks are relatively short, segment-based vibration localization is sufficient to identify events at various locations within the network, eliminating the need for the complex location calibration process required for typical DFS system installations.

[0039] In PON systems, there are potential situations where additional interference and noise may occur in phase interferometry measurements. Because the signal bandwidth used for the measurement is very narrow compared to normal data traffic (over 1 MHz compared to over 10 GHz), the OSNR of the reflected optical signal will be very high even if there is a round-trip loss of the optical signal due to the splitter. The main cause of signal degradation may be due to other reflections in the PON system that cannot be controlled. The power levels of the signal and interference are shown below.

number

[0040] Fiber loss = -0.2 dB / km, N = 32, L D =L D_avg =1km, C conn =-50dB, C Ray Using the parameters of P = -72 dB / m, applicants sig was found to be -31 dB lower than P0 (L F = 1 km). Also, P interf It was found that the four terms in (P0) are -42 dB, -47 dB, -57.5 dB, and -62.5 dB lower than P0. Therefore, the last two terms, which are more than 10 dB lower than the first two terms, can be omitted. In this case, the signal-to-interference ratio (SIR, P sig vs. P interf ) is about 10 dB. F If the length of the ONU is long or if there is high reflection due to a connector or poor connection before the splitter, the phase measurement performance will be degraded due to a reduced SIR. In such an event, it is recommended to place a gain element at the ONU to reduce the Psig It is possible to increase the SIR, so that the gain provided adds directly to the SIR. An example of such a gain element is the semiconductor optical amplifier (SOA). In fact, a reflective SOA (RSOA) provides all three functions: reflection, TDM switching, and gain.

[0041] The phase measurement system employs continuous signal interferometry with a PON architecture, allowing it to accommodate optical power levels similar to those used in data communications. Unlike DFS, there are no high-intensity pulses that can cause harmful nonlinear interference to other communication channels in the same fiber. Therefore, the forwarding phase interferometric sensing system can be combined with data communications systems over the same fiber using a WDM approach.

[0042] 4 shows an example deployment illustrating a forward phase recovery sensing overlay for 5G C-RAN using WDM, and is a schematic diagram illustrating an example of how a standard wavelength diplexer can be used to integrate the sensing system with the C-RAN system. In this case, the diplexer is placed at the OLT and ONU immediately after the fiber connection to multiplex and demultiplex the data and sensing signals.

[0043] Integration between the disclosed forward phase interference system and data communication channels can also be implemented. As mentioned earlier, optical phase information can be obtained from a typical coherent receiver DSP module. In a PON architecture, if a coherent receiver is implemented, the optical phase change due to fiber vibration can also be obtained in the same way. The increased cost of ultra-low phase noise lasers and coherent receivers can be absorbed if the hardware cost can be shared among all ONUs.

[0044] Figure 5(A) shows an example in which a laser light source is shared by all ONUs for US communications and a coherent receiver is used at the OLT. Shared laser US communications has previously been proposed for PONs, providing a low-cost solution for the optical components required at the ONUs. In fact, the R-SOA is one of the integrated solutions available for low-speed data modulation. If US data communications use TDM, the vibration source identification follows the same TDM control frame as shown in Figure 2. However, FDM can be implemented in the US communications channel, with frequency subbands used at each ONU. In this case, the coherent receiver DSP can simultaneously acquire phase information for all frequency subbands without the need for TDM frame adjustment, as shown in Figure 5(B). In this case, the phase sampling rate is no longer limited to the preset frame rate.

[0045] In actual cable installation and connection of PON systems in 5G applications, the physical route of each drop fiber does not extend beyond the splitter. As shown in Figure 6, to minimize cable installation costs, drop fibers can share adjacent fiber cores within the same cable at the beginning of the route. Each time, several fibers are dropped at a drop box, while the remaining fibers continue to the next drop box (i.e., within the same cable). In this configuration, the segment-based vibration identification of the present invention can also provide the ability to identify vibration sources in different sections along the shared cable, provided that each section is separated by at least one drop box.

[0046] For example, if vibration occurs between drop box 1 and drop box 2, it is expected that the vibration will be seen at (N-4) of the phase signal acquired after drop box 1. Therefore, by comparing the correlation between the two-phase signal acquired from drop box 1 with the correlation of another signal dropped later, the system can identify whether the vibration occurred before or after drop box 1. The same operation can be performed step-by-step for all drop boxes, allowing the source of vibration due to the shared cable between each drop box to be identified.

[0047] Identifying the vibration source after a fiber drop works similarly to that shown in Figures 1 and 2. One useful case scenario is identifying a stray cable that has separated or fallen from its original pole fixture, as shown in Drop Box 2 in Figure 6. A stray cable can pose a potential safety hazard if it falls onto the roadway or is blown away by high winds.

[0048] While the present disclosure has been illustrated herein using certain specific examples, those skilled in the art will recognize that the present teachings are not limited thereto. Accordingly, the present disclosure should be limited only by the scope of the claims appended hereto.

Claims

1. 1. A system for vibration sensing over a passive optical network (PON), comprising: an optical line terminal (OLT); a plurality of optical network units (ONUs); a passive optical network optically connecting the OLT to the ONUs; and the plurality of ONUs include an optical switch other than a reflective semiconductor optical amplifier that is in optical communication with the PON, and an optical reflector that is in optical communication with the optical switch; The OLT includes an interrogator configured to provide a continuous, non-pulsed optical phase interference signal to the PON, receive reflected signals reflected by a plurality of the optical reflectors, and detect mechanical vibrations within the PON from measurements of phase changes in the continuous optical phase interference signal and the received signal.

2. The system of claim 1 , wherein the OLT includes a coherent receiver for receiving / detecting the reflected signal.

3. The system of claim 2 , wherein the OLT includes a single laser for both sensing and local oscillation.

4. The system of claim 3 , further configured to selectively activate a plurality of said optical switches such that at a given time, only one of said plurality of ONUs provides said reflected signal.

5. The system of claim 1 , wherein the PON carries telecommunications traffic simultaneously with the continuous optical phase interference signal and any of the reflected signals.

6. 6. The system of claim 5, wherein the PON includes N ONUs, a 1xN splitter, a single feeder fiber optically connecting the OLT to the 1xN splitter, and N drop fibers optically connecting the 1xN splitter to individual ONUs.

7. The system of claim 6 , wherein the OLT determines a source of reflection other than the ONU.

8. The system of claim 6 , wherein the OLT determines the detected mechanically vibrating section of the PON.

9. The system of claim 6 , wherein the OLT distributes a clock signal to the ONUs.

10. The system of claim 6 , wherein at least one of the ONUs includes a gain element that directly boosts the reflected signal.

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