WO2008116309A1 - Procédé et système de détection dans de défauts dans un réseau optique à trajet multiple - Google Patents

Procédé et système de détection dans de défauts dans un réseau optique à trajet multiple Download PDF

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WO2008116309A1
WO2008116309A1 PCT/CA2008/000571 CA2008000571W WO2008116309A1 WO 2008116309 A1 WO2008116309 A1 WO 2008116309A1 CA 2008000571 W CA2008000571 W CA 2008000571W WO 2008116309 A1 WO2008116309 A1 WO 2008116309A1
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Prior art keywords
light
signal
decoding
network
return signal
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PCT/CA2008/000571
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English (en)
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Habib Fathallah
Leslie Ann Rusch
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Universite Laval
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/071—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using a reflected signal, e.g. using optical time domain reflectometers [OTDR]

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  • the present invention relates to the field of optical tests and measurement and more particularly concerns a method, a system and devices providing for the identification and localisation of defects in one of multiple light paths of an optical network.
  • Optical network testing, monitoring and management is an important issue for a multitude of telecommunication applications, such as fiber-to-the-home FTTH, access and metro passive optical networks PONs, and wavelength division multiplexing WDM communication networks.
  • OTDR optical time domain reflectometry
  • FIG. 1 PRIOR ART
  • the concept of standard optical time domain reflectometry consists of detecting the Rayleigh and/or Brillouin backscattered and reflected signals from one transmitted short, high-power optical pulse.
  • the OTDR theoretically measures the impulse response of the waveguide at the wavelength of the transmitted OTDR pulse.
  • This principle is widely used in the installation and monitoring of traditional point-to-point optical fiber networks.
  • the OTDR testing is achieved for all standard bands to verify the node-to-node (or end-to-end) loss budget. All events that involve a refractive index change in light path, hence contributing to a power loss, gain, reflection ghost or combination of them, could be measured.
  • FIG. 2 a hypothetical example of an optical multi-path waveguide is illustrated with PRIOR ART standard OTDR.
  • the OTDR pulse enters the multi- path waveguide (DUT: device under test) from a port P 0 , is split into multiple sub- pulses (N in FIG. 2), each of which travels through a specific path.
  • N different paths from port Po to Pj where i 1 to N, are shown with N optical paths h o, i to ho ,N - Every sub-pulse will generate backscattered power in addition to reflected depending on the path traveled.
  • the OTDR receives backscattered and reflected power that is a sum of all scattered and reflected powers from all the N paths traversed by the N sub-pulses.
  • I 0 ⁇ t, ⁇ be the impulse response of the multi-path waveguide observed from the IN/OUT port Po. This is equal to the sum of all individual impulse responses of the N paths.
  • PS Passive splitters
  • TDM time division multiplexing
  • FTTH fiber-to-the- home
  • PONs passive optical networks
  • the OTDR trace at the central office (CO) is a linear sum of the backscattered and reflected power from all the network branches. It is difficult, and even impossible, for the CO Manager even with great expertise to distinguish events in one branch from events in other branches.
  • FTTH-PONs seem to be the ultimate winning solution for tomorrow's last/first mile bottleneck. Important FTTH deployments have been carried out in North America, Europe and Japan, over the last decade. Starting from 1 :1 (1 fiber to 1 customer) in the early 90s, PS together with TDM technologies have enabled up to 1:128 for the GPON standard (ITU G. 984) with forward error correction (FEC). It is recently reported a test bed with 1 :256 PS, and future extra large XL-PON systems are aimed at splitting factors of up to 1024. Many FTTH management problems result from the very little information available to the CO manager about the network. This directly affects the quality of service and dramatically increases the administration, the maintenance and the provisioning costs.
  • ITU G. 984 forward error correction
  • FIG. 3 illustrates all the PRIOR ART approaches described in the following.
  • FIG. 3a shows one PRIOR ART approach [Tanaka, K., Tateda, M. and Inoue, Y. "Measuring the individual Attenuation Distribution of Passive Branched Optical Networks," IEEE Photonics Technology Letters, Vol. 8, No. 7, July 1996.] that addresses the problematic using a passive splitter assembled with a WDM de-multiplexer and an active optical switch PS-R (i.e., passive splitter-router).
  • PS-R active optical switch
  • each waveband is dedicated for the monitoring of one specific branch of the PS.
  • the active switch PS-R cyclically selects the branch to be monitored, and a tuneable OTDR laser is used to generate the wavelength appropriate for the branch.
  • One optical reflector (not necessarily wavelength selective) is placed at every branch's end in order to alleviate the OTDR dynamic range requirements.
  • FIG. 3b shows another PRIOR ART approach [Chan, Chun-Kit, Tong, Frank, Chen, Lian-Kuan, Ho, Keang-Po and Lam Dennis, "Fiber-Fault Identification for Branched Access Networks Using a Wavelength-Sweeping Monitoring Source," IEEE Photonics technology Letters, Vol.
  • FIG. 3c shows another PRIOR ART approach [Ye, Chien-Hung, Chi, Sien Optical fiber-fault surveillance for passive optical networks in S-band operation window" OPTICS EXPRESS, Vol. 13, No. 14, 11 July 2005.].
  • the monitoring principle in this approach consists of a tuneable very long ring laser, made by an EDFA as an amplification medium, placed in the central office, and Bragg gratings at the end of the branches that close the ring.
  • the laser is tuned using a tuneable passband filter. When a branch is healthy, the ring is closed and the lasing process works well, indicating the good state of the branch.
  • FIG. 4a shows another PRIOR ART approach [Park, S.-B., Jung, D.K., Shin, H.S., Shin, D.J., Hwang, S., Oh, Y. and Shim, C 1 "Optical fault monitoring method using broadband light source in WDM-PON," Electronic Letters, Vol. 42, No. 4, 16 th February 2006 ] proposed for a WDM-PON system where every user is dedicated one wavelength for data and another for monitoring. The system assigns the C band for data and the L band for monitoring.
  • FIGs. 4a), b) and c) describe three different settings of reflectors to be placed at the branches' ends.
  • the first uses a fiber loop with a circulator
  • the second uses a wavelength splitter with standard reflector
  • the third uses an in-line Bragg grating before the fiber termination.
  • a major drawback of this system is the use of 50 % of the wavelength resources for monitoring.
  • the capacity in terms of number of branches
  • a method for testing an optical network for defects between a network node and a plurality of network locations, each network location being connected to the network node through a corresponding light path said method includes the steps of: a) transmitting a pulsed monitoring light signal from the network node to each of the network locations through the corresponding light path; b) providing an encoder in each of the light paths proximate the corresponding network location, each encoder reflecting the monitoring signal back towards the network node and encoding the reflected monitoring light signal according to an encoding function, the encoding function of each encoder having a low cross-correlation to the encoding function of all other encoders; c) receiving a return signal at the network node corresponding to the reflection of the monitoring light signal in said light paths; and d) for at least one of said light paths: i.
  • decoding the return signal according to a decoding function correlated to the encoding function associated with said light path, thereby obtaining a decoded return signal; and ii. analysing the decoded return signal to determine therefrom if any defect is present in said light path.
  • - transmitting means for transmitting a pulsed monitoring light signal from the network node to each of the network locations through the corresponding light path;
  • each encoder associated with each of the light paths and provided therein proximate the corresponding network location, each encoder reflecting the monitoring signal back towards the network node and encoding the reflected monitoring light signal according to an encoding function, the encoding function of each encoder having a low cross-correlation to the encoding function of all other encoders; - receiving means for receiving a return signal at the network node corresponding to the reflection of the monitoring light signal in said light paths;
  • - decoding means for decoding the return signal according to any one of a plurality of decoding functions, thereby obtaining a decoded return signal, each decoding function being correlated to the encoding function associated with one of the light paths; and - analysing means for analysing the decoded return signal to determine therefrom if any defect is present in the light path associated with the corresponding decoding function.
  • a set of devices for installation in an optical network for testing the same for defects between a network node and a plurality of network locations, each network location being connected to the network node through a corresponding light path
  • said set of devices comprising: - a light generating module for generating a pulsed monitoring light signal, said light generating module having a light output connectable to said optical network for transmitting the monitoring light signal from the network node to each of the network locations through the corresponding light path;
  • each encoder having an encoding function for encoding the reflected monitoring light signal, the encoding function of each encoder having a low cross-correlation to the encoding function of all other encoders; - a processing module having a light input for receiving a return signal at the network node corresponding to the reflection of the monitoring light signal in said light paths, said processing module having decoding means for decoding the return signal according to any one of a plurality of decoding functions, thereby obtaining a decoded return signal, each decoding function being correlated to the encoding function associated with one of the light paths, and analysing means for analysing the decoded return signal to determine therefrom if any defect is present in the light path associated with the corresponding decoding function.
  • FIG. 1 is a diagram illustrating standard optical time domain reflectometer OTDR based on light backscattering and reflection principle, conceived for characterisation of single light-path waveguides only and point-to- point optical fiber links.
  • FIG. 2 is a diagram illustrating the problematic faced by standard OTDRs when applied to characterizing multi-path optical waveguides such us optical passive splitters in standard FTTH, and general architectures of passive optical networks.
  • An optical network is presented here like a complex and large multi-path optical waveguide.
  • FIGs. 3A to 3C show different prior art approaches for addressing the problematic illustrated in FIG. 2;
  • FIG. 3A shows one prior art approach, proposed in 1996, that addresses the problematic using passive splitter concatenated with an active optical switch PS R;
  • FIG. 3B shows another prior art approach, proposed in 1999, that makes use of Bragg gratings placed in the ends of the branches to help identify the unhealthy branches;
  • FIG. 3C shows another prior art approach to the problematic similar to the of b) but suggesting the use of the under employed S-band.
  • FIG. 4A shows another prior art approach for a WDM based PON, where every client is dedicated two wavelengths, one for data and the other for monitoring;
  • FIGs. 4B and 4C show two different possible reflecting schemes provided at the clients of such a network.
  • FIG. 5 is a diagram illustrating the general principle of a method and system according to a preferred embodiment of the present invention.
  • FIG. 6 is a block diagram of a system for testing an optical network for defects according to an embodiment of the invention, where the decoding functions are implemented using optical components.
  • FIG. 7 is a state diagram showing the step of a method for testing an optical network for defects according to an embodiment of the invention, which may be implemented using the system of FIG. 6.
  • FIG. 8 is a block diagram of a system for testing an optical network for defects according to another embodiment, where the decoding functions are also implemented using optical components.
  • FIG. 9 is a state diagram showing the step of a method for testing an optical network for defects which may be implemented using the system of FIG. 8.
  • FIG. 10 is a block diagram of a system for testing an optical network for defects according to an embodiment of the invention, where the decoding functions are ) is performed on the electrical field using waveform functions selected to perform the function similar to that in optics.
  • FIG. 11 is a state diagram showing the step of a method for testing an optical network for defects according to an embodiment of the invention, which may be implemented using the system of FIG. 10.
  • FIG. 12 is a block diagram of a system for testing an optical network for defects according to an embodiment of the invention, where the decoding functions are ) is performed in electronics using modified decoding technique.
  • FIG. 13 is a state diagram showing the step of a method for testing an optical network for defects according to an embodiment of the invention, which may be implemented using the system of FIG. 12.
  • FIG. 14 is a block diagram of system similar to that of FIG. 12 but illustrating the importance of pseudo-noise coding prior to the pulse transmission in order to increase the signal to noise ratio at the receiver.
  • FIG. 15 is a state diagram showing the important processing and decision steps according to the embodiment of FIG. 14.
  • FIG. 16 is a schematic representation of a system according to an embodiment of the invention implemented in a TDM/TDMA FTTH optical network.
  • FIG. 17 is a schematic representation of a system according to an embodiment of the invention implemented in a WDM optical network.
  • FIG. 18 is a schematic representation of a system according to an embodiment of the invention implemented in a hybrid TDM/TDMA over WDM passive optical network.
  • FIGs. 19 and 20 are schematic representations of systems according to an embodiment of the invention implemented in multipath optical waveguide composed of WDM passive ring networks.
  • FIGs. 21 A to 211 show alternative embodiments of encoders for use in a system according to the present invention, all for time-dependent encoding functions.
  • FIG 22 shows design charts for time-only dependent waveforms inspired from optical orthogonal code families.
  • FIG. 23 shows one embodiment of a coding function with time and wavelength dependent coding waveforms.
  • FIG. 24 shows one embodiment of a coding function with wavelength-only dependent coding waveforms.
  • the present invention relates to a method and system for testing an optical network for defects between a network node and a plurality of network locations.
  • optical network is understood to refer to any collection of light paths connecting together different points or locations.
  • optical and “light” are used herein to refer to any appropriate portion of the electromagnetic spectrum, and are not limited to the visible spectrum only.
  • the method and system disclosed herein may be particularly advantageous for characterizing multi-path optical waveguides and managing FTTH, PONs, and WDM optical networks or the like.
  • the description below refers mainly to such embodiments, one skilled in the art will understand that the present invention may easily be adapted to different applications without departing from the scope of protection.
  • the optical network may be embodied by a multipath waveguide.
  • defects it is understood a break in a light path, or any other event, damage, alteration, wear, etc which may interrupt or affect light transmission therein.
  • each network location is here schematized as a port (P 1 , P 2 , ... PN) connected to the network node P 0 through a corresponding light path (h o, i, ho , 2, ⁇ • • ⁇ IO.N).
  • each network location may be embodied by a customer site, and the network node by the Central Office.
  • the method and system of the invention in this context would allow the monitoring of the entire network.
  • the invention may alternatively be applied between any plurality of network locations linked to a single network node, as will be easily understood by one skilled in the art.
  • the method of the present invention first includes transmitting a pulsed monitoring light signal 34 from the network node Po to each of the network locations (Pi, P2, ... PN) through the corresponding light path (h o ,i, h o ,2, ... ho,N).
  • the transmitted pulse function is represented as u(t, ⁇ ).
  • An encoder OCM is provided in each of the light paths, at or proximate the corresponding network location. In practice, the encoder OCM may for example be positioned at the premises of each customer. It will be understood that the encoders need not be at the extremity of any given light path, but that the placement of an encoder will determine the portion of the corresponding light path being monitored as part of the method and system of the invention.
  • Each encoder OCM reflects the monitoring signal 34 back towards the network node P 0 , and encodes the reflected monitoring light signal according to an encoding function e,(t, ⁇ ).
  • the encoding function of each encoder has a low cross- correlation to the encoding function of all other encoders, as will be explained in more detail further below.
  • the encoding functions may for example be designed to be orthogonal or pseudo-orthogonal.
  • the coding waveforms embodying the encoding functions could be time-only, wavelength-only, or time and wavelength dependent functions, as well as phase dependent, polarisation dependent or any combination of these parameters.
  • the encoders themselves may be embodied by reflective components such as multi- structure Bragg gratings, or any other coding technology. Examples of encoders for time modulation of the monitoring signal are shown at FIGs. 21 A through 211 and are explained further below.
  • the method next involves receiving a return signal 36 at the network node, corresponding to the reflection of the monitoring light signal 34 in said light paths.
  • the return signal will include the encoded monitoring signal from every light unbroken light path, as well as any reflections occurring within the network.
  • the return signal may be expressed as follows:
  • Information on defects in any given one of the light paths may be obtained by decoding the return signal 36 according to a decoding function correlated to the encoding function associated with this light path.
  • the decoded return signal thereby obtained is analysed to determine therefrom if any defect is present in the light path, and possibly, where along this path.
  • the decoding of the return light beam may be performed optically, or electronically. Different manners of performing such a decoding and analysis are explained below in connections with the embodiments of FIG. 6, FIG. 8, FIG. 10, FIG.12 and FIG. 14.
  • FIG. 6 is a block diagram illustrating a first preferred embodiment of a system 32 in which the decoding functions are designed using optical components and performed on the optical field of the return signal before any detection.
  • the system 32 is shown as having a light generating module 46 including a laser 42, or other light source, generating the pulsed monitoring signal 34.
  • the laser may be embodied by any appropriate device known in the art, and is driven by a laser control 44 as also well known in the art.
  • a first circulator 48 directs the monitoring light signal 34 from the laser 42 to the optical network, where it is distributed along multiple light paths and reflected by an encoder at the end of each, and directs the return signal 36 received from the network to a processing module 50 including both decoding and analysing means.
  • a bank of optical decoders 38 each the mirror image of one of the encoders of the system, is provided.
  • An optical switch 40 preferably directs the return signal 36 to each decoder 38, either as needed or successively.
  • a processor 52 is preferably provided and incorporates a detector 56 for transforming the decoded signal into an electronic decoded signal, and appropriate applications to analyse this electronic decoded signal.
  • a second circulator 54 connects the first circulator 48, optical switch 40 and processor 52.
  • the decoded signal is therefore expressed as:
  • FIG. 7 is a state diagram explaining the workings of the decoding and analysing of the return signal using the system of FIG. 6.
  • This method includes sending 100 the return signal to the processing module, and selecting 102 one of the light path h 0
  • the selected light path is correlated 104 using the decoding function associated with the same light path. In FIG. 6, this is accomplished by setting the switch 40 to close the circuit to the appropriate optical decoder 38.
  • a decoded signal for this particular light path is thereby obtained.
  • the method then preferably includes a step of identifying 106 an autocorrelation major peak in this decoded signal, corresponding to the position of the encoder along the light path. This is preferably done electronically by the processor.
  • the return signal does not includes a component encoded by the corresponding encoder, indicating that the light path is broken; a message to this effect can be generated 110.
  • the processor searches 114 the decoded return signal for at least one reflection peak. If one or more such peaks are detected 116, they are each interpreted 118 as a potential defect along said light path (splice, bending, faulty connection, etc).
  • the profile of the light path may be traced 120 as the time delay corresponding to each minor peak can be correlated to a position along the light path.
  • FIG. 8 is a block diagram illustrating a system according to a second preferred embodiment.
  • the de-correlating (decoding) is also performed on the return signal's optical field before the TE detector.
  • the first path (a-b-e-f) connects the decoder directly to the photodetector, avoiding the re-coding block; this makes the scheme identical to that of FIG. 6.
  • the second path (a-c-g-f) feeds the decoded signal to the re-encoding block tuned to the respective encoder, and then sends the re- encoded signal to the photodetector.
  • the switch is tuned to this second path when a fault is detected in a specific light path.
  • the re-encoding simply regenerates the return signal prior to the decoder.
  • FIG. 8 focuses on the case where one (e.g, branch k) among the N light paths is interrupted.
  • the interruption reflects back a pulse with a reflectivity ⁇ .
  • the return signal at the monitoring equipment called OC-OTDR is as follows:
  • R k (t, ⁇ ) S(t, ⁇ )*d k (t, ⁇ )
  • the first term of the decoded signal is a correlation of the /f" 1 decoder function and the impulse response of the remaining part of the A* 1 light path; no autocorrelation function exists in this term.
  • the second term is a sum of cross-correlations of the kth decoder with all the healthy paths' encoders convolved with their respective paths' impulse responses.
  • the OC-OTDR TE due to the prior measurements and identifications, already knows the second term. The OC-OTDR TE could use this prior information to derive the first term. Otherwise, the OC-OTDR TE can achieve a re-encoding of that decoded signal using the re-encoding block described in FIG. 8, tuned to the A* h encoder.
  • the re-encoded signal is composed of two terms; the first is a correlation of the autocorrelation function with the impulse response of the remaining part of the interrupted /c" 1 path.
  • the second terms are re-coded (or re-spread) cross- correlation functions, which represents the interference.
  • the OC-OTDR TE extracts the impulse response J 04 (t, ⁇ ).
  • the position (or the delay) of the autocorrelation function AUT k (t, ⁇ ) indicates the fault position in the /c" 1 path.
  • This embodiment is capable of not only identifying the faulty light-path (or fiber) but also determining its distance from the central office.
  • FIG. 9 is a state diagram showing the key process steps of the multi-path waveguide characterisation or optical network characterisation and monitoring, according to the embodiment of FIG 8.
  • FIG. 10 is a block diagram of a system 32 according to the another preferred embodiment: the de-correlating (decoding) is performed on the electrical field using waveform functions selected to perform the function similar to that in optics.
  • electronic decoders 39 are provided after the detector 52.
  • electronic re-encoders 59 may be provided and perform electronically the same function as the re-encoding block of FIG. 8.
  • Performance of OCDM decoding in electronics after the detection is extremely rare in data communications.
  • For data communications we typically try to transfer to optics as many electronic functions as possible. In the case of this application, it is not necessary to achieve very high transmission bandwidth. Also, electronics of 1 or 2 GHz speed is no longer expensive. The decoding function could be performed at that speed using low cost electronic devices. This leads to significant cost savings by eliminating optical components to separately decode each client.
  • electronic processing of the decoding and the re-encoding of FIG. 6 and FIG. 8 reduces the power loss budget of the system by 10 dB or more since it removes all the decoding loss. Furthermore, this enables developing and applying complex detection algorithms and performing the decoding of all the branches impulse responses simultaneously instead of cyclically.
  • the state diagram of FIG. 11 shows the important processing and decision steps according to the embodiment of FIG 10.
  • FIG. 12 is a block diagram of a system according to the yet another embodiment: the de-correlating (decoding) is performed in electronics using modified decoding technique.
  • the decoder weights or otherwise takes into account, the whole or a part of the available information, including all the encoders' properties and the calculated transfer functions of the other paths.
  • the system of FIG. 12 is a block diagram of a system according to the yet another embodiment: the de-correlating (decoding) is performed in electronics using modified decoding technique.
  • the decoder weights or otherwise takes into account, the whole or a part of the available information, including all the encoders' properties and the calculated transfer functions of the other paths.
  • a processor 52 incorporating a decoding application for parallely decoding the electronic return signal according to each of the decoding functions, thereby electronically obtaining the decoded return signal for each of the light paths, and a correlation application searching each of the decoded return signal for an autocorrelation major peak corresponding to a position of the encoder along the corresponding light path, and identifying this corresponding light path as broken if the autocorrelation major peak is not located.
  • FIG. 13 is a state diagram showing the important processing and decision steps according to the embodiment of FIG. 12.
  • This method includes detecting and sending 121 the return signal to the processing module, and electronically correlating 122 the detected signal with all the decoding waveforms. A plurality of decoded signals, one for each light path is thereby obtained. The method then preferably includes a step of analysing 124 all the decoded signals to identify an autocorrelation major peak, corresponding to the position of the encoder along the light path. This is done electronically by the processor.
  • the return signal does not includes a component encoded by the corresponding encoder, indicating that the light path is broken.
  • the method then preferably includes analysing 128 the decoded signal for this light path using the information from all the other paths, information from prior measurements (i.e. the results of the same analysis performed at a previous time), or both. This may allow the identification 130 of an reflection peak corresponding to the break location along the light path, and consequently an estimation 132 of the location of the break along the path.
  • a message 134 is then displayed, to the effect that a break has been located and where along the path. In the event that reflection peak corresponding to the break location is not identified, a message to this effect can also be displayed 136.
  • the processor searches 114 the decoded return signal for at least one reflection peak. If one or more such peaks are detected 116, they are each interpreted 118 as a potential defect along said light path, it will be noted that not all reflection peaks are the result of a defect, as a splice, a connector, etc may also generate such a peak.
  • the correlation application however knows the location of fixed network reflection sources, and can differentiate them from reflection peaks associated to the potential defects.
  • the profile of the light path may be traced 120 as the time delay corresponding to each minor peak can be correlated to a position along the light path.
  • FIG. 14 is a block diagram of a system according to another embodiment similar to that of FIG. 12 but illustrating the importance of pseudo-noise modulation or coding of the monitoring signal prior to transmission through the network, in order to increase the signal to noise ratio at the receiver.
  • FIG. 15 is a state diagram showing the important processing and decision steps according to the embodiment of FIG. 14. Pseudo-noise noise modulation is well known in the art, and reference can for example be made to [DONNER EXEMPLE DE REFERENCE]
  • FIG. 10, FIG. 12 and FIG. 14 Electronic based processing embodiments illustrated in FIG. 10, FIG. 12 and FIG. 14 are clearly less cumbersome and expensive than that of FIG. 6 and FIG. 8.
  • the monitoring equipment should cost not more than 10% more than traditional OTDR equipment. We expect the cost of the monitoring equipment to be 1500 to 2500 $ depending on the processing functions to be included.
  • the cost of the encoders to be placed at the fiber ends should range from 10 to 100 $ each, depending on the volume and the selected components as shown in FIG. 23.
  • the present invention also relates to a set of devices for installation in an optical network for testing the same for defects.
  • the set of devices includes a light generating device, a plurality of encoders and a processing module which integrates the decoding and analysing components of any of the system embodiments described above. These devices may be provided as a kit to service providers for installation in an already existing network or in a new one.
  • Optical-Coding based Optical-Time-Domain-Reflectometry (OC-OTDR) since this fulfills the traditional OTDR like-role, but in the context of complex multi-path waveguides.
  • Traditional OTDR suffers from cumulated multi-path backscattered and reflected signals, making it impossible to discriminate the impairments of one specific optical path from others.
  • FTTH and PON applications are applied for specific cases of multi path waveguides like FTTH, PON 1 or more complex ring WDM networks.
  • the present OC-OTDR technique could be straightforwardly applied to the characterization, installation, monitoring and management of FTTH, PON, and more complex ring WDM networks. Using time domain codes, this requires only one wavelength, preferably in the 1650 nm wavelength already reserved for standard FTTH TDM- PON monitoring, or the non-standard but inexpensive 1625 nm wavelength.
  • the present technique is also scalable for more advanced and complex PON networks, always using a single wavelength. As illustrated in FIG.
  • every network branch is terminated by a standard passive wavelength selector (WS), widely used in PONs, isolating the standard monitoring U band from the other data bands at the front of every ONT and at the CO as well. If two dimensional or wavelength-only dimensional codes are used, a wider wavelength band is required for monitoring; the standard U band could be also effective for these kinds of codes.
  • the transmission section of the OC-OTDR TE at the CO preferably consists of a U band pulsed laser driven by a processor to transmit short pulses with a predetermined low frequency rate (a few megahertz or lower) and adequate power to support the back and forth cumulative loss. Any of the embodiments of FIG. 6 to 15 could be considered.
  • Every pulse propagates through the tree network, is split at the PS, is coded by encoders OCM 1 , i 1 to N, and is then reflected back to the CO.
  • the encoder may consist of a passive device that fragments an incoming pulse into a number of p sub-pulses distributed in time according to a specific code, i.e., direct sequence coding in the time domain. Every encoder at the branch termination implements a unique code.
  • the proposed network management system is a modified form of direct sequence DS-OCDM, and the codes used are also the so-called optical orthogonal codes (OOC).
  • OOC optical orthogonal codes
  • the PS (coupler) combines the upstream encoded pulses together as in standard OCDM.
  • a tunable DS-decoder such as, for example, the optical switch 40 and a bank of fixed decoders 38 of FIG. 6 (similar to encoders but introducing delays in reverse order), discriminates responses coming from different branches of the tree network. Every healthy branch in the network contributes an autocorrelation peak. A missing autocorrelation peak indicates the corresponding network branch is broken or exhibits abnormal power loss.
  • the height of the detected autocorrelation peak in the normally working branches indicates the cumulative end-to-end loss of the fiber link, including that attributed to the fiber, connectors, splitters, splices, fiber bending, etc.
  • the present technique provides the network manager the currently missing information about the loss specifically incurred by the fiber link.
  • cyclic communication alone between the OLT and ONTs is not sufficient to provide the network manager with information specific to the fiber link.
  • the receiver of the monitoring equipment also acquires a cumulative reflections (or impulse responses) coming from all branches, in addition to that of the feeder.
  • impulse responses are discemable by means of the decoder and detection process, i.e., the use of orthogonal codes results in the individual impulse responses being overlaid orthogonally.
  • the correlation process is made using the reflected powers and not the Rayleigh backscattered powers.
  • the dynamic range requirement is very much alleviated, as the backscattered power is about 40 dB lower than that reflected.
  • the present system allows the CO Manager to have real-time, full information about all network elements. It does not rely on customer help or calls to diagnosis the network state. The manager will no longer be confused between fiber and ONT faults. Recall that when a fiber fault occurs the carrier is responsible; the proposed technique allows troubleshooting without involving the customer. An ONT fault depends in most cases on the customer himself and is not considered a service interruption. Without complete and in-service live PON monitoring, an outside intervention by technicians is necessary in either case: fiber and ONT faults.
  • the encoders of the present invention are preferably passive components that can be placed outside the customer premises (i.e., home); the ONT however could be located inside the customer home, i.e., it is under his control, (sometimes his property), and is his total responsibility.
  • the encoder delimits the service provider control (or ownership) and responsibility from that of the customer.
  • Fig. 17 shows a WDM-PON network architecture based on WDM multiplexer (upstream) and demultiplexer (downstream).
  • WDM-PON every customer is served by a dedicated wavelength; this allows fixed high bandwidth delivery to all users.
  • Previously proposed PON management uses half of the available wavelengths for data and the other half for monitoring, i.e., every customer is assigned two wavelengths, one for data and another for monitoring.
  • FIG. 17 there is also shown an architectural solution that allows the technique of the present invention to manage this WDM-PONs with much fewer wavelength resources.
  • a WS is placed before the 1 :K WDM demultiplexer input in order to isolate the monitoring U band from the data bands.
  • a 1 :K PS splits the monitoring signal into K copies that are coupled again with data fibers at the demultiplexer K outputs. Upstream and downstream data is assumed here to share the same fiber; other variants could be similarly derived.
  • any other bi-directional bypass assembly coupling the monitoring signal between the input and output branches on either sides of each WDM demultiplexer in the network could equally be used, as one skilled in the art will readily understand.
  • the present technique eliminates all the monitoring wavelengths and makes them available for data, hence doubling the network capacity. Only a single monitoring wavelength is split and distributed to all clients.
  • FIG. 18 shows typical TDM/WDM-PON network architecture where a separate TDM system is built over every WDM wavelength. This takes advantages from TDMA and WDM technology in order to dramatically increase the number of users served although per client bandwidth is reduced. Similar to the WDM/PON case, a bi-directional bypass assembly, such as (1 +K) wavelength selectors and a 1 :K passive splitter to create an alternate path for the OCDM monitoring signal. It should be noted that this method does not control 100% of the fiber paths since the monitoring signal avoids going through the de multiplexers. The architecture, however, controls most of the sensitive segments of these PONs.
  • FIG. 19 shows another implementation possibility of the present OC-OTDR technology in the case of a multipath optical waveguide composed of a WDM passive ring network.
  • An all-optical node architecture is shown with egress distribution fibers and ports.
  • FIG 20 shows another implementation possibility of the present OC-OTDR technology in the case of a multipath optical waveguide composed of a WDM passive ring network.
  • a hybrid electronic/optic node architecture is shown with egress distribution fibers and ports.
  • FIG 21 shows eight alternatives of implementation embodiments of the coding function, all for time-dependent coding waveforms.
  • Eight designs are proposed for the modified OCM, all appropriate for the disclosed application, in FIG. 21 B to 211. All exploit splitters and delay lines to fragment the incident pulse into sub-pulses, disperse them, gather them back to the fiber and return them to the CO.
  • Each of these encoder designs has its own advantages and drawbacks.
  • the first, FIG. 21 B is the straightforward modification of the standard one of FIG. 21A, and entails 2 ⁇ 1 :2 q splitters and one circulator.
  • the coming pulse enters to the first splitter exhibits first splitting into 2 q sub-pulses, each of which travels a separate fiber delay line with different length.
  • the number of delay lines is equal to the number of ones in the optical code; their lengths correspond to the relative delays between the ones of the code. Since every branch termination is connected to an encoder with a specific code, the delay line lengths change from one encoder to another corresponding to the positions of ones.
  • the second implementation in FIG. 21 C requires one 1 :2 q+1 splitter and eliminates the circulator.
  • the delay lines should be selected in the same way as in FIG. 21 B. A number of 2 q delay lines are used in this embodiment, each delay line is connected to a separate pair of ports of the splitter.
  • FIG. 21 D requires only one 1 :2 q splitter.
  • the ends of the splitter branches are used as Fresnel reflectors.
  • the length of the branches is adjusted corresponding to the half of the delays between the ones in the code.
  • FIG. 21 E is an implementation using a multiple Bragg grating (MBG), i.e., a series of discrete gratings at the same wavelength, but with different reflection intensities and physical locations.
  • the number of gratings in the series is equal to the number of ones in the optical code.
  • the relative physical position between the gratings corresponds to the time delay between ones in the optical code.
  • the encoders may include a resonant cavity dividing the monitoring signal into the sub-pulses, a path length within this resonant cavity being different for each of the encoders.
  • the designs of Fig. 21 F and 21 G are based on this principle. From an input pulse, they generate a series of periodic pulses with a period equal to twice the length of the cavity. In FIG. 21 F the cavity is made by two gratings separated by a delay line with a length equal to T-,/2. The reflectivity of both gratings could be easily calculated in order to have the highest power and the most power-equalized pulses in the series. In this design, we consider only the first few pulses as a part of the code and neglect the remaining low power pulses.
  • Prime-Periodic codes It consists in a family of zero/one sequences where the ones are periodically positioned inside long series of zeros with a period equal to prime number (3,5,7, 11... ) .
  • prime codes For weight (number of Ones) equal to 4, these prime codes could be written as: (codei corresponding to prime number 3: 1001001001, code2 corresponding to prime number 5: 1000010000100001 , code 3 corresponding to prime number 7: 1000000100000010000001 , and so on).
  • FIG. 21 H is a way to make a cavity using passive splitter and delay line.
  • the designer optimize the selection of splitting ratio in the splitter in order to generate the periodic signal with the highest power and more equalized pulses, at least the beginning of the pulses train.
  • the length of the delay line fixes the spacing between the ones in the sequence hence differentiating between different codes.
  • FIG. 21 H and 211 combine two cavities with respective lengths Tiand T 2 .
  • we combine two cavities with different lengths we have more degrees of freedom in determining codes.
  • Designing a system according to the present invention preferably takes into consideration the three following parameters: 1 ) high coding capacity (the number of different codes should be a minimum of 128 for standard GPON with FEC); 2) the modified DS-encoders to be located at network terminations should be low cost and low component count; and 3) the power loss between the incident pulse and reflected coded signal fed back to the fiber should be minimized in order to reduce the required dynamic range of the monitoring system.
  • the encoder in FIG. 21 B is the most expensive because of the circulator (with 1.5 dB loss per pass) and exhibits the highest power loss.
  • the encoder in FIG. 21 D is less expensive, however exhibits very high loss due to the reflection coefficient ⁇ , typically about 4% (13 dB). Increasing ⁇ to 50%, using special mirrors at the end of the fiber, will make this setting attractive.
  • the encoder in FIG. 21 C is less expensive than that in FIG. 21 E because of the Bragg gratings compared to PSs, however MBGs has less insertion loss.
  • Tab. 1 we outline the component count and derive the power loss equations of each of the proposed DS-OCDM encoders. The coding loss illustrated is Tab.
  • the coding loss is the ratio between the sum of powers of all the returned pulses and the power of the incident pulse.
  • the coding loss is a relevant factor in the system analysis since this directly affects the correlation and detection performance.
  • the MBG could be inserted directly in the data fiber without wavelength selector (WS) because it operates out of band. Note however, that this MBG could not be written with high reflectivity; only 40-60 % reflectivity was previously reported.
  • the encoders of FIG. 21 C and FIG 21 E look very competitive in terms of cost and reflectivity.
  • ⁇ be a collection of binary n-tuplets from the specification (n, w, ⁇ a , ⁇ c ) where n is the code length, w is the Hamming weight, ⁇ ⁇ and ⁇ c are constraints defined as
  • the code weight w i.e., the number of logical ones per code
  • the code weight w is equal to the number of delay lines in the encoding devices of FIG. 21 B and 21 D, and the number of Bragg gratings in the MBG encoder of FIG. 21 E.
  • OOCs could never be made truly orthogonal, especially in an asynchronous case.
  • the autocorrelation will generally be perturbed by interference coming from other branches. It is known that the performance of most OCDMA systems is limited by interference. However, in this application, the interference problem is much lower than that in traditional OCDMA system because: (i) the repetition rate of the incident monitoring pulse can easily be reduced, avoiding interference between reflections resulting from sequential pulses; (ii) the distance between customers will affect the interference statistics since only close customers will cause appreciable interference.
  • the correlation process of the present invention uses the reflected power instead of the Rayleigh backscattered power, thus alleviating the dynamic range requirement compared to the standard OTDR system, by about 40 dB.
  • the existence of an encoder, acting as a mirror at a fiber end increases the reflected power, thus additionally alleviating the required dynamic range.
  • the decoder introduces an additional loss, but this remains quite limited compared to the gain we achieve in the dynamic range.
  • the importance of the loss/power budget in the present system should be considered in a monitoring (or OTDR) context rather than in data communication context.
  • OTDR and monitoring researchers usually use special photodetectors with extremely high photosensitivity and large dynamic rage.
  • Commercial OTDRs currently work with received power that ranges from -90 to -100 dBm and supports a dynamic range of about 12O dB.
  • FIG. 22B presents codes that could support TDM/WDM-PON of FIG. 2OB. In our analysis here we assumed the optical pulses sent from the monitoring unit were normalized in power and duration. More extensive analysis would include the detector sensitivity, the absolute pulse power and duration. Applications with arbitrary dimension codes
  • FIG. 23 shows a two dimensional (time and wavelength) OCM coding embodiment using a series of Bragg gratings.
  • Other known methods could be used to implement two dimensional coding, especially those previously proposed for optical fast frequency hopping CDMA (or two-dimensional OCDMA) etc. See for example Fathallah, H., L. A. Rusch and S. LaRochelle, "Fast Frequency Hopping
  • FIG. 24 shows one wavelength-only dimension OCM coding embodiments using series of pass-band Bragg gratings.
  • Other known methods could be used to implement wavelength-dimensional coding, especially those previously proposed for frequency encoded OCDMA (or spectral amplitude coding SAC-OCDMA) etc.
  • the OTDR receiver observes the power that is the sum of Rayleigh backscattered and reflected light powers from all individual branches. As the number of branches in the network increases, the OTDR trace complexity increases proportionally. The network manager still has no efficient tools to recognize which event corresponds to which connector, splice, fusion or branch end.
  • the management task is even more difficult when an observed peak event corresponds to simultaneous or close-in-time events in the network.
  • Our OCDM encoders will introduce codes inside the reflected powers from every branch end, allowing the CO receiver to decode reflections from each branch separately. Identifying the code in a reflected train of pulses is the equivalent of identifying a specific branch.
  • Passive splitters with a high number of ports induce very high loss (15 dB, 21 dB and 27 dB), rapidly calling for much higher dynamic range in future OTDRs. It was previously recommended to use of a reflective element at every fiber end to increase the amount of power returning to the OTDR.
  • the OTDR receiver measures the reflected power to detect the end of a branch instead of the Rayleigh backscattering power that is typically tens of dBs lower (about -40 dB).
  • the OCDM encoder already performs this reflection role, in addition to introducing signatures inside the reflected power. Note also that the system and method herein consider the reflected powers only, in contrast to the standard OTDR that measures the reflected powers and the Rayleigh backscattered powers as well. This greatly alleviates the loss/power budget constraints of the system. Limits of the ONT intelligence
  • the ONTs are intelligent enough to cyclically communicate with the optical line terminal (OLT) at the CO. Status cells are exchanged almost every 100 ms to inform the network manager about the quality of the received signal at the ONT. When the quality of this signal degrades, the ONT information is not sufficient for the network manager to localize the origin of the degradation.
  • ONT optical line terminal
  • the quality of the signal delivered to the customer depends on three different independent elements: (i) the transmission quality at the central office, (ii) the fiber link quality and (iii) the receiver quality at the ONT (including detection and all signal processing steps). Constant communication between the OLT and the ONT is essential for the network manager to estimate the total quality of the transmission link and many other aspects. When the transmission quality degrades, it is not possible to localize the impairment, nor to determine if the impairment is due to (i), (ii) or (iii).
  • the network manager needs information about every segment of the network. Information about the OLT transmission quality is already available to the network manager through integrated tapped detection at the OLT laser. Information about the total quality of the system is available through direct communication between the OLT and the ONT. The missing information is that of the fiber link between the OLT and the ONT. Recall that the focus of this proposed solution, is to provide a technique for the network manager to obtain this missing information. Our technique allows the network manager to identify the faulty branch and to in- service monitor the total loss exhibited by any working fiber.
  • FTTH service provisioning Fixing the RN to a customer's distance: During FTTH installation, some of the OTDR suppliers recommend that no customers be connected to the remote node (RN) with equal length fibers. This recommendation is made to ensure that every network branch will contribute to a discrete, distinguishable event in the OTDR trace. Note that events coming from equidistant customers overlap in the OTDR trace and confuse the network manager. This installation criterion reduces the chance that equidistant events overlap in an OTDR trace, but it cannot guarantee that other reflection sources such as connectors, splices, fusions etc. will not cause ambiguity in the OTDR trace. Note that this recommendation is not a part of the FTTH standard.
  • Unpredictable customer demand Customer demand for FTTH services is unpredictable, thus the network ramification and distances to clients is unpredictable.
  • the previous fiber length recommendation increases the complexity and the cost of provisioning new customers. This problem becomes even more evident when the number of branches in a TDM-PON increases. This increases the number of discrete and overlapping events in the CO OTDR trace, dramatically aggravating the task of the network manager.
  • Unused RN Ports In typical PON roll-out, some unused capacity remains at the splitters, resulting in unused ports, as illustrated in Fig 1.
  • the unconnected ports in the RN simultaneously reflect back a power and contribute to a high peak event that is an accumulation of many overlapping, indiscernible events.
  • the OCDM encoders could be installed at the RN output ports, preventing the CO manager to be confused by the events occurring from the unused ports of the RN.
  • Fiber cut identification When a fiber cut occurs at a distance that coincides or is close to an event of another branch, the CO manager will observe no event in the OTDR trace. For an isolated fiber break, a new peak event will be observed, but no indication that helps to determine the faulty branch. Extensive prior knowledge of all elements of the network could help the manager guess the nature of the event; however, this is an inefficient control of the network. Some OTDR suppliers propose that the network manager wait until receiving a call from the disconnected customer! If the number of customers claiming service interruption increases, the manager will understand that the fault occurred in the feeder instead of the distribution segment. This is intuitive, but provides no systematic solution to the PON management issues.
  • the monitoring equipment when a fiber branch is cut, the monitoring equipment will miss the encoded signal that is specific to the cut branch. Note that when one customer calls for an interruption, the manager cannot determine if the problem is coming from the fiber or from the ONT itself. In that case, the OTDR suppliers recommend that a technician go to the customer premise and start testing by checking the ONT first. We will see in the end of this section how the OCDM passive encoding device delimits the service provider responsibility from that of the customer. In effect, the service provider has direct control and responsibility of the OLT and the fiber network, but the ONI is almost always under the customer control.
  • Quality of Service The service provider cannot rely on customer calls to assure its quality of service. It is crucial for any telecom system to be able to guarantee a level of service that is defined by the number of interruption per year and the mean duration for these interruptions. Efficient monitoring requires automated fiber fault location and preventive in-service and live network measurements. In addition, customers will request Service Level Agreements with service providers to ensure an acceptable quality of service. Loopback information, including an estimate of the total loss, received end power, the ONT state, etc. is necessary to ensure adequate management of the network, etc. Our encoding system could be used to have an accurate estimate of the total loss exhibited through the network for every termination.
  • the demarcation point is the location in a network where control (or sometimes ownership), and thus responsibility, switches from the service provider to the customer.
  • the carrier maintenance responsibility starts from the central office and stops at these so called demarcation points.
  • the demarcation point is adjacent to the customer premises, in the front of the ONT connector.
  • the demarcation device could be installed outside the customer premise, thus accessible to the technicians without the customer presence.
  • the customer controls or sometimes owns), hence takes care of the ONT that is installed inside his premises or home (see Fig 16.).
  • the manager cannot determine if this is caused by a fiber cut that is his maintenance responsibility, or if simply the ONT is disconnected or turned OFF, which is the customer responsibility.
  • PONs need a device, preferably passive, to be installed at every ONT, outside the premises, and that identifies exactly the demarcation point for that branch.
  • OCDM encoder fulfills this need and serves as the demarcation point device.

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Abstract

L'invention concerne un procédé et un système de détection de défauts dans des trajets lumineux multiples d'un réseau optique. Un signal lumineux de surveillance pulsé est transmis dans le réseau et réfléchi par des codeurs dans chaque trajet lumineux. Les codeurs codent le signal lumineux conformément à des fonctions de codage à faible intercorrélation. Le signal lumineux de retour provenant de tous les trajets lumineux est décodé, soit optiquement soit électroniquement, conformément à des fonctions de décodage corrélées à chacune des fonctions de codage, et analysé pour déterminer si un défaut est présent dans un quelconque trajet lumineux donné. Conformément à certains modes de réalisation, la position du défaut le long du trajet lumineux peut également être déterminée.
PCT/CA2008/000571 2007-03-26 2008-03-26 Procédé et système de détection dans de défauts dans un réseau optique à trajet multiple WO2008116309A1 (fr)

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WO2013066605A1 (fr) * 2011-11-01 2013-05-10 Plexxi Inc. Hiérarchie de commande dans un réseau de centres de données
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US8842988B2 (en) 2011-06-20 2014-09-23 Plexxi Inc. Optical junction nodes for use in data center networks
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US9288555B2 (en) 2011-11-01 2016-03-15 Plexxi Inc. Data center network architecture
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EP4224747A4 (fr) * 2020-09-30 2024-04-03 NEC Corporation Système de communication et procédé de commande du système de communication
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EP3386210A1 (fr) * 2010-06-30 2018-10-10 ECI Telecom Ltd Technologie pour l'affectation de défauts dans des réseaux optiques passifs (pon)
EP2611047A1 (fr) * 2010-08-25 2013-07-03 ZTE Corporation Procédé et système de détection d'une défaillance d'une fibre sur un réseau optique passif
EP2611047A4 (fr) * 2010-08-25 2014-05-28 Zte Corp Procédé et système de détection d'une défaillance d'une fibre sur un réseau optique passif
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US9301026B2 (en) 2011-11-01 2016-03-29 Plexxi Inc. Affinity modeling in a data center network
US9337931B2 (en) 2011-11-01 2016-05-10 Plexxi Inc. Control and provisioning in a data center network with at least one central controller
US9942623B2 (en) 2011-11-01 2018-04-10 Plexxi Inc. Data center network architecture
US9204207B2 (en) 2011-11-01 2015-12-01 Plexxi Inc. Hierarchy of control in a data center network
WO2013066605A1 (fr) * 2011-11-01 2013-05-10 Plexxi Inc. Hiérarchie de commande dans un réseau de centres de données
US9876572B2 (en) 2011-11-01 2018-01-23 Plexxi Inc. Configuring a computer network to satisfy multicast dispersion and latency requirements using affinity and network topologies
US9887777B2 (en) 2011-11-01 2018-02-06 Plexxi Inc. Affinity modeling in a data center network
US9450815B2 (en) 2013-07-11 2016-09-20 Plexxi Inc. Network node connection configuration
US10063426B2 (en) 2013-07-11 2018-08-28 Plexxi Inc. Network node connection configuration
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CN109560866A (zh) * 2018-12-28 2019-04-02 东南大学 基于可调谐激光器的无源光网络链路监测系统和方法
CN109560866B (zh) * 2018-12-28 2024-01-19 东南大学 基于可调谐激光器的无源光网络链路监测系统和方法
JP7557556B2 (ja) 2020-06-30 2024-09-27 華為技術有限公司 スプリッタ、光分配ネットワーク、および光フィルタ構造に対応する波長を決定する方法
EP4224747A4 (fr) * 2020-09-30 2024-04-03 NEC Corporation Système de communication et procédé de commande du système de communication
CN113489535A (zh) * 2021-08-25 2021-10-08 交大材料科技(江苏)研究院有限公司 光纤故障检测模块与方法
CN115967445A (zh) * 2022-06-29 2023-04-14 中兴通讯股份有限公司 光网络拓扑的生成方法、设备和系统
CN115967445B (zh) * 2022-06-29 2024-03-19 中兴通讯股份有限公司 光网络拓扑的生成方法、设备和系统

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