PASSIVE OPTICAL NETWORK OPTICAL TIME-DOMAIN REFLECTOMETRY

Abstract

Optical Time-Domain Reflectometer (OTDR) troubleshooting of a passive optical network (PON) can be enhanced by deploying cascaded splitters, at least some of which have multiple inputs. That is, at least some of the splitters in the PON have not only a first input coupleable to the optical line terminator (OLT) or output of another splitter but also a second input directly coupleable to an Optical Time-Domain Reflectometer (OTDR). Optical time-delay reflectometry can be performed upon a selected portion or segment of the PON by selecting a splitter and transmitting an optical test signal from the OTDR directly to the input of the selected splitter and analyzing the reflected signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a network diagram of a conventional (prior art) arrangement for troubleshooting a passive optical network (PON) using an Optical Time-Domain Reflectometer (OTDR).

FIG. 2 is a network block diagram of an arrangement for troubleshooting a PON using an Optical Time-Domain Reflectometer (OTDR)-based system in accordance with an exemplary embodiment of the present invention.

FIG. 3 is a flow diagram illustrating a method for troubleshooting a PON using the arrangement illustrated in FIG. 2.

DETAILED DESCRIPTION

As illustrated in FIG. 2, in an exemplary embodiment of the invention, an Optical Time-Domain Reflectometer (OTDR) system 20 has a plurality of connections to a passive optical network (PON). Although in the illustrated embodiment OTDR system 20 provides a plurality of connections that can be selectably operated (i.e., selected and operated independently of one another), in other embodiments a splitter or other suitable passive or active device (not shown) can be used to multiplex or switch a single OTDR output onto a plurality of fibers, including a manually operated device such as a switch or patch panel. The PON includes an optical line terminator (OLT) 22, a first-level (2:8) splitter 24, eight second-level (2:8) splitters 26, and 64 optical network terminals (ONTs) 28. Note that there is a cascade arrangement between first-level splitter 24 and each of second-level splitters 26. The number of ONTs 28 and splitters 24 and 26 and their arrangement in this embodiment is for purposes of illustration only, and other embodiments can have any other suitable number of ONTs and, accordingly, any suitable number, size (i.e., 2:N) and arrangement of splitters. Also, the term “splitter” is intended to encompass within its scope not only a unitary passive optical device that splits or distributes signals over two or more outputs but also any suitable system of two or more devices that performs an equivalent function.

More specifically, in the exemplary embodiment first-level splitter 24 has an input (which can be referred to as a PON input for convenience) coupled to OLT 22, another input (which can be referred to as an OTDR input for convenience) coupled to OTDR system 20, and a plurality of outputs, while each of second-level splitters 26 has a PON input coupled to an output of first-level splitter 24, an OTDR input coupled to OTDR system 20, and a plurality of outputs. Each ONT 28 is coupled to one of the outputs of one of splitters 26. As noted above, in other embodiments of the invention the splitters can be arranged in other suitable ways to service more or fewer ONTs, and in such an embodiment an ONT can be coupled to an output of a first-level splitter, second-level splitter, or any other level splitter. As no more than two levels are needed to illustrate the invention, an embodiment having only two levels is described for purposes of convenience.

As illustrated in FIG. 3, in accordance with an exemplary method of operation, in which optical time-domain reflectometry (“OTDR method” ) is used to troubleshoot a problem that the PON (FIG. 2) is believed to be experiencing, at step 30 one of splitters 24 or 26 is selected. Although in the exemplary embodiment of the invention OTDR system 20 can by used to select a single splitter 24 or 26, in other embodiments any other means can be used to couple an OTDR output to a selected splitter, such as a multiplexer or switching device (not shown). In some embodiments, a person can manually connect the OTDR output to a splitter input, e.g., through a patch panel, individual fiber plug-and-jack connectors or similar means, resulting in its selection.

At step 32, OTDR system 20 transmits an optical test signal to the input of the selected splitter 24 or 26 to which it is connected. Note that, as described above, an output of OTDR system 20 is directly connected (i.e., without any intervening splitters or other lossy elements) to an input of each splitter 24 and 26. As the other input is connected to either an output of splitter 24 or OLT 22, it is preferred that OTDR system 20 transmit a wavelength that does not interfere with the wavelengths on which OLT 22 operates. By operating OTDR system 20 and OLT 22 on non-interfering wavelengths, OTDR system 20 can be used to troubleshoot the network or portions thereof without interfering with normal network communication between OLT 22 and ONTs 28.

At step 34, OTDR system 20 is used to analyze the reflected optical signals in response to the transmitted optical test signal. The manner in which an OTDR is used to transmit a signal on an optical network and analyze the reflected signals to test whether a fault exists or otherwise evaluate the optical performance of the network is well-understood in the art and therefore not described herein. The term “fault” is used herein to refer to any optical abnormality, such as a broken or damaged fiber, faulty connector, splice loss, etc.

Note that the signal loss is minimized when a second-level splitter 26 is selected at the above-described step 30, thereby bypassing first-level splitter 24. For example, if the fiber contributes five decibels (dB) of loss, and each splitter 24 and 26 contributes 10 dB, by selecting a second-level splitter 26, the total one-way loss is only 15 dB, and the total two-way loss is only 30 dB. The OTDR method can be performed on the entire PON by performing it separately on each of its constituent portions. The signal loss experienced when any one of those portions is selected will not be so great as to exceed the dynamic range of OTDR system 20.

The method can be performed iteratively on different branches or portions of the PON to facilitate fault isolation, as indicated by step 36. For example, the method can first be performed by selecting a first one of splitters 26 at step 30. If the method indicates that there is no fault on the PON branch serviced by that splitter 26, the method can be performed again by selecting a second one of splitters 26 at step 30, and so on, until a fault is detected on one of the branches. In networks having more levels than the two shown in FIG. 2, once a fault is detected on one of the branches, the method can be continued by selecting splitters that service sub-branches. Of course, if a PON branch is known to be experiencing a problem (e.g., the network operator receives reports from subscribers experiencing problems in a residential neighborhood or office building or other localized area serviced by a portion of the PON), the method can begin with that branch.

Note that in the exemplary embodiment of the invention the OTDR method can be performed while the PON is in normal use because OTDR system 20 and OLT 22 operate at mutually non-interfering wavelengths and because each of splitters 24 and 26 has two inputs. Nevertheless, in other embodiments some or all of splitters 24 and 26 can have only one input, and a person can (e.g., manually) disconnect the splitter input from the PON and couple it to OTDR system 20 to perform the OTDR method, even if the OTDR and OLT operate at the same wavelength. For example, in an embodiment of the invention in which the selected splitter 24 were to have only a single input, a person could disconnect it from OLT 22, connect it to OTDR system 20, perform the OTDR method, disconnect it from OTDR system 20, and re-connect it to OLT 22.

It will be apparent to those skilled in the art that various modifications and variations can be made to this invention without departing from the spirit or scope of the invention. For example, although the specification above refers to various standards, such as GPON, the invention encompasses variations and alternatives that will occur readily to persons skilled in the art in view of the teachings herein. Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of any claims and their equivalents. With regard to the claims, no claim is intended to invoke the sixth paragraph of 35 U.S.C. Section 112 unless it includes the term “means for” followed by a participle.

Claims

1. A system for performing optical time-delay reflectometry in a passive optical network (PON), comprising: an optical line terminator (OLT);an Optical Time-Delay Reflectometer (OTDR);a plurality of splitters including a first splitter having a plurality of outputs and an input coupleable to the OLT, and a second splitter having a plurality of outputs, an input coupled to an output of the first splitter, and an input coupleable to the OTDR; anda plurality of optical network terminals (ONTs), each ONT coupled to one of the outputs of one of the splitters.

2. The system claimed in claim 1, wherein the OTDR and OLT operate at mutually non-interfering optical wavelengths.

3. A method for performing optical time-delay reflectometry in a passive optical network (PON) comprising an optical line terminator (OLT), a plurality of splitters including a second splitter having an input coupled to an output of a first splitter, and a plurality of optical network terminals (ONTs) coupled to outputs of the splitters, the method comprising: selecting one of the splitters of the plurality of splitters;an Optical Time-Domain Reflectometer (OTDR) transmitting an optical test signal directly to an input of the selected splitter; andthe OTDR analyzing reflected optical signals in response to the transmitted optical test signal.

4. The method claimed in claim 3, wherein the selecting step comprises selecting a splitter on a faulty branch of the PON.

5. The method claimed in claim 4, further comprising iteratively repeating the method until a fault is isolated to a sub-branch of the PON, wherein the selecting step in further iterations comprises selecting a splitter on a sub-branch of a faulty branch of the PON.

6. The method claimed in claim 4, wherein the transmitting step comprises transmitting an optical test signal having a wavelength not interfering with a wavelength on which the OLT operates.

7. A system for performing optical time-delay reflectometry in a passive optical network (PON) having a plurality of selectable, cascaded splitters, comprising: means for coupling an optical line terminator (OLT) to the PON;means for coupling an Optical Time-Delay Reflectometer (OTDR) directly to an input of a selected splitter of the plurality of cascaded splitters; anda plurality of optical network terminals (ONTs), each ONT coupled to an output of one of the splitters.