U.S. patent application number 15/098806 was filed with the patent office on 2016-11-03 for device, remote node and methods for pon supervision.
The applicant listed for this patent is Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Patryk URBAN, Gemma VALL-LLOSERA.
Application Number | 20160323033 15/098806 |
Document ID | / |
Family ID | 47437277 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160323033 |
Kind Code |
A1 |
VALL-LLOSERA; Gemma ; et
al. |
November 3, 2016 |
DEVICE, REMOTE NODE AND METHODS FOR PON SUPERVISION
Abstract
A remote node in a passive optical network (PON), the remote
node comprises a filter arrangement and a sequential splitter
arrangement, where the filter arrangement is arranged to receive a
feeder signal including data communication content and optical time
domain reflectometry (OTDR) pulses, and where the filter
arrangement is adapted to transmit the data communication content
to the sequential splitter arrangement. Furthermore, the invention
involves a method for determining the location of a fault section
in a drop section.
Inventors: |
VALL-LLOSERA; Gemma;
(Jarfalla, SE) ; URBAN; Patryk; (Vallingby,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget LM Ericsson (publ) |
Stockholm |
|
SE |
|
|
Family ID: |
47437277 |
Appl. No.: |
15/098806 |
Filed: |
April 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14130316 |
Dec 30, 2013 |
9344188 |
|
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PCT/SE2011/050897 |
Jul 1, 2011 |
|
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15098806 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01M 11/3127 20130101;
G01M 11/3136 20130101; H04B 10/0791 20130101; H04J 14/0246
20130101; H04J 14/0282 20130101; H04J 14/0227 20130101; H04B 10/071
20130101; H04J 14/0247 20130101; H04J 14/0242 20130101; H04J
14/0272 20130101; H04L 12/44 20130101; H04J 14/0252 20130101 |
International
Class: |
H04B 10/071 20060101
H04B010/071; H04J 14/02 20060101 H04J014/02; H04B 10/079 20060101
H04B010/079 |
Claims
1. A remote node in a passive optical network (PON), the remote
node comprising: a filter arrangement and a sequential splitter
arrangement, wherein the filter arrangement is arranged to receive
a feeder signal including data communication content and optical
time domain reflectometry (OTDR) pulses, and wherein the filter
arrangement is adapted to transmit the data communication content
to the sequential splitter arrangement.
2. The remote node according to claim 1, wherein the filter
arrangement includes a first filter and secondary filters arranged
to allow passage of a pre-selected wavelength to a pre-selected
drop section, and wherein the secondary filters include
sequentially arranged filters to allow passage of a wavelength
shifted signal of the pre-selected wavelength to the pre-selected
drop section.
3. The remote node according to claim 2, wherein the sequential
splitter arrangement comprises a multi-stage splitter and wherein a
second splitter stage of the multi-stage splitter involves at least
two splitters and is arranged on a downlink end of the remote
node.
4. The remote node according to claim 3, wherein the sequential
splitter arrangement is a de-multiplexer arrangement.
5. The remote node according to claim 2, wherein the pre-selected
wavelength is passed to the pre-selected drop section without using
an active components.
6. The remote node according to claim 1, wherein the filter
arrangement includes a plurality of filters, and each filter
comprises one input link and two output links.
7. The remote node according to claim 1, wherein the sequential
splitter arrangement includes a plurality of splitters, and each
splitter comprises at least one input link and two output
links.
8. The remote node according to claim 1, wherein the feeder signal
is received through a fiber that carries both the data
communication content and the OTDR pulses.
9. The remote node according to claim 1, wherein the feeder signal
is received through a first fiber that carries the data
communication content and a second fiber that carries the OTDR
pulses.
10. The remote node according to claim 9, wherein the sequential
splitter arrangement includes a plurality of splitters, and each
splitter receives the data communication content through a first
link and the OTDR pulses through a second link.
11. The remote node according to claim 1, wherein the remote node
is configured without utilizing a power supply.
12. A method in a remote node in a passive optical network (PON)
for distributing a wavelength shifted OTDR signal to at least one
drop section in a passive optical network (PON), comprising:
receiving a wavelength shifted optical time domain reflectometry
(OTDR) signal having a pre-selected wavelength; and outputting the
wavelength shifted OTDR signal to at least one dedicated drop
section.
13. The method according to claim 12, wherein the outputting the
wavelength shifted signal to the at least one dedicated drop
section comprises: filtering the wavelength shifted OTDR signal
through a filter arrangement, which allows the wavelength shifted
OTDR signal to the at least one dedicated drop section.
14. The method according to claim 12, further comprising: receiving
data communication content; and outputting the data communication
content to the at least one dedicated drop section.
15. The method according to claim 14, wherein the data
communication content is received from an optical line
terminal.
16. The method according to claim 12, wherein the outputting the
wavelength shifted OTDR signal to at least one dedicated drop
section is performed without a power supply in the remote node.
17. The method according to claim 12, wherein the outputting the
wavelength shifted OTDR signal to at least one dedicated drop
section is performed without using an active component.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
14/130,316, filed Dec. 30, 2013, which is hereby incorporated by
reference. This application is a continuation of U.S. application
Ser. No. 14/130,316, which is the National stage of International
Application No. PCT/SE2011/050897, filed Jul. 1, 2011, which are
hereby incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to optical layer monitoring in
a passive optical network (PON).
BACKGROUND
[0003] A passive optical network (PON) is a network architecture
employing fiber cables from a central office to local premises. It
employs passive optical components to enable a single optical
feeder fiber to serve multiple premises. A PON consists of a
central office node, where the optical line terminal (OLT)
equipment is located, one or more termination nodes at customer
premises, called optical network terminations (ONT) or optical
network units (ONU), and), and the infrastructure such as fiber,
splitters, filters, etc. connecting the central office node to the
termination nodes, called the optical distribution network (ODN).
In a passive optical network a single optical fiber, feeder fiber,
guides the light towards the remote node (RN) where it is delivered
to the different drop sections by means of data splitters, arrayed
waveguide gratings (AWGs), filters, or any other passive equipment.
From the RN the light is guided towards the customer premises: ONT
if the unit serves one single home, ONU if the unit serves multiple
homes. On the uplink, the ONT/ONU sends user data back to the OLT
using the same or a different wavelength.
[0004] The primary reason for the PON choice has been its cost
effectiveness because of the efficient use of the fiber and because
of that most equipment outside the central office can be passive
equipment that does not consume power. Compared to active optical
networks (AON) PON can lower both operational expenditure (OPEX)
and capital expenditure (CAPEX). Different forms of PON including
Broadband PON (BPON), Ethernet PON (EPON), Gigabit Ethernet PON
(GEPON), and Gigabit PON (GPON) have been deployed in different
markets. Even though PONs have relatively low OPEX compared to
active solutions, there is still room for the operators to save
significant amount of OPEX using effective preventive maintenance
of the physical infrastructure.
[0005] In today's PON systems, the physical infrastructure is
usually not entirely visible to a network management system (NMS).
A physical failure cannot be detected before creating service
outage in upper layers which may lead to loss in business for the
operators. The aim of preventive maintenance is to detect any kind
of deterioration in the network that can cause suspended services
and to localize these faults.
[0006] Supervision of monitoring of PONs should provide continuous,
remote, automatic, and cost effective supervision of the physical
layer. It should provide rapid and accurate detection of
performance degradation as well as service disruption. The testing
should not affect normal data transmission (non-intrusive testing).
It should distinguish between a failure in the end-users' own
equipment and a failure in the operator's network. It should be
interoperable with many network variants (bit rate, protocol,
etc.).
[0007] A common maintenance tool employed for monitoring or
supervision of PONs is an optical time domain reflectometer and the
technique used is called optical time domain reflectometry (OTDR).
The OTDR injects a series of optical pulses into the fiber under
test. Backscattered (Rayleigh) and back-reflected (Fresnel) light
from points along the fiber is detected and analyzed. The magnitude
of the backscattered signal is dependent on the Rayleigh
backscattering coefficient, attenuation, fiber imperfections and
splices, and optical power level in the fiber. The strength of the
return pulses is measured, integrated as a function of time, and
evaluated as a function of fiber length. The OTDR may be used to
estimate the fiber's length and overall attenuation, including
splice and mated-connector losses. It may also be used to locate
faults, such as breaks, and to measure optical return loss. The
optical dynamic range of an OTDR is limited by a combination of
optical pulse output power, optical pulse width, input sensitivity,
and signal integration time. Higher optical pulse output power, and
better input sensitivity, combine directly to improve measuring
range, and are usually fixed features of a particular
instrument.
[0008] OTDR monitoring technique is commonly used in PON systems.
The same can be outlined for the Raman assisted OTDR, commonly
planned to gain higher resolution in system fault detection.
Off-the-shelf OTDR market equipment uses a fixed wavelength in the
U-band of the ITU-T grid but for future access technologies as WDM
PON, tunable OTDR devices would be required. Tunable OTDR enables
selection of a specific drop section to be monitored without the
need of an optical switch at the RN. However, many operators have
already invested into OTDR equipment in the central office and are
not interested in making further investments to upgrade the OTDR
equipment.
[0009] Monitoring should not influence regular data communication,
i.e. it should be non-invasive. This is achievable by utilization
of a dedicated optical bandwidth for the measuring function.
Further, the technique should be sensitive to relatively low power
fluctuations detectable in on-demand or periodic modes. Still
further, it should not require any high initial investment. This
mainly yields that no additional monitoring functionality on the
customer premises side should be needed and PON monitoring
functionality should be shared over a complete PON system or a
group of PON systems.
[0010] Today's existing solutions for providing supervision or
monitoring do only satisfy some of the above requirements. An
overview of some existing solutions is given in the article K.
Yuksel, V. Moeyaert, M. Wuilpart, and P. Megret "Optical Layer
Monitoring in Passive Optical Netorks (PONS): A Review", ICTON
2008, Tu.B1.1. Most of the solutions existing today significantly
increase capital expenditures because they require either a
customized OTDR device, which is expensive, wavelength specific
components in the fiber links (drop section) towards the ONTs,
which causes power budget reduction, advanced OLT transmitter
upgrades, e.g. light path doubling. Still further, most of today's
existing solutions to provide supervision or monitoring can only
detect a fault in a fiber link which introduces significant loss of
more than 5 dB, far above an expected threshold of 1 dB.
SUMMARY
[0011] It is an object of the present invention to address at least
some of the problems outlined above. In particular, it is an object
of the exemplifying embodiments to provide a wavelength shifter
module and a method therein for adapting an optical time domain
reflectometry, OTDR, signal for supervision of drop sections in a
passive optical network, PON, wherein the wavelength of the OTDR
signal is shifted to a pre-selected wavelength to enable a data
splitter in a remote node to forward the OTDR signal to an
individual drop section in the PON, thereby supervising the fiber
links in the drop section between the remote node and the ONT.
[0012] It is also an object of the present invention to provide a
network node and a method therein for receiving a wavelength
shifted OTDR signal from the wavelength shifter module for
outputting to a dedicated drop section. Furthermore, it is an
object of the invention to provide a system for determining the
location of at least one fault section in a drop section of a
passive optical network (PON) and a method for supervising
individual drop sections in the PON.
[0013] These objects and others may be obtained by providing a
wavelength shifter module and a method in a wavelength shifter
module; a remote node and a method in a remote node; and a system
for determining a location of a fault section in a drop section of
the PON and method in a PON according to the independent claims
attached below.
[0014] According to an aspect of the invention, a wavelength
shifter module configured to wavelength shift an OTDR signal for
supervision of individual drop sections in a PON is provided. The
wavelength adaption module is configured to receive an OTDR signal
comprising at least one wavelength, to filter the received OTDR
signal, and to shift the wavelength of the filtered OTDR signal to
a pre-selected wavelength, preferably in a Raman wavelength
shifter. The module is further configured to filter the wavelength
shifted signal in a tunable filter to output a wavelength shifted
optical signal having the pre-selected wavelength towards a remote
node in the network. The wavelength of the optical signal has been
preselected to enable a data splitter in the remote node to forward
the wavelength shifted OTDR signal to a drop section or a group of
drop sections in the PON, thereby enabling supervision of the fiber
links in the drop sections.
[0015] According to another aspect of the invention, a method in a
wavelength shifter module is provided for adapting an OTDR signal
for supervision of a drop section in a PON. The method comprises
receiving an OTDR signal having at least one wavelength, filtering
the signal in a filter arranged to discriminate optical signals
within at least one pre-selected wavelength interval. The method
further comprises inputting the filtered signal to a wavelength
shifter, wherein the wavelength of the OTDR signal is shifted to a
pre-selected wavelength. Filtering the wavelength shifted signal in
a tunable filter tuned to allow passage of optical signals of
pre-selected wavelengths and inputting the signal from the tunable
filter to a circulator arranged to output the wavelength shifted
signal to a remote node. The wavelength of the optical signal has
been pre-selected to enable a data splitter in the remote node to
forward the wavelength shifted signal to one or a group of drop
sections in a PON.
[0016] According to yet an aspect of the invention a remote node is
provided for a passive optical network, wherein the node comprises
a first filter arrangement and at least one sequential splitter
arrangement. The remote node is adapted to receive a wavelength
shifted OTDR signal, to filter the wavelength shifted signal and to
output a selected wavelength of the wavelength shifted signal to a
drop section in a group of optical network terminals.
[0017] In accordance with a further aspect of the invention, a
method in a remote node is provided for distributing a wavelength
shifted OTDR-signal to at least one drop section in a passive
optical network (PON), including the steps of receiving a
wavelength shifted OTDR-signal having a pre-selected wavelength and
outputting the wavelength shifted signal to at least one dedicated
drop section.
[0018] According to another aspect of the invention, a system for
determining a location of at least one fault section in a drop
section in a passive optical network (PON) is provided. The system
comprises an OTDR equipment in a central office node of the system,
and at least one remote node comprising a data splitter
arrangement. A wavelength shifter module is arranged at the output
of the OTDR equipment, the wavelength shifter module being arranged
to shift the wavelength of a received OTDR signal to a pre-selected
wavelength and to circulate the wavelength shifted signal to a
remote node in the passive optical network.
[0019] According to a further aspect of the invention, a method in
a passive optical network (PON) is provided for determining the
location of at least one fault section in a drop section from a
remote node to an optical network termination (ONT) or an optical
network unit (ONU). The method comprises receiving a fault
indication in a network management plant in the optical line
terminal in a central office from at least a transceiver in an
optical network termination (ONT) or optical network unit (ONU).
The method further includes initiating fault detection by
transmitting an optical time domain reflectometry (OTDR) signal of
at least one wavelength in the downlink. Shifting the wavelength of
said optical OTDR signal to a predetermined wavelength, forwarding
the wavelength shifted signal to a remote node and distributing the
wavelength shifted signal to at least one drop section
corresponding to the received fault indication.
[0020] A significant advantage of the inventive wavelength shifter
module, the method in the wavelength shifter module, the remote
node and the method in the passive optical network is the ability
to monitor individual drop sections with high accuracy and fault
detection sensitivity limited only by the performance of the
applied OTDR. This means that there is usually a high sensitivity
to detect low power fluctuations.
[0021] Another advantage is that the solution offers a possibility
of configuring the network node without components requiring power
supply. Specifically, the remote node can be configured as a
passive node incorporating passive filters and power splitters.
Furthermore, the solution is an all optical solution providing the
benefit of not having to perform optical/electrical/optical
conversions.
[0022] Furthermore, the inventive solution offers the advantage of
being cost effective since the required additional hardware
components are few and on the shelf components. The solution does
not require additional monitoring functionality on the optical
network termination side, i.e., all added functionality may be
included in the central office.
[0023] Yet another advantage is that the OTDR monitoring is
non-invasive and does not influence the regular data
communication.
[0024] The solution is applicable to any known type of passive
optical network, including WDM PONs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The invention may best be understood by referring to the
following description and accompanying drawings that are used to
illustrate embodiments of the invention. In the drawings:
[0026] FIG. 1A is a prior art PON configuration common OTDR
feeder;
[0027] FIG. 1B is a prior art PON configuration dedicated OTDR
feeder;
[0028] FIG. 2A is a basic configuration of a wavelength shifter
module;
[0029] FIG. 2B is a further embodiment of a wavelength shifter
module;
[0030] FIG. 2C is a further embodiment of a wavelength shifter
module;
[0031] FIG. 3A is a PON configuration of a common OTDR feeder, N
ONT branches;
[0032] FIG. 3B is a PON configuration of a dedicated OTDR feeder, N
ONT branches;
[0033] FIG. 3C is a PON configuration of a common OTDR feeder, N
remote nodes;
[0034] FIG. 4A is a block diagram illustrating splitter
arrangements, common feeder, K=1, N=4;
[0035] FIG. 4B is a block diagram illustrating splitter
arrangements, dedicated feeder, K=1, N=4;
[0036] FIG. 4C is a block diagram illustrating splitter
arrangements, common feeder, K=1, N=8;
[0037] FIG. 4D is a block diagram illustrating splitter
arrangements, dedicated feeder, K=1, N=8;
[0038] FIG. 5 is a flowchart illustrating an exemplifying
embodiment of a method in a wavelength shifter module; and
[0039] FIG. 6 is a flowchart illustrating an exemplifying
embodiment of a supervision method in a PON.
DETAILED DESCRIPTION
[0040] FIG. 1 discloses a block diagram of an exemplifying
embodiment of a prior art passive optical network (PON). The PON
comprises a central office (CO) 100 having an optical line
termination (OLT) 101 and an OTDR device 102. The central office
100 exchanges information with optical network terminations (ONT)
120 through an intermediate remote node (RN) 110 including at least
one splitter arrangement 113. The central office 100 is connected
to the RN by a feeder link 130. The ONTs 120 are connected to the
RN 110 via fiber links 140, which are also referred to as drop
sections. In the embodiment illustrated in FIG. 1A a common fiber
130 connects the OLT 101 and the OTDR device 102 to the remote node
110. A filter or switch on the connection side to the remote node,
offers the ability to direct the relevant signals to the remote
node. An alternative configuration is presented in FIG. 1B, wherein
a dedicated feeder fiber 150 connects the OTDR device to the remote
node 110. In order to achieve the ability to direct the OTDR signal
to different drop sections 140 from the remote node to the ONTs, a
switch 112 requiring power supply is included in the remote node
110, thus converting the passive optical infrastructure into an
active structure requiring power supply in the remote node 110.
[0041] FIGS. 3A-C disclose block diagrams of exemplifying
embodiments of passive optical networks including the inventive
wavelength shifter module 200. The wavelength shifter module 200 is
included in the central office 300 on an outgoing side of the OTDR
device 302. The wavelength shifter module 200 is arranged to
receive a signal from the OTDR device 302 and to perform a
wavelength shifting operation on the OTDR signal prior to
outputting a wavelength shifted signal to a remote node 310. With
the wavelength shifter module 200 introduced in the central office
300, it is possible to achieve the benefits of a tunable OTDR
without having to replace OTDR devices 302 already in place in
present networks. The wavelength shifter module 200 tunes the
outgoing OTDR signal to a preselected wavelength within a range of
tunable wavelengths within a Raman gain bandwidth. The selection of
wavelength depends on information from drop sections 340 in the
PON, wherein transceivers in ONTs 320 are configured to signal
malfunction. The OTDR device 302 is activated when information is
received from an ONT transceiver that there is a malfunction in the
corresponding drop section 340.
[0042] A network management plant in the optical terminal device in
the central office 300 receives a malfunction indication from a
transceiver at the downlink of a drop section 340. The optical line
terminal 301 decides one drop section or a group of drop sections
for OTDR monitoring and initiates the OTDR signaling in the OTDR
device 302. The OTDR device 302 launches a short pulse of light
into the fiber. The backscattered light is monitored as a function
of time or distance along the fiber. The OTDR pulse from the
central office 300 is directed to the remote node 310 via a common
feeder fiber 330 (FIG. 3A) or by means of a dedicated fiber 350
(FIG. 3B). If more than one PON tree is to be monitored, the
central office 300 is configured to include a switch 305 (FIG.
3C).
[0043] FIGS. 4A-D disclose different configurations of the
arrangements in the remote node 310. The inventive embodiments are
discussed based on splitter arrangements 311, but it should be
noted that arrangements wherein the splitters are substituted by
wavelength de-multiplexers are foreseen within the scope of the
invention. Arrangements with wavelength de-multiplexers are
particularly useful for passive optical networks working according
to wavelength division multiplexing (WDM) access technology.
[0044] FIG. 4B is a block diagram illustrating an exemplifying
splitter arrangement in a remote node in accordance with the
passive network configuration of FIG. 3B, wherein K=1 and N=4. The
splitter arrangement 312 comprises a first 1.times.2 splitter stage
412 having an input connected to the data communication feeder
fiber. The two outputs from the 1.times.2 splitter are each
connected to an input of a respective second 2.times.2 splitter.
The second input of the respective second 2.times.2 splitter is
connected to an output of a filter 411 which is arranged to receive
the OTDR signal having a pre-selected wavelength on a dedicated
fiber link. The filter is configured to forward the OTDR signal
having the pre-selected wavelength to the splitter 2.times.2 if the
pre-selected wavelength has a second value, illustrated by "Monitor
2". The two 2.times.2 splitters 413 constitute the last splitter
stage in the disclosed embodiment. For the person skilled in the
art, it is obvious that corresponding splitter arrangements are
possible for splitter configurations of a higher order of K and
N.
[0045] However, in order to clearly illustrate the scalability of
the splitter arrangements, FIG. 4D discloses a splitter arrangement
312 wherein K=1 and N=8. A data communication feeder is connected
to a filter, which is configured to forward a received data
information signal from the OLT to a 1.times.4 splitter in a first
splitter stage. The dedicated fiber link for the OTDR signal, here
illustrated as "Monitor", is connected to a first filter configured
to forward the OTDR signal having a pre-selected wavelength to a
first 2.times.2 splitter in case the pre-selected wavelength has a
first value. This is illustrated by "Monitor 1". The first
2.times.2 splitter 434 may be connected to a first group 2 ONTs,
but may also be connected to a further splitter arrangement. If the
pre-selected wavelength does not correspond to the first value,
then the first filter 432-1 is configured to forward the OTDR
signal having the pre-selected wavelength to a second filter 432-2.
The second filter forwards the OTDR signal to a second 2.times.2
splitter in case the pre-selected wavelength has a second value,
illustrated by "Monitor 2". It the preselected wavelength does not
correspond to the second value, the second filter is configured to
forward the OTDR signal having a pre-selected wavelength to a third
filter 432-3. Corresponding steps are applied in the third and
fourth filters. The four 2.times.2 splitters constitute a secondary
splitter stage in the disclosed embodiment.
[0046] It will be clear to the person skilled in the art that the
embodiments illustrated in FIGS. 4A-D are non-limiting to the
invention and that it is possible to scale the splitter stage to
handle as many drop section branches as output wavelengths as the
number of pre-selected wavelengths from the wavelength shifter
module. The significance in the configuration of the remote node
lies in the lack of active components required in the remote node
for distribution of the pre-selected OTDR wavelength to the correct
drop section; thus the remote node may be configured without power
supply.
[0047] FIGS. 2A-C illustrate different embodiments of the inventive
wavelength shifter module 200, wherein FIG. 2a depicts a basic
configuration of such a wavelength shifter module.
[0048] An OTDR signal is received in an incoming wavelength
discriminating filter 210 of the wavelength shifter module 200.
This filter 210 is preferably a red/blue (R/B) filter, but other
types of wavelength discriminating filters are also foreseen within
the scope of the invention and the illustrated embodiment. The
filtered signal having a wavelength within a predetermined
wavelength interval is introduced into a wavelength shifter 220,
preferably a Raman wavelength shifter (RWS). In the wavelength
shifter light the wavelength of the incoming light is tuned to a
selected range of wavelengths. The parameters of the RWS may be set
so that it is possible to obtain several wavelengths at the output
of the Raman wavelength shifter depending on the reflectivity value
of the fiber Bragg grating. Based on stimulated Raman scattering
and using the fact that when the Stokes power becomes large enough
it can act as a pump to the next order Stokes, it is possible to
greatly expand the range of wavelengths possible to use for
monitoring of different drop sections in the network.
[0049] The tunable filter 230 following the Raman wavelength
shifter 220 enables selection of an appropriate generated Stokes
wavelength for monitoring of a specific drop section. Tuning of
this filter 230 is enabled through control signals from the network
management system. A circulator 240 on the output of the wavelength
shifter module 200 is configured such that the wavelength shifted
OTDR signal having the pre-selected wavelength fed to the
circulator will be transmitted towards the remote node. The
circulator will also allow received back-scattered light resulting
from the wavelength shifted OTDR signal to be transmitted towards
the wavelength discriminating filter 210 on the up-link side of the
RWS. The filter will allow passage of the back-scattered light
through the filter and towards the OTDR device 302 so that
evaluation of the back-scattered signal is enabled.
[0050] FIG. 2B discloses a configuration of the wavelength shifter
module 200, wherein a pulse generation arrangement 250 is
introduced following the tunable filter. The pulse generation
arrangement 250 enables reshaping of the optical carrier from the
RWS in the case that the generated Stokes wavelengths do not follow
the main carrier envelope. An example of implementation of the
pulse generation box involves saturating the light (constant output
power) and then amplitude modulating the signal.
[0051] FIG. 2C discloses an embodiment of a wavelength shifter
module 200 including an isolator 260 on the output of the incoming
wavelength discriminating filter 210. The isolator 260 prevents
light-leakage from the RWS in the backward direction.
[0052] Due to the fiber loops in the Raman wavelength shifter and
the Bragg cavities, a time delay will be imposed on the outgoing
signal. This may be corrected with post-processing techniques in
the central office 300.
[0053] It should be noted that FIGS. 2A-C illustrate various
functional units in a wavelength shifter module 200 that may be
implemented using any suitable software and/or hardware
means/circuits. The inventive wavelength shifter module 200 is not
limited to the disclosed embodiments.
[0054] FIG. 5 discloses the inventive method in a wavelength
shifter module 200 wherein the wavelength shifter module 200 in a
first step 510 receives an OTDR signal from an OTDR device 302,
preferably arranged in the central office 300 of the PON. The OTDR
signal is a short pulse of light with at least one wavelength
launched into the fiber optical signal. In the discussed
embodiment, the OTDR signal is of a predetermined, fixed
wavelength. However, the invention need not be limited to fixed
wavelength OTDR signals but could also be applicable to any type of
situation where a wavelength outside the available range of
wavelengths is required.
[0055] The method further involves filtering of the received OTDR
signal in a wavelength discriminating filter, preferably a Red/Blue
(R/B) filter, in a second step 520. The filtered signal is
subjected to wavelength shifting 530, wherein the wavelength
shifting may be achieved through stimulated Raman scattering and
use of the fact that when the Stokes power becomes large enough it
can act as a pump to the next order Stokes. The output Stokes
wavelengths will depend on the Stokes shift in the wavelength
shifter and the characteristics of the fiber Bragg grating. In
order to obtain several wavelengths following the wavelength
shifting, part of the signal from the wavelength shifter will be
subjected to feedback or amplification and part of the signal will
be further processed in subsequent method steps. Such subsequent
method steps involve filtering of the Stokes wavelengths to allow
passage of a pre-selected wavelength in a fourth step 540. The
wavelength is pre-selected to enable a data splitter in a remote
node to forward the wavelength shifted signal to a dedicated group
of ONTs in the PON. A concluding step 550 in the method in the
wavelength shifter module 200 involves outputting the wavelength
shifted signal of a pre-selected wavelength.
[0056] In another embodiment of the invention, the method in the
wavelength shifter module 200 also involves the step 560 of erasing
the envelope of a main carrier in the filtered wavelength shifted
Stokes wavelengths and to remodulate the light signals to conform
to OTDR pulse signaling.
[0057] FIG. 6 discloses the inventive method for determining the
location of at least one fault section in a drop section from a
remote node to an optical network termination (ONT) or an optical
network unit (ONU). A network management plant in the central
office 300 receives a fault indication from a drop section in the
optical distribution network in an initial step 610. The fault
indication may be based on transceiver signaling form the
transceiver in the ONT or ONU, but the inventive method is not
limited to the method of assessing the existence of mal-functioning
drop section. Based on the drop-section to be evaluated, a tunable
filter in the wavelength shifter module 200 may be adjusted in a
following step 630 to allow passage of the pre-selected wavelength
signal. OTDR signaling into the optical distribution network is
initiated so that an OTDR signal is transmitted in a following step
640. The OTDR signal is wavelength shifted in a subsequent step 650
to the pre-selected wavelength prior to injection into the optical
distribution network. The wavelength shifted signal is distributed
to the relevant drop section, preferably by filtering and power
splitting of the wavelength shifted signal. However, in a WDM-PON
embodiment, the power splitting is substituted by wavelength
de-multiplexing. The distance to the fault section in the drop
section is determined from the back-reflected light from the drop
section.
[0058] FIG. 6 discloses a method in a passive optical network for
determining a location of an existing fault section in a drop
section. However, the inventive method may also be used for
supervising different drop sections without the prior knowledge of
the existence of a fault. In such an instance the pre-selection of
the wavelengths from the wavelength shifter will involve possible
OTDR wavelengths corresponding to the number of drop sections to
supervise. By directing OTDR signals of specific wavelengths to
specific drop sections, the existence and location of a fault in a
drop section may be established without prior fault
indications.
[0059] While the invention has been exemplified by means of the
embodiments given above, the invention is not limited to these
specific embodiments but include any alternatives, modifications
and varieties that would fall within the wording of the attached
claims.
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