Special Feature

Optical Fiber Network Operation Technologies
for Expanding Optical Access Network Services
Masahito Arii, Yuji Azuma, Yoshitaka Enomoto†,
Katsuaki Suzuki, Noriyuki Araki, Shigenori Uruno,and Tsunekazu Watanabe
Abstract
This article introduces NTT’s R&D trends in optical distribution management and identification technologies and in optical fiber line testing technologies designed to reduce operating costs. Reducing optical fiber network operating costs related to service demand, cable removal, and maintenance work isessential if we are to achieve NTT’s target of 30 million optical fiber access customers by 2010.

1. BackgroundIn addition to reducing optical fiber network construction costs, we must also reduce operating costs,
especially those related to service demand, cableremoval, and maintenance work (Fig. 1), if we are toachieve 30 million customers by 2010.
Service demand work, which involves deploying
optical services to customers’ houses, can be divided
into operations performed in the central office andthose performed outside. In the central office, workers connect transmission equipment to a distribution

frame that terminates outside optical cables. Outside
work involves connecting a drop fiber from an aerial
closure to the customer’s house and installing optical
network units (ONUs). Once this work has been completed, tests must be performed to ensure that it has
been done correctly.
The cable removal work needed to change a cable’sroute, as a result of road construction for example,
also involves various operations. First, the new cable
is installed. Then, workers identify the active routefiber and the new route fiber. Next, they cut the active
route fiber and reconnect it to the new route fiber toConstruction work

Service demand work

Cable removal work

(1) Optical fiber distribution incentral office(2) Outside optical fiber installation(drop cable, inside customer’shouse)(3) ONU installation(4) Testing(1) Installation of new route cable(2) Identification of active and newroute fibers(3) Transfer of active route to newroute(4) Testing

Maintenance work
(1) Fault isolation and location(2) Repair(3) Periodic test
(preventive maintenance)

Fig. 1. Optical fiber network operation work.
† NTT Access Network Service Systems LaboratoriesTsukuba-shi, 305-0805 Japan
Email: enomoto@ansl.ntt.co.jp

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Central office
Optical cable for central office(before installing IDM)
Customers’ houses

OLT

FTM
ID tags (post-attached)
IDM-B
IDM-A
ID tags
ID tags
Optical cables for central officeOutside optical cable

Fig. 2. Configuration of optical fiber distribution in a central office.

transfer the active route to the new route. Then, they
perform tests to determine whether or not the connection has been completed successfully.
Maintenance work involves performing routine
maintenance and responding to breakdowns andcomplaints. When a customer reports a fault, it mustbe isolated quickly. The fault’s location must bedetermined and the fault must be promptly repaired
and the repairs must be tested. Periodic testing as
preventive maintenance allows repairs to be madebefore a fault occurs.
To reduce the cost of all the operations needed in an
optical fiber network, we must make components easier to use and must improve the efficiency of the management and identification of the central office andoutside optical distribution facilities and optical fiberline testing. In this article, we introduce the trends ofNTT’s research and development related to optical
distribution management and identification technologies and optical fiber line testing technologies.
2. Optical distribution management andidentification technologies
2.1 Optical fiber distribution technologies in acentral office
The configuration used for optical fiber distribution
in a central office is shown in Fig. 2. Optical fiber distribution facilities include transmission equipment
such as optical line terminals (OLTs) that provide IP(Internet protocol) services, distribution frames thatterminate outside optical cables, and optical cables
for connecting OLTs to distribution frames. At present, the distribution frames are being changed overfrom fiber termination modules (FTMs) to integrated
Vol. 5 No. 2 Feb. 2007

ID tags

Optical splitters

Optical testing Size: 1600 × 600 × 1800 mm3module
No. of fibers accommodated: 4000

Fig. 3. Integrated distribution module.

distribution modules (IDMs). A single IDM (Fig. 3)
can accommodate 4000 optical fibers because wedeveloped compact connectors and thinner opticalcables. It can also contain optical splitters for passive
optical network (PON) systems and optical testing
equipment (described later).
Distribution management is based on the use of IDtags, as shown in Fig. 4. An ID tag is attached to theeach end of each optical cable connected to the IDMs
and information about the fibers in the cable is stored
in a database. This information can be accessed when
the tag is read using an ID code scanner. NTT has
chosen to use QR Codes*, which are a two-dimensional bar codes, for these ID tags. We have also
developed ID tags that can be attached to older opti* QR Code is a registered trademark of Denso Wave Incorporated.

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ID code scanner
5 mm

Todatabase(a) Post-attached ID tag

Control terminal

QR code5.6 mmAttachment tool

5.6 mm

ID tag(b) Tag attachment process in an FTM

Fig. 4. Distribution management using ID tags.

Fig. 5. Post-attached ID tag.

Central office
Optical coupler

Optical fiber
ID testerCommunication light

ONU

OLT
Identification light (1650 nm)
Identification
light source

Communicationlight

Fiber ribbon
Bending part

Bend

Identification light
Detector

Optical fiber ID testerFiber bending partFig. 6. Optical fiber identification technologies using an optical fiber ID tester.

cal cables connected to an FTM that were deployed
before the installation of the IDMs [1]. This postattached ID tag and the tag attachment process areshown in Fig. 5. This retrofitting makes it possible tostore information about every optical cable in thedatabase, which makes it easy to manage all the central office distribution facilities.
2.2 Optical fiber identification techniques
To reduce the risk of cutting or disconnecting thewrong optical fiber during optical fiber operations
inside and outside the central office, a worker canidentify optical fibers non-destructively using an
optical fiber ID tester (Fig. 6). He or she selects alikely optical fiber and places it in the optical fiber ID
tester. An identification light, which has a longer
wavelength than the communication light, islaunched into the target optical fiber from the central
office. The tester bends the optical fiber and detects34

the light that consequently leaks from it. If the tester
detects the identification light, then the worker knowsthat the target fiber has been correctly selected. Otherwise, he or she can repeat the process to find the target fiber. This process does not interfere with thecommunication light, so in-service testing without
affecting data transmission is possible. By optimizing
the shape of the fiber bending structure and increasing the sensitivity of the detector, we have enabled
identification to be performed even for long-wavelength-band optical communications and for opticalfiber cords whose thick cover makes it difficult todetect leaking light [2].
3. Optical fiber line testing technology
3.1 Optical fiber line testing system
Optical fiber lines must be tested to assess whether
service demand, cable removal, and maintenance
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work has been performed correctly. Our optical fiberline testing system performs various types of optical
test remotely, so it can reduce the operation time andcost. Its functions are shown in Table 1. In addition totesting for construction work, this system can perform various tests for service demand, cable removal,
and maintenance work such as fault isolation, fault
location, and periodic tests. The system configurationis shown in Fig. 7. An operator in a maintenance center sends orders via a data network to the optical testing module (OTM) telling it to perform various tests.
The OTM retrieves optical fiber information from a
database server, measures the target optical fiber, andreturns the test results to the control terminal.
The OTM, which is installed in an IDM, contains
various types of optical measurement equipment suchas an optical time domain reflectometer (OTDR), alight source to provide the identification light, and apower meter. An optical switch called a fiber selector
selects the target fiber and an optical coupler introTable 1. Functions of the optical fiber line testing system.
Construction(service demandand cableremoval work)

MaintenanceFiber identification

OTDR
test

Connection loss and reflection
value

Loss
test

Optical cable span loss

Fault
test

Fault isolation and location

Periodic
test

Detection of water penetration
into closure

Insertion of identification light

DatabaseserverCentralofficeduces the test light into the target fiber. An optical filter installed in front of the ONU at a customer’s house
cuts and reflects the test light from the OTM whileallowing the communication light from the OLT topass. Therefore, this system can carry out automatic
measurements, obtain fiber characteristics, and determine the nature and location of faults without affecting the transmission quality.
3.2 Water penetration detection technology
If water penetrates into an underground closure
over time, it can increase the optical loss and degrade
the mechanical strength. For this reason, water sensor
modules are attached to an optical fiber for maintenance use in each underground closure. The watersensor module and the detection mechanism areshown in Fig. 8. If water penetrates the undergroundclosure, an absorbent material in the sensor module
expands and applies a bending force to the opticalfiber. This induces bending loss, which can be detected by an OTDR. Moreover, the OTDR can determine
the distance at which the bending loss is occurring, so
it is possible to identify which closure has the problem. Therefore, water penetration in each underground closure can be monitored by periodic tests
with the optical fiber line testing system.
3.3 Standardization of wavelengths formaintenance use
The wavelength bands defined by ITU-T (International Telecommunication Union, Telecommunication Standardization Sector) are shown in Fig. 9.

IDM
Customer’shouse

Jumper unit Opticalcoupler
OLT
OLT

ONU
FS

Datanetwork

Optical filter
ONU

OTM
ClosureTest
OK
Controlterminal

Optical fiber cable

Maintenancecenter

Water sensor module
FS: fiber selector

Fig. 7. Configuration of the optical fiber line testing system.

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ITU-T Recommendation L.41 calls for the 1650-nm
window to be either vacant or used for maintenancerather than to be actively used. NTT has chosen to use
the 1650-nm window for maintenance purposes inaccordance with this recommendation.

Bending loss

Distance

3.4 Advanced measurement technique
When optical splitters are installed in optical closures and cabinets near a customer’s house, it is difficult to isolate a fiber fault in a branched optical fiberusing an OTDR because the reflection from one optical filter hides the reflection from another optical filter. We have developed a fiber fault isolation technique using a high-resolution OTDR (H-OTDR), as
shown in Fig. 10. For this technique, we narrowed thepulse width of the OTDR and broadened the bandwidth of the OTDR receiver circuit. As a result, theCentral
officeOTDR
ClosureExpansion

Optical fiber
NormalWater penetration

Fig. 8. Water sensor module.

O-band1260–1360 nm

E-band1360–1460 nm

1300

1400

S-band1460–
1530 nm
1500

C-band L-band U-band
1530–
1565–
1625–
1565 nm 1625 nm 1675 nm
1550
1600 1650 (nm)(a) Definition of wavelength bands in ITU-T G. Supplement 391310-nm window
Case 11550-nm window

1625-nm window

1650-nm window

Vacant or maintenance Vacant or maintenance Vacant or maintenance

ActiveCase 2 Vacant or maintenance

ActiveVacant or maintenance Vacant or maintenanceCase 3

ActiveActiveVacant or maintenance Vacant or maintenanceCase 4

Active or vacant

ActiveVacant or maintenance

Active(b) Assignment of wavelengths for maintenances in ITU-T Recommendation L.41

Fig. 9. Standardization of wavelength bands.
#8
Central officeIDM

8-branch opticalsplitter

#7

Fault

#6
#5
#4
#3
#2
#1

OLT
FS
OTM
H-OTDR

Wider bandwidth(10 ns)

Reflector(optical filter)

40

Narrow pulse width(20 → 10 ns)
Highresolution Light source
Receiver
OTDR

ONU

38dB

363m
Initial343230

FaultDistanceFig. 10. Fault isolation technique using a high-resolution OTDR.

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Facilityinformation

Test results

Facility information
database server-Facility information-Test results
Test order
Mobile phonewith camera-Search for information-Test order

QR CodeTest results
IDM/FTM
OTM

Field

Central office

Fig. 11. Facility management and testing system using mobile phones.

reflection from each optical filter can be isolated
when the difference in path lengths is more than 3 m.
Using the H-OTDR, we can isolate a fiber fault bycomparing the initial reflection level with measured
reflection levels. This technique is also applicable toservice demand and cable removal work.

phone containing a camera and forward that code to adatabase. This makes it easy to store and retrieve optical facility information in the field and perform tests.
We are also investigating facility management andtesting technologies that combine RFID tags andadvanced optical measurement techniques.

4. Future plans

References

We are developing a facility management and testing system based on the use of mobile phones (Fig.
11) with the aim of simplifying the management andtesting of outside optical facilities. With this system,a worker in the field can capture the QR Code of anID tag attached to an outside facility using a mobile

Vol. 5 No. 2 Feb. 2007

[1]

[2]

S. Uruno, H. Izumita, and M. Arii, “Optical Wiring Technology toPrevent Congestion of Indoor Optical Wiring,” NTT Technical Journal, Vol. 17, No. 8, pp. 55–57, 2005 (in Japanese).
Y. Aoyagi and H. Yamamoto, “Fiber or Fiber-cord Identification
Technique for Long-wavelength-band Communications Light,” NTT
Technical Review, Vol. 2, No. 6, pp. 67–69, 2004.

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Masahito Arii

Noriyuki Araki

Senior Manager, NTT COMWARE Corporation.
He received the B.E. degree in electrical engineering from Waseda University, Tokyo, and theM.E. degree in computer science from Tokyo
Institute of Technology, Tokyo, in 1983 and1985, respectively. He joined NTT in 1985 andengaged in the development of optical accessnetwork systems. The work described in this article was performed when he was in NTT AccessNetwork Service Systems Laboratories. Hemoved to NTT COMWARE in 2006 and is currently responsible for the development of opticalaccess network operation support systems.

Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in electrical and electronic engineering from Sophia
University, Tokyo, in 1993 and 1995, respectively. He joined NTT Access Network System Laboratories (now NTT Access Network Service
Systems Laboratories) in 1995. Since then, hehas been engaged in R&D of devices for opticalfiber line testing systems. He is a member ofIEICE.

Yuji Azuma

Shigenori Uruno

Senior Research Engineer, Supervisor, NTTAccess Network Service Systems Laboratories.
He received the B.E. degree in electrical engineering from Doshisha University, Kyoto, in
1984. He joined the Ibaraki Electrical Communication Laboratories of Nippon Telegraph andTelephone Public Corporation (now NTT) in1984. He has researched the characteristics ofhigh-density optical fiber cables, evaluated thereliability of optical devices, and developed optical fiber maintenance equipment. Recently, hehas been working on an optical fiber distribution
system for central offices. He is a member of theInstitute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Research Engineer, NTT Access Network Service Systems Laboratories.
He joined the Ibaraki Electrical Communication Laboratories of Nippon Telegraph and Telephone Public Corporation (now NTT) in 1969.
Currently, he is engaged in research on cable
jointing, fluoride fiber transmission, remotely
controlled optical fiber switches, and the development of an optical fiber distribution system forcentral offices. He is a member of IEICE.

Yoshitaka Enomoto

Tsunekazu Watanabe

Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.E. degree in electrical engineering from Meiji University, Tokyo, in 1992.
He joined NTT Telecommunication Field Systems R&D Center (now NTT Access NetworkService Systems Laboratories) in 1992. Since
then, he has been engaged in R&D of an opticalfiber line testing system and an optical fiber distribution system for central offices. Currently, heis engaged in work on a compact optical splitterfor outside plant. He is a member of IEICE.

NTT Access Network Service Systems Laboratories.
He joined NTT in 1996. Since then, he has
been engaged in R&D of an optical fiber linetesting system.

Katsuaki Suzuki
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in systems engineering from Nagoya Institute of Technology, Aichi, in 1993 and 1995, respectively.
He joined NTT in 1995. Since then, he has been
engaged in R&D of an optical fiber ID tester andan optical fiber distribution system for central
offices.

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