An overview of optical measurements in FTTX networks

Dr Andrew McGrath Director – Business DevelopmentSterlite Technologies

Mike HarropBusiness Development ManagerEXFO

Contents

• Optical fibre in the Access Network

• What are we measuring & why?Network Loss BudgetInsertion lossReturn lossSplice loss

• When do we test?

• How do we test? Insertion lossReturn lossUni & bi-directional splice loss

Bend Insensitive SMF for the Access Network

• Millions of km of ITU-T G.652 installed

• ITU-T G.657 for decreased bending loss in Access Networks

G.657.A category fully compliant with G.652

G.657.B not necessarily compliant & intended for use in or near buildings

• Minimum specified bend radius at 1550nm & 1625nm

G.657.A1 = 10 mm

G.657.A2.B2 = 7.5 mm

G.657.B3 = 5 mm

Access Network Applications

G.652.D G.657.A1

G.652.D G.657.A1 G.657.A2

G.657.B3

G.652.D G.657.A1 G.657.A2

What & Why do we measure?

Network Loss 

Budget

Network Design

Insertion loss (IL)

Optical Return Loss (ORL)

Optical Attenuation

What & Why do we measure?

Verify & record the network build & loss budget

• Insertion Loss (IL) is optical signal power loss (dB) due to the insertion of an optical component in the network or connection between optical fibres.

The lower the value the lower the signal loss e.g. 0.1dB

Intrinsic loss – optical cable loss

Extrinsic loss – connectors, splices, components

• Optical Return Loss (ORL) measures the portion of light that is reflected back to the source from a connection (dB).

The higher the ORL the lower the reflection

E.g. Connector requires small insertion loss & high optical return loss

0

0,2

0,4

0,6

0,8

1

1,2

1000 1100 1200 1300 1400 1500 1600

(dB

/km

)

WAVELENGTH (nm)

1310nm 1490nm 1550nm

Intrinsic Optical Fibre loss – Spectral Attenuation

Bi‐directional Splice Loss

• OTDR measures Rayleigh backscattered light

• Mode field (MFD) differences can cause ‘gain’ or ‘loss’ at a joint

• When possible measure from both directions and average

Splice loss & MFD

OTDR trace from large to smaller MFD: ‘gain’ 

OTDR trace from smaller to larger MFD: exaggerated ‘loss’

Splice

Use average value to calculate actual loss

When do we test?

• Network construction• Acceptance testing• Service activation• Network troubleshooting

• How much testing? Testing philosophy?- Measure everything vs. measure minimum- Build & design quality- Cost!- Testing & service activation time- Measurement uncertainty- Contractual requirement & warranty issues

Point to Multipoint

Point to Point 

FTTH Architectures

Connector InspectionInsertion loss & Optical Return LossOTDR (Bidirectional)

Connector InspectionInsertion loss & Optical Return LossOTDR (Unidirectional)

Cleaning Connectors

• Cleaning is one of the most neglected, but important, operations• Large number of network problems are related to connectors• Permanent damage can occur on dirty connectors with a high power signal• All connectors MUST be clean before they are inserted.• This also applies to the patch-cord used by the tester.

Dirty connectorClean connector Permanently damaged

Insertion Loss & Optical Return Loss Testing

• Check loss & ORL are within budget also check connectivity• Different practices adopted, whole link, up to FDH or drop

Once first customer live can only be tested after the splitter.

OTDR Testing downstream

OLT

3m

30m350m

500m

Backscatter from multiple sources

OTDR Testing Upstream

•Testing upstream allows the links to be fully characterized•Once PON live

•Need to test ‘out of band’at 1625 or 1650nm

Real life uni-directional OTDR expectations

• Network operators- Define a maximum allowable OTDR one way loss for possible MFD mismatch- G.657.A fibres fully compatible with G.652

• Knowledge of where the splice is in the network essential- Risk of ignoring bad splice or trying to re-make splice unnecessarily.

Summary

• Bend insensitive fibres developed for access networks

• Testing used to verify & record the network build & loss budget

• However, need to decide what is measured & why: cost, time, requirements etc

• Insertion loss, Optical Return loss & OTDR typically tested

• PON uni-directional OTDR measurements may impact acceptance specs

FTTH Testing Overview

http://www.ftthcouncil.eu/EN/home/form-handbook#

Thank you for your attention!

www.ftthcouncil.eu