Advanced Fibre Testing: Paving the Way for High-Speed Networks · 2017. 10. 26. · Advanced Fibre...

Trevor NordApplication SpecialistJDSU (UK) Ltd

Advanced Fibre Testing:Paving the Way for High-Speed Networks

Singlemode Optical Fibre

Fibre Review

© 2011 JDS Uniphase Corporation | JDSU CONFIDENTIAL AND PROPRIETARY INFORMATION

Elements of Loss

Fibre Attenuation

- Caused by scattering & absorption of light as it travels through the fibre

- Measured as function of wavelength (dB/km)

Bending Losses

- Microbending losses are due to microscopic fibre deformations in the core-cladding interface

- Macrobending losses are due to physical bends in the fibre that are large in relation to fibre diameter


Light scattered

Impurities

Absorption and Scattering

� Absorption: Light is absorbed due to chemical properties or natural impurities in the glass. Accounts for about 5% of total loss.

� Scattering is the loss of a light signal from the fiber core caused by impurities or changes in the index of refraction of the fiber. Accounts for about 95% of total loss.

Pure Glass=Si O2

Si

SiO O O

Si

SiO

Si

CuO

Imperfections


Useful optical phenomena

OTDR depends on two types of phenomena:- Rayleigh scattering - Fresnel reflections.

Rayleigh scattering and backscattering effect in a fibre

Light reflection phenomenon = Fresnel reflection


Optical Return Loss (ORL)

- The ORL is the amount of transmitted light reflected back to the source

- The ORL is measured in dB and is a positive value.

- The higher the number, the smaller the reflection. Examples:– +50dB is a good value– +17dB is a bad value!!

SC - PC

SC - APC

Angled connectors (APC) reduce the back-reflection

- Increase transmitter noise– Reducing the OSNR in analog video

transmission– Increasing the BER in digital transmission

systems- Increase light source interference

– Changes central wavelength and output power

- Higher incidence of transmitter damage


Chromatic Dispersion

In telecom transmission, a pulse of light is composed by multiple wavelengths (colors) which travel at different speed, causing chromatic dispersion

λ1

λ2

λ3

λ4

λ5 Chromatic

Dispersion

In DWDM transmission, each wavelength travels at a different speed. This can have an impact on number of channels being carried, the channel spacing and the total link length

Pulse Spreading


Effect - Transmission Speed and Distance Limitations

Data Rate Max. Distance for G.652 at 1550nm

Max. Distance for G.655 at 1550nm

2.5 Gbit/s 980km 2770km

10 Gbit/s 61km 175km

40 Gbit/s 3.8km 10.8km

Higher Data Rate

0 11 0 1 0 1 0 11 0 1 0 1

Digital pulses overlapping

1 0 1 1 0 1

Low Data RateInput pulses Output pulses

The variation of index according to the wavelength broadens the pulse and causes it to interfere with neighboring pulses

CD starts being a problem at 10 Gbit/s especially for G652 fibers and is critical at 40Gbit/s


Fibre Types and Compensation

Examples of Chromatic Dispersion values at 1550nm:- G.652 : ~ 17 ps/(nm.km) - G.653 : 0 ps/(nm.km) - G.655 : ~4 ps/(nm.km)

2010

20

-20

-10

0

1200 14001300 1500 λλλλ (nm)

Dis

pers

ion

(ps/

nm.k

m)

1300 nm 1550 nm

λλλλ (nm)

DCF

SMF + DCF

Example of a dispersion compensation result on a SMF fiber

As CD is a linear parameter, intrinsic to the fibre, it can be compensated. There are different techniques which enable compensating CD (such as Dispersion Compensating Fibre with negative CD values)


Chromatic Dispersion – Fibre Classification

ITU-T Fibre Type

description Zero wavelength Dispersion at 1550nm

Dispersion slope @1550nm

G.652 Non Dispersion Shifted fibre

1310 – 1324 nm 17 ps/nm.km 0.057 ps/nm².km

G.653 Dispersion Shifted fibre

1500 – 1600 nm 0 ps/nm.km 0.07 ps/nm².km

G.655 Non zero Dispersion Shifted

fibre

Not specified but ~1450-1480 nm

4 ps/nm.km 0.045 to 0.1 ps/nm².km

Negative Non zero dispersion shifted

fibre

-5ps/nm.km 0.05 to 0.12 ps/nm².km

G.656 Broadband NZDSF 8 ps/nm.km


Polarization Mode Dispersion

- Although we refer to fibres as singlemode, in reality the light energy is carried along in different modes known as polarization modes

- PMD is caused because the fibre is birefringent (Silica, the base material of optical fibres is birefringent):

– Two principal polarization modes travel at different speed

Fast axis

Slow axis

Optical fiber

Elliptical fiber designCore stress

Claddingeccentricity

Fiber twist Fiber stress Fiber bend

PMD depends on mechanical stress- Bends and Twists

PMD is caused by geometrical irregularitiesof the fiber- Elliptical and Assymetries


Birefringent Crystal



V1

V2

V1

V2

DGD

DGD

v1

v2

Fast

Slow

Stress !!! ( bending, twisting)Temperature



- DGD is a randomly varying quantity. It varies significantly with wavelength and times (changes may be introduced by vibration, movement or changes in temperature…)- DGD varies so much that a parameter has been defined: the PMD coefficient (average value of DGD along a range of conditions)

� PMD is directly linked to bit interval error

1 1 0 1

1 1 0? 1

� PMD puts limits on the transmission speed of the networks� PMD puts limits on transmission distance of the networks

For a PMD value of 0.5ps/√km, max distance:- 6400 km @2.5Gbit/s- 400km @10Gbit/s - 25km @40Gbit/s


Limiting Fibre Parameters

The standards recommend that the maximum admissible PMD delay is 10% of the bit length.

STM-4 622 Mb/s 1600 ps 160 ps

STM-16 2.50 Gb/s 400 ps 40 ps

STM-64 10 Gb/s 100 ps 10 ps

STM-256 40 Gb/s 25 ps 2.5 ps

SONET Transmission Rate Bit Time PMD Limit

PMD sensitivity

- If the PMD measurement is bad, then the fibre is considered as PMD sensitive. Simple transmission shall apply to this fibre, as no compensation exists so far

- If the PMD measurement is good, then the fibre cannot be considered as “PMD insensitive” or “non-PMD sensitive”


DGD variance over time and wavelength


Coping with PMD

� OFC 2007 – survey data showed that about 25% of UK fibre had PMD greater than 0.5ps/√km

� Testing and record keeping– If you do not know the size of the issue, it should be measured

� Agile Optical Network - Dynamic Channel Allocation– If a specific wavelength is suffering because of excessive DGD then it

may be possible to move the traffic to a different channel with better DGD performance or even to a different route

� Polarisation scrambling and FEC– Covers all channels– Scrambling reduces the impact of the DGD to reduce outages– Remaining errors “fixed” by FEC

� Optical PMD Compensators– Use polarisation controllers with a control/feedback loop


Why measure the Spectral Attenuation Profile of a f ibre?

� The purpose of the spectral attenuation (AP) measurement is to represent the attenuation as a function of the wavelength.

� Historically, this measurement was required mainly for long-haul applications. With the increase of CWDM deployment and the extension of the DWDM wavelength range, it is becoming necessary to have a clear picture of the fiber attenuation other the wavelengths intended to be loaded with traffic.


Characterising the full wavelength range

� In long distance transmissions, as well as at very high bit rate (10G, 40G systems), Raman amplifications are becoming more widely used across the whole spectrum.

� In addition, distal pumping of Erbium amplifiers at 1480 nm are currently deployed.

� Characterising fibre at the pump wavelengths (1420, 1450 nm, 1480 nm, etc.) is of high interest to ensure amplification will occur along the required distance.

Understanding Fibre Link and Network Characterisation

Characterising Fibre Plant


What is Fibre Characterisation?

� Simply the process of testing optical fibres to ensure that they are suitable for the type of transmission (i.e. WDM, SDH, OTN, Ethernet) for which they will be used.

� The type of transmission will dictate the measurement standards used

Transmission type

Speed PMD Max CD Max

SDH 10 Gbs 10 ps 1176ps/nm

Ethernet 10 Gbs 5 ps 738 ps/nm

SDH 40 Gbs 2.5 ps 64 ps/nm


When should I consider testing ?

Std CD PMD

- New cable Installs (10G+) √ √ √

- Planning / Upgrading existing plant √ √ √

- Following purchase or lease of existing plant √ √

- Periodic tests over time √ √

- After cable maintenance √ √

- Determine if dispersion compensation required √ √


Link & Network Characterisation

� Link Characterisation– It measures the fibre performance

and the quality of any interconnections

– The suite of tests mostly depend on the user’s methods and procedures

– It could be uni- or bi-directional– Tests – Connector Inspection, IL,

ORL, OTDR, PMD, CD, AP

� Network Characterisation– It provides the network baseline

measurements before turning the transmission system up.

– Network Characterisation includes measurements through the optical amplifiers, dispersion compensators, and any elements in line.

– It is a limited suite of tests as compared to Link Characterization

Point BPoint A

CWDM/DWDM Optical Network

Optical Amp.Video Headend

DWDM Optical Networ

k

ROADM

Optical AmplifierRouter

� Connector inspection� Insertion Loss� Optical Return Loss� OTDR� Polarization Mode Dispersion� Chromatic dispersion� Spectral Attenuation

Testing the Fibre Plant@ On@ Charge


Contamination and Signal Performance

Fibre Contamination and Its Effect on Signal Perfor manceCLEAN CONNECTION

Back Reflection = -67.5 dBTotal Loss = 0.250 dB

1

DIRTY CONNECTION

Back Reflection = -32.5 dBTotal Loss = 4.87 dB

3

Clean Connection vs. Dirty ConnectionThis OTDR trace illustrates a significant decrease in signal performance when dirty connectors are mated.


Measuring Insertion Loss (IL)

� The insertion loss measurement over a complete link requires a calibrated source and a power meter.

� This is a unidirectional measurement, however could be performed bi-directionally for operation purposes

Calibrated Light Source

dBm WMenu Ca

nc

el

dB

>2s

Perm

Optical Power Meter

d B mW d B

Pt Pr

It is the difference between the transmitted power and the received power at each end of the link


OCWR Method

Measuring Optical Return Loss (ORL)

� The 2 predominant test methods are available:– Optical Continuous Wave Reflectometry (OCWR)

• An Optical Return Loss Meter (composed of a laser source and a power meter on the same test port)

– Optical Time Domain Reflectometry (OTDR)• The OTDR is able to measure not only the total ORL of the link but also

section ORL (cursor A – B)

OTDR Method


OCWR vs OTDR

Accuracy (typ.) ±±±± 0.5dB

Typical Application

- Total link ORL & isolated event reflectance measurements during fiber installation & commissioning

Strengths - Accuracy- Fast & real time info- Simple & easy results (direct value)

Weaknesses - No localization

Pro

cess

Con

trol

ler

Dis

play

Coupler

Photodetector

Pulsed Light Source

Optical Continuous Wave Reflectometer

Optical Time Domain Reflectometer

Termination Plug

Pro

cess

Con

trol

ler

Dis

play

CW Stabilized Light Source

Power Meter

Coupler

Accuracy (typ.) ±±±± 2dB

Typical Application

- Perfect tool for troubleshooting-Spatial characterization of reflective events & estimation of the partial & total ORL

Strengths - Locate reflective events- Single-end measurement

Weaknesses - Accuracy- Long acquisition time


Measuring Loss, ORL and Distance with an OTDR

It’s the single most important tester used in the installation, maintenance & troubleshooting of fibre plant

Most versatile of Fibre Test Tools�Identifies events & impairments(splices, bends, connectors, breaks)� Provides physical distance to each event/ impairment� Measures fibre attenuation loss of each event or impairment� Provides reflectance / return lossvalues for each reflective event or impairment� Manages the data collected and supports data reporting.


Bi-Directional OTDR Analysis

Bi-directional OTDR Analysis � True splice loss measurement (gives the

average loss which eliminates for example the effect of fibre mismatch).

� Reveals events that are hidden by dead zones in one direction.


Wavelength Tunable Filter

ReferenceFilter

Fibre under test

� Reference test method defined by IEC 60793-1-42 and ITU-T G650.1

� The modulated light is sent over the Fibre Under Test. The phase of the test signal is compared to the phase of the reference signal used to modulate the input signal.

� The measured value is the group delay corresponding to a wavelength interval. It is calculated using an approximation formula. The chromatic dispersion is then calculated by taking the derivative of the group delay with respect to wavelength.

λn

Ref Detection

Measurement Signal Detection

λ ref

λn

Phase Shift ∆φ∆φ∆φ∆φ

Phase detection & comparison

Mod

ulat

ed

Bro

adba

nd s

ourc

e

λ ref (1550nm)λ1260-1640nm

@ On@ Charge

Measuring CD


� A polarized light is sent over the Fibre Under Test and the transmitted spectrum is analyzed with an Optical Spectrum Analyzer

� A reference power level is taken without the polarizer in the optical path

� The scan is then repeated with the polarizer inserted. There will be some fluctuations in the received optical power. The system calculates the ratio of the 2 power levels scans.

� It’s possible to shift to the time domain the analysis of the fixed-analyzer response by taking the Fourier transform of the power fluctuations with wavelength.

OSAPolarizerFUTBroadband

SourcePolarizer

@ On@ Charge

Measuring PMD


Measuring Spectral Attenuation

1300 1400 1500 1600700 900 1100

Reference

After Fibre Loss

Spectrum Comparison

- Shows the spectrum of the fibre (e.g. see if the water peak is present)

- Provides the total loss at each wavelength (equivalent to a light source and power meter)

Spectrum Analyzer

FUTBroadbandSource

Isolator

Spectrum AnalyzerBroadbandSource

Isolator

Water Peak


Fibre Characterisation Results

Advanced Fibre Testing: Paving the Way for High-Speed Networks · 2017. 10. 26. · Advanced Fibre...

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