© 2010 jdsu. all rights reserved.jdsu confidential & proprietary information1 optical loss...

© 2010 JDSU. All rights reserved. JDSU CONFIDENTIAL & PROPRIETARY INFORMATION1

Optical Loss Budget (Example 2)

1000BASE-ZX GBIC– Ptmax = 5 dBm

– Ptmin = 0 dBm

– Prmax = -3 dBm

– Prmin = -23 dBm

Questions:– Can you connect one GBIC to

another with only a patchcord?

– How can you ensure that the fiber system does not exceed the maximum loss?

Optical Loss Budget:Bmax = Ptmin – Prmin

Bmin = Ptmax -Prmax

Opt

ical

Pow

er L

evel

(dB

m)

Maximum Loss (dB) = 23 dB

Minimum Loss (dB) = 8 dB

Ptmax 5 dBm

Prmax -3 dBm

Ptmin 0 dBm

Prmin -23 dBm


Test!

Basic Tests– Visual Fault Locator (VFL)– Optical Insertion Loss– Optical Power Levels

Advanced Tests– Optical Return Loss (ORL)– Optical Time Domain Reflectometer (OTDR)– Chromatic Dispersion (CD)– Polarization Mode Dispersion (PMD)– Optical Spectral Analysis (OSA)


Visual Fault Locator

VFLs provide a visible red light source useful for identifying fiber locations, detecting faults due to bending or poor connectorization, and to confirming continuity.

VFL sources can be modulated in a number of formats to help identify the correct VFL (where a number of VFL tests may be performed).

FFL-050 FFL-100


Advanced Tests

Optical Return Loss (ORL) Optical Time Domain Reflectometer (OTDR)

– Detect, locate, and measure events at any location on the fiber link

Fiber Characterization– Determines the services that the fiber can be carry– Basic tests plus:

• Chromatic Dispersion (CD)

• Polarization Mode Dispersion (PMD)

Optical Spectrum Analysis (OSA)– Spectral analysis for Wavelength Division Multiplexing (WDM)

systems


Introduction to OTDR

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

T-BERD 4000 FTTx / Access OTDR Most versatile of Fiber Test Tools Detect, locate and measure events at any location on the fiber link Identifies events & impairments (splices, bends, connectors, breaks) Provides physical distance to each event/ impairment Measures fiber attenuation loss of each event or impairment Provides reflectance / return loss values for each reflective event or impairment Manages the data collected and supports data reporting.


Background on Fiber Phenomena

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

Rayleigh scattering and backscattering effect in a fiber

Light reflection phenomenon = Fresnel reflection


How does it work ?

The OTDR injects a short pulse of light into one end of the fiber and analyzes the backscatter and reflected signal coming back

The received signal is then plotted into a backscatter X/Y display in dB vs. distance

Event analysis is then performed in order to populate the table of results.

OTDR Block Diagram Example of an OTDR trace


Dynamic Range & Injection Level

Dynamic Range determines the observable length of the fiber & depends on the OTDR design and settings

Injection level is the power level in which the OTDR injects light into the fiber under test

Poor launch conditions, resulting in low injection levels, are the primary reason for reductions in dynamic range, and therefore

accuracy of the measurements

Effect of pulse width: the bigger the pulse, the more backscatter we receive


What does an OTDR Measure ?

Distance– The OTDR measurement is based on “Time”:

The round trip time travel of each pulse sent down the fiber is measured. Knowing the speed of light in a vacuum and the index of refraction of the fiber glass, distance can then be calculated.

Fiber distance = Speed of light (vacuum) X time 2 x IOR



Attenuation (also called fiber loss)Expressed in dB or dB/km, this represents the loss, or rate of loss between two events along a fiber span



Event LossDifference in optical power level before and after an event, expressed in dB

Fusion Splice or Macrobend

Connector orMechanical Splice


ReflectanceRatio of reflected power to incident power of an event, expressed as a negative dB value

The higher the reflectance, the more light reflected back, the worse the connection

A -50dB reflectance is better than -20dB value


Typical reflectance values

Polished Connector ~ -45dB

Ultra-Polished Connector ~ -55dB

Angled Polished Connector ~ -65dB



Optical Return Loss (ORL)Measure of the amount of light that is reflected back from a feature: forward power to the reflected power. The bigger the number in dBs the less light is being reflected.

The OTDR is able to measure not only the total ORL of the link but also section ORL

Distance (km)

Att

enu

atio

n (

dB

)

ORL of the defined section


Optical Return Loss (ORL)

Light reflected back to the source

PT: Output power of the light source

PAPC: Back-reflected power of APC connector

PPC: Back-reflected power of PC connector

PF: Backscattered power of fiber

PB: Total amount of back-reflected power

ORL (dB) = 10Log > 0)(B

T

P

P

PAPCPPC PAPC PAPC

PT

PF PF PF

Light Source

Photo-diode


Effects of High ORL Values

All laser sources, especially distributed feedback lasers, are sensitive to optical reflection, which causes spectral fluctuation and, subsequently, power jitter. Return loss is a measure of the amount of reflection accruing in an optical system. A -45dB reflection is equivalent to 45dB return loss (ORL). A minimum of 45-50dB return loss is the industry standard for passive components to ensure normal system operation in singlemode fiber systems.

Increase in transmitter noise– Reducing the OSNR in analog video transmission– Increasing the BER in digital transmission systems

Increase in light source interference – Changes central wavelength and output power

Higher incidence of transmitter damage

The angle reduces the back-reflection of the connection.

SC - PC SC - APC

OTDR Events

How to interpret a trace


How to interpret an OTDR Trace


Front End Reflection

Connection between the OTDR and the patchcord or launch cable

Located at the extreme left edge of the trace

Reflectance: Polished Connector ~ -45dB Ultra-Polished Connector ~ -55dB Angled Polished Connector up to ~ -65dB

Insertion Loss: Unable to measure


Dead Zones

Attenuation Dead Zone (ADZ) is the minimum distance after a reflective event that a non-reflective event can be measured (0.5dB)

In this case the two events are more closely spaced than the ADZ, and shown as one event

ADZ can be reduced using shorter pulse widths

Event Dead Zone (EDZ) is the minimum distance where 2 consecutive unsaturated reflective events can be distinguished

In this case the two events are more closely spaced than the EDZ, and shown as one event

EDZ can be reduced using shorter pulse widths


Connector

A connector mechanically mates 2 fibers together and creates a reflective event

Reflectance:

Polished Connector ~ -45dB

Ultra-Polished Connector ~ -55dB

Angled Polished Connector up to ~ -65dB

Insertion Loss: ~ 0.5dB

(loss of ~0.2dB w/ very good connector)


Fusion Splices

A Fusion Splice thermally fuses two fibers together using a splicing machine

Reflectance: None

Insertion Loss: < 0.1dB

A “Gainer” is a splice gain that appears when two fibers of different backscatter coefficients are spliced together (the higher coefficient being downstream)

Reflectance: None

Insertion Loss: Small gain


Fusion Splices

Direction A-B Direction B-A


Macrobend

Macrobending results from physical bending of the fiber.

Bending Losses are higher as wavelength increases.

Therefore to distinguish a bend from a splice, two wavelengths are used (typically 1310 & 1550nm)

Reflectance: None

Insertion Loss: Varies w/ degree of bend & wavelength


Mechanical Splice

A Mechanical Splice mechanically aligns two fibers together using a self-contained assembly.

Reflectance: ~ -35dB

Insertion Loss: ~ 0.5dB


Fiber End or Break

A Fiber End or Break occurs when the fiber terminates.

The end reflection depends on the fiber end cleavage and its environment.

Reflectance: PC open to air ~ -14dB

APC open to air ~ - 35dB

Insertion Loss: High (generally)


Ghosts

A Ghost is an unexpected event resulting from a strong reflection causing “echos” on the trace

When it appears it often occurs after the fiber end.

It is always an exact duplicate distance from the incident reflection.

Reflectance: Lower than echo source

Insertion Loss: None


Typical Attenuation Values

0.2 dB/km for singlemode fiber at 1550 nm 0.35 dB/km for singlemode fiber at 1310 nm 1 dB/km for multimode fiber at 1300 nm 3 dB/km for multimode fiber at 850 nm 0.05 dB for a fusion splice 0.3 dB for a mechanical splice 0.5 dB for a connector pair (FOTP-34) Splitters/monitor points (varys with component)

Best Practices with OTDRs


Performing an OTDR Test

1. Inspect & Clean connector end faces (patch cords & bulkheads (including test instrument)

2. Set up instrument for test environment

3. Test

4. View trace/table of results

5. Store / Report Results

6. Further analysis optional (for advanced users)


Key OTDR Setup Parameters for Manual Operation

Pulse Width– Controls the amount of light injected into the fiber– A short pulse width enables high resolution and short dead zones,

but limited dynamic range– A long pulse width enables high dynamic range but less resolution

and longer dead zones

Short Pulse:• More Resolution• Shorter Dead Zones• Less Dynamic Range• More Noise

5ns

1µs

100ns Long Pulse:• Less Resolution• Wider Dead Zones• More Dynamic Range• Less Noise


5s 30s

20s


Acquisition Time (Averaging)– Length of time the OTDR takes to acquire and average the data

points– Increasing acquisition time improves the dynamic range w/o

affecting the resolution or dead zones.



Index of Refraction (IOR)– The IOR converts time, measured by the OTDR, to

distance, which is displayed on the trace– Entering the appropriate value into the OTDR will

ensure accurate length measurements for the fiber.


How to select the right OTDR Test Module

OTDR modules are primarily specified in terms of dynamic range

Select the optimum test module as follows: 1. Determine the longest span you will be testing w/ this

module

2. Determine the expected link loss budget this will translate to

3. Select the module by subtracting 6 dB from the rated dynamic range of the module (this is the range of the unit to view backscatter signal or measure a splice loss)


Example: Link Loss / OTDR Module selection calculation

Calculation Factors

Link example & calculations

Longest span length 75km

Avg fiber span loss 0.33dB/km @ 1310nm x 75 = 24.75dB

0.20dB/km @ 1550nm x 75 = 15dB

Connector Loss Typically 2 connectors per span

2 x 0.5dB each = 1dB

Splice Loss Typically < 0.1dB per splice w/ 1 splice per 5 km of fiber

75 / 5 = 15 splices x 0.1dB each = 1.5dB

dB adjustment OTDR module DR

Recommend allowing 6 dB for splice loss measurement

1310nm 1550nm

dB dB

24.75 15

+ 1 + 1

+ 1.5 + 1.5

+ 6 + 6

= 33.25 = 23.5Dynamic Range requirement for Module


Tools to Optimize OTDR testing

Launch Cable Using a launch cable allows the characterization of the connector at the origin of the link. This shifts the first connector outside the dead zone of the OTDR connector The last connector can also be measured by using a receive cable

About Launch Cables

Launch cables are typically 100 – 1,000 meters in length.

The length required depends upon the dead zone performance of the OTDR. A minimum 2x the attenuation dead zone length is recommended, although in practice, most are much longer


TB6000/8000 OTDR Distance Chart

Fiber Characterization

Step-by-step review

© 2010 jdsu. all rights reserved.jdsu confidential & proprietary information1 optical loss...

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