Fibre Optic Basics - fia-online.co.uk · section of the fibre, is called the “Mode Field”or...

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Fibre Optic Basics

FIA Summer Seminar 2014Andrew Cole

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Theory Basics – Characteristics of Light

What is Light?•An electromagnetic wave, which like other waves, has a frequency (f), velocity (v)and a wavelength (λ). •The velocity v, the speed of light is represented by c.

c = f λ.


Lasers and EyeSafety


Lasers and Eye Safety• Visible Light Wavelength: 400nm to 700nm• Typical Telecom Wavelength: 850nm to 1650nm• Light used for communications is not visible light,

but it will harm your eye

Unless you are 100% sure that the light is switched off

Never look into the end of an optical fibre


Unless you are 100% sure that the light is switched offNever look into the end of an optical fibre


Lasers and Eye Safety


Video Inspection Probe / Indirect Viewing Aid

Lasers and Eye Safety


Unless you are 100% sure that the light is switched off

Never look into the end of an optical fibre


Units of Measure

Frequency: THz (x1012 Hz or x106 GHz)(1550nm is approx. 292THz)

Wavelength: um or nm (x10-6 m/s) nm (x10-9 m/s)

Optical Power: dBm

Attenuation: dB

Pulse Width: us or ns (x10-6) or ns (x10-9)



The Speed of Light

c = 299,792,458 m/s(3 x 108 m/s Approximately)

in a vacuum



Index of Refraction (IOR), n (in fibre)

n = c (c = speed of light in a vacuum)

v (v = speed of light in the fibre)

IOR is specified by the fibre manufacturer

But, light travels more slowly in any other material



d = c tn

Where:d = distancec = the speed of lightt =durationn = the Index of Refraction (IOR)

The distance that light travels through an optical fibre, in a given time can be calculated



Buffer / Coating900 microns

Cladding (glass)125 microns

Core (glass)8-10 microns SM62.5 microns MM

• In a single mode fibre, not all the light energy is fully contained in the core of the fibre. Some of the light energy travels in the cladding, close to the core.

• The area of the beam of light, across a section of the fibre, is called the “Mode Field” or “Effective Area” and its diameter the “Mode Field Diameter”.

• The Core acts as a waveguide.

Fibre Type Mode Field Diameter @ 1550nm MFD mid-range Effective Area (nominal)

Dispersion compensating fibre 6.0-6.2um 6.1um 30um2

G.653 7.8-8.5um 8.15um 52um2

G.655 TW-RS 7.8-9.0um 8.4um 55um2

G.655 LEAF 9.2-10.0um 9.6um 72um2

G.652 9.7-10.7um 10.2um 83um2

G.654.B 9.5-13.0um 11.25um 100um2



How doesan OTDR measure?


BackscatterParticle

Light pulse hits dopant particle Weaker after backscattering

OTDR Basics – Rayleigh Scattering

Rayleigh Scattering


Fibre Core

Light Pulse

Reflection

OTDR Basics – Time of Flight


d = t c2 n

Light Pulse

Reflection

Where:d = distancec = speed of light in a vacuumt = elapsed timen = Index of Refraction

OTDR Basics – Time of Flight


OTDR Basics – OTDR Parameter Settings


OTDR BasicsPulse Width and Dynamic Range

Too short(short deadzones but

insufficient DR)

Too long (good DR but large

deadzones)

Just right (good DR, short

deadzones)


If the OTDR stops measuring here...…this data won’t be acquired!

Range must be at least 25% greater than the fiber under test.

OTDR Basics – Range Setting


OTDR Basics – OTDR Parameter Settings


OTDR Basics – OTDR Trace


Launch

Reflective Event

Non-ReflectiveEvent

End/Fault

NoiseDistance

Ret

urn

Sign

al L

evel

OTDR Basics – Typical OTDR Trace


Fusionsplice

Event Deadzone Backscatter Deadzone

OTDR Basics - Deadzones


Reflectance

OTDR Basics – Reflectance


OTDR Basics – Loss & Reflectance

P1P3 P2

• Insertion Loss P1 – P2 (expressed in dB)

• Reflectance P3 – P1 (expressed in negative dB, i.e. -40dB)

• Return Loss P1 – P3 (expressed in positive dB, i.e. +40dB)


ORL

Backreflectionsfrom RayleighScattering

FresnelReflections

TransmittedLight

OTDR Basics – Optical Return Loss(ORL)


Fibre OpticCleanliness


Always clean before you connect!!

Fibre Optic Basics - fia-online.co.uk · section of the fibre, is called the “Mode Field”or...

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