Post on 07-Oct-2020
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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.
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Theory Basics – Characteristics of Light
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Lasers and EyeSafety
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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
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Unless you are 100% sure that the light is switched offNever look into the end of an optical fibre
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Lasers and Eye Safety
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Video Inspection Probe / Indirect Viewing Aid
Lasers and Eye Safety
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Unless you are 100% sure that the light is switched off
Never look into the end of an optical fibre
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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)
Theory Basics – Characteristics of Light
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The Speed of Light
c = 299,792,458 m/s(3 x 108 m/s Approximately)
in a vacuum
Theory Basics – Characteristics of Light
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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
Theory Basics – Characteristics of Light
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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
Theory Basics – Characteristics of Light
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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
Theory Basics – Characteristics of Light
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How doesan OTDR measure?
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BackscatterParticle
Light pulse hits dopant particle Weaker after backscattering
OTDR Basics – Rayleigh Scattering
Rayleigh Scattering
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Fibre Core
Light Pulse
Reflection
OTDR Basics – Time of Flight
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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
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OTDR Basics – OTDR Parameter Settings
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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)
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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
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OTDR Basics – OTDR Parameter Settings
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OTDR Basics – OTDR Trace
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Launch
Reflective Event
Non-ReflectiveEvent
End/Fault
NoiseDistance
Ret
urn
Sign
al L
evel
OTDR Basics – Typical OTDR Trace
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Fusionsplice
Event Deadzone Backscatter Deadzone
OTDR Basics - Deadzones
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Reflectance
OTDR Basics – Reflectance
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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)
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ORL
Backreflectionsfrom RayleighScattering
FresnelReflections
TransmittedLight
OTDR Basics – Optical Return Loss(ORL)
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Fibre OpticCleanliness
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Always clean before you connect!!
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Always clean before you connect!!
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