Fiber Optics FOA - Class Feb 13

8/8/2019 Fiber Optics FOA - Class Feb 13

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Fiber Basics

EMS Name used to refer to a range of different types or forms of radiation.

*We look at these different forms of radiation as waves.

*What makes each region different from the other is the wavelength.

*the shorter the wavelength the higher the frequency, the longer the wavelength

the lower the frequency

*white light consists of many different electromagnetic waves

*All electromagnetic waves travel at the speed of light (180,000m/s)

*A given wavelength travel at different speeds in different materials (for a given

material different wavelengths will travel at different speeds in that material)

*The length of a wave indicates its colour

*important thing in fiber optics is to have total internal reflection. Once there is

refraction at a much low level there maybe wave refraction outside the cladding

affecting the signal as well as the bandwidth (attenuation).

Efficient wavelengths which exist in the infrared region


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850nm, 1300nm, 1310nm, 1550nm

As frequency of a wavelength increases, the wavelength gets shorter, so that it can

occur more often in the same period of time.

As we are at a wavelength lower than 850nm we have high attenuation, as we

approach 850nm the attenuation decreases, passing 850nm attenuation rises again

to a peak then decreases once again as it approaches 1300nm and continues to

1550nm. (**view dotted line**)

*850nm & 1300nm are associated with the use of multimode fiber

*1310nm & 1550nm are associated with the use of single mode fiber

Spectral width the amount of wavelengths for a given light source.


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*No light source emits a single wavelength

LIGHT SOURCES

Light emitting diode (LED)

Low cost

Low power

Wide spectral width

Laser Diode (less prism effect taking place separation of the different

wavelengths)

High cost

Medium power

Narrow spectral width

Table 1 - Comparison of LEDs and Lasers


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Characteristic LEDs Lasers

Output PowerLinearly proportional to drive

currentProportional to current above the

threshold

Current Drive Current: 50 to 100 mA Peak Threshold Current: 5 to 40 mA

Coupled Power Moderate High

Speed Slower Faster

Output Pattern Higher Lower

Bandwidth Moderate High

Wavelengths Available 0.66 to 1.65 m 0.78 to 1.65 m

Spectral Width Wider (40-190 nm FWHM)Narrower (0.00001 nm to 10 nm

FWHM)

Fiber Type Multimode Only SM, MM

Ease of Use Easier Harder

Lifetime Longer Long

Cost Low ($5-$300) High ($100-$10,000)

Critical angle angle that exist at the point where total internal reflection begins to

take place

Numerical aperture NA an indication of how many light at different angles will

accommodate total internal reflection (light at different angles

larger aperture accommodates more angles at which the light reflects

smaller aperture accommodates less angles

for acceptance light angle must at least be equal or runs parallel to the

acceptance angle or run closer to the path of the core


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A single fiber can provide multiple paths for light travel

These paths are called modes

Over 100,000 modes can exist

Material make-up of fibers

Ultra pure, ultra transparent silicon dioxide or fused quartz

Impurities are added during manufg. to create the required refractive index

Refractive index profile There are two types

Step index profile There is a sudden change in the refractive index value

when you look at the core related to cladding. (like from air to glass)

Graded index profile There is no longer a sudden change of refractive

index between the core and the cladding, a gradual changes takes place.

Note the core has a higher refractive index value than the cladding

Lot simpler to create step index fiber

Step index multimode was 1st fiber designed but is too slow for most uses due to the

dispersion caused by the different path lengths of the various modes


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Problems with the efficient transmission of light

Pulse spreading (Dispersion) Affects the bandwidth (limits bandwidth)

Modal dispersion is a bandwidth limiter for multimode fiber, happens in

single mode but has no great impact.

Chromatic dispersion affect bandwidth in single mode fiber

Attenuation Affects the power/strength of the signal (weakens signal)

The decrease in the power of an optical signal from input to output (loss of

light measured in decibels dB)

Example 3dB = 50% loss of light

10dB = 90% loss

20dB = 99% loss


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Typical Attenuation - Loss of light / optical power loss

Fiber type 850nm 1310nm 1550nm

Single mode NA 0.35 dB/Km 0.25 dB/Km

Multimode 3.5 dB/Km 1.0 dB/Km NA

Factors that cause attenuation

Intrinsic causes Internal to the fiber

Absorption - longer wavelength less absorption, shorter wavelength more

absorption

Scattering - major cause of attenuation. (In multimode fiber)

*Note some light will scatter and enter back to the source interfering with the signal

transmitted.

Extrinsic causes external to fiber

Micro-bending pressure on the fiber

Macro-bending bending the fiber more than specified messing with the angles

resulting in refraction

Fiber curl has to do with spicing (one end curls resulting in fibers not being aligned

correctly)

Core concentricity how centered is the core in the entire fiber

Ellipticity How round the core is

Structural Elements & Functions

Buffer Tube

Strength members Kevlar (Aramid Yarn)

Outer jacket

Inner jacket

Gel Filling Compound used for high vibration type environment


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Water Blocking Compound Gel found outside of the buffer tube to assist in keeping

water out

Binding tape or yarn to keep the assembly together

Armour

Ribbon referring directly to the fiber itself

BUFFER TUBE DESIGNS

2 types of buffer tube leads to 7 designs, 5 lose tube designs and 2 tight tube

designs

Loose tube design

Single fiber per tube

Multiple fibers per tube

Central buffer tube

Ribbon

Start or slotted

Tight tube

Break out (each fiber has its own kevlar)

Distribution (premise)

Cable types: Zipcord, Distribution, Loose tube, Break out

Tight Buffer cables can be used for indoors as well as outdoor.

Typical components

900um buffer fiber

Fibers are additionally buffered

Tight bound design

Core locked design


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Fiber glass central strength members

Kevlar strength members

Inner/outer jackets

No Gel

Variation & improvement to this cable

Typical components

Individual fibers break out like a patch cord

Flexible non-memory breakout

Kevlar strength members

Amoured jackets

Central tube only style

Quick to break out

Pull in by standard mesh grip

More flexible to pull in

Adv of loose tube over tight tube

Loose tube is made for the outdoor environment

Proven for long outdoor runs with wide temp range

Less expensive above 24-fiber count cables

Have better packing density

Available in armored for direct buried. All dielectric for aerial and ducted

applications & riser rated construction for use in riser applications

Note loose tube cables shouldnt be carried into a building more than 50

feet, due to electrical standards, has materials that could be toxic

250m found in the loose tube type cables & cannot be terminated directly

with connector


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Adv of tight buffered/tube over loose tube

Can be used in intra-building backbone, Horizontal distribution, for patch cords &

equipment cables

Increased physical flex

Smaller bend radius for lower fiber count cables

Easier handling characteristics in low fiber count

Fiber ends can be terminated directly with connectors (900m found in these

Tight buffer)

Important performance specs. Installation specs

Maximum recommended installation load (pulling force being exerted on the

cable)

Minimum recommended short-term (during installation) bend radius

Minimum recommended long term (after installation, at rest) bend radius

Temp ranges (both high n low limits) for installation and temp ranges when being

stored

Moisture/water resistance how much pressure under water the cable canwithstand before its resistance is breached.

Cable diameter if running in ducts already containing cables the size of the fiber

needs to be known to match the space remaining in that duct.

Important performance specs. Environmental specs

Temperature range of operation

Compliance with NEC (national electrical code) or local electrical codes

Radiation resistance

Vertical rise distance cable has weight the further you go upwards the more

pressure on the cable at the farthest end

Flame resistance how well the cable resist fire


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Crush loads

Toxicity should not give off a certain amount of toxic fumes for a given period of

time

Vibration

In fiber technology splicing is the continuity of total internal reflection

Must have:

Precise alignment

Fiber retention ensures whatever clamping mechanism used will properly restrain

the fiber over a period of time

End face protection the use of dust caps

Splices

Permanent or semi-permanentmechanical spice

Used when rerouting of optical path is not required or expected

Mid-span splices- connects 2 lengths of fiber (with a third in the middle)

Eg ______1___________3_____________2_______

Pigtail splicing used at the fiber ends length of cable or fiber terminated at oneend only

The other end of the pig tail would be connected (spliced) to the oncoming

cable

Connection Loss Factors

End-face quality

Polishing clean fiber core, no cracks, scratches, pits

Cleaving:

Good Cleave smooth, mirrored surface

Bad Cleave chips and shards


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Preparation for fusion splicing:

1. Remove the coating

2. Cleave/Cut to proper lengths

3. Place in the fusion device. Align & Gap

4. The fiber cores are melted together. 2 ends are now fused together

5. And heat-shrunk

Fusion splicing vs Mechanical splicing

Fusion splicing Mechanical Splicing

Capital Investment Expensive Cost effective

Tech knowledge/training Very high low

Splicing time Long Quick

Construction applications Yes Yes

Maintenance application Yes Yes

Affected by environment Yes No

Requires power source Yes No

Air condition required Yes No

Attenuation


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*Note for patch cables Yellow signifies single mode while Orange signifies

multimode

Connectors 5 basic features

Method of coupling

Keying can only be plugged in, in one particular way

Contact of fiber cores

Style SC, ST, etc

Installation technique

Connectors 3 Part connector system

Connector or plug

Receptacle

Adapter

Connector installation methods 4 General types

Method 1 Epoxy, Crimp & Polish (Epoxy)

Method2 preloaded, Preheated & Polish (3M Hot Melt Proprietary)

Method3 Epoxyless, Crimp/Crimp/Polish (Mechanical)

Method4 Anaerobic Adhesive, Crimp & Polish

Variation of Method 3 No Polish (Cleave & Polish)

Attenuators Variable & Fixed


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Fiber optic systems

Can be grouped into 4 categories

Point-to-point

Point-to-multipoint

Network

Switched network

Point-to-point

Simplest form

2-way communication between systems

Each end has permanent linked TX and Receiver Pair

Distance is of little or no concern

Concept can be imposed on different types of transmission systems

Point-to-Multipoint

Signals sent from 1 TX to many terminals

Sometimes called broadcasting

Terminals may or may not send signals to transmitter

If they do, the return signals are often at a lower speed than the broadcast signal

Fixed with permanent connections

Typically include multiple levels of signal distribution

If the TX dies, the system dies


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Network

2-way transmission between any pair or terminals

Different physical topologies exist: Ring, Bus, Star, Mesh, etc

Permanent connection to each terminal

Switched

Creates flexibility

Allows any pair of terminals to send/receive signals directly to/from each other

Adds a level of complexity that makes and breaks connection

*note. In practice, most switching is performed electronically with fiber optic point

to point systems linking electronic switches

It is difficult to switch optically

Cost is a factor

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Fiber Optics FOA - Class Feb 13

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