gn fiber optic theory 32704

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Revision APart # 32704

FIBER OPTICBASICS

FIBER OPTIC BASICS TOC-1

Table of Contents

Chapter 1 The Nature of LightClassifying Light.................................................................... 1-3Power .................................................................................... 1-4Wavelength .......................................................................... 1-5Reflection & Refraction ........................................................ 1-6Rayleigh Scattering ............................................................ 1-11

Chapter 2 Optical Fiber CharacteristicsTypical Optical Fiber Parameters ......................................... 2-3Optical Fiber Types ............................................................... 2-4Singlemode vs. Multimode Fiber ......................................... 2-5Fiber Geometry Problems..................................................... 2-6Index of Refraction (n) ......................................................... 2-7Attenuation .......................................................................... 2-9Causes of Attenuation ....................................................... 2-10Dispersion ........................................................................... 2-11Splice Loss Due to Core Mismatch ..................................... 2-12Causes of Connector Loss ................................................... 2-13

Chapter 3 Optical MeasurementsTesting Fiber - Why? ............................................................. 3-3Testing Fiber - When? .......................................................... 3-4Testing Fiber - What? ........................................................... 3-5Attenuation Measurement .................................................. 3-6OTDR Block Diagram ............................................................ 3-7How an OTDR Works ............................................................ 3-8

TOC-2 FIBER OPTIC BASICS

Reflected Light ..................................................................... 3-9Returned Light ................................................................... 3-10OTDR Distance Measurements ........................................... 3-11OTDR Loss Measurements .................................................. 3-13OTDR Trace Basics ............................................................... 3-15Measuring Optical Return Loss .......................................... 3-16Wavelength - Scattering Loss............................................. 3-17Wavelength - Bending Loss ............................................... 3-18Deadzones .......................................................................... 3-19Event Deadzone ................................................................. 3-21Attenuation Deadzone ...................................................... 3-22Fresnel Characteristics ........................................................ 3-23Fusion Splices ...................................................................... 3-24Why Losers and Gainers? ................................................... 3-25Dynamic Range ................................................................... 3-26Backscatter vs. Dynamic Range .......................................... 3-27Resolution ........................................................................... 3-28Data Sampling Resolution ................................................. 3-29Spatial Resolution - Long Pulse ......................................... 3-30Spatial Resolution - Short Pulse ......................................... 3-31Spatial Resolution - Long vs. Short Pulse Widths ............. 3-32

FIBER OPTIC BASICS Page 1-1

Chapter 1

The Nature of Light


Classifying Light

Optical Power is measured in dBm.

Color (Wavelength) 380nm (blue) to 750nm(red) is visible to humans. Fiber optic systemstypically transmit at 850, 1300, 1310 and1550nm.

Increased demand for bandwidth is drivingDense Wave Division Multiplexing (DWDM)systems which transmit from 1520 to 1580nmusing 4, 8,16, or 32 wavelength channels ormore over a single fiber.

4 FIBER OPTIC BASICS

Power

Like a light bulb:more wattage = brighter light

FO transmitters:about 1mw (0 dBm)

Power ranges:+20 dBm to -70 dBm


Wavelength

Measure of Color of light

Units in nanometers (nm) or microns (mm)

Different colors (wavelengths) exhibit differentcharacteristics. ex: red & orange sunsets; yellowfog lights


Reflection & Refraction

Reflection is a light ray BOUNCINGoff of the interface of two materials

Refraction is the BENDING of the light rayas it changes speed going from one materialto another


Refraction & Reflection

A ray of light in glass will bend (refract) away from thedirection of travel as it escapes to the surrounding air.The amount of this refraction angle is constant for any singlewavelength. Some light is reflected off the glass-air surfaceback into the glass.



When the angle becomes shallow enough, the reflected raydoes not leave the glass at all, but travels along the air-glassinterface. This angle is called the Critical Angle.



At angles beyond the Critical Angle, all light is Reflected backinto the fiber at the same angle it strikes the air-glass interface.This condition is known as Total Internal Reflectance



As long as the light ray stays at the Critical Angle or lesswhen it hits the air-glass interface, it will remain in the fiberuntil it reaches the other end.

In a glass fiber the light remains in the core because lightreflects at core/cladding boundary.


Rayleigh Scattering

As light passes through a particle, part of it is scattered inall directions. The part that returns to the source is calledBACKSCATTER.


Chapter 2

Optical FiberCharacteristics


Typical Optical FiberParameters

Most Transmission Glass Fiber has the Following Properties:

The denser Core is centered within the Cladding. Lighttravels in the core only. The Buffer protects the glass fiber.


Optical Fiber Types

Multimode fiber has alarge core relative tothe cladding diameter.

Singlemode fiber has asmaller core relative tothe cladding diameter.


Multimode vs.Singlemode Fiber

Multimode allows many paths (modes) for light to travel

Singlemode allows only one single path for light to travel


Fiber Geometry Problems

All fibers are allowed a certain tolerance in the core/claddinggeometry. This can cause light loss at joints between fibers.


Index of Refraction (n)

C is a constant. V dependson the density of the glass.More dense glass causes light to goslower (smaller v =>larger n).


Index of Refraction (n)

How IOR Affects Fiber Characteristics

The core of the fiber has an Index of Refraction (n1) which isdifferent than the Index of Refraction of the cladding (n2).These two values determine the acceptance angle and criticalangle Q of the fiber.


AttenuationAs light is guided through the core, four properties can causeattenuation:

1. ABSORPTION occurs when light strikes impuritiesin the core glass and is absorbed.

2. SCATTERING occurs when light strikes an areawhere the material density changes.

3. MACROBENDING is large-scale bending of thefiber which exceeds the fiber bend radius and causeslight to leave the core and travel in the cladding(usually an installation problem).

4. MICROBENDING is microscopic distortion of thefiber which causes light to leave the core and travel inthe cladding (created during manufacturing).


Causes of Attenuation


DispersionDISPERSION is the spread of a pulse of light as it is guidedthrough the fiber.

There are 3 types of dispersion:

1. MODAL dispersion occurs when various modes oflight follows different paths through the fiber andarrive at the far end at different times. It occurs onlyin multimode fibers.

2. MATERIAL (or CHROMATIC) dispersion occursbecause different wavelengths (colors) of lighttravel at different velocities through the fiber.

3. WAVEGUIDE dispersion occurs because light travelsin both the core and cladding at slightly differentspeeds. It is most significant in singlemode fibers.

4. POLARIZATION MODE DISPERSION occurswhen the X and Y polarization states of a light signaltravel at different speeds through a fiber. This issimilar to MODAL dispersion except that it can beof significance in singlemode fibers.


Splice Loss Due to CoreMismatch

Off-center core in the second fiber does not receive all thelight from the first fiber. The amount of light lost is theSplice Loss.


Causes of Connector Loss

End-Face Separation

Angular Separation(end-face not cleaved to perpendicular)

Core Misalignment


Chapter 3

Optical Measurements


Testing Fiber - Why?

Verify specs

Check handling

Record best condition

Detect defects

Locate faults

Troubleshoot problems


Testing Fiber - When?

At Factory

When Received

After Placed

After/During Splicing

System Acceptance

Periodic (Annual)

Troubleshooting


Testing Fiber - What?

Continuity

Average Loss (dB/Km)

Splice Loss & Location

Reflectance / ORL

End-to-End Attenuation

Overall Length


Attenuation Measurement

The simplest and most accurate method of measuring theend to end loss of an optical span is with a light source and apower meter. Shown here is the proper method for storing areference and then measuring loss.

Multimode optical measurements require a mandrel wrap atthe source end. Bi-directional testing further improves lossaccuracy.


OTDR Block Diagram

The OTDR sends out a pulse of light and measures the levelof light that is reflected back. An optical coupler allows bothoptical source and optical receiver to be connected to thesame fiber.


How an OTDR Works

The modern OTDR knows how fast light will travel throughthe core of the fiber under test because of the index of refraction(IOR) setting. The OTDR knows how far it needs to measurebecause of the fiber length setting. With this information, theOTDR will repeatedly sample the level of reflected light.

GN Nettests CMA4000 takes up to 16,384 samples of re-flected light per pulse of transmitted light. This means that ifthe fiber length setting was 128 km, sampling would occurevery 8 meters (128 km/16384).

In REAL TIME mode the CMA4000 sends out between 64 to256 pulses of light per trace, depending on the currentpulsewidth. In Average mode (FAST, MED, SLOW) the unitwill send out either 2048, 32768, or 261,288 pulses of light.The sample points collected are in this way averaged to presentscreen trace.


Reflected Light

Rayleigh Backscatter = 5 log (Po WS)-10ax(log e)

Where: Po is the launched optical power in watts

W is the transmitted pulsewidth in seconds

S is the scattering factor expressed in watts/Joule

a is the fiber attenuation coefficient in Nepers/m

x is the distance along the fiber


Returned Light

An OTDR relies on returned light for making measure-ments. There are two forms of returned light: reflected andscattered.

Reflected light is called FRESNEL REFLECTION. Whenlight passes from one index of refraction to another somelight is always reflected back.

There is an air gap between fiber ends joined with mechani-cal connectors. As light passes from the index of the core tothe lower index of air, a high amount of reflection can takeplace. Initial and End Fresnel Reflections are good examplesof events resulting from the glass-to-air transition.

The backscattering of light is called RAYLEIGH SCATTERING.This scattering is the result of variations in the density of thecore glass. Glass density is not uniform. As light passes fromone density to another, there is some scattering of the light, anda small amount returns to the OTDR.


OTDR DistanceMeasurement

Where: d is distance

c is the speed of light

t is the time

n is the Index of Refraction

As shown in the formula above, if n is incorrect, then thedistance measured will also be wrong!


OTDR DistanceMeasurements

Index of Refraction is set for a wavelength tomeasure fiber distance

More fiber than cable (approx. 2 to 6%)

Ground location is most important. Need tocompensate for loops & slack in fiber and cable

Use landmarks to correlate OTDR to grounddistances


OTDR LossMeasurements

OTDR measures BACKSCATTER,and detects REFLECTIONS

Compares BACKSCATTER levels todetermine loss between points in fiber

Splice losses determined by amount ofshift in backscatter


OTDR LossMeasurements

Backscatter is directly related to the signal in the test pulse.As the signal decreases, so does the backscatter. The differ-ence in strength between two points of backscatter is thesame as the difference in strength between the test pulse atthe same two points.


OTDR Trace Basics


Measuring ORLOptical Return Loss

ORL is calculated as the total amount of light returningfrom the area between the cursors below the trace line to thenoise level. It includes total Backscatter and all Reflections.


WavelengthScattering Loss Difference


WavelengthBending Loss Difference

Higher wavelengths are more sensitive to bend losses.


Deadzones

Specified as a DISTANCE

Determines how CLOSE to the OTDR you can detect and measure a splice loss

Determines how CLOSE TOGETHERtwo events (splices) can be measured

Directly related to PULSE WIDTH:larger pulse widths produce larger dead zones


Deadzones

A dead zone is the portion of a trace where an OTDR cannottake accurate measurements because it is in a recovery ortransitional state.


Event Deadzone

An Event Deadzone is the area between two points 1.5 dBfrom the top of an unsaturated reflection.

An Event Deadzone ends 1.5 dB down after pulse recovery,where another pulse could be distinguished if present.


Attenuation Deadzone

An Attenuation Deadzone is measured from the beginning of apulse to a point 0.5 dB above the extrapolated backscatter line.


Fresnel Characteristics


Fusion Splices

A fusion splice exhibits a deadzone approximately equal tothe pulsewidth.


Why Losers and Gainers?

TBs equals the total backscatter

W1 equals the field radii of transmitting fiber

W2 equals the field radii of receiving fiber

The loss or gain in backscatter power across a splice due todifferent mode field radii is calculated using the above formula.

The loss or gain at the splice can appear much larger thanthe actual transmission loss.

Refer to the diagrams on page 3-24.

This happens when the fiber is only measured in one direc-tion. Therefore, for greater accuracy, take loss measurementsin both directions, add the results, then divide by two.


Dynamic Range

Measured in dB. Typical range is 20-40dBor more.

Describes how much loss an OTDR canmeasure in a fiber, which in turn describeshow long of a fiber can be measured

Directly related to Pulse Width:larger pulse widths provide largerdynamic range

Increase by using longer PW andby decreasing noise through averaging


Backscatter vs. DynamicRange

Backscatter range is from the bottom of the screen at zero dB tothe highest point of normal trace backscatter. This does notinclude initial Fresnel reflection and recovery.

Dynamic Range (SNR=1 method) is measured down from thehighest point of normal backscatter to approximately 70.7% ofthe peak noise floor. This is the usable portion of the trace.


Resolution

Described as a DISTANCE

Two Types:Data SamplingSpatial Resolution (from Dead Zones)

Determines:Accuracy of event location. If you canmeasure two closely spaced splices inthe fiber.


Data SamplingResolution

The above graph charts resolution at both 8 meters and 16meters, illustrating the effective accuracy in splice location.Pulsewidth is the same in both cases, and is not affected bysampling.


Spatial Resolution Dead Zone Effects From Using

Long Pulse Width

Long Pulse Width produces a longer Dead Zone preventingthe detection and measurement of individual splices. ThePulse Width strikes the second connection before clearingthe first connection.

It is impossible to tell by looking at the trace which splice iscausing the high loss. The OTDR reports this as a GroupedEvent as it can not determine where one splice ends and thenext begins.


Spatial Resolution Dead Zone Effects From Using

Short Pulse Width

Short Pulse Width produces a shorter Dead Zone allowingeach splice to be measured individually. The Pulse Widthclears the first connection before striking the second. Thisproduces Rayleigh Scattering between the splices allowingindividual measurement.

It is now quite easy to determine which splice is causing thegreater loss.


Spatial ResolutionLong vs. Short Pulse Widths

By using a Long Pulse Width takes longer to make the tran-sition from backscatter of the first fiber to backscatter of thesecond fiber. Short Pulse Width makes a sharper transition.

gn fiber optic theory 32704

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