2.0 Fiber Optic Characteristic

7/27/2019 2.0 Fiber Optic Characteristic

http://slidepdf.com/reader/full/20-fiber-optic-characteristic 1/51



UNIT OF LENGTH BASED ONSPEED OF LIGHT

The measured speed of light in vacuum in the

experiments mentioned above uses a standard meter

for the distance traveled by light. The 1983

Conference Generale des Poids et Mesures turned the table around, and adopted an exact value of

speed of light to be 299,792,458 m/s,

Exact value of speed of light in vacuum:c = 299,792,458 m/ s (exact)



OPTICS



Electromagnetic waves in opticalpropagation

Visible light waves are the only

electromagnetic waves we can see.

We see these waves as the colors of therainbow.

Each color has a different wavelength. Red has the longest wavelength and violet

has the shortest wavelength.

When all the waves are seen together, they

make white light.



Continue…

When white light shines through a prism,

the white light is broken apart into thecolors of the visible light spectrum.

Water vapor in the atmosphere can alsobreak apart wavelengths creating a

rainbow.



Light as an EM Wave and the EM Spectrum

Light is known to be an electromagnetic wave.Like all waves, electromagnetic waves have a wavelength and frequency,

related by:

f c





EM waves can be produced by the acceleration of

charged particles, such as electrons accelerating in theantenna.

Another example of EM waves: X-rays, which areproduced when fast-moving electrons are rapidlydecelerated upon striking a metal target.

Production of Electromagnetic Waves

The electric and magnetic waves are perpendicular to each other,

and to the direction of propagation.



Since a changing electric field produces a magnetic field, and a changing

magnetic field produces an electric field, once sinusoidal fields are

created they can propagate on their own.

These propagating fields are called electromagnetic waves.



Maxwell’s prediction of the speed of light in a vacuum,

ε o: permittivity of free space or electric constant

μ o: permeability of free space or magnetic constant



Light as an EM Wave and the EM Spectrum

Light is known to be an electromagnetic wave.

Cahaya dikenali sebagai gelombang elektromagnet.

Like all waves, electromagnetic waves have a wavelength and

frequency, related by:

Seperti ombak, gelombang elektromagnet mempunyai panjang

gelombang dan frekuensi, yang berkaitan dengan:

f c



Fundamentals of light

2.1.3 The characteristics of light

A light characteristic to be useful in an optical link :

i) Light travel in a straight line

ii) It must be possible to operate the device continuously at a variety of

temperatures for many years.

iii) It must be possible to modulate the light output over a wide range of modulating frequencies.

iv) For the links, the wavelength of the output should coincide with one of

transmission windows for the fiber type used.

v) To couple large amount of power into an optical fiber, the emitting area

should be small.

vi) To reduce material dispersian in an optical fiber link, the output spectrum

should be narrow.

vii) The power requirement for its operation must be low.



Vii) The light source must be compatible with the modern solid state

devices.

viii) The optical output power must be directly modulated by varying

the input current to the device.

ix) Better linearity to prevent harmonics.

x) High coupling efficiency.

xi) High optical output power.

xii) High reliability.

xiii) Low weight and low cost.



Reflection and Refraction

The ‘Speed of Light’ is simply thevelocity of an electromagnetic wave ina vacuum.

Light travels slower in materials.

As light passes from one material toanother, its direction changes.

Refraction is the deflection of light.

Different wavelengths of light travel atdifferent speeds in the same material.



Reflection

The angle of incidence (from the incidence

ray to the normal) is equivalent to theangle of reflection (from the reflective rayto the normal).



Refraction The bending of waves, such as light waves,

when they pass from one substance toanother.

Refraction occurs because waves travel atdifferent speeds through different

substances. The angle between the light ray and the

normal as it leaves a medium is called theangle of incidence.

The angle between the light ray and thenormal as it enters a medium is called theangle of refraction.



Diffraction is a wave effect . It refers to the fact

that light, like other waves, bends around

objects it passes, & spreads out after passing

through narrow slits or around an edge.

This bending gives rise to a diffraction pattern

due to interference between rays of light that

travel different distances.

The resulting pattern of light & dark stripes is

called a di f f ract ion pattern .

Diffraction



Huygens’ principle is consistent with diffraction:

(a) around the edge of an obstacle

(b) through a large hole(c) through a small hole whose size is on the

order of the wavelength of the wave



The extent of the diffraction increases as the ratio of the

wavelength to the width of the opening increases.



Angle of incidence and refraction



Refraction

sm1000.3 8c

DEFINITION OF

THE INDEX OF REFRACTION

The index of refraction of a material is the

ratio of the speed of light in a vacuum tothe speed of light in the material:

v

cn

materialin thelightof Speed

in vacuumlightof Speed

Light travels through a vacuum at a speed

Light travels through materials at a speed

less than its speed in a vacuum.



Refraction: Snell’s Law

Light changes direction when crossing a

boundary from one medium to another. This iscalled refraction. The angle, which the outgoing

ray makes with the normal is called the angle of

refraction.



SNELL’S LAW OF REFRACTION

When light travels from a material

with one index of refraction to amaterial with a different index of

refraction, the angle of incidence is

related to the angle of refraction by

2211 sinsin nn

SNELL’S LAW



Snell’s Law

The relationship between the incident ray and

refracted ray is:n1 sin Ѳ1 = n2 sin Ѳ2

n1 and n2 are two values of refractive index.

Ѳ1 and Ѳ2 are the angle of incidence and

refraction.



Example: Determining the Angle of Refraction

A light ray strikes an air/water surface at an angle

of 46 degrees with respect to the normal. Find the

angle of refraction when the direction of the ray is

(a) from air to water and (b) from water to air.



Example: Determining the Angle of Refraction

A light ray strikes an air/water surface at an angle of 46 degrees

with respect to the normal. Find the angle of refraction when thedirection of the ray is (a) from air to water and (b) from water to

air.

54.033.1

46sin00.1sinsin2

112

nn

(a)

(b)

332

96.000.1

46sin33.1sinsin2

112

nn

742



Refraction is what makes objects half-submerged

in water look odd.

APPARENT DEPTH



Example: Finding a Sunken Chest

The searchlight on a yacht isbeing used to illuminate a

sunken chest. At what angle

of incidence should the light be

aimed?



Example: Finding a Sunken Chest

The searchlight on a yacht isbeing used to illuminate a

sunken chest. At what angle

of incidence should the light be

aimed?

313.30.2tan 1

2

1

221

sinsin

n

n

441

69.000.1

31sin33.1



When light passes from a medium of larger refractive

index into one of smaller refractive index, the refracted

ray bends away from the normal.

Crit ical angle

1

2sinn

nc

Total Internal Reflection

There is an angle of incidence for which the angle of

refraction will be 90°; this is called the critical angle:

21 nn



Total Internal Reflection

With the angle of incidence greater than

the critical angle, total reflection occurs. With material with indices on both sides

(cladding), the light will be continuallyreflected and follow the core.



Critical Angles

The critical angle is measured from the cylindrical axis of

the core. An angle of incidence for which the angle of refraction will

be 90°; this is called the critical angle:

For reflection to occur, angle of incidence must exceed thecritical angle -Ѳc. The critical angle Ѳ2 may be found by:

ѳc = arc sine (n2 / n1)



Example:

if n1 = 1.446 and n2= 1.430

Ѳc = arc sine (n2 / n1)

= 80.6⁰



If the angle of incidence is larger than the cr i t ical

angle , no transmission occurs and the light is

reflected.

This is called total internal ref lect io n .



Example:



A beam of light is propagating through diamond and strikes the

diamond-air interface at an angle of incidence of 28 degrees.(a) Will part of the beam enter the air or will there be total

internal reflection? (b) Repeat part (a) assuming that the

diamond is surrounded by water.

Example: Total Internal Reflection



A beam of light is propagating through diamond and strikes the

diamond-air interface at an angle of incidence of 28 degrees. (a)

Will part of the beam enter the air or will there be total internalreflection? (b) Repeat part (a) assuming that the diamond is

surrounded by water.

Example: Total Internal Reflection

4.2442.2

00.1sinsin

1

1

21

n

n

c (a)

(b)

3.3342.2

33.1sinsin

1

1

21

n

n

c

2

11

2

sinsin

n

n

42.2

28sin00.1

194.02.112

2

112

sinsinn

n

42.2

28sin33.1

258.0

0.152

E l



Example:

Measure the angle of incidence - the anglebetween the normal and incident ray. It isapproximately 60 degrees.

List known Values:

ni =1.00 nr =1.52

ѳ i = 60 degreesList Unknown: Find ѳ r

Substitute into Snell's law equation and

perform the necessary algebraicoperations to solve:



Continue…

1.00 • sine 60⁰ = 1.52 • sine ѳr

0.8660 = 1.52 • sine ѳr

0.570 = sine ѳr

34.7 degrees = ѳ r

Now draw the refracted ray at an angle of 34.7 degrees from the normal.



Acceptance Angle And Numerical Aperture

Numerical Aperture (NA)

• The Numerical Aperture (NA) of a fiber is defined as the sineof the largest angle an incident ray can have for total internalreflectance in the core.• NA can be determined by measuring the divergence angle of the light cone it emits when all its modes are excited.•Qualitatively, NA is a measure of the light gathering ability of a fiber. It also indicates how easy it is to couple light into a

fiber.

http://assets.newport.com/web600w-EN/images/1381509.gif



Numeral Aperture (NA)

The acceptance angle is typically related to

fiber numerical aperture (NA) :NA = sin (Ѳ fiber) Ѳ fiber = arc sin(NA)

The numerical aperture (NA) is a

measurement of the ability of an opticalfiber to capture light.

The NA is related to the acceptance anglea, which indicates the size of a cone of light

that can be accepted by the fibre.



Acceptance Angle

The maximum angle within which light will

be accepted by an element, such as adetector or waveguide.

In the latter, it is quantified as half theVertex Angle of the cone within which

Optical Power may be coupled into boundModes of a fiber. Also called acceptancecone.

http://www.timbercon.com/Vertex-Angle.html

http://www.timbercon.com/Optical-Power.html

http://www.timbercon.com/Modes.html

http://www.timbercon.com/Modes.html

http://www.timbercon.com/Optical-Power.html

http://www.timbercon.com/Vertex-Angle.html



Figure: Acceptance angle of an opticalfibre

Both numerical aperture and acceptanceangle are linked to the refractive indexvia:

NA = na

Sin a

= (n1

2 – n2

2)1/2

Where n1 = refractive index of core

n2 = refractive index of cladding

na = refractive index of air (1.00)

cladding

n1

n2

core

n2

cladding

air

a

n2

n1

core

Continue:



Continue:

The light-gathering ability of an opticalfiber, as determined by the square root of

the difference of the squares of therefractive indexes of the core (n1) and thecladding (n2).

A light source naturally injects some lightrays into the core at angles less than thecritical angle, which is perpendicular tothe plane of the core/cladding interface.

The numerical aperture essentially is anindication of how well an optical fiberaccepts and propagates light.

Continue



Continue…

Figure: The relationship between the acceptance angle and therefractive indices

E l



Example:

A step index fiber has a core diameter of

100μm and a refractive index of 1.480. Thecladding has a refractive index of 1.460.Calculate the numerical aperture of the fiberand acceptance angle from air.

Solution:The numerical aperture is

NA = (n12 – n2

2)1/2 = (1.4802 - 1.4602)

= 0.2425



Continue…

From sinαmax = NA / n0

= 0.2425 / 1

The acceptance angle is αmax = 14⁰

The total acceptance angle is 28⁰



How to change h

Index of

refraction

N1

N2

Index of

refraction

N1

N2

step index gradient index

Modes



single mode

multi mode - step index

Modes

note high and low order modes

multi mode - gradient index

cladding

core

50

125

10

55km range

20km range



Single-Mode Step Index

Advantages:

Minimum dispersion(one path only)

Larger bandwidth

Disadvantages:

Difficult to couple light(small core)

Small source needed

Expensive and difficultto manufacture



Multi-Mode Step Index

ADV:

Inexpensive to manufacture,and simple

Easy to couple light into

DIS:

Different paths, more dispersion

Info rate and BW is less

Documents

2.0 Fiber Optic Characteristic