Chapter 3 Fiber Optics and Integrated Optics

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Chapter 3 Fiber Optics and Integrated Optics

• Gradient-index opticsthe refractive index is the
  
• 
  function of space
• Fiber opticsOptical wave-guide,
  tele-communication
• Integrated opticsminiaturized optical system

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True or false statement
The light travels in the straight line in the
air. (1) True (2) False

n refractive index
?-- density of the air
Ttemperature of the air
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How does light travel?
If nconstant Light
travels in straight line If nvarying in space
Light travels in
curved line! It follows
the law of refraction!
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3.1 Gradient Refractive Index

• 1.Atmospheric refraction
• The light is bending towards
• the higher index side
• Sun rising setting
• The true position of the sun is
• lower than what you see
• Looming
• Lift up the image
• Mirage
• Images formed as if
• there is a pool of water!

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• 2. Gradient index lenses
• Conventional lens
• refraction takes place only at the surface
  of the lens.
• Gradient lens
• refraction takes place within the lens.
• Advantages of gradient lens
• Correct some aberrationreplace the aspherical
  lens.
• Can produce very small lenshard to manufacture
  in traditional way.
• Simplify the optical system a gradient lens can
  replace a number of homogeneous lenses.

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a. Radial gradient lenses

• The index of refraction varies as a function of
  distance from the optical axis.
• Cylindrical symmetry
• Positive lens n higher in the center
• Negative lens n higher in the periphery
• The end surface can be plane or spherical for
  additional power.

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For a positive radial gradient lens, when the
shape of the lens is a cylinder, what will be the
distribution of the refractive index?
(1) n higher in the center
(2) n higher in the periphery

• What about a negative lens?

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b.Axial gradient lenses

• The surfaces of constant index are planes and
  normal to the axis.
• Correction of spherical aberration
• Conventional lens
• marginal ray bends more
• center ray bends less
• gradient lens
• index is higher near the front
• higher index material is removed in
  periphery
• marginal ray bends less

r
z
O
Optical axis
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c. Spherical gradient lens

• The index of refraction varies symmetrically
  about a point.
• The surfaces of content index are spheres.
• Example Crystalline lens of the human eye

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GRIN
GRIN is short for graded-index or gradient index.
It refers to an optical element in which the
refractive index varies. More specifically (from
the Photonics Dictionary) a GRIN lens is a lens
whose material refractive index varies
continuously as a function of spatial coordinates
in the medium. Also, a graded-index fiber
describes an optical fiber having a core
refractive index that decreases almost
parabolically and radially outward toward the
cladding.
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GRIN lenses come in two basic flavors RADIAL or
AXIAL which are sometimes refered to as RGRIN and
AGRIN respectively. RGRINS are usually used where
you want to add optical power to focus light. An
RGRIN with flat surfaces can focus light just as
a normal lens with curved surfaces does. Thin
RGRIN lenses with flat surfaces are known as WOOD
lenses, named after the American physisist R.W.
Wood who did a lot of experimental work with
radial gradients from about 1895 to 1905 and
included descriptions of how to make them in his
physics text book (available from OSA).
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d.Manufacture of gradients

• Methods available
• neutron irradiation
• chemical vapor deposition
• polymerization
• ion stuffing
• ion exchange
• Ag diffuse into the glass replace Na ? n?(40h)
• Theoretically ?n0.15
• Practically ?n0.05

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3.2 Fiber Optics
What is an Optical Fiber?
An optical fiber is a waveguide for light
consists of core inner part where wave
propagates cladding outer part used to keep
wave in core buffer protective
coating jacket outer protective shield
can have a connector too
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• Two types of fiber step-index gradient fiber
• Structure core(higher n) cladding(lower n)
• Total internal reflection

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Types of Fibers
nc
step-indexmultimode
nf
nc
nc
step-indexsinglemode
nf
nc
nc
GRIN
nf
nc
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1. Step-Index fiber
NA of a Fiber
The NA defines a cone of acceptance for light
that will be guided by the fiber
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nc
nf
ni
90-?
?t
?max
NA in air
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NA changes with n
air
water
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NA is sensitive to n
1 change
5 change
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NA and Acceptance angle
water
air
?i
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• Two types of fiber with different propagation
  modes
• single-mode fiber
• only single mode is permitted
• small core diameter 8.3(core) /125(cladding) ?m
• Multi-mode fiber
• several modes are permitted
• large core diameter 5062.5(core)
  /125(cladding)?m

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Types of fiber ends
beam patterns can be spherical cylindrical
bundles
90 degree
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• Fiber-optic Cable
• Many extremely thin strands of glass or plastic
  bound together in a sheathing which transmits
  signals with light beams
• Can be used for voice, data, and video

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Angle Preservation
In an ideal fiber, the angle of incidence will
equal the exit angle.
2?
?
?
??
?
2?
?
2?
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example critical bend radius
Rough surfaces, bending, and other real-world
imperfections will case a change in the exit cone.
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Fiber Tapers
?2
d1
d2
?1

• way to change the acceptance angles of a fiber
• sometimes used to collimate light

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2. Gradient-Index Fiber Simplification
continuous n change ? discrete layers of n From
Snells refraction law At the nth boundary, at
the distance R from the axis Therefore With
n(R)?? Sin I(R)? ? I(R) ? Until Sin I (R ) 1 ?
I(R) 90?, The ray return back to the center(
optical axis)
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Additional Fiber Types
(All single mode)
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• 3. Applications
• A. Transmission of light image
• to illuminate hard to reach places to conduct
  light out of small places
• Inside
• heart, digestive tract, stomach, respiratory
  tract, lung, etc.

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B. Tele-communication
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• advantages
• light in weight, efficient use of space in
  conduits
• less expensive
• Free from electrical interference, aircraft,
  military, security
• Flexible
• Secure to interception
• Low power lost
• enormous capacity of transmission WDM/
  DWDM(Dense Wavelength Division Multiplexing),
  Higher data rates over longer distances-- more
  bandwidth for internet traffic

Problem remained Attenuation power lost (
minimum at 1.55?m) Dispersion modal, material
(minimum at 1.31?m),
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Types of Dispersion in Fibers
modal - time delay from path length
differences - usually the biggest culprit in
step-index material - n(?) different times to
cross fiber -(note smallest effect 1.3
?m) waveguide - changes in field distribution
-(important for SM) non-linear - n can become
intensity-dependent
NOTE GRIN fibers tend to have less modal
dispersion because the ray paths are shorter
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Effect of Modal Dispersion
initial pulse
farther down
farther still
time
time
time
modal example step index 24 ns km -1
GRIN 122 ps km-1
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• 4. Bel Decibel (dB) Comparative unit
• Input ?1 output ?2
• Attenuation
• Bel ? Decibel (dB) ?
• ?2 10?1 ? 1Bel ? 10dB
• ?2 100?1 ? 2Bel ? 20dB
• ?2 ?1 ? 0Bel ? 0dB
• ?2 ?1/2 ? 10lg0.5 ? -3dB

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Fibers are made of glass - commonly
high-quality fused silica (SiO2) - some trace
impurities (usually controlled) Losses due to -
Rayleigh scattering ( ??-4) - absorption -
mechanical stress - coatings
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Attenuation Profiles
page 297
IR absorption
Rayleigh Scattering
89 transmission
absorption and scattering in fiber
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Fiber loss
(dB/km) Where, L1,L2
distance from the start of the fiber, L1 ? ?1,
L2 ? ?2 20years ago -20dB/km was thought to be
the limit Now -0.2dB/km fiber is
commonly used Single-mode fiber 50100KM
Multi-mode fiber 24KM
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Dispersion The Basics
Light propagates at a finite speed
fastest ray
slowest ray
fastest ray one traveling down middle (axial
mode)
slowest ray one entering at highest angle (high
order mode)
there will be a difference in time for these two
rays
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Coupling with Lenses
n
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n
1
n
2
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a

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s
.
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Coupling with Prisms
Commercial applications? (sensors) Research
labs Optical fiber tap
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Review

• Optical fibers carry modes of light
• Step-index, GRIN, single mode multimode
• NA is related to acceptance cone and ns.
• How Step-index and GRIN fibers propagate light.
• Factors that change light propagation in fibers
• mechanical aspects (bending, tapers, etc)
• attenuation
• dispersion

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3.3 Integrated Optics

• Integrated optics ?Integrated circuit

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Integrated optics offers a particularly
interesting candidate for implementing parallel,
reversible computing structures

• Integrated optics (integrated wave-guide)
  Miniature dimension of fiber optics
• usually manufactured in the way of thin film
• ( thickness in the
  order of wavelength)
• planar guideswide
• strip guidesnarrow
• Beam couplers guide light to enter the thin film

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PLC Planar Lightwave Circuits
Top Cladding
SiO2
Bottom Cladding
SiO2
Si Wafer
Si
Waveguide Core
Guided Light
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Coupling with Prisms
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Grating Couplers (Input and Output)
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• 1.Integrated prisms
• Thin film prisms
• thinner film effective velocity of light ?
• thicker filmeffective velocity of light ?
• Refractive gradient prisms
• light bend towards the high index side.

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• 2. Thin film lenses
• Luneburg lens
• a flat circular mound
• Index being highest in the center, decreasing
  towards periphery
• Geodesic lens
• dome shaped film uniform thickness
• rays follow the shortest path between two
  points on a surface

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• 3.Other Integrated Elements
• Light modulators
• operate on amplitude,phase, frequency,state of
  polarization
• Electrical signal change ? Light direction change
• Light switches, deflectors, Light scanners
• 4. Manufacture
• Earlier way vacuum depositionTaO, LiNb coated
  on a substrate
• Modern ways diffusion techniques
• ion implantation
• proton bombardment
• electron or laser writing

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• Monolithic integrated optics
• Light source, light guiding,
• modulating, detection
• are performed
• in a single crystal
• GaAs gallium arsenide
• semiconductor material
• Fiber optical gyroscope

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Advantages of Integrated-Optic Circuits

• Small size, low power consumption
• Efficiency and reliability of batch fabrication
• Higher speed possible (not limited by inductance,
  capacitance)
• parallel optical processing possible (WDM)

Substrate platform type

• Hybrid -- (near term, use existing technology)
• Monolithic -- (long term, ultimately cheaper,
  more reliable)
• quartz, LiNbO , Si, GaAs, other III-V
  semiconductors

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homework

• 2,3,5
• Translation(E to C)