Basics of fiber optics
and optical sensors
BME 4562/5560
Fall 2007
Florida International University
Outline
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Basic principles
Parameters
Cables
Connecting
Applications
Special fibers
Optical fiber
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Individual fiber
Refractive index n determines
the velocity of light in medium v,
relative to velocity in vacuum c:
n=
c
v
c ≈ 3 ⋅ 108
m
s
•Refraction is the change in direction of
a wave due to a change in velocity.
•It happens when waves travel from a
medium with a given refractive index to
a medium with another.
•At the boundary between the media the
wave changes direction.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
θ1
normal
Snell’s law
n1
boundary
n2
θ2
n1
sin θ 2
=
n2
sin θ 1
Total internal reflection
The ray normal
to the boundary
is not bent
Reflection and
transmission
coefficients can
be calculated
from the Fresnel
equations
Light striking a medium with
a lower refractive index can
be totally reflected
n1 < n2
θc
Part of the
normal ray is
reflected
Light source
θc - critical angle
n 
θ c = arcsin 1 
 n2 
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
n2
Light incident at
any angle > θc
is totally reflected
Examples of total internal reflection
Bulk material:
Wikipedia, the free encyclopedia
Optical fiber:
www.i-fiberoptics.com/IFO_69-78_hi.pdf
Can all rays enter or leave the fiber?
Numerical aperture:
2
Refracted ray
ncl
2
NA= n1 sinθmax = nco − ncl
n1
θc
Totally reflected ray
nco
θmax
θmax - acceptance angle
ncl
Numerical aperture in practice
For the same fiber the acceptance angle
in the air is higher than in the water:
θ max 1 > θ max 2
Ray and wave representation
Ray theory - each ray at any angle higher that the critical angle can propagate along the fiber.
When the phase of the plane wave is taken into account, only rays at certain angles can
propagate along the fiber.
Modes
•Mode is an available distribution of
electromagnetic field in a plane transverse
to the direction of light propagation.
•Available patterns are derived from
Maxwell’s equations and boundary
conditions.
Higher order modes:
Wikipedia, the free encyclopedia
Single-mode and multimode fibers
Single-mode fiber guides only
fundamental mode LP01:
Multimode fiber guides fundamental mode
and higher order modes:
Single mode operation occurs above the cutoff wavelength λc:
2πa
λc =
NA
V
V- normalized frequency
a - core radius
NA – numerical aperture
Fibers with higher core diameter guide higher number of modes and have
higher numerical aperture
Core diameter and mode field diameter
In multimode fibers the boundary between
core and cladding corresponds to the diameter of
the light spot.
In single-mode fibers not all the light is
propagated in the core. The fundamental mode
LP01 has a Gaussian distribution.
Core diameter describes performance of the
multimode fibers.
Mode field diameter is the diameter at which
power is reduced to 1/e2 of the maximum power
Light power losses in fibers
Attenuation:
L – fiber length
Pin – input power
Pout – output power
Pin
1
α = 10 log
[dB/km]
L
Pout
L1
α1
L2
α2
α3
L3
α4
α5
αtotal [dB] = α1·L1 + α2 + α3·L2 + α4 + α5·L3
Attenuation is caused by:
Absorption
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atomic defects in glass composition
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impurity atoms in glass materials
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constituent atoms of the fiber material
Scattering
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microscopic variations in material density
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compositional fluctuations
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structural inhomogeneities and defects that occur during fabrication
Radiative losses
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macroscopic bends
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microbending
Gerd Keiser, Optical Fiber Communications
Attenuation depends on wavelength
Important for
spectroscopic
applications
Spectroscopic and
telecommunications
applications
Bare fibers break easily
The diameter of a bare fiber is from few tens to few hundred microns.
Bare fiber on spool:
Buffer that strengthens
the fiber
Cladding
Core
http://www.oceanoptics.com/
Buffer does not fully protect the fiber
For use indoor (buildings, laboratories)
Patchcord
For use outdoor (ground, air, water)
Cables
http://www.arcelect.com/fibercable.htm
Fiber ‘ends’ for special applications
Flame-resistant fiber probe consists of gold
buffer and nickel sleeve
http://www.oceanoptics.com/
Fibers have to be connected
Proper connection requires:
polished end-face.....
...... and good alignment
axial
end separation
angular
How to connect the fibers?
Different types of connectors.....
...and adapters
www.jdsuniphase.com
http://www.telebyteusa.com/foprimer/foch2.htm#2.6
From theory to applications
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Telecommunications
Imaging
Connection between optical elements
Spectroscopy
Couplers/splitters
Sensors
Most widely used in network and
telecommunications
Sergiusz Patela, Wroclaw University of Technology
Recently fibers have been very popular in
biomedicine
Fiber optic imaging allows to observe inaccessible areas:
Coherent fibers are individual fibers that are grouped together to form a bundle.Each fiber is
in exact relationship to the other fiber’s position in the bundle from the beginning of the bundle to the end.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Text - from Schott Fiber Optics
Fibers for light delivery
From light source to
spectrometer
Pigtailed light source
Pigtailed fiber with collimating lens
http://www.oceanoptics.com/
Fiber-optic splitters for reflection, backscattering
and fluorescence measurements
Pigtailed
light source
Sample
Spectrometer
I.Kasik & V.Matejec, M.Chomat, M.Hayer, D.Berkova, www.ure.cas.cz
http://www.oceanoptics.com/
Example - in vivo spectroscopy
The interaction of light with tissue can be
used to interrogate tissue biochemistry,
circulation and structure. The near infrared
wavelength range is ideally suited for in
vivo spectroscopy due to the absorption and
scattering properties of tissue in this region
of the electromagnetic spectrum.
http://www.ibd.nrc.ca/english/spec_e_inVivo.htm
Fiber-optic setup for absorption measurements
Spectrometer
http://www.oceanoptics.com/