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Transcript
may be regarded as a form of electromagnetic
radiation, consisting of interdependent, mutually
perpendicular transverse oscillations of an electric and
magnetic field. It forms a narrow section of the
the wavelength range being approximately 390nm
(violet) to 740nm (red). According to the quantum
theory, light is absorbed in packets of light quanta, or
photons.
Source: Dictionary of Physics
Oscillations and Waves
Oscillation – a periodic variation of any physical quantity
Wave – oscillation of an extended medium which transmits a disturbance
Some definitions:
Amplitude - the difference between the maximum displacement and minimum
displacement of the wave.
Cycle (Period), T - one complete oscillation of a periodic wave, after which the
wave is returned to its original form. (measured in sec)
Frequency, f - the number of cycles that a periodic wave undergoes per
second. ( measured in Hz = 1/sec)
Wavelength,  - the distance from one peak to the next of a periodic wave.
(measured in m)
Electromagnetic (EM) Waves
These are produced by vibrating charges, either positive (protons) or
negative (electrons).
EM waves are described as all other waves –
Amplitude – magnitude of the electric (or magnetic) field
Intensity – proportional to (Amplitude)2
Frequency – color
Wavelength
Definition: Spectrum – a range of frequencies
EM travel in empty space at the speed of light –
c = 299,792,457 m/sec  3×108 m/sec
Source: http://micro.magnet.fsu.edu/primer/java/polarizedlight/emwave/index.html
Polarization
The wave on the left has vertical polarization and
the wave on the right has horizontal polarization.
Light in transparent media
Glass and other transparent media transmit light, which travels at different
speeds inside of various materials (media). The speed is given in terms of a
parameter called the refractive index, denoted by n, of the medium. The
wavelength of a light wave inside a medium also depends on the refractive
index.
The refractive index, n:
c  3×108 m/sec
n1
c
n
speed in medium
.
In air n  1
medium, n = 2
air, n  1
Light rays bend when traversing boundaries between media
with different refractive index:
in
n1
n2


n1

out
Snell’s Law
n1 sin   n2 sin 
See http://micro.magnet.fsu.edu/primer/java/scienceopticsu/refraction/index.html
Light refraction
When a wave moves from one medium into another in which the light’s
speed is different, the direction of the wave’s travel bends. The wavefronts
remain continuous across the boundary between the two media.
n1
wavefront
n2 > n1
n2 > n1
n1
Some values for the refractive index of common optical materials
MEDIUM
n(visible)
vacuum
1
air
1.0003
water
1.3
glass
1.5
diamond
2.4
gallium
arsenide
3.5
Total internal reflection
n > n’
If light traveling inside a medium with a higher refractive index than the
surrounding medium, and it hits the inner surface of the medium at a steep
enough angle, then the light is reflected completely. This angle is known as
the “critical angle”. This is the basis of optical fiber, which is used to transmit
light over long distances.
Angle smaller than
the critical angle
Angle equal to the
critical angle
Angle greater than the
critical angle:
Total Internal Reflection
See http://micro.magnet.fsu.edu/primer/java/refraction/criticalangle/index.html
Optical Waveguides and Fibers
n > n’ always
Light is guided by total internal reflection
Slab waveguide
n’
out
in
n
n’
Confines light by total internal reflection only along one direction in space
n > n’ always
Optical fiber
size
n’
n’
Cladding
n
Core
~ 100 μm
1 – 10 μm
n’
n’
n’
n
n
n’
n’
Optical fibers are cylindrical waveguides,
providing light confinement by total
internal reflection along all directions
which are perpendicular to the
propagation direction. These are
essentially bendable “light pipes”.
Cross-section
Optical loss in fiber-quality fused silica. (circa 1995)
Fibers are made of ultrapure
SiO2 glass (silica). Different
dopants are added both to the
core and cladding, such that the
refractive index of the core is
slightly larger than that of the
cladding.
Communications
window
Optical loss in fiber-quality fused silica. (circa 2001)
To optimize fibers for
telecommunications applications it
was necessary to purify them to a
very high degree and remove all
traces of water. This eliminated the
high absorption losses in the
“communications window”.
Fiber-Optic Communications Systems
Laser
Light pulses travel in fiber (short or long)
Input electric pulses ~10Gb/sec
Output electric pulses
Example of fiber-optical communication link. Electrical current pulses
representing digital data drive a semiconductor laser. The emitted light
pulses pass through a fiber and are detected by a photo-detector at the
far end.
Amplifying optical signals
How far can an optical signal (light) travel in fiber before absorption causes
significant losses and signal deterioration?
Fibers can typically transmit
information over a distance of
80km, after which signals require
amplification and/or regeneration.
Fibers also have a very large
bandwidth – the communications
window where absorption losses in
the fiber are small is broad. This
allows transmitting many
wavelengths (frequencies)
simultaneously.
Communications window
EDFA – Erbium Doped Fiber Amplifier
Amplifiers can be integrated into the fiber, by doping fibers with Erbium atoms.
Pump
laser
Laser
In the amplifier, Erbium atoms are pumped by a separate pump semiconductor
laser (PSCL). Once in the excited state, these atoms will undergo stimulated
emission when the signal pulses arrive at the EDFA. In this way, energy from the
EDFA is added to the signal pulses, leading to their amplification.
Connecting fibers – optical communications systems
Different frequency
for each channel
MUX = Multiplexing
DEMUX = Demultiplexing
SCL = semiconductor laser
Mod = modulator
Det = detector
Multiplexing and Demultiplexing optical signals
prisms
diffraction
gratings
Techniques for multiplexing and demultiplexing. Prisms or diffraction
gratings deflect light beams into different angles depending on their
frequencies.
Some useful applets: http://mapageweb.umontreal.ca/hamamh/Fiber/FibNet.htm
For tutorials about light refraction and total internal reflection see
http://micro.magnet.fsu.edu/primer/java/refraction/index.html
To visualize injection of light into optical fibers and fiber networks see
http://mapageweb.umontreal.ca/hamamh/teach.htm