Fundamentals of Optoelectronics

Ideen Taeb
Jon Mah
Combination of photonics and
Advantages: Capacity to generate, transport
and manipulate data at very high rate
Photonics/Optoelectronics refer to
coexistence of electron and photons in the
same system
First transmission trunk using glass fibers
in 1983
Photon law is tripling the bandwidth every
Compared to copper wire, optical fibers cost
less, weigh less, have less attenuation and
dispersion and provide more bandwith.
 Highly used in electronic
Growing every year, 30% growth every year
since 1992
Combined market for optoelectronic
components and final end-products
currently stands at $30 billion
Electron vs. Photon
Pauli Exclusion Principle
LED- Advantage of ease of use, 160 degrees
circular cone light emission, but low in power
LD- Advantage of high power around 30 mW,
but emission in elliptical cone rather than
VCSEL- Have both high power as wells as
emission into circular cone, furthermore they
can be produced in uniform arrays on wafers
Forward biased junction
Current flows from n side to the p side
Band Gap or Energy Gap (EG): Difference of
energy between the conduction band and
valance band
Wavelength of emitted light depends on EG
Most widely used material for visible
spectrum: GaAs, GaP, and GaAsP
Forward biased p-n junctions where emmited
photons are confined in an optical cavity
Two types
◦ Edge Emitting: Wide, astigmatic emission
◦ Surface Emitting: Narrower Beam Emission
Different from LDs
and LEDs, light
emission occurs in
a direction
perpendicular to
the active region
Have a potential to
be operated at
orders of Gb/s
P-i-n Photodiodes
◦ A p-n junction with a
sandwiched intrinsic
◦ Operated in the
reverse-biased mode
◦ Response times are
in order of 10 ps.
MSM Detectors
◦ Consists of two
electrodes which
form back to back
schottky diodes.
◦ Very fast, can be
switched completely
on or off with an
applied bias
◦ Response time in in
order of 1ps
Free-Space Channels
High-speed communication (>1Gbs)
Wide BW, elimination of impedance mismatch problem
Potential for high density interconnects
Decreased interconnection delays and so on
Disadvantages caused by:
◦ Potentially require a significant change in the way system
architectures are designed
◦ Laser wavelength stabilities in the order of 1nm can be
◦ Physical size of some proposed architectures are
◦ Power inefficiencies can be limiting
◦ Dependent on weather
Guided Wave Channels
◦ Can be classified according to the interconnection
medium employed and the level of interconnection
hierarchy they target
Speed of propagation in a medium
v c
n 
• Photon Energy
E  hc
• Frequency
  2f  2c 
Speed of EM waves
in a medium
depends on
interactions with
Electric Field and
Magnetic Field
  
n1 sin(1 )  n2 sin(2 )
Critical Angle (Φ1) occurs at Φ2=90˚
critical  a sin n2 n 
• For angles larger than the critical angle, have
total internal reflection (TIR)
– Principle behind traditional waveguides
• different from photonic crystal waveguides
– Phase changes with the angle
n1 > n2, but just
Then NA is small
NA  n sin  acc  n  n
Light of different frequencies propagate at different
speeds through the medium
◦ Typical units of ps/nm-km
Due to both material (n = n(λ)) and waveguide
effects (effective n1, n2)
Birefringence caused by Polarization Effects (fiber
cross section not perfectly circular).
Higher order effects (Kerr effect)
n  npar  nperp  KE
Due to imperfections in fabrication as
well as Rayleigh Scattering
◦ Scattering due to particles smaller than λ (why
is the sky blue?)
◦ Units of dB/km
◦ For GeO2-doped single-mode silica fiber
~0.2dB/km at λ=1.55μm
 P( L) 
  log10 
 P(0) 
• Also get attenuation due to bending
Time-Division Multiplexing (TDM)
◦ E.g. Telephone lines
Frequency-Division Multiplexing
◦ E.g. FM radio
◦ Optical effect
◦ E.g. Prism
Superprisms made
from Photonic
Crystals (large
dispersion in
periodic media)
Fused Silica (SiO2) Fiber
 Can be made extremely pure, then doped to attain
desired n
 Exhibits very low loss and dispersion at λ=1.55μm
Plastic Fiber
 Lossy (~102 dB/km)
 Flexible, inexpensive, lightweight
Other Glass Fiber
 Chalcogenide, fluoroaluminate, etc. for longer
Major problems in coupling fiber
1. The fibers must be of compatible types
Dispersion effects, single mode/multi mode
Matching of NA
2. The ends of the fiber must be brought
together in close proximity
3. The fibers must be accurately aligned with
 Diamreceiving_ core
Loss  20 log10 
 Diam
launching _ core
Bragg Gratings:
constructive interference
where d=distance
between gratings
n  2d sin 
Optoelectronics market is growing every year
Optoelectronics provide a high bandwidth for
Utilize TIR for light propagation in waveguides
Dispersion and attenuation are main drivers in
optical fiber design
Interconnections and coupling require precise
alignment of optical elements
A number of inter- and intra-chip connection
schemes exist and are being explored