Modulators and
Semiconductors
ERIC MITCHELL
Acousto-Optic Modulators
• Based on the diffraction of light though means of sound waves travelling
though a median
• The quartz crystal has a piezoelectric transducer attached at the end that
propagates strong acoustic waves within the crystal.
• acoustic waves generated by applying an RF signal to the transducer
Common Materials
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Lithium Niobate LiNbO3
Tellurium Dioxide TeO2
KTP – Potassium Titanyl Phosphate KTiOPO4
Lead Molybdate
Glass
Germanium
Quartz
Acousto-Optic Modulators
Scattering:
• Ultra sound waves cause Bragg-scattering
• scattering efficiencies >90
𝐼𝐷𝑖𝑓𝑓𝑟𝑎𝑐𝑡𝑒𝑑
𝜔𝑙
𝜋𝑙
𝑛6 𝑝2
2
2
= sin
∆𝑛 = sin
𝐼𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡
2𝑐
2λ 𝜌𝑣𝑠3 𝐼𝑎𝑐𝑜𝑢𝑠𝑡𝑖𝑐
0.6328
= sin 1.4
𝑙 𝑀𝜔 𝐼𝑎𝑐𝑜𝑢𝑠𝑡𝑖𝑐
λ𝜇𝑚
n: index of refraction
Δn: change induced by acoustic wave
l: path length in the medium
ω: frequency
ρ: material density
λ: light wavelength
vs: speed of sound in the medium p: photoelastic constant of the medium
Mω: diffraction figure of merit of the material relative to water
2
𝐼𝐷𝑖𝑓𝑓𝑟𝑎𝑐𝑡𝑒𝑑
𝐼𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡
is the fraction of power of the incident beam and diffracted
beam
Optical Electronics, Yariv
Semiconductors
Basic Characteristics:
•
Small energy gap between valence and
conduction band (~1.1eV)
• Insulating material have a very large energy
gap (~5.0 eV)
• Semiconductors allow for control of
conductivity with impure atoms called
dopants
• Electrons can easily be excited to
conduction band leaving behind holes
• Absorption of light increases strongly with
frequency
Semiconductors
Laser diode:
•Electrical bias across the laser diode causes
electrons and holes to be injected from opposite
sides of the p-n junction into depletion region
•If electron and holes appear in the same region
they may recombine and result in spontaneous
emission
• The difference between the photon emitting
semiconductor laser and the photon emitting
(nonlight emitting) semiconductor junction
diodes lies in the use of different types of
semiconductors
• These are the "direct bandgap" semiconductors
Examples:
• Gallium arsenide
• Indium phosphide
• Gallium nitride
Semiconductors
Photodiodes:
• Capable of detecting radiation energy hv > Eg
• Photodiodes have an intrinsic high resistivity (i) region in which
most of the potential drop occurs
• Photons are absorbed in the intrinsic high resistivity (i) region
• Carriers can then efficiently contribute to the photo current
• Thicker depletion region = more efficient collection of carriers
=larger quantum efficiency = higher detection bandwidth
• Most p-i-n junctions are based on silicon
Semiconductors
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Laser amplification:
When excited electrons in the active region return to
ground state, amplification occurs
R1 and R2 are responsible for the optical feedback in
amplifier
Advantages:
Integrated optical amplifiers are smaller in length than
fiber amplifiers
Possibility of very large gains ( >20db) in short
semiconductor chip ( <400 micrometers)
Disadvantage:
The presence of residual reflections and the resulting need
for optical isolators
reflections can give rise to instabilities and noise
Electro-Optic Modulation
• Combination of properly oriented polarizers and birefringent crystals
to control the amount of light transmitted in a optical system
The electro-optic effect:
• When a voltage is applied across a crystal it induces
birefringence
• Birefringence crystal: an incident light rays will separate into
two rays and scatter in different directions.
• Birefringence will be increasing function of applied voltage
• Transmission will be a an oscillatory function of applied voltage
Consists of rotating retardation plate
example: “Quarter wave” (Γ = π/2)
Electro-Optic Modulation
Transmission factor:
𝐼𝑜
𝛤
= 𝑠𝑖𝑛2 = 𝑠𝑖𝑛2
𝐼𝑖
2
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𝜋 𝑉
2 𝑉𝜋
Amplitude modulation:
modulator is biased with retardation (Γ )
50 percent transmission corresponds too
applied voltage of V = Vπ/2 or Γ = π/2
small applied sinusoidal voltage modulates
transmitted intensity about the bias point
maximum transmission known as half-wave
voltage
Γ is related to V in the equation Vsin(ωt)
𝐼𝑜
𝛤
= 𝑠𝑖𝑛2 = 𝑠𝑖𝑛2
𝐼𝑖
2
𝜋 𝑉
π Γ
= sin2 [ + sin(ω𝑡)]
2 𝑉𝜋
4 2
Electro-Optic Modulation
Intensity modulation:
• can be achieved using crossed polarizers or by means of
interference
• the interference of modulated and unmodulated light can be
achieved with a waveguide interferometer.
• i.e. Mach-Zehnder Modulator
• high speed telecommunication employ the use of a phase
modulator in conjunction with a Mach-Zehnder Interferometer
References
Yariv, Optical electronics
Oshea, Rodes An Introduction to Laser and their Applications
Silfvast, T. William Laser Fundamentals
Simcick, John ELECTRO-OPTIC AND ACOUSTO-OPTIC DEVICES
What When how, Optical Time-Division Multiplexed Communication Networks Part 5