6. Optoelectronic Devices

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6. Optoelectronic Devices
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Optical Waveguides
(a) A buried-in rectangular waveguide, (b) a
buried-in rib waveguide, (c) a strip-loaded
waveguide, and (d) a diffused waveguide
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Some Fabrication Processes of Optical Waveguides
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Basic Theory of Waveguides
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Theory of Planar Optical Waveguides
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Approximate Theory of Rectangular Optical
Waveguides Surrounding by a Uniform Medium
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Approximate Theory of Rectangular Optical
Waveguides Surrounding by a Uniform Medium (Cont)
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Approximate Theory of Rectangular Optical
Waveguides Surrounding by a Uniform Medium (Cont)
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Applications of Y-Branches and Bends of
Conventional Optical Waveguides
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Multimode Interference (MMI) Devices
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Example of Optical Performance of MMI Device
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1n MMI Optical Splitters
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All-optical Logic Gate Based on MMI Waveguide
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All-optical Logic Gate Based on MMI Waveguide
(Cont)
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All-optical Logic Gate Based on MMI Waveguide
(Cont)
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Photonic Crystals
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Square-lattice and Triangular-lattice Photonic
Crystals
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Band Structures of Photonic Crystals
Eg. The band structures of the 2D square-lattice
photonic crystal with the lattice constant is
a0.5µm. The radius of the pillar is Rc225nm.
And the refractive index of the pillar is
3.16227766.
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Photonic Crystals Improving LED Efficiency

• Incorporating a photonic crystal into an
  indium-gallium-nitride (InGaN) LED increases both
  the internal quantum efficiency and the amount of
  light extracted. The light is produced in the
  quantum-well (QW) active region.

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Photonic Crystals Improving LED Efficiency (Cont)
Far-field emission patterns from a conventional
(left) and a photonic-crystal LED (right) are
very different. The latter has a
strongly-modified emission pattern due to the
scattering of waveguided modes out of the LED
chip.
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Photonic Crystal Waveguides (PCWGs)
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Comparison between the Conventional Waveguides
and the PCWGs

• The conventional optical waveguides are weakly
  guided. There exist large power losses in the
  wide-angle bends/branches. However, the same
  structures made of line-defect photonic crystals
  give little losses because the lights were
  trapped by the defects of the photonic crystals.
• Most of the conventional optical waveguide
  devices can be easily modulated by EO effect, AO
  effect, and so on. But only a few photonic
  crystal waveguide devices can be modulated.

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Periodical Dielectric Waveguides (PDWGs)
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Electro-Optic (EO) Effect

• The electro-optic (EO) effect is a nonlinear
  optical effect that results in a refractive index
  that is a function of the applied electric field
  (voltage)
• Examples of Pockels effect Ammonium dihydrogen
  phosphate (ADP), Potassium dihydrogen phosphate
  (KDP), Lithium Niobate, Lithium Tantalate, etc.
• Examples of Kerr effect Most glasses, gases, and
  some crystals

Pockels effect
Kerr effect
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Phase Modulators

• Phase shift
• , where Vp (the half-wave voltage) is the voltage
  applied to achieve a phase shift of p radians.

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Mach-Zehnder Modulator to Modulate Amplitude of
Light
Output Intensity
Consider the case of f00. If VVp, then PoutPin
is the maximum, else if V0, then Pout0 is the
minimum.
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Characteristics of Optical Modulators/Switches

• Extinction Ratio ?(I0-Im)/I0 if Im?I0 and
  ?(Im-I0)/Im if Im?I0, where Im is the optical
  intensity when the maximum signal is applied to
  the modulator and I0 is the optical intensity
  with no signal applied.
• Insertion Loss Li10log(It/Im), where It is the
  transmitted intensity with no modulator and Im is
  the transmitted intensity when the maximum signal
  is applied to the modulator.
• Bandwith ?f2p/T, where T is the switching time.

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Optical Directional Coupler as a Channel Switch
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A Complicated Optical Directional Coupler
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3dB-Directional Coupler as a Beam Splitter

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Coupled-Mode Equations to Analyze Directional
Coupler
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Coupled-Mode Equations (Cont)

• The coupling length is Lcp/2?. Both Lc and ?
  depend on the refractive index distribution of
  guide.
• While the waveguiding mode traverses a distance
  of odd multiple of the coupling length (Lc, 3Lc,
  , etc), the optical power is completely
  transferred into the other waveguide. But it is
  back to the original waveguide after a distance
  of even multiple of the coupling lengths (2Lc,
  4Lc, , etc). If the waveguiding mode traverses a
  distance of odd multiple of the half coupling
  length (Lc/2, 3Lc/2, , etc), the optical power
  is equally distributed in the two guides.

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Acousto-Optic (AO) Modulators
Bragg-type Width gtgt ?2/? Raman-Nath-type Width
ltlt ?2/? ? wavelength of light ? wavelength of
acoustic wave
Bragg-type AO modulator sin?B?/2?
Raman-Nath type AO modulator sin?mm?/2?, m
integer
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Bragg-type AO Modulator as Spectrum Analyzer
Acousto-optic materials Visible and NIR Flint
glass, TeO2, fused quartz Infrared Ge High
frequency LiNbO3, GaP
Operations of Bragg-type AO modulator Bragg
diffraction effect Driving frequency 1MHz
1GHz Rise time 150 ns (1-mm diameter laser)
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Direct Coupling from Laser/Fiber to Waveguide

• Direct Coupling Efficiency

where is the laser/fiber mode and
is the waveguide mode.
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Coupling Efficiency from Laser/Fiber to Waveguide
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Coupling Efficiency from Laser/Fiber to Waveguide
(Cont)
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Coupling Efficiency from Laser/Fiber to Waveguide
(Cont)
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Simulation Results Coupling Efficiency from
Laser/Fiber to Waveguide
For given waveguides fundamental mode, one can
obtained the optimal coupling efficiency by
selecting the values of w and c.
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Typical Optical Disks
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DVD Disks
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Lasers in DVD Players
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Optoelectronic Devices in DVD Players
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Band Theory of Semiconductor Devices

• Metal The conduction band and the valence band
  may overlap.
• Semiconductor The bandgap between the conduction
  band and the valence band is very small. The
  electron can be easily excited into the
  conduction band to become a free electron.
• Insulator The bandgap between the conduction
  band and the valence band is very large. The
  electron is hardly excited into the conduction
  band to become a free electron.

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Semiconductor
Fermi energy level, EF the highest energy level
which an electron can occupy the valance band at
0k
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Bandgap Theory of Diode
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Bandgap Theory of Tunnel Diode
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Bandgap Theory of n-p-n Transistor
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• Radiation from a Semiconductor Junction

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Homojunction Laser Diode
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Formation of Cavity in Laser Diode
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Threshold Current
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Heterostructure Laser Diodes
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Stripe AlGaAs/GaAs/AlGaAs LD

• Advantages of stripe geometry
• 1. reduced contact area ? Ith?
• 2. reduced emission area, easier coupling
  to optical fibers
• Typical W a few µm, Ith tens of mA
• Poor lateral optical confinement of photons

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Buried Double Heterostructure LD

• Good lateral optical confinement by lower
  refractive index material ?stimulated emission
  rate ?
• Active region confined to the waveguide defined
  by the refractive index variation ? index guided
  laser diode
• Buried DH with right dimensions compared with the
  ? of radiation ? only fundamental mode can exist?
  single mode laser diode
• DH AlGaAs/GaAs LD
• ? 900 nm LD
• DH InGaAsP/InP LD ? 1.3/1.55 µm LD

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Output Modes of LD

• Output spectrum depends on
• 1. optical gain curve of the active medium
  
• 2. nature of the optical resonator
• L decides longitudinal mode separation. W H
  decides lateral mode separation
• With sufficiently small W H?only TEM00 lateral
  mode will exist ( longitudinal modes depends on L
  )
• Diffraction at the cavity ends ?laser beam
  divergence ( aperture ??diffraction ?)

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Current Dependence of Power Spectrum in LD

• Output spectrum depends on
• 1. optical gain curve of the active
  medium, and
• 2. nature of the optical resonator
• Output spectrum from an index guided LD
• low current ?multimode
• high current ?single mode

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Light Detectors

• Classification by spectral response
• wide spectral response
• narrow spectral response

• Principles of photodetection
• ? External photoelectric effect
• Eg. vacuum photodiode
• photomultiplier
• Internal photoelectric effect
• Eg. p-n junction photodiode
• PIN photodiode
• avalanche photodiode

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• Characteristics of Light Detectors

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• External Photoelectric Detector ? Vacuum
  Photodiode

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• External Photoelectric Detector? Photomultiplier

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• Internal Photoelectric Detector (Semiconductor
  Photodiode)

P-N photodiode
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• PIN and Avalanche Photodiodes

Operating modes (1) photoconductive mode
(reverse biased) (2) Photovoltaic mode (forward
biased)
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Typical Characteristics of Photodetectors
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• Principle of OP Circuit for Photodiodes

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Light Emitting Diode (LED)
Construction
Optical design
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Choice of LED Materials
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Typical Choice of Materials for LEDs
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Radiative Transition Through Isoelectronic
Centers

• For indirect band-gap semiconductors?use
  recombination of bound excitons at isoelectronic
  centers to generate radiative recombination
• Isoelectronic center produced by replacing one
  host atom in the crystal with another kind of
  atom having the same number of valence electrons
• Isoelectronic center attract electron and hole
  pair ? exciton radiative recombination can occur
  without phonon assistance ? h?slightly smaller
  than bandgap energy Eg
• Common isoelectronic centers
• N in GaP ? 565 nm
• N in GaAs0.35P0.65 ? 632 nm
• N in GaAs0.15P0.85 ? 589 nm
• ZnO pair in GaP ( neutral molecular center ) ?
  700 nm

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Choice of Substrates for Red and Yellow LEDs
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Material System for High Brightness Red/Yellow
LEDs
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Choice of Substrates for Blue LEDs

• Choices of light emitting material for blue LEDs
  ( before 1994 ) GaN system, ZnSe system, SiC,
  etc. And the winner is GaN

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Earlier LED Structures
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Basic Structures of High Brightness Visible LEDs
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High Brightness Blue LEDs
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Output spectra
Radiation pattern
Note response time 90ns (yellow
and red LED) 500ns (green LED)