Ch6-smith, thin film deposition principles and practice.ppt
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Transcript of Ch6-smith, thin film deposition principles and practice.ppt
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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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1×n 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 CrystalsEg. The band structures of the 2D square-lattice photonic crystal with the lattice constant is a=0.5μm. The radius of the pillar is Rc=225nm. 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 Vπ (the half-wave voltage) is the voltage applied to achieve a phase shift of π radians.
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Mach-Zehnder Modulator to Modulate Amplitude of Light
2
cos1 0
VV
II inoutOutput Intensity:
Consider the case of φ0=0. If V=Vπ, then Pout=Pin is the maximum, else if V=0, then Pout=0 is the minimum.
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Characteristics of Optical Modulators/Switches
• Extinction Ratio: η=(I0-Im)/I0 if Im≦I0 and η=(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: Li=10log(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: △f=2π/T, where T is the switching time.• Isolation: Isolation=10log(I2/I1), where I1 is the optical
intensity in the driving port and I2 is the optical intensity at the driven port when the switch is in the off state with respect to port 1 and 2.
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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 Lc=π/2κ. While the waveguiding mode traverses a distance of odd multiple of the coupling length (Lc, 3Lc, 5Lc, …, etc), the optical power is completely transferred into the other waveguide. But it is back after a distance of even multiple of the coupling lengths (2Lc, 4Lc, 6Lc, …, etc). If the waveguiding mode traverses a distance of odd multiple of the half coupling length (Lc/2, 3Lc/2, 5Lc/2, …, etc), the optical power is equally distributed in the two guides.
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Acousto-Optic (AO) Modulators
Bragg-type AO modulator:sinθB=/2
Raman-Nath type AO modulator:sinθm=m/2, m: integer
Bragg-type: l >> 2/Raman-Nath-type: l << 2/: wavelength of light: wavelength of acoustic wave
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Bragg-type AO Modulator as Spectrum Analyzer
Bragg angle:
2sin 1
d
: wavelength of light: wavelength of acoustic wave
Operations of Bragg-type AO
modulator:
— Bragg diffraction effect
— Driving frequency: 1MHz ~ 1GHz
— Rise time: 150 ns (1-mm diameter laser)
Acousto-optic materials:
Visible and NIR — Flint glass, TeO2,
fused quartz
Infrared — Ge
High frequency — LiNbO3, GaP
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Direct Coupling from Laser/Fiber to Waveguide
dxdyyxdxdyyx
dxdyyxyx
22
2
),(),(
),(),(
• Direct Coupling Efficiency:
where is the laser/fiber mode and is the waveguide mode.
)(x)(x
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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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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 0°k
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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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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 λ 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
Principles of photodetection
external photoelectric effect vacuum photodiode photomultiplier
internal photoelectric effect p-n junction photodiode PIN photodiode avalanche photodiode
Classification by spectral response
wide spectral response
narrow spectral response
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Characteristics of Light Detectors
(A)detector thefromcurrent output :
(V)detector thefrom tageoutput vol :
lm)or (W flux liminousor radiant applied :
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External Photoelectric Detector Vacuum Photodiode
ntPhotocurre
hc
Pei
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External Photoelectric Detector Photomultiplier
dynodes ofnumber :N
dynodeeach at gain :
ntPhotocurre
hc
Pei N
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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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Radiation from a Semiconductor Junction
wavelength of radiation:
where : energy gap (ev)
: wavelength of radiation (nm)
e.g. GaAs =1.43 ev, find the radiation wavelength
(nm) )ev(E
1240
(NIR) Infrared Near(nm) 87643.1
1240
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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 → hυslightly 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
Note : response time
~ 90ns (yellow and red LED)
~ 500ns (green LED)
Radiation pattern
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Optoelectronic Devices in DVD Players
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DVD Disks
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Lasers in DVD Players
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Typical Optical Disks