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Opto-Electronics and
Photonics
Woo-Young Choi
Dept. of Electrical and Electronic EngineeringYonsei University
Lecture 21 : Optical Gain
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Simulated emission for optical gain
è Population inversion: ‘Pump’ carriers into N2 so that N2 > N1
Optical pumping and electrical pumping possible
(Optical amplifier)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Optical pumping
Consider 3-level systems
2-level system not practical
Pump signal has the same wavelength as input/output signal
In Out
Pump
E
E1
E2
3
(1) carriers at E3 quickly come down to E2
Requirements:
(2) Carriers at E2 radiativelycome down to E1
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Optical gain materials for 3-level systems
Erbium
One of rare earth metals
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Erbium
E10
1.27 eV
0.80 eV E2
E3
1550 nm 1550 nm
InOut
980 nm
Non-radiative decay
Pump
-Pump light (l=980nm) absorbedgenerating carriers at E3
- When N2>N1 (population inversion),stimulated emission > absorption
for 1550nm light
- Carriers at E3 rapidly transfer to E2
building up N2
- Er can be easily added to core of Silica fiber
- High-power semiconductor lasers easily available for 980nm pumping source
- Very useful for optical communication applications
è EDF (Er-Doped Fiber)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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EDFA: Er-Doped Fiber Amplifier
A key component for long-distance optical communication systems
Roughly, one EDFA for every ~100km fiber
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Other optical gain materials
- Crystals doped with impurities: Ruby doped with Cr (Al2O3:Cr3+)
Used for first laser demonstration
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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- Gases:
(1s2)
(1s12s1)
0
20.61 eV
He
(2p6)Ground states
(2p55s1)Ne
(2p53p1)
(2p53s1)
Collisions
Lasing emission632.8 nm
~600 nm
Collisions with the walls
Fast spontaneous decay
20.66 eV
Electron impact
The principle of operation of the He-Ne laser. He-Ne laser energy levels(for 632.8 nm emission).
?1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Other optical gain materials
Mixture of He and Ne gases (9:1)
è Used for HeNe lasers
High voltage applied
è He plasma (He ion + electrons)
èEnergetic electrons collide with ground-state He electronics
è Collision between He and Ne
Electrical pumping
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Semiconductors with direct bandgap
- Continuous energy levels across the band gap
Conduction band: more holes than electrons
Valence band: more electrons than holes
(E-k diagrams: Dispersion diagram for electron waves)
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Interaction with photons if hv > Eg
Unlike 2-level, 3-level systems, continuous spectrum for absorption and emission
Absorption SpectrumFor GaAs12 2 1 12 1 1 2 2( ) ( ) ( ) ( )R h E E B N E P E hn r n= - = × × ×
èMore absorption for larger photon energies
More electrons as E1 becomes smaller
More holes as E2 becomes larger
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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12 12 1 1 2 2( ) ( ) ( ) ( )R h B N E P E hn r n= × × ×
Absorption
21 21 2 2 1 1( ) ( ) ( ) ( )R h B N E P E hn r n= × × ×
Stimulated Emission
( )1 1 2 2 2 2 1 1( ) or ( ) ( )?N E P E N E P E× ×Which is larger?
Depends how much pumping and photon energy (E2-E1)
Spontaneous emission not considered
è Noise
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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How to electrically pump electrons and holes?
hu Eg
Eg
V
p n+
Electron in CBHole in VB
Forward-biased PN junction
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Optical Amplifier
Pout = G Pin
Pin Pout
E1
E2
pumpOptical Gain Material withlength z = exp(gz) Pin
Eout = exp(-jkz)Ein
k = ,β jα-
outE = exp( )exp( ) inj z z Eb a- -
outP = exp( 2 ) inz Pa-
2g a= -
Opto-Electronics and Photonics (2020/2) W.-Y. Choi
Lecture 21: Optical Gain
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Homework ( Due on 12/4)
(a) Determine the expression for N2 – N1 at the steady-state in which dN2/dt = - dN1/dt = 0.
(b) Determine R required for transparency (N2 =N1)
Assume t32 is very small so that N3 = 0, t21 = t,
2 22 1
dN NR WN WNdt t
= - - +
1 22 1
dN NR WN WNdt t
= - + + -
N1 + N2 = N