External Modulators
EECS 290Q
Fall, 2001
October 9, 2001
10/9/01
EE290Q Only (CCH Lecture 3)
1
Optical Modulation
n
Direct modulation on semiconductor lasers:
n
Output frequency shifts with drive signal
n
n
carrier induced (chirp)
temperature variation due to carrier modulation
n
n
Limited extinction ratio
à
because we don’t want to turn
off laser at 0-bits
Impact on distance*bit-rate product
Electro-optical modulation
Electroabsorption (EA) modulation
n
n
n
External modulation
n
n
Chirp can still exist
Facilitates integration
n
Always incur 6-7 dB insertion loss
10/9/01
EE290Q Only (CCH Lecture 3)
2
1
Modulator Basics
n
Direct
n
n
n
n
External
n
n
n
n
n
n
Simple
Cost-effective
Compact
Additional component
Additional loss
Higher speed
Large extinction ratio
Low chirp
Low modulation distortion
10/9/01
EE290Q Only (CCH Lecture 3)
3
Electroabsorption (EA) Modulator
n
EA modulator is a semiconductor device
n
Typically has the same structure as the laser diode.
n
n
Use an applied electric field (reverse bias) to
change the absorption spectrum.
Typically no carriers are injected into the active
region. However, carriers are generated due to
absorption of light.
10/9/01
EE290Q Only (CCH Lecture 3)
4
2
Advantages of EA modulators
n
n
n
n
n
Zero biasing voltage
Low driving voltage
Low/negative chirp
High speed
Integrated with DFB
10/9/01
EE290Q Only (CCH Lecture 3)
5
Schematics of an EA modulator
n
n
Waveguide type is more commonly used.
Transmission type does not lead to high enough extinction
ratio.
10/9/01
EE290Q Only (CCH Lecture 3)
6
3
Outline
n
Absorption spectrum changes with applied
electric field
n
Franz -Keldysh Effect
n
Extinction ratio enhancement in quantum well
structures
n
Quantum confined Stark effect
“Introduction to Semiconductor Optics” by N.
Peyghambarian et al. (1993).
n
Recommended reference book
n
10/9/01
EE290Q Only (CCH Lecture 3)
7
Physics behind EA Modulators
n
How absorption spectrum in semiconductors can be
changed?
n
Physical model: effective-mass equation
n
n
Single-particle representation
Two-particel representation
n
n
n
n
Coulomb interaction between electrons and holes: Excitons
Electric field effect: Franz-Keldysh effect
Coulomb+Electric field: DC Stark effect
Coulomb+Electric field+QW: QCSE
10/9/01
EE290Q Only (CCH Lecture 3)
8
4
Absorption Spectrum Change under
Applied Electric Field
n
Franz-Keldysh Effect
n
Neglect Coulomb interaction between electrons and
holes.
Franz -Keldysh effect plus Coulomb interaction between
electrons and holes (excitons).
DC Stark Effect in quantum wells
Excitons been confined in quantum well. Stark effect
enhanced.
n
DC Stark Effect
n
n
Quantum confined Start effect (QCSE)
n
n
10/9/01
EE290Q Only (CCH Lecture 3)
9
Density of States
n
0D
ρ(E)
1D
3D
n
2D
n
E
10/9/01
For a given energy range,
the number of carriers
necessary to fill out these
density of states:
3D>2D>1D>0D.
Absorption is integral of
density of states *(fv-fc)
Where fv and fc are the
fermi probabilities of a
conduction and valance
band states are occupied
by an electron,
respectively.
EE290Q Only (CCH Lecture 3)
10
5
京公网安备 11010802033920号
Copyright © 2005-2024 EEWORLD.com.cn, Inc. All rights reserved
评论