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3.1 Introduction
3.1.1 Components of Optical Transmitters
Binary to single
Coding/line coding
Modulator
Optical Source
Driving Circuit
PCM
Channel coupler
Optical signal output
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Biased current
Modulation current
(≥10Gb/s)
Modulation current
Biased current
(≤2.5Gb/s)
(a) Direct Modulation
(b) External Modulation
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1. stability : power & wavelength
2. reliability > 25 years (Pout to Pout /2)
3. small emissive area compatible with fiber core dimensions
4. right wavelength range
0.85 µm : GaAlAs/GaAs
1.31 µm, 1.55 µm : InP/InGaAsP
5. narrow linewidth → Dispersion
6. direct modulation
7. high efficiency: MQW-LD Ith ~ 10 mA
3.1.2 Requirements for Optical Source
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3.2.1 Three fundamental transition processes
1. Spontaneous Emission → LED
2. Stimulated Emission → LD, SOA
3. Stimulated Absorption → PIN / APD
3.2 Basic Concepts
Light Emission
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3.2.2 Emission and Absorption Rates
E2
N2
N1
E1
spectral density of the
electromagnetic energy
In thermal equilibrium :
kB: Boltzmann Constant
T: Absolute Temperature
According to Boltzmann Statistics :
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In the visible or near-infrared region at room temperature
thermal equilibrium laser operation
N2>N1, Rstim>Rabs (population inversion)
Conclusion:
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Energy bands in semiconductor
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The occupation probability for electrons in the conduction and
valence bands is given by the Fermi-Dirac distributions:
Efc, Efv are the Fermi levels in conduction band and valence band respectively
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joint density of states :
the number of states per unit
volume per unit energy range
Eg: bandgap
mr: reduced mass
mc, mv: effective masses of electrons & holes in
conduction and valence bands, respectively
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Population-inversion condition:
in thermal equilibrium:
pumping energy into the semiconductor from an external energy source
a forward-biased p-n junction
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1. Type of semiconductor
Intrinsic semiconductor:
undoped, Fermi level, lying in the middle of the bandgap
n-type semiconductor:
Fermi level moves toward the conduction band as the dopant concentration increases
p-type semiconductor:
Fermi level moves toward the valence band as the dopant concentration increases
3.2.3 p-n junctions
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n-type
Intrinsic
p-type
forward biased
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Forward biased p-n junctions
(a) in thermal equilibrium
(b) under forward biased
2.
under forward biased:
built-in electric field is reduced
diffusion of electrons and holes across the junction
generate light through spontaneous emission or stimulated emission
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P
N
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3. (1) Homojunction:
the same semiconductor material
wide region for electron-hole recombination
difficult to realize high carrier densities
(2) Heterojunction: different bandgaps
(3) Double-heterojunction: sandwiching a thin layer between the
p-type and n-type layers such that the bandgap of the sandwiches layer is smaller than the layer surrounding it.
Homojunction & heterojunction
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Double-heterojunction
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Active layer: light is generated inside it as a result of electron-hole recombination
higher density of carriers →higher index → waveguide (1D)
Heterojunction: confinement of charge carriers & the optical field
0.85µm: cladding/active: GaAlAs/GaAs
1.31µm, 1.55µm: cladding/active: InP/InGaAsP
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1. electron-hole recombination
3.2.4 Nonradiative Recombination
Trap of defects
Surface recombination
Auger
Nonradiative recombination
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2. internal quantum efficiency
Rrr : radiative recombination rate
Rnr : nonradiative recombination rate
Rtot : totale recombination rate
τ : recombination time
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Nonradiative recombination is harmful to devices!
positive feed back
E
0
E
0
k1
k2
(1) direct-bandgap (GaAs, InP)
(2) indirect-bandgap (Si, Ge)
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3. carrier lifetime
A : defects & traps
B : spontaneous radiative
C : Auger
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Quality of the heterojunction interface depends on the lattice constant of the two materials. (matching !)
3.2.5 Semiconductor Materials
ternary compound
quaternary compound
0.85µm: GaAlAs/GaAs (cladding/active)
1.31µm, 1.55µm: InP/InGaAsP (cladding/active)
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3.4 Semiconductor lasers (Laser Diodes)
Advantages of stimulated emission compared with spontaneous emission of semiconductor materials
emitting high power (to 100mW)
narrow angular spread
narrow spectral width
direct modulation at high frequency (to 10GHz, because is small)
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Components of Semiconductor Lasers
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z=0
z=L
Injection current
Gain medium
Resonant cavity
Resonant cavity
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3.4.1 Optical Gain
Peak gain of medium:
when
: differential gain (gain cross section)
: injection carrier density
: transparent carrier density
: threshold carrier density
NT is equal to Nth ?
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Figure 3.9: (a) Gain spectrum of a 1.3-μm InGaAsP laser at several carrier densities N. (b) Variation of peak gain gp with N. The dashed line shows the quality of a linear fit in the high gain region.
Blue or red shifting of peak wavelength vs. injected current ?
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3.4.2 Feedback and Laser Threshold
Feedback
R1
R2
n0=1
n
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Threshold
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Amplitude condition
Phase condition
spacing of oscillating frequency
oscillating frequency
threshold gain
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MLM
Loss
SLM
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3.4.3 LD Structures
Broad-area LD
Figure 3.12: A broad-area semiconductor laser. The active layer (hatched region) is sandwiched between p-type and n-type cladding layers of a higher-bandgap material.
light-confinement mechanism in the direction perpendicular to the junction plane introduced by double heterostructure
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no such light-confinement mechanism in the lateral direction parallel to the junction plane.
the light generated spreads over the entire width of the laser.
relatively high threshold current and a spatial pattern that is highly elliptical and that changes in an uncontrollable manner with the current.
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Gain-guided semiconductor lasers
Figure 3.13: Cross section of two stripe-geometry laser structures used to design gain-guided semiconductor lasers and referred to as (a) oxide stripe and (b) junction stripe.
Stripe lasers
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solve the light-confinement problem by limiting current injection over a narrow stripe.
the spot size is not stable as the laser power is increased
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Index-guided semiconductor lasers
Figure 3.14: Cross section of two index-guided semiconductor lasers: (a) ridge-waveguide structurefor weak index guiding; (b) etched-mesa buried heterostructure for strong index guiding.
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3.5 Laser Characteristics
3.5.1 CW Characteristics
For a single-mode laser, the rate equations:
P, N: number of photons & carriers
Net rate of stimulated emission—optical gain:
Photon lifetime:
CW operation
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threshold current
Spontaneous emission
Stimulated emission
P-I Curves
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Ith (T)
T0: characteristic temperature
GaAs: T0=120K, InGaAsP: T0=50 ~ 70K
Bending of P-I curves
Rnr: mainly depending on Auger recombination in InGaAsP LD
Solution: built-in thermoelectric cooler used to deal with temperature
sensitivities of InGaAsP Lasers
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Threshold of current & carrier
For I>Ith
threshold
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3.5.2 Small-Signal Modulation
:amplitude-phase coupling parameter, ex. bulk material: 4~8; MQW: ~3
Frequency response
small-signal modulation:
Frequency chirp
Modulation:
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Figure 3.21: Measured (solid curves) and fitted (dashed curves) modulation response of a 1.55-μm DFB laser as a function of modulation frequency at several bias levels.
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Frequency chirp
3.5.3 Large-Signal Modulation
leading edge: mode frequency
shifts toward the blue side
trailing edge: mode frequency
shifts toward the red side
External modulation for high speed transmission!
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Electro-optical Delay & Relaxation Oscillation
Pre-biased to reduce delay time!
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Self-pulsation
不同于张弛振荡,没有阻尼,脉动频率范围为0.2~4GHz
容易发生在阈值附近和P-I特性的扭曲区
造成自脉动的机理涉及量子噪声效应、有源区的缺陷及温度感应的变化等因素
抑制这种现象主要靠控制材料的质量,尽量减少有源区的缺陷。
Operated far from kink zone!
O
P
I
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Pattern effect
TB
I
P
biased above threshold!
“11”
“11”
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3.6.1 Basic concept
Analog & Digital Modulation
3.6 Transmitter Design
(a) LED analog modulation (b) LED digital modulation (c) LD digital modulation
for LD, biased above threshold!
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Digital Logic Electrical Level
0 1
TTL: 0 ~ 0.8V 2.0 ~ 5.0V
(-5V) ECL: -1.75 V -0.85 V
(+5V) PECL: +3.25 V +4.15 V
Extinction Ratio
P
P1
P0
0
t
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Source-fiber coupling
packaging
source
fiber
Rf
coating
lensed fiber
die
submount
PD
heat sink
TEC cooler
fiber
metal shell
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Butterfly packaged LD
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External Modulator
LiNbO3 modulator in Mach-Zehnder configuration
V
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EA
0
V=0
V(t)
T
T1
T2
λ
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3.6.2 Driving circuit
Digital modulation circuit with APC for LD
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射极耦合电路
三极管VT1和VT2是轮流截止和导通的,避免了载流子恢复时间的影响,因而可工作于更高的速率
射极耦合电路为恒流源,总电源电流可以保持不变,所以电源电流噪声小
D1和D2是温度补偿二极管,由于D1、D2、VT2和VT3的导通电压分别有-2.5mV/C的负温度特性,利用D1、D2对VT2、VT3的温度特性进行补偿,使温度变化时驱动电流保持恒定。
LD
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Ib & Im
LD的驱动电路中偏置电流和调制电流大小的选取至关重要,偏置电流的选择合适与否直接影响激光器的高速调制输出特性。
加大直流偏置,使其接近阈值,可以减小电光延迟时间,也可使张驰振荡得到一定程度的抑制。
当激光器偏置在阈值附近时,较小的调制电流就能得到足够高的输出光脉冲,调制效率较高,而且由于偏置电流与最大电流相差不大,可以大大减小码型效应和结发热效应的不良影响。
过大的直流偏置电流会使消光比恶化,影响接收机灵敏度。
激光器恰好偏置在阈值时,散粒噪声会增强,直接影响信号的信噪比。
因此偏置电流的选取应兼顾上述各种影响,根据所用光源的特性与具体系统的要求适当选取。