IV. Laser Diode (LD) or Semiconductor Laser
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Operation Mechanism
Characteristics of LD
LD Design (1): control of electronic properties
LD Design (2): control of optical properties
Advanced LD Structures
Applications of LD
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Introduction to the Semiconductor Laser
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LASER — Light Amplification by Stimulated Emission of Radiation
The Laser is a source of highly directional, monochromatic, coherent light.
The Laser operates under a “stimulated emission” process.
The semiconductor laser differs from other lasers (solid, gas, and liquid lasers):
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small size (typical on the order of 0.1 × 0.1 × 0.3 mm3)
high efficiency
the laser output is easily modulated at high frequency by controlling the junction current
low or medium power (as compared with ruby or CO2 laser, but is comparable to the
He-Ne laser)
 particularly suitable for fiber optic communication
 Important applications of the semiconductor lasers:
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optical-fiber communication, video recording, optical reading, high-speed laser printing.
high-resolution gas spectroscopy, atmospheric pollution monitoring.
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From LED to LD: Improvement by an Optical Cavity
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Comparison between an LD and LED
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Laser Diode
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Stimulated radiation
narrow linewidth
coherent
higher output power
a threshold device
strong temperature dependence
higher coupling efficiency to a fiber
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LED
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Spontaneous radiation
broad spectral
incoherent
lower output power
no threshold current
weak temperature dependence
lower coupling efficiency
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Stimulated Emission
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Stimulation emission
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The two basic requirements for a stimulated emission process to occur:
(1) providing an optical resonant cavity to build up a large enough photon field
– a very large photon field energy density (12) will enhance the stimulated emission
over spontaneous emission
(2) obtaining population inversion condition
– under the population inversion condition (n2 > n1) the stimulated emission is to
dominate over absorption of photons from the radiation field
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Optical Resonant Cavity
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Optical resonant cavity
– parallel reflecting mirrors to reflect the
photons back and forth, allowing the
photon energy density to build up.
The Fabry-Perot faces (cavity)
– The reflecting ends of the laser cavity
– The gain in photons per pass between
the Fabry-Perot faces must larger than
the losses (such as the transmission
at the ends, scattering from impurities
absorption, and others)
In the semiconductor laser, optical resonant
cavity is made by cleaving.
– Cleave the oriented sample (GaAs)
along a crystal plane (110), letting the
crystal structure itself provide the
parallel faces.
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Resonant modes of a laser cavity
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Longitudinal modes
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Lateral modes
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determine the output-light wavelength
leading to subpeaks on the sides of the
fundamental modes, and resulting in
“kinks” in the output-current curve.
suppressed by the “stripe-geometry”
structure
Transverse modes
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generating “hot spots”
suppressed by “thin active layer “ design
 Suppressing lateral and transverse mode
is necessary to improve the performance
of lasers.
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Single-mode laser: the laser operates in the
fundamental transverse and lateral modes but
with several longitudinal modes.
Single-frequency laser: the laser operates in
only one longitudinal mode.
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Longitudinal modes of a laser cavity
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For stimulated emission, the length L of
the cavity must satisfy the condition (for
resonant):
m [ 0 / 2n ] = L or m 0 = 2 n L
m is an integral number and  is the
refraction index in the semiconductor
corresponding to the wavelength 0 (n is
generally a function of  0)
The separation 0 between the allowed
modes in the longitudinal direction is
0 2 
1
 dn
1  0
 m
 o 
2 Ln 
n d0 


Since dn/d0 is very small,
 0  02 / 2Ln (for m = 1)
For typical GaAs laser of  0 = 0.94 nm,
n = 3.6 and L = 300 m,   0 = 4 Å.
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Population Inversion (1)
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Forward biasing a p-n junction formed
between degenerate semiconductors
under high-injection condition.
 Population inversion appears about the
transition region
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The condition necessary for population
inversion is (EFC - EFV) > Eg
where EFC, and EFV are the quasi-Fermi
levels
 In the figure shows then energy diagrams
of a degenerate p-n junction
(a) at thermal equilibrium
(b) under forward bias
(c) under high-injection condition
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Population Inversion (2)
(a) incoherent (spontaneous) emission
 EFC - EFV > h > Eg
(b) laser modes at threshold
 There modes correspond to
successive numbers
of integral half-wavelengths fitted within
the cavity
(c) dominant laser mode above threshold
 h = Eg
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Carrier and Optical Confinement
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Carrier and Optical Confinement can be
obtained by using the heterostructure
design in the LD
Carrier Confinement
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reduce the threshold current density
laser can operate continuously at room
temperature
Optical Confinement
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confinement factor  : the ratio of the
light intensity within the active layer to the
sum of light intensity both and outside the
active layer
 = 1 - exp ( - C n d )
n : the difference in the reflective index
d : the thickness of the active layer
 the larger the n and d are, the higher
the  will be
 Optical confinement provides effective
wave-guide for optical communication
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Homojunction and Heterojunction Laser
 Homojunction Laser
 pulse mode output
 large threshold current density
 operated at low temperature
 broad spectral width of output light
 Improvement  Heterojunction Laser
 Heterojunction Laser
(1) Single-Heterojunction Laser (SH Laser)
(2) Double-Heterojunction Laser (DH Laser)
(3) Stripe-geometry DH Laser
(4) Single quantum well (SQW) Laser
(5) Multiple quantum well (MQW) Laser
(6) Strained layer superlattice (SLS) structure
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Double-Heterojunction (DH) Laser
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Threshold Current Density
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Gain (g)
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the incremental optical energy flux per unit
length
Threshold Gain
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the gain satifies the condition that a light
wave makes a complete traveral of the
cavity without attenuation
1 1
g     n 
L R
 is the confinement factor,  is the loss per
unit length, L is the length of the cavity, R is
the reflectance of the ends of the cavity
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Threshold Current Density (Jth)
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the minimum current density required for
lasing to occur

1  1 
  ln  


g 0 
L  R 
To reduce Jth, we can increase , , L, R
and reduce d, 
J th 
–
J 0d
 J0
d
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Characteristics of the DH laser
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Threshold current density vs. active
layer thickness
– The threshold current density decreases
with decreasing d, reaches a minimum,
and then increases. The increase of Jth at
very narrow active thickness is caused
by poor optical confinement.
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Output power vs. diode current
– The light-current characteristics is quite
linear above threshold.
Temperature dependence
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The threshold current increases exponentially
with temperature  Jth ~ exp [ T/T0 ]
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Emission Spectra of the typical DH laser
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Emission spectra of a perfect laser
– above the threshold, the laser may
approach near-perfect monochromatic
emission with a spectra width in the
order of 1 to 10 Å.
High-resolution emission spectra
(of a typical stripe-geometry DH laser)
– Sub-peaks, which are evenly spaced
with a separation of  = 7.5 Å, appear
in the spectra. belong to the longitudinal
modes.
– Because of these longitudinal modes,
the stripe geometry laser is not a
spectrally pure light source for optical
communication.
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Design considerations for laser diode performance
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Low threshold current
– low threshold can be generated by electronic devices which can be modulated at high
speed to provide a high speed modulation in the output
(1) reducing the active layer thickness (d)
↣ Quantum-Well (~ 50 - 100 Å), Strain Quantum-Well
(2) N-doped active region
(3) Stripe geometry
Lateral confinement
– to avoid the “kink” effect, which produces noise in the optical transmitter
 reduce the lateral dimension of the Fabry-Perot cavity
 (1) Stripe geometry (Gain-guided cavity)
(2) Buried heterostructures
Selective Optical Cavity
– to reduce the laser linewidth
 (1) Distributed Feedback (DFB) structures
(2) Buried heterostructures
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Stripe Geometry Laser
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Using the “gain-guided cavity” to carry out the
lateral confinement
Advantages of a stripe geometry structure
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Removing side peaks from the main modes by
suppression of the lateral mode.
Reducing the threshold current
less stringent demands on fabrication (because
of the smaller active volume and the greater
protection offered by isolating the active region
from an open surface along two sides)
Fundamental mode operation is valid for all stripe
widths below 10 - 15 µm.
Different types of stripe-geometry structure:
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oxide stripe
implantation
selective diffusion
Mesa stripe
buried heterostructures
ridge structures
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Single Frequency Laser
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Single frequency lasers is desirable in the optical
fiber communication system to increase the
bandwidth of an optical signal.
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This is because light pulses of different
frequencies travel through optical fiber at
different speeds thus causing pulse spread.
 Dispersion mechanisms for a step-index fiber:
(1) intermodal dispersion
(2) waveguide dispersion
(3) material dispersion
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Dispersion effects can be minimized by using
long wavelength sources of narrow spectral
width (a single frequency laser) in conjunction
with single mode fibers.
 Methods to achieve the single frequency lasers:
(1) Frequency Selective Feedback
– External Grating, Distributed-Feedback
(DFB), Distributed Bragg Reflector (DBR)
(2) Coupled Cavity
– Cleaved Coupled Cavity (C3) laser
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Distributed Feedback (DFB) Laser
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In periodic structures, special effects occur when
the wavelength of the wave approaches the
wavelength of the periodic structure. In
semiconductor crystals, this leads to bandgaps
and Bragg reflections.
The wavelength selective periodic grating with a
corrugated structure, made by E-beam lithography
and RIE, is incorporated into to the laser.
The period of the grating is d = 2qB /2n
where B is the Bragg wavelength give by
0  B
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
m  1 
2
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2
B
2nL
where 0 is the oscillating wavelength
DFB lasers have been made with sawed end
facets or with antireflection coating to suppress
the Fabry-Perot modes.
The DFB laser’ main advantage is its very small
temperature dependence.
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Distributed Bragg Reflector (DBR) Laser
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In the DBR laser, the period reflecting mirror
stack is placed outside the active lasing region.
The advantages of the DBR lasers:
– high coupling efficiency between the
active lasing region and the passive
waveduide structures.
– the wavelength of the output light is
tunable.
The reflective index of the stack is alterable by
current injection.
The wavelengths that get the highest feedback
must satisfy
B = 2 q (nr1 d1 + nr2 d2)
where is a positive integer
The values of nr1 d1 and nr2 d2 can be altered
electronically, therefore can have a certain
degree of wavelength tunability
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Cleaved-Coupled-Cavity (C3) Laser
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The C3 laser consists of two standard FabryPerot cavity laser diodes which are selfaligned and very closely coupled to form a
two-cavity resonator.
Because the laser light has to travel through
an additional cavity (modulator), the only
radiation that is reinforced is at a wavelength
that resonates both in the laser’s cavity and
also in the modulator.
The two cavities can have their currents
controlled independently and this is the main
advantage of the C3 laser.
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Quantum Well Laser
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If the thickness of the active region Ly is
made small enough (Ly ~ the “de Broglie
wavelength”  = h/p < 500Å, depending on
the materials for GaAs, Ly ~ 20 nm), the
carriers are confined in a finite potential well
in which the energy band splitting into a
“staircase” of discrete levels (the quantization
effect)
E-h recombination can only occur with
“n = 0 transition” in the quantum well.
In a quantum well (QW), a large number
electrons all of the same energy can
recombine with a similar block of holes.
Hence, a QW laser should gives a much
narrower output wavelength, unlike the other
lasers with the bulk effect, where
recombining carriers are distributed in energy
over a parabolically varying density of states
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Multiple Quantum Well (MQW) Laser
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Several single quantum wells are coupled into a
“multiple quantum well (MQW)” structure.
The significantly reduced temperature sensitivity
of MQW lasers has been related to the staircase
density of states distribution and the distributed
electron and photon distributions of the active
region.
This optical confinement helps to contain the
otherwise large losses from a narrow active
region, leading to low threshold currents.
An MQW is the active region of a laser that can
emit a single frequency at several different
wavelengths, known as a multiple array grating
integrated cavity (MAGIC) laser.
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Graded Index Separate Confinement Heterostructure
(GRINSCH) Laser
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GRaded INdex Separate Confinement
Heterostructure (GRINSCH) Laser
A narrower carrier confinement region (d) of
high recombination is separated from a wider
optical waveguide region
Optical confinement can be optimized
without affecting the carrier confinement
GRINSCH-SQW and GRINSCH-MQW
The threshold current for a GRINSCH is
much lower than that of a DH laser
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For a standard DH laser, both mirror and
absorption losses increase rapid for thin active
region, leading to very high threshold current.
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GRINSCH Laser
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Vertical Cavity Surface Emitting Laser (VCSEL)
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The structure of an VCSEL is very much like a
standard heterojunction LED.
Advantages of the VCSEL:
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the possibility of single frequency operation due
to the short cavity
the removal of the fragile cleavage process that
creates the end mirrors in a standard laser.
The success of the VCSEL depends on
incorporating high reflectivity mirrors in the
structures
The incorporations of DBR and MQW
structures highly improve the performance of
the VCSEL.
Various DBRs in the VCSEL:
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crystalline BRD
amorphous DBR stacks
MgF/ZnSe DBR
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