4 laserdiodestruct

Dublin Institute of Technology
Dr. Gerald Farrell
Optical Communications Systems
School of Electronic and
Communications Engineering
Unauthorised usage or reproduction strictly prohibited
Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Semiconductor Laser Diodes
27/02/02 2.4 Semicdr Laser diode structures and characteristics.prz

Optical Communications Systems, Dr. Gerald Farrell, School of Electronic and Communications Engineering
Unauthorised usage or reproduction strictly prohibited, Copyright 2002, Dr. Gerald Farrell, Dublin Institute of Technology
Laser Structures
Source: Master 4_3

Semiconductor Laser Structures
A wide variety of laser structures have evolved, with the aim of reduced
thresholds, improved efficiency and narrow spectral output:
Basic broad area laser
Stripe geometry laser
Gain guided laser
Index guided laser
Single frequency laser
Multi-section laser
Source: Master 4_3

Double Heterostructure
The double heterostructure is one of the most basic Laser structures.
Typical 5 layer structure is shown below.
Bandgap energy is higher in the confinement regions, resulting in a concentration of radiative
recombination in the lower bandgap energy active region, improving efficiency.
Refractive index in the confinement region is lower, resulting in optical confinement within the
active region.
Contact region
Contact region
p-GaAs
p-AlGaAs
Active Layer
n-GaAs
n-AlGaAs
n-GaAs
Electrode
Heterojunctions
Light output normal to
page
Confinement
regions
Electrode
Refractive
index profile
Source: Master 4_3

Broad Area DH Injection Lasers
Roughened sides
n-AlGaAs
Light Output
Cleaved Mirror
n+ -GaAs
p -AlGaAs
n+ -GaAs
Confinement Layers
Contact metallization
p -GaAs
Active Layer
In this simple early laser structure the DH structure confines the light to the active region
in the vertical direction.
Lasing still takes place across the whole width of the device, hence it is called a broad
area laser.
Low quantum efficiency, by comparison with more advanced designs, resulting in high
threshold current values.
Output light geometry is unsuitable for coupling to fibre.
Source: Master 4_3

Gain Guided Lasers
Laser structures are designed to keep the threshold as low as possible, with a high efficiency
and a narrow output beam.
Two basic design approaches are gain guiding and index guiding.
In a gain guided laser the current flow is restricted to a narrow stripe by placing high resistivity
regions within the contact regions.
Gain guiding is not very successful, thresholds are high, >100mA, with low differential quantum
efficiencies and non-linear kinks in the output characteristic.
p-GaAs
p-AlGaAs
Active Layer
n-GaAs
n-AlGaAs
n-GaAs
Electrode
Heterojunctions
Confinement
regions
Electrode
High resistivity
region
Source: Master 4_3

DH Stripe Geometry Lasers
Stripe formed by inclusion of insulation layers, thus most of the current enters the active
region in a narrow stripe that runs the length of the device.
Result is a narrow emission region, with a lower lasing threshold and a narrower output
beam.
Source: Master 4_3

Index Guided Lasers
Index guiding overcomes most of the disadvantages of gain guided designs.
In an index guided structure the active region is surrounded by a region of lower
refractive index, confining the photons to a narrow stripe, in both the transverse and
vertical directions.
Several designs have emerged including the ridge waveguide (weakly index guided)
and buried heterostructure (BH) (fully index guided) designs.
Typically the threshold currents lie in the region of 10-20 mA for BH lasers, with active
regions a couple of microns wide.
Buried heterostructure laser
diode
Source: Master 4_3

Twin Section Lasers
Gain Section
Absorber Section
Active Region
Two distinct sections, based on split anode contacts.
Forward biased section is so-called gain section.
Other section is left unconnected or reversed biased, called the absorber.
Produces hysteresis in the light-current characteristic and repetitive self-pulsation.
Numerous optical signal processing applications, including all optical frequency changing.
Basic Fabry-Perot twin section laser
Source: Master 4_3

Twin section Laser
Characteristics
0 10 20 30 40 50 60 70
0
2
4
6
8
10
Light
Intensity
(a.u..)
Gain section current (mA)
Twin section laser light-current curve,
displays hysteresis
Results in two distinct states, potentially
useful for optical memory and logic
5 mV/div
1 ns/div
O/P
I/P
Twin section lasers can also exhibit
repetitive on-off behaviour, called
self-pulsation.
Proposed applications include all-optical
synchronisation for frequency
multiplication / division and clock
extraction.
Trace shows all-optical frequency
multiplication by 2:1
Source: Master 4_3

Laser
Characteristics
Source: Master 4_3

Laser Efficiency
Basic internal laser quantum efficiency ηηi is defined as:
ηηi =
number of photons produced in the laser cavity
number of injected electrons
Defined in a number of ways:
Laser differential efficiency ηηd is defined as the ratio of the increase in the
photon output for a given increase in the number of injected electrons:
ηηd =Approximate
expression
dPe
dI.(Eg)
where dPe is the change in the optical power emitted
from the device, dI is the change in input current and Eg
is the bandgap energy.
Total laser efficiency ηηt is defined as (with approximate expression):
ηηt =
total number of output photons
total number of injected electrons
Pe
I.(Eg)
≈≈
Source: Master 4_3

Laser Characteristics: Threshold
Spontaneous
emission regime
Stimulated
emission regime
Light output
Injection current
Laser threshold current
Saturation
All Semiconductor laser diodes have a
light current characteristic, with a defined
threshold current.
Below the threshold spontaneous
emission dominates
Beyond the threshold, where stimulated
emission dominates, the differential
quantum efficiency increase dramatically.
The threshold current by convention is
the intercept on the current axis of a line
drawn along the characteristic, as shown
Source: Master 4_3

Temperature Dependence (I)
The threshold current is highly
temperature dependent.
The temperature dependence of the
laser threshold is proportional to
T/To.
T is the absolute temperature in
degrees Kelvin
To is the so called characteristic
temperature
To depends on the active region
material.
Light output versus input current
characteristic at various temperatures for
an InGaAsP laser
0 10 20 30 40 50 60 70 80
10 mW
7.5 mW
5 mW
2.5 mW
0 mW
DC current
(mA)
10 20 30 40 50 60
Laser temperature in
degrees C
Light
Output
Source: Master 4_3

Temperature Dependence (II)
In general the threshold current density Jth
temperature dependence is :
Light output versus input current characteristic at
various temperatures for an InGaAsP laser
0 10 20 30 40 50 60 70 80
10 mW
7.5 mW
5 mW
2.5 mW
0 mW
DC current
(mA)
10 20 30 40 50 60
Laser temperature in
degrees C
Light
Output
Jth is proportional to exp
T
To
To is about 120 to 190 degrees K for AlGaAs devices,
InGaAsP devices have a stronger dependence with To values of 40 to 75 degrees K
Source: Master 4_3

Temperature Dependence
Problem
1. The threshold for an InGaAsP laser diode is measured and is found to
be 31 mA and 34 mA for a device temperatures of 20 °C and 25 °C
respectively.
2. Show clearly how the above information can be used to derive an
approximate value for the characteristic temperature of the laser.
3. If this laser diode is used in system which drives the laser with a
constant current of 50 mA, what is the maximum device temperature
permissible if the laser is to operate above threshold?
Source: Master 4_3

Solution (I)
Source: Master 4_3
Solution: In general the threshold current density Jth temperature dependence is given by:
Jth = A exp (T/To)
where A is a constant. Assuming that the distribution of current within the laser is not strongly
temperature dependent then the laser threshold (Ith) temperature dependence can be approximated by:
Ith = B exp (T/To)
where B is some constant. Assuming that at two temperatures T1 and T2 the laser threshold currents are
I1 and I 2 respectively then:
[ ]ln
I
I
T T
To
1
2
1 2


 =
−

Solution (II)
Source: Master 4_3
Based on the measurements provided the value of To, the characteristic temperature is 54.1 °K. If the
device is to lase at 50 mA, then the threshold must be less than 50 mA. If the maximum temperature at
which lasing will occur is Tx then (temperatures in °K) :
[ ]
o
x
T
T
mA
mA 298
34
50
ln
−
=





Substituting for To and solving for Tx gives Tx = 318.8 °K or 45.8 °C.

Laser Diode Optical Spectrum
Laser diodes generally display multiple longitudinal modes (multimode)
Gain guided lasers are multimode at all drive currents levels
With index guided lasers several modes exist near threshold, but as current increases
one or two modes dominate.
True singlemode lasers have only one mode
Index guided
laser diode
Sharp LT022
Gain guided
laser diode
Sharp LT023
Source: Master 4_3

Single Frequency Lasers
Demand for ultra-narrow, so called single frequency, laser diodes is increasing for a
number of reasons, including low dispersion and frequency division multiplexing.
One of the most popular types is the Distributed Feedback Laser (DFB).
Instead of feedback from the cleaved ends of the laser, an internal diffraction grating
is fabricated within the laser, the period of which sets the operating
frequency/wavelength. Linewidths of 10-50 MHz have been demonstrated.
Multisection lasers have been developed which are tunable by electrical bias.
Distributed Feedback
Laser diode
Bias Tuning
of a
Multisection
DFB
Source: Master 4_3

Laser Modulation Bandwidth
All semiconductor laser diodes exhibit a so-called relaxation oscillation
Current pulse injected into the laser produces an optical output pulse exhibiting
relaxation oscillation
Relaxation oscillation can be seen as a resonance frequency for the interchange of
energy between photons and carriers
Relaxation oscillation normally sets the limit on the modulation frequency of the laser
(0.5 to 10 GHz)
time
Current pulse input to laser
time
Optical pulse, with relaxation oscillation
Source: Master 4_3

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