Download Ch 4 Optical source

General requirements for a light source for use
in optical communications
1. The emission wavelength compatible with the loss spectrum of glass fibers,
820nm, 1300nm & 1550nm.
2. The sources should be capable of modulation at rates in excess of 1GHz for
high data rate transmission.
3. The spectral width of the sources should be narrow in order to minimize the
bandwidth limiting pulse dispersion in the fibers.
4. The average emitted power of the source that is needed is typically few milliwatts,
although higher power values are needed for very long continuous fiber links or
if high loss fibers are used.
5. The radiance of the source should be as high as possible for effective coupling into
the low-loss fiber with small NA ( ~0.2). This means that the beam spread of
the sources must be minimized.
6. The sources must have long lifetime and it must be possible to operate the device
continuously at room temperature.
7. The sources must be highly reliable.
8. The sources should be reasonably low cost.
The principal light sources used for fiber optic communications applications are
heterojunction-structured semiconductor laser diodes or injection laser diodes
(ILDs) and light-emitting diodes (LEDs).
TYPES OF OPTICAL SOURCES
Wideband continuous spectra sources
(Incandescent Lamps).
Monochromatic incoherent sources (Light
Emitting Diodes - LED).
Monochromatic coherent sources (Light
Amplification by Stimulated Emission of
Radiation - LASER)
SPECTRAL LINEWIDTHS
Transmitter
The starting point of the optical communication system is
the transmitter where the electrical signal convert to the
optical signal by modulate the optical source. Converting
the electrical signal into the optical signal is using an
electronic circuit. The circuit is a driving circuit.
The most common devices used as the light source in
optical transmitters are the light emitting diode (LED) and
the laser diode (LD).
These devices are mounted in a package that enables
an optical fiber to be placed in very close proximity to the
light emitting region in order to couple as much light as
possible into the fiber.
Transmitter (Cont’ 1)
 In some cases, the emitter is even fitted with a tiny spherical
lens to collect and focus “every last drop” of light onto the fiber
and in other cases, a fiber is “pigtailed” directly onto the actual
surface of the emitter.
The most important part on this circuit is the light source,
because the design of the circuit is depending on the source.
There are few types of semiconductor sources in
communication system. The most commonly use in the
communication system is Light Emitting Diode (LED) and
Laser Diode (LD). These two types of sources has different
characteristic and the driving circuit for each type of sources
should be different.
The difference between LEDs and laser diodes is that LEDs produce
incoherent light, while laser diodes produce coherent light.
OPTICAL TRANSMITTERS
On-OFF Modulation
Linear Modulation
Figure 1. Basic Optical Modulation Methods Intensity
The basic optical transmitter converts electrical input signals into
modulated light for transmission over an optical fiber. Depending on
the nature of this signal, the resulting modulated light may be turned
on and off or may be linearly varied in intensity between two
predetermined levels, as shown in Figure 1.
Component of Transmitter
Light source
Driving Circuit
Protection and voltage supply
Light Source and Semiconductor
Laser
The sources for transmitter is
semiconductor laser
Most common semiconductor laser in the
transmitter are Light Emitting Diode (LED)
and Laser Diode (LD)
Semiconductor Laser
Semiconductor laser is developed by two of
semiconductor materials that are p-type and ntype materials in which n-type material contains
more electrons and p-type material contains
more holes.
The materials producing the energy gap
between the two materials that is called band
gap.
Semiconductor Laser (Cont’1)
Semiconductor material have conduction
properties between insulator and metals
The conduction properties creates the energyband that call band gap
There two energy bands: valance – lower,
meaning less energy and conduction – upper,
meaning higher energy
There are separate by energy gap as shown in
figure 1 below
Semiconductor Laser
(Cont’2)
Figure 1: Electron-hole recombination
process
Semiconductor Laser (Cont’3)
This band gap is the place where the recombination and
excitation process occur.
The electron can be either at the valance band or
conduction band
When some external energy – either through
temperature or by an external electric – is provided to
the electron at the valance band, some electron acquire
enough energy to leap over energy gap and occupy
energy level at the conduction band
This process call excitation process where electron
leaves holes (positive charge carries) at the valance
band
Semiconductor Laser (Cont’4)
As a result, the electron-hole recombination process
releases the quantum of energy-a photon that called
laser (Mynbaev, 2001).
The concentration of electron and holes is known as the
intrinsic carries concentration ni
Doping can increase the conduction and the doped
semiconductor is called an intrinsic material
Doping can increases the conduction by two method:
first, doping to adding electron concentration and create
donor level as shown in figure 2 below
Semiconductor Laser
(Cont’5)
Electron energy
Electrons
concentration
distribution
Conduction
band
Ec
No. of electron states
Ep
Donor level
No. of hole states
Ev
Valance band
Figure 2: Donor level and electron concentration at
conduction band
Semiconductor Laser (Cont’6)
Donor level will provide an electron to the conduction
band, thus increase the electron concentration at the
conduction band
Second by increase the hole concentration at valence
band by creating the acceptor level as shown on figure 3
Accepter level will receive the electron from the valance
band, thus increase the conduction and leave high hole
concentration at valance band
The energy produce by the semiconductor laser is
depend on the surface of the material where there are
indirect band gap and direct band gap.
Band Gaps
Semiconductor material have two types of band gaps:
Direct band gap and Indirect band gap
Direct band gap, where the electron and hole have the
same momentum value as shown on figure 4
Indirect band needs a thirds particle to conserve
momentum where the conduction band has minimum
energy and valance band has maximum energy level as
shown on figure 5
Band Gaps (Cont’ 1)
Conduction band
Direct band
gap energy
Edir
λ3
λ2
Electron
trasnsition
λ1
Photon energy
hv = Edir
Valance band
Figure 4: Direct band gap material
Band Gaps (Cont’ 2)
Conduction band
Photon of energy
Eph
λ2
Electron
trasnsition
λ1
Photon energy
hv = Eind + Eph
Indirect
band gap
energy Eind
Valance band
Figure 5: Indirect band gap material
Direct and Indirect Bandgap
Semiconductors
Number of photons generated
Number of e - crossing the junction
Rate of Rad. Rec.

Rate of radiative recomb.  Rate of Non - Rad. Rec.
1 /  rr

1 /  rr  1 /  nr
int 

 nr rr /  rr
 nr   rr

1
Think of rr or nr as the ave. time
e-h recombines
1   rr /  nr
But 1/nr varies drastically for direct or indirect bandgap material.
1 injected into LED is I then,
If
current
But
varies for different material.
Rr Rnr  I / q
Very different for direct bandgap/indirect
rr
Rr is totalmaterial
no. of photon generated
bandgap
/ sec,
thus optical power generated internally to LED
I
hcI
Pint   int hv   int
q
q
By assuming outside medium is air, total power emitted by LED
Pint
P
n(n  1) 2
Example
How semiconductor laser works
The pn junction on semiconductor material create the
depletion region
This region create when electron from n material filled
holes at p material.
When the equilibrium state achieved its prevent from net
movement of charges
The junction or the depletion now has no mobile carrier,
since electron and holes locked into covalent structure.
If external battery is connected to the junction in
reversed biased the depletion region will increase and
the minority carrier flow across the junction
How semiconductor laser works
(Cont’ 1)
Minority carrier
flow
p side
n side
Widened
depletion region
+
-
Figure 6: Reverse biased widen the depletion region
How semiconductor laser works
(Cont’ 2)
This will act as photodiode as shown on figure 6
If the external battery supply connected to the junction in
forward biased, then the magnitude of the barrier
potential reduced and allow the electron to diffuse
Conduction band electron on the n side material across
the and recombine with the opposite charge
Once across, they significantly increase the minority
carriers concentration and the access carrier then
recombine with the opposite charge majority carriers
The recombination process of excess minority carriers is
the mechanism which optical radiation is generated
How semiconductor laser works
(Cont’ 3)
p side
n side
narrowed
depletion region
-
+
Figure 7: Forward biased lowering the barrier
potential
Light Emitting Diodes (LED)
LED is a semiconductor diode; the construction of the LED is same
as other diode but the other regular diode is loss the recombination
energy in the thermal.
LED is used the recombination energy into radiation spectrum of
light.
LEDs have relatively large emitting areas and as a result are not as
good light sources as LDs.
However, they are widely used for short to moderate transmission
distances because they are much more economical, quite linear in
terms of light output versus electrical current input and stable in
terms of light output versus ambient operating temperature.
Light Emitting Diodes (LED)
(Cont’1)
LEDs are of interest for fiber optics because of
five inherent characteristics:





They are small
They possess high radiance (i.e., They emit lots of
light in a small area).
The emitting area is small, comparable to the
dimensions of optical fibers.
They have a very long life, offering high reliability.
They can be modulated (turned off and on) at high
speeds.
LED Structures and Configuration
There are two possible structure in LED:
Homostructure and heterostructure
Homostructure configuration have drawback
where the active region is too defuse which
makes the device’s efficiency very low
Homostructure makes the device radiates a
broad light beam and make coupling light into
fiber inefficient.
LED Structures and Configuration
(Cont’1)
Most LED is design using heterostructure
because its gives good confinement of
recombination process
Two type LED configurations


Edge Emitting LED (ELED)
Surface Emitting LED (SLED)
Energy Gaps in LEDs
Eg=hc/l = 1240eV-nm/l
(1)
Equation 1 defines the bandgap energy Eg:
Where:
h = Plank's Constant = 4.13 x 10-15 eV•s
c = speed of light = 2.998 x 108 m/s
l = wavelength in nm
LED Configuration
(a)
(b)
Figure 8: LED structures a) Edge emitters b) Surface emitter
LED Configuration (Cont’1)
The Full Half Wave Maximum (FHWM) of
LED is depending on the emitting surface
of LED where surface emitting will give
more FWHM compare to the edge
emitting.
In the optical communication system, LED
source is using by slicing into small
spectrum width by using the grating.
Edge Emitter
More complex and expensive devices, offer high
output power levels and high speed
performance
The output power is high because the emitting
spot is very small, typically 30-50 µm, allowing
good coupling efficiency to similarly sized optical
fibers.
Relatively narrow emission spectra. The fullwidth, half-maximum (FWHM) is typically about
7% of the central wavelength
Edge Emitter (Cont’1)
The super-radiant LED: a cross between a
conventional LED and a laser. Have a very high
power density and possess some internal optical
gain like a laser, but the optical output is still
incoherent, unlike a laser. Very narrow emission
spectra, typically 1-2% of the central wavelength
and offer power levels rivaling a laser diode.
These devices are popular for fiber optic
gyroscope applications.
Surface Emitter
Comparatively simple structure, relatively inexpensive,
offer low-to-moderate output power levels, and are
capable of low-to-moderate operating speeds.
Optical output power is as high or higher than the edgeemitting LED, since the emitting area is large, causing
poor coupling efficiency to the optical fiber.
Surface-emitting LEDs are almost perfect Lambertian
emitters. This means that they emit light in all directions.
Surface Emitter (Cont’1)
The radiant intensity is maximum normal to the
surface and decreases in proportion to the
cosine of the angle from the normal.
N = N0cosA (Lambert’s cosine law )
Thus very little of the total light goes in the
required direction for injection into an optical
fiber.
LED Materials
There are many material in construct LED, for
example GaAlAs (gallium aluminum arsenide)
for short-wavelength devices. Long-wavelength
devices generally incorporate InGaAsP (indium
gallium arsenide phosphide).
These material gives different energy gap as
shown in table 1 below
Different material also will gives different
wavelength for different application
LED Materials (Cont’1)
Material
Energy Gap Eg (eV)
Wavelength (nm)
Si
1.17
1067
Ge
0.775
1610
GaAs
1.424
876
InP
1.35
924
InGaAs
0.75-1.24
1664-1006
AlGaAs
1.42-1.92
879-650
InGaAsP
0.75-1.35
1664-924
Table 1: Energy gap and wavelengths
LED Materials (Cont’2)
The first materials, GaP and AlAs, are used to make emitters in the
visible portions of the spectrum.
The next three materials, GaAs, InP, and AlGaAs, are used to make
emitters in the near infrared portion spectrum generally referred to
as the “first window” in optical fiber.
The last material, InGaAsP is used to make emitters in the infrared
portion spectrum referred to as the “second and third windows” in
optical fibers.
The energy gap corresponds to the energy of the emitted photons
and also is indicative of the voltage drop associated with a forward
biased LED.
LED Optical Spectrum
Figure 9: LED Spectrum
Light Emitting Diode
For optical communication systems requiring bit rates less than approximately 100200 Mb/s together with multimode fiber-coupled optical power in the tens of
microwatts, semiconductor light emitting diodes (LEDs) are usually the best light
source choice. These LEDs require less complex drive circuitry than laser diodes
since no thermal or optical stabilization circuits are needed, and they can be
fabricated less expensively with higher yields.
To be useful in fiber transmission applications an LED must have
1. A high radiance (or brightness)- a measure in watts, of the power radiated into a
unit solid
angle per unit area of the emitting surface.
2. A fast response time - the time delay between the application of a current pulse
and the onset of optical emission.
3. A high quantum efficiency - the fraction of injected electron-hole pairs that
recombine
radiatively.
In a heterostructure LED, carrier confinement is used to achieve a high level of
radiative recombination in the active region of the device, which yields a high
quantum efficiency. Optical confinement is of important for preventing absorption of
the emitted radiation by the material surrounding the pn junction. This dual
confinement leads to both high efficiency and high radiance.
LED structures
The two basic LED configuration being used for fiber optics are surface emitters (also
called Burrus or front emitters) and edge emitters.
Surface Emitter LED (SLED)
In the surface emitter, the plane of
the active light-emitting region is
oriented perpendicularly to the axis
of the fiber. Normally, a well is
etched through the substrate of the
device, into which a fiber is then
cemented in order to accept the
emitted light. The circular active area
in practical surface emitters is
nominally 50μm in diameter and up
to 2.5mm thick. The emission pattern
is essentially isotropic with a 1200
half-power beam width and is called
a lambertian pattern.
In this pattern, the source is equally bright when viewed from any direction, but the power diminishes
as cosθ, θ is the angle between the viewing direction and the normal to the surface. The power coupled
Pc into a multimode step index fiber may be estimated from the relationship ,
where r is the Fresnel reflection coefficient at the fiber surface, A is the smaller of the fiber core cross
section or the emission area of the source and RD is the radiance of the source. The power coupled into
the fiber is also dependent on other factors such as the distance and alignment between the emission
area and the fiber, the SLED emission pattern and the medium between the emitting area and the fiber.
Edge emitter LED (ELED)
The high radiance ELED used
in optical communications is
similar to a conventional
contact stripe injection laser.
This structure forms a
waveguide channel that
directs the optical radiation
toward the fiber core.
Most of the propagation light
is emitted at one end face
only due to a reflector on the
other end face and an
antireflection coating on the
emitting end face. The
emission pattern of this ELED
is more directional than that
of the SLED. In the plane
parallel to the junction, where
there is no waveguide effect,
the emitted beam is lambertian with half-power width of
. In the plane perpendicular to the
junction, the half-power beam width has been made as small as 25-350 by a proper choice of
the waveguide thickness.
LED Optical output power
Intrinsically the LED is a very linear device in comparison with the majority of
injection lasers and hence it tends to be more suitable for analog transmission.
The surface emitter radiates significantly
more optical power into air than the edge
emitter, and that both devices are
reasonably linear at moderate drive
currents.
Output spectrum
The spectral linewidth of an LED operating at
room temperature in the 0.8 to 0.9 mm wavelength
band is usually between 25 and 40nm at the half
maximum intensity points (FWHP). For materials
with smaller bandgap energies operating in the 1.1
to 1.7mm wavelength region the linewidth tends to
increase to around 50 to 160nm.
Typical spectral output characteristics for
InGaAsP devices. The output spectral
widths of SLEDs tend to be broader than
those of edge-emitting LEDs because of
different internal-absorption effects.
The output spectra tends to broaden at a
rate of between 0.1 and 0.3 nm0C-1 with
increase in temperature due to the
greater energy spread in carrier
distributions at higher temperatures. The
peak emission wavelength is also shifted
by +0.3 to 0.4nm0C-1 for AlGaAs
devices and by +0.6nm0C-1 for InGaAsP
devices.
Analog LED Driver Circuits
Figure 12 below is the simplest of the three
configurations. It uses a transistor, Q1, and a limited
amount of resistors to convert an analog input voltage
into a proportional current flowing through the LED, D1.
Also referred to as a transconductance amplifier, this
configuration converts a voltage into a current.
In LEDs, the light output equates proportionally to the
drive current, not the drive voltage. LEDs exhibit a peak
drive current at about 100 mA, and the voltage drop is
typically 1.5 Volts.
Analog LED Driver Circuits (Cont’1)
Figure 12: Simple LED circuit
Analog LED Driver Circuits (Cont’2)
Figure 13 eliminates most of the nonlinearities
associated with Q1. In this case, U1 forms a feedback
loop that drives the base of Q1 in such a way that
assures that VR2 equals VIN.
The circuit still experiences some lesser nonlinearities
associated with Q1 and also is limited by the delay
associated with the feedback signal in the servo loop
formed by U1. Thus allowing the circuit to only achieve a
bandwidth of about 10-100 MHz. The circuit in Figure 3b
work well in application transmitting DC coupled analog
signals
Analog LED Driver Circuits (Cont’3)
Figure 13:Linear, Low Frequency LED driver circuits
Digital LED Drive Circuit
The main concern on the circuit is where the
circuit can support the maximum speed.
The current input to the circuit or the signal
current is not the main concern since the circuit
can detect the current
Digital circuit and the analog circuit is does not
have much different where the only data signal
is in digital where the threshold level is fixed
Digital LED Drive Circuit (Cont’1)
Figure 14: Digital LED drive circuits.
Digital LED Drive Circuit (Cont’2)
Figure 14 is a simple series driver circuit. The input voltage is
applied to the base of transistor Q1 through resistor R1. The
transistor will either be off or on. When transistor Q1 is off, no
current will flow through the LED, and no light will be emitted. When
transistor Q1 is on, the cathode (bottom) of the LED will be pulled
low.
Transistor Q1 will pull its collector down to about 0.25 Volts. The
current is equal to the voltage across resistor R2 divided by the
resistance of R2. The voltage across R2 is equal to the power
supply voltage less the LED forward voltage drop and the saturation
voltage of the drive transistor. The key advantage of the series driver
is its low average power supply current. This type of driver circuit is
rarely used at data rates above 30-50 Mb/s.
Laser Diode (LD)
Laser diode is different to LED, even though the
material used in construction lased diode is
similar to the LED, but the radiate light is came
from the other process, stimulated emission
process.
Laser diode light is monochromatic and the
spectral width of the light is small.
LD is a semiconductor that emits coherent light
when forward biased
Laser Diode (Cont’1)
Laser diode also produces coherent light where all
oscillations are in phase and provide better detection for
receiver of an information signal.
Since the laser diode is monochromatic, the light is easy
directed especially directed into the fiber.
Compare to the LED, laser diode is highly intense and
power efficient. LED need 150mA of current to achieve
power radiate at 1mW but laser diode only need 10mA
current to achieve same power level.
Laser Diode (Cont’2)
Five inherent properties make lasers attractive for use in fiber optics:



They are small.
They possess high radiance (i.e., They emit lots of light in a small
area).
The emitting area is small, comparable to the dimensions of optical
fibers.

They have a very long life, offering high reliability.

They can be modulated (turned off and on) at high speeds.
Laser Diode Working Principle
Laser from LD is create from the stimulated emission
radiation
Stimulated radiation has four properties


External photon forces a photon with similar energy (Ep) to be
emitted, where the external photon stimulates radiation with the
same frequency. This will ensure the spectral width of the light is
radiated narrowed
Since all photon propagate in the same radiation, all of them
contribute to output light. Current to light conversion occur at the
high efficiency and produce high power laser output. Then,
ensure the efficiency of power output where only 10mA current
for 1mW power
Laser Diode Working Principle
(Cont’1)


The stimulated photon propagates in the
same direction as the photon that stimulated
it; hence, the stimulated light will be well
directed.
Since a stimulated photon is radiated only
when external triggers and both photon are
said to be synchronized. This make photon
are in phase and so the stimulated radiation is
coherent
Laser Diode Working Principle
(Cont’2)
To achieve stimulate emission, the number of
photon should be large enough
To get the large number of photon, two mirror is
used to double up the number of photon
This two mirror provide positive optical feedback
Population inversion will provide the number of
electron to excites at much higher rate
Laser Diode Working Principle
(Cont’3)
Population inversion is occur when the high density
forward current is passed through the small area active
area
For laser action (lasing) we need to have more electron
at the higher-energy conduction band than at the lowerenergy valance band
Thus we need population inversion where provide
necessary condition for lasing effect because the greater
the number of excited electrons, the greater the number
of photon stimulated
Laser Diode Working Principle
(Cont’4)
The number of excited electron also determine the gain
of semiconductor diode
The loss of diode is at the photons’ absorb by
semiconductor material before they can escape to create
radiation
Other loss is at the mirror, where mirror do not reflect
100% of incident photon
The gain is increasing by increasing the forward current
The loss and the gain become equal at threshold level
where the current at this level called threshold current.
Laser Diode Working Principle
(Cont’5)
At this threshold condition, laser diode start to
act as laser, where when the forward current
increase the number of emitted stimulated
photon increase which means the intensity of
radiated light also increase
Then we will get the well directed, highly intense,
monochromatic and coherent light
Laser Diode Working Principle
(Cont’6)
Lasing effect and input-output
characteristic occur when below process
met:



Population inversion
Stimulated emission
Positive feedback
Output Power (mW)
Laser Diode Characteristic
Ith
Forward current (mA)
Figure 15: input-output characteristic of LD
Laser Diode Characteristic (Cont’1)
Laser diode light that can characterized as
below


Monochromatic : The spectral width of the
radiated light is very narrow. The line width of
a laser diode can be in tenth or hundred of
nanometer
Well directed: A laser diode radiates narrow ,
well directed beam that can be easily
launched into optical fiber
Laser Diode Characteristic (Cont’2)


Highly intense and power efficient: A laser
diode can radiate hundreds of milliwatts of
output power. LD making the current to light
conversion 10 times more efficient than it is in
the best LEDs.
Coherent: Light radiates by a laser diode is
coherent; where all oscillation are in phase.
Laser Diode Characteristic (Cont’3)
Threshold current of laser diode depend on the
temperature
The threshold current will increase when the
temperature increase as shown in figure 16
below
There are two types of laser diode design that
provide solution for the temperature: Cooled
laser diode and uncooled laser diode
Laser Diode Characteristic (Cont’4)
Uncooled laser diode means, laser diode does not
require any cooling
This means also laser diode consumes less overall
power and heat generated at nonradiative transition of
exicited electrons is small.
Cooled laser diode means, laser diode needs a heat
pump to transfer heat from the one place to another
Cooled laser diode always include a thermoelectric
cooler (TEC) which function to keep laser diode at
operating temperature
Laser Diode Characteristic (Cont’5)
Figure 16: The threshold current at different temperature
Laser Diode Characteristic (Cont’6)
Laser diode is linear in terms of light output versus
electrical current input, but unlike LEDs, they are not
stable over wide operating temperature ranges and
require more elaborate circuitry to achieve acceptable
stability
The other effect of temperature is changes in the slope
efficiency.
The slope efficiency is the number of milliwatts or
microwatts of light output per milliampere of increased
drive current above threshold. Most lasers show a drop
in slope efficiency as temperature increases.
Laser Diode Characteristic (Cont’6)
Emission Pattern: The pattern of emitted light
affects the amount of light that can be
coupled into the optical fiber. The size of the
emitting region should be similar to the
diameter of the fiber core.
Figure 17: Laser emission pattern and fiber core
Structure of Laser Diode
Laser diode structure is quite similar to the
construction of the edge emitting LED
Two ends surface are cleaved to make them
work as mirror for positive feedback
The thickness of active region in laser diode is
very small
Latest technology using quantum well technique
to reduce the current threshold
Types of Laser Diode
There ere two types of laser diode
commonly user in communication system


Fabry-Perot laser diode
Distributed feedback (DFB) laser diode
Fabry-Perot laser diode
The arrangement for Fabry-Perot laser diode is where
the active medium is place between two mirror
Two mirror in the arrangement to provide positive
feedback for laser diode
The wave or the photon travel from light hand mirror into
the right hand mirror where each wave reflected produce
180° phase shift than continue to propagate
Thus, will produce the standing wave for Fabry-Perot
laser diode as shown in figure 16
For lasing action, the medium must reach the stage of population inversion.
Although the population inversion is necessary to achieve lasing, it is not the only
required condition. In addition, a minimum threshold gain within the amplifying
medium must be also attained so that laser oscillations are initiated and sustained.
This threshold gain may be determined by considering the change in energy of a
light beam as it passes through the amplifying medium.
The light gets amplified by the process of optical feed back inside the active
medium, i.e. the medium with population inversion. The active medium has two
mirrors – ideally one of the mirrors should have 100% reflectivity, and the other
Amplifying medium
should be partially reflecting so that the laser light can be extracted from the active
medium. The structure essentially acts as a Fabry-Perot resonator. A stable output
is obtained when the optical gain is exactly matched by the losses in the
amplifying medium. However, there are some factors that contribute to the losses
inside the active medium; these are as follows:
• Absorption and scattering in the amplifying medium.
• Absorption, scattering and diffraction at the mirrors.
• Non-useful transmission through the mirrors.
The cavity gain should be sufficient to overcome the losses, and finally to amplify
the light.
Gain / loss
Fabry-Perot laser diode (Cont’1)
λN+2
λN+1
λN
λN-1
λN-2
λ (nm)
Figure 18: Gain-loss curve for possible longitudinal modes
Fabry-Perot laser diode (Cont’2)
Fabry-Perot lasers are the most economical, but
they are generally noisy, slower devices.
Fabry-Perot lasers further break down into:


Buried Hetero (BH) and
Multi-Quantum Well (MQW) types
BH and related styles ruled for many years, but
now MQW types are becoming very widespread.
Fabry-Perot laser diode (Cont’3)
MQW lasers offer significant advantages over all
former types of Fabry-Perot lasers.
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They offer lower threshold current, higher slope
efficiency, lower noise, better linearity, and much
greater stability over temperature.
The performance margins of MQW lasers are so
great, laser manufacturers get better yields, so laser
cost is reduced.
One disadvantage of MQW lasers is their tendency to
be more susceptible to backreflections.
Output power, mW
Fabry-Perot laser diode (Cont’4)
Δλ
λ (nm)
Figure 19: Fabry-Perot laser diode and standing wave
Distributed Feedback Laser Diode
In distributed feedback laser diode, the
Bragg grating is used to reduce the
spectral width of the laser spectrum
The Bragg grating only allow selective
wavelength propagate
This Bragg grating also acts as mirror
Output power, mW
Distributed Feedback Laser Diode
(Cont’1)
Gain
λ
Δλ
λ (nm)
Figure 20: The DFB laser diode spectrum
Distributed Feedback Laser Diode
(Cont’2)
The radiated wavelength is depend on the Bragg
condition given by below formula
2 Λ neff = λB
The spectral width of the laser spectrum is
extremely narrow and suitable for
communication system especially in WDM
system
DFB laser were proposed in early 1960s but
were not developed commercially until the
1980s
Distributed Feedback Laser Diode
(Cont’3)
DFB lasers are quieter devices (e.g., high signal-tonoise), have narrower spectral widths, and are usually
faster devices.
DFB lasers offer the highest performance levels and also
the highest cost of the two types.
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They are nearly monochromatic (i.e. they emit a very pure single
color of light.) while FP lasers emit light at a number of discrete
wavelengths.
DFB lasers tend to be used for the highest speed digital
applications and for most analog applications because of their
faster speed, lower noise, and superior linearity.
Distributed Feedback Laser Diode
(Cont’4)
Figure 21: An injection laser diode which has a Bragg reflection
grating in the active region in order to suppress multiple
longitudinal modes and enhance a single longitudinal mode
Laser Diode Material
The material inside laser diode will present the
wavelength of laser diode
Different material will present different
wavelength
Different material also provide different energy
gap
Material in laser diode will provide how long the
transmitter can support and the wavelength
mode and type
Laser Diode Material (Cont’1)
InGaAs:Abbreviation for indium gallium arsenide.
Generally used to make high-performance longwavelength detectors
InGaAsP: Abbreviation for indium gallium arsenide
phosphide. Generally used for long-wavelength light
emitters
Injection Laser Diode (ILD): A laser employing a forwardbiased semiconductor junction as the active medium.
Stimulated emission of coherent light occurs at a PIN
junction where electrons and holes are driven into the
junction
Laser Diode Material (Cont’3)
The material construction in laser diode are include
quantum well, graded index confinement layer, contact
layer, cladding layer and substrate
These material are combined and put in together with
different type of construction such as shown in figure 21.
The other types of construction are include:



Basic structure of broad-area
Gain-guided
Ridge-waveguide (RWG)
Laser Diode Material (Cont’4)
Different configuration of arrangement of
material will give different band gap energy
The arrangement that available in constructing
the laser diode are:
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
Single-quantum well LD
Multiple quantum well LD
Graded-index separate-confinement hetrostructure
Laser Diode Driver Circuit
The design of laser diode driver circuit should be
suitable to the characteristic of laser diode
The laser diode has certain threshold level when
its start in linear region
The laser diode driver should make the laser
operate in linear region where the output power
will be linear to the input current
Laser Diode Driver Circuit (Cont’1)
Therefore, the circuit of laser diode should
provide certain level of dc current when
the system not operate
This bias current for the threshold level
should suitable to the laser diode used
because different laser diode has different
threshold current level
Laser Diode Driver Circuit (Cont’2)
The modulation of the laser signal is the done by
the driving circuit
The driving circuit will modulate the electrical
signal into optical source.
The driving circuit will make the laser flash
according to the data modulation
The rise time and fall time of the signal is
depend on the driving circuit and the source
itself
Laser Diode Driver Circuit (Cont’3)
The driving circuit is design according to
the source (as mention before) and also
the signal receive.
If the signal is in the form of analog the
analog driving circuit is to modulate used
and if the signal is in digital the digital
driving circuit is used to modulate
Analog Driving Circuit
Figure 22: (a) Analog Laser Diode Drive Circuit
Analog Driving Circuit (Cont’1)
Figure 22a, shows Laser drive circuit with moderate
linearity and good performance in frequencies up to 500
MHz.
The analog signal path only involves C1, R1, Q1, R2,
and D1, the laser diode. Q1 acts as a transconductant
stage in which voltage flows in and current flows out.
C1 passes only the AC portion of the analog input signal.
R1, usually only a few tens of Ohms, squelches any
possible oscillations in Q1.
Analog Driving Circuit (Cont’2)
The AC portion of analog input voltage VIN
appears at the base of Q1 and also at the
emitter of Q1. VIN, the AC voltage at the emitter
of Q1, imposes across R2 to create a
modulation current VIN/R2.
U1 supplies DC current to the laser through R3
and R1. U1 creates a servo loop that maintains
a constant photodiode current through the rear
facet monitor PIN diode.
Analog Driving Circuit (Cont’3)
Figure 22: (b) Analog Laser Diode Drive Circuit
Analog Driving Circuit (Cont’4)
Figure 22b, shows a more advanced analog
laser circuit, offering excellent linearity at very
high frequencies (GHz).
The signal path of this circuit only involves U2,
Z1, C1, and the laser diode, D1.
Amplifier U2 provides input matching, gain and
isolates the laser from outside conditions. The
block labeled Z1 can take on many functions.
Analog Driving Circuit (Cont’5)
At a minimum, it interfaces the output of
the amplifier U2, usually 50 or 75Ohms, to
the laser that has an impedance ranging
from 5 Ohms to 25 Ohms. Sometimes the
laser package incorporates this
impedance matching.
Digital Laser Drive Circuit
Figure 23: Digital Laser Diode Drive Circuit
Digital Laser Drive Circuit (Cont’1)
Figure 12, illustrates common discrete
component circuit configurations that function to
drive lasers for digital applications.
However, a wide variety of highly integrated ICs
exist because of the high demand for digital
laser drivers. The discrete component circuit
configurations illustrate the most commonly used
principles in commercially available laser driver
ICs.
Vertical Cavity Surface Emitting
Laser (VCSEL)
VCSEL: are a new laser structure that emits
laser light vertically from its surface and has
vertical laser cavity. Figure 24 illustrates the
structure of a VCSEL.
VCSEL: Lasers that emit light perpendicular to
the plane of the wafer they are grown on. They
have very small dimensions compared to
conventional lasers and are very efficient.
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’1)
Figure 24: Digital Laser Diode Drive Circuit
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’2)
The vertical cavity is design vertically, where the
laser feedback is arranged in the vertical
direction
Several quantum well are made in the
arrangement of the VCSEL within the active
region to enhance the light gain
The region is placed between Bragg reflector
which work as highly refractive mirrors
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’3)
Closely resembles those of conventional edgeemitting semiconductor lasers. The heart of the
VCSEL is an electrically pumped gain region,
also called the active region, emits light.
VCESEL operate when layers of varying
semiconductor materials above and below the
gain region create mirrors. Each mirror reflects a
narrow range of wavelengths back into the cavity
causing light emission at a single wavelength
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’4)
The principle operation is same as other laser
diode where the emission of photon will produce
after stimulated emission process and the
photon will experience positive feedback cause
by the mirror.
VCSELs are typically multi-quantum well
(MQW), with lasing occurring in layers only 2030 atoms thick. Bragg-reflectors with as many as
120 mirror layers form the laser reflectors.
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’5)
The advantages of the VCSEL are:
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The VCSEL is operate in single mode regime
where the size of the cavity is very small that
allow only one regime
The VCSEL diode has very small dimension
which allow manufacturer to fabricate many
diodes on one subtrate.
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’6)

The size of the VCSEL also provide low
power consumption VCSEL can radiate 3mW
output power for 10 mA current. Since the
VCSEL is small the active area is also small
which bring the VCSEL high switching speed
and make VCSEL can support high
modulation rate up to 200GHz
Vertical Cavity Surface Emitting
Laser (VCSEL) (Cont’7)

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A VCSEL diode radiates a circular output
beam in contrast to that radiated by edgeemitting laser
The VCSEL fabrication can be said as the
merger between optic and electronic. The
fabrication of the VCSEL is very similar to the
electronic chips where gives VCSEL
enormous range of advantage that chips
have.
The difficulties in DH laser are overcome by a further development of a large-optical
cavity (LOC) laser and uses regions of AlGaAs of varying composition.
Some common material systems used in fabrication of heterojunction lasers.
Each of the advances described has lowered the operating threshold of GaAs lasers.
The typical current densities necessary to achieve the lasing threshold of the various
junction types at 300° K.
Homojunction
Single heterojunction
Double heterojunction
Double heterojunction, large optical cavity
40,000 A/cm2
10,000 A/cm2
1,300 A/cm2
600 A/cm2
Laser Lifetime
The operating lifetime of a laser
diode is reduced significantly by
operation at elevated
temperature.
The lifetime is reduced by a
factor that varies with absolute
temperature T as exp(Ea/kT),
where Ea is an activation energy,
typically around 0.5 to 0.7 eV,
and k is Boltzmann’s constant.
According to this dependence,
an increase in operating
temperature of 40 Celsius
degrees will decrease the
lifetime by a factor around 30.
function of operating time, for various operating temperatures. At 20° C, the
mean time The percentage of a typical 5-mW Al1–xGaxAs laser diodes laser that
have failed as a before failure is 770,000 hours, but at 70° C, it has fallen to
27,000 hours.
Vertical Cavity Surface Emitting Lasers
(VCSELs)
Advantages over the edge-emitting lasers:
Its design allows the chips to be manufactured and
tested on a single wafer.
Large arrays of devices can be created exploiting
methods such as 'flip' chip optical interconnects
optical neural network applications to become
possible.
In the telecommunications industry, the VCSEL's
uniform, single mode beam profile is desirable for
coupling into optical fibres.
The cavity length of VCSELs is very short typically
1-3 wavelengths of the emitted light. As a result, in a
single pass of the cavity, a photon has a small
chance of a triggering a stimulated emission event at
low carrier densities. Therefore, VCSELs require
highly reflective mirrors to be efficient. For VCSELs,
the reflectivity required for low threshold currents is
greater than 99.9%. Such a high reflectivtiy can not
be acheived by the use of metalic mirrors. VCSELs
make use Distributed Bragg Reflectors. (DBRs).
These are formed by laying down alternating layers of
semiconductor or dielectric materials with a difference
in refractive index.
R