ee-403 optical source
TRANSCRIPT
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Optical Fiber
Communicat ion
OpticalSources
Engr.Mujeeb [email protected]
Lecturer ( Dep of Electrical
Engineering)
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OPTICAL SOURCES
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Optical Sources
There are two types of light sourcesused in fiber optic:
Light Emitting diode (LED)
LASER diodes (LD’s)
LED’s may have two types of source:
Surface-emitting LED
Edge-emitting LED
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• A light-emitting diode (LED) is a semiconductor light source.
• LEDs are used as indicator lamps in many devices and are
increasingly used for other lighting.
• Early LEDs emitted low-intensity red light, but modern versions are
available across the visible.
• The LED consists of semiconducting material doped with impuritiesto create a p-n junction.
• As in other diodes, current flows easily from the p-side or anode, to
the n-side or cathode, but not in the reverse direction.
• Charge-carriers—electrons and holes—flow into the junction from
electrodes with different voltages. When an electron meets a hole, it
falls into a lower energy level, and releases energy in the form of a
photon.
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LASER diodes are: Fabry- Perot (FP-LD)
Distributed feed back (DFB- LD)
Vertical cavity surface-emitting Laser
(VCSE-LD)
All light emitters are complex
semiconductors that convert an
electrical current in to light.
Optical Sources
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The conversion process is fairly
efficient as it creates very little heat
compared to the heat generated byincandescent lights.
Optical Sources
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Small size
High radiance(Emit a lot of light in a small area)
Small emitting area(Area is comparable to the dimensionof optical fiber cores)
LED’s and Laser diodes are of interestfor fiber optic because of five inherentcharacteristics.
Optical Sources
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Very long life(offer high reliability)
Can be modulated at high speed
LED’s are used as visible indicator in
most electronics equipment.
LASER diodes are most widely used
in compact disk players.
Optical Sources
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Intentionally Left Blank
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How an atom can be stimulated ?
Atom is stimulated by using light energy.
Energy can also be imparted to an
atom by heat, Electrical force or by
collision with other particles.
How Light is Generated
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Atoms contain two specific types of
energy:
Binding energy of nucleus. Electron energy.
How Light is Generated
Electrons have different specificenergy levels in which they move
around the nucleus.
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Forbidden zone between each energy
level is known as “Energy gap”.
For an electron to move from oneenergy level to another, it must gain
or lose the exact energy difference
between the two levels.
How Light is Generated
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When the electron is orbiting in the
nucleus at its lower energy level E1 ,
it is said to be in its unexcited or ground state.
Bohr model of the hydrogen atom
How Light is Generated
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+ r E1
E2
E3 Ground state
E5E4
E3
E2
E1
Energy level diagram of the
hydrogen atom.
Bohr model of the hydrogen atom
showing permissible electron energy
levels.
How Light is Generated
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Direct Transition.
When the stimulated electron dropsdirectly back to its ground state, its
called direct transition.
Indirect Transition.
When the electron returns to itsground state in two or more steps,its called indirect transition.
How Light is Generated
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The result of direct transition is
emission of radiations of a single
wavelength.
The result of indirect transition is the
emission of radiations of two or
more wavelengths.
How Light is Generated
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Electron Energy Absorption-Direct transition
l =122nm
E1
E2
E3
E4
E5Excited state
How Light is Generated
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Direct transition
l =122nm
E1
E2
E3
E4
E5Excited state
How Light is Generated
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Electron Energy Absorption-Indirect transition.
l =103nm (UV)
E1
E2
E3
E4
E5Excited state
How Light is Generated
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Indirect transition
E1
E2
E3
E4
E5Excited state
How Light is Generated
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Semiconductor Material
• Semiconductors
– Conduction properties lie between conductor and insulator
– Conduction properties can be described byEnergy Bands
– At low temperature conduction band iscompletely empty
– By raising the temperature, electron exitedacross the band gap.
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Li ht E itti Di d (LED )
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LED is a solid-state component andhas nearly unlimited life
expectancy.
It is physically more sturdy than its
glass-encapsulated predecessors and
requires far less operating power.
Light-Emitting Diodes (LEDs)
Best Light source for bit rates less
than approx 100 – 200 Mb/s and
multimode fiber.
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LED is a PN junction diode that
emits light through the recombination
of electrons and holes when current is
forced through its junction.
LED Operation
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+-
Recombined
electron and hole
Emitted Photon
P-Type Region
(Anode)
N-Type Region
(Cathode)
Basic operation of a light emitting diode
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+
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--
-
--
--
--
-
--
---
+
++
++
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+ -
+ -
+ -
+ -
+ -
+ -
+ -
+ -
Emitted Photon
LED Operation
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How does it work?
P-n junction ElectricalContacts
A typical LED needs a p-n junction
Junction is biased to produce even more
e-h and to inject electrons from n to p for recombination to happen
There are a lot of electrons and holes at
the junction due to excitations
Electrons from n need to be injected to p
to promote recombination
Recombination
produces light!!
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Injection Luminescence
in LED
Under forward bias – majority carriers from both sides of the junctioncan cross the depletion region and entering the material at the other side.
Upon entering, the majority carriers become minority carriers
For example, electrons in n-type (majority carriers) enter the p-typeto become minority carriers
The minority carriers will be larger minority carrier injection
Minority carriers will diffuse and recombine with the majority carrier.
For example, the electrons as minority carriers in the p-region willrecombine with the holes. Holes are the majority carrier in the p-region.
The recombination causes light to be emitted
Such process is termed radiative recombination.
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Recombination and Efficiency
eVo
Eg
p n+
h =Eg
Eg
p n+ (a) (b)
Electrons in CB
Holes in VB
EC
EV
EF
◘Ideal LED will have all injection electrons to take part in the recombination process
◘In real device not all electron will recombine with holes to radiate light
◘Sometimes recombination occurs but no light is being emitted (non-radiative)
◘Efficiency of the device therefore can be described
◘Efficiency is the rate of photon emission over the rate of supply electrons
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Calculate
• If GaAs has Eg = 1.43ev• What is the wavelength, lg it emits?
• What colour corresponds to the
wavelength?
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Joseph C. Palais
6.1
32
The released energy takes the form of a photon.
-19 -341 1.6 10 , 6.626 10eV J h J s
(6.1)
h f W g
g W
c h
l
c h W g
LIGHT EMITTING DIODES
Now
Wg is in joules and l is in meters
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Construction of Typical LED
Substrate
n
Al
SiO2
Electrical
contacts
p
Light output
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LED Construction
Efficient light emitter is also an efficient absorbers of
radiation therefore, a shallow p-n junction required.
Active materials (n and p) will be grown on a lattice
matched substrate.
The p-n junction will be forward biased with contacts
made by metallisation to the upper and lower surfaces.
Ought to leave the upper part ‘clear’ so photon can
escape.
The silica provides passivation/device isolation and
carrier confinement
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Efficient LED
Need a p-n junction (preferably the samesemiconductor material only different dopants)
Recombination must occur Radiativetransmission to give out the ‘right coloured LED’
‘Right coloured LED’ hc/l = Ec-Ev = Eg so choose material with the right Eg
Direct band gap semiconductors to allow efficientrecombination
All photons created must be able to leave thesemiconductor
Little or no reabsorption of photons
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CandidateMaterials
Direct band gapmaterials
e.g. GaAs not Si
UV-ED l ~0.5-400nm
Eg > 3.25eV
LED - l ~450-650nm
Eg = 3.1eV to 1.6eV IR-ED- l ~750nm- 1nm
Eg = 1.65eV
Readily doped n or p-typesMaterials with refractiveindex that could allow lightto ‘get out’
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Typical Exam Question
Describe the principles of operation of an LED and state the
material’s requirements criteria to
produce an efficient LED.
(50 marks)
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Visible LED
Definition:LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = 390- 770nm. This coincides with the energy value of 3.18eV- 1.61eV whichcorresponds to colours as stated below:
Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV Yellow ~ 2.15eV Orange ~ 2.08eV
Red ~ 1.62eV
Colour of an LED
should emits
The band gap, Eg thatthe semiconductor must
posses to emit eachlight
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LED Driver Circuits
The optical output of LED isapproximately proportional to the
drive current.
LEDs are usually driven with either a
digital signal or an analog signal.
The key concern is driving the LED
so that maximum speed is achieved.
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LED Structures
• LED for optical used should have
– High radiance out put
– High quantum efficiency
– High confinement of the charge carrier to the
active region of PN junction
– For this double hetrostructure configuration isused
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LED Structures
Radiance or brightness:
It is a measure ,in watts, of the optical power radiated into
a unit solid angle per unit area of emitting surface
Quantum efficiency:
It is related to fraction of injected electron-hole pairs that
recombine radiatively.
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LED Structures
Charge carrier confinement:It is used to achieve a high level of radiative
recombination in the active region of the device.
Optical confinement:
It is used to prevent absorption of the emitted radiation by
the material surrounding the pn junction
*By confining the charge carrier and optical emission high
radiance and high quantum efficiency can be achieved
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LED Structures
To achieve carrier and optical confinement doubleheterostructure of two different alloy layers on each side of
active region is used.
The band gap differences of adjacent layers confine the
charge carriers, while the difference in the indices of
refraction of adjoining layers confine the optical field to the
central active region.
This dual confinement leads to both
high efficiency and high radiance.
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• Two different alloy layers on each side of
the active region
• Recombination region is maximumconfined to 0.3 um.
• Two LED configuration
– Surface emitter
– Edge emitter
LED Structures
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*Energy Bands in a Double Heterojunction
Double-heterostructure configuration
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– Emitting region is perpendicular to the axis of the fiber
– The circular active area in practical surface
emitters is normally 50 um in diameter and upto 2.5 um thick.
– The emission pattern is essentially isotropic
with a 120 half power beam width.
Surface-emitting LED
S f E itti LED’
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Surface-Emitting LED’s
The surface-emitting LED (S-LED) emits light
over a vary wide angle. This type of light source
is often called a Lambertian emitter, because of
the nature of the emission pattern
This broad emission angle is attractive for use as
an indicating LED, therefore it is difficult to
focus more than a small amount of the total light
output in the fiber.
S f E itti LED’
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Surface-Emitting LED’s
The key advantage of surface-emittingLED’s is their low cost, making these
light emitters the dominant type in use.
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High radiance surface emitting LED. The active region is limited to a
circular section having an area compatible with the fiber core end
face
Ed E itti LED’
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Edge-Emitting LED’s
This type has a much narrower angleof light emission and also has asmaller emitting area.
This allows a larger percentage totallight output to be focused into thefiber core.
The disadvantage of E-LED’s is thatthey are very temperature sensitivecompared to S-LED’s.
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Edge emitting double heterojunction LED.
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Light emission cone
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Light-emission cone
Only light falling within a cone defined by the critical angle will be
emitted from an optical source.
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Some LED and Laser Diode Material Mixtures and
their Characteristics
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Quantum Efficiency
The active region of an ideal LED emits one photon for
every electron injected.
Each charge quantum-particle (electron) produces one
light quantum-particle (photon).
Thus the ideal active region of an LED has a quantumefficiency of unity.
The internal quantum efficiency is defined
ηint =number of electrons injected into LED per second
number of photons emitted from active region per second
Quantum Efficiency
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Quantum Efficiency
The external quantum efficiency is defined as
ηext =number of electrons injected into LED per second
number of photons emitted into free space per second
The external quantum efficiency gives the ratio of the
number of useful light particles to the number of
injected charge particles.
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Coupling light output to a fibre is the most difficult and
costly part of manufacturing a real LED or laser device
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Intentionally Left Blank
Laser Diodes
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“LASER” is an acronym of
Light Amplification by
Stimulated Emission of R adiations.
Laser Diodes
Advantages:
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g
Ideal laser light is single-wavelength only. This is not
exactly true for communication lasers.
Lasers can be modulated (controlled) very precisely.
Lasers can produce relatively high power . Indeed some
types of laser can produce kilowatts of power. In
communication applications, semiconductor lasers of
power up to about 20 milli watts are available
Because laser light is produced in parallel beams, a high
percentage (50% to 80%) can be transferred into the
fibre.
Disadvantages:
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g
Lasers have been quite expensive by comparison with
LEDs.Reason: Temperature control and output power control is
needed.
The wavelength that a laser produces is a characteristicof the material used to build it and of its physical
construction. Lasers have to be individually designed for
each wavelength they are going to use.
Amplitude modulation using an analogue signal is
difficult with most lasers because laser output signal
power is generally non-linear with input signal power
A Id l B f L R di i
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An Ideal Beam of LASER Radiation
Properties of LASER
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LASER has three main characteristics:
Monochromatic
Coherent
Collimated
p
Properties of LASER
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Radiation which occurs at a single wavelength or color.
Monochromatic
p
Properties of LASER
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It means all waves vibrate in step, there byconstructively reinforcing each adjacent
wave.
Constructive interference Coherent light
Destructive interference Incoherent light
=
=
Coherent
A.
B.
OPTICAL INTERFERENCE
p
Properties of LASER
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It means rays are travelling in the samedirection on parallel paths.
Source
Target area
A
Source
Collimating lens
B
Collimated
p
LASER Principles
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• Laser action is the result of three keyprocess
1. Photon absorption
2. Spontaneous emission
3. Stimulated emission
p
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Laser transition processes
LASER Principles
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LASER Principles
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Spontaneous emission
When an electron is elevated to a high energy state thisstate is usually unstable.
Electron will spontaneously return to a more stable state
very quickly (within a few picoseconds) emitting a photon as
it does so.
When light is emitted spontaneously:
•Its direction and phase will be random
•Wavelength will be determined by the amount of energy
that the emitting electron must give up.
LASER Principles
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Stimulated emission
In some situations when an electron enters a high energy
(excited) state it is able to stay there for a relatively long
time (a few microseconds) before it changes state
spontaneously.
When an electron is in this semi-stable (metastable) high
energy state it can be “stimulated” by the presence of a
photon of light to emit its energy in the form of another
photon.
Stimulated emission
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**The photon that triggered (stimulated) the
emission itself is not absorbed and continues along
its original path accompanied by the newly emitted
photon.
**It is of fundamental importance to understand that
when stimulated emission takes place the emitted
photon has exactly the same wavelength, phase and
direction as that of the photon which stimulated it.
Stimulated emission
LASER Principles
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• Photon absorption
E2
E1
LASER Principles
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Absorption
l =122nm
E1
E2
E3
E4
E5Excited state
LASER Principles
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–Spontaneous emission
E2
E1
LASER Principles
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Spontaneous Emission
l =122nm
E1
E2
E3
E4
E5Excited state
LASER Principles
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• Stimulated emission
E2
E1
LASER Principles
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Stimulated Emission
l =122nm
E1
E2
E3
E4
E5Excited state
Population Inversion
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Under normal conditions, thepopulation of unexcited atoms greatly
exceeds the population of excited atoms.
This result, predominance of the
spontaneous emission and the resultingradiation is largely incoherent.
Population Inversion
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For a LASER to operate, situationmust be reversed, called population
inversion.
Once the majority of the atoms in a
material are in an excited state,
stimulated emission will be greaterthan spontaneous.
Population inversion is achieved by various
“Pumping” techniques.
Energy Pumping
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Energy level diagram for a typical laser mater ial.
X Y
Energy Pumping
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The energy population in a Lasingmedium is inverted by a methodknown as energy pumping.
Energy can be pumped, into amaterial in a number of ways,depending upon the composition of the material
Optical pumping.
Electronic pumping.
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The amount of light leaving the end mirror (partially
reflected) equals the amount of energy being pumped into
the cavity ,minus losses .
The amount of reflectivity needed in the end
mirrors is a function of the amount of gain in the
cavity.
Types of LASER Diodes
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Fabry-Perot LASER Diode
Distributed Feedback LASER Diode
Vertical Cavity Surface-emitting LASER
Diode
Fabry-Perot LASER Diode
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– Radiation is generated by the cavity
– Cavity size is very small like 250-500 Um long, 5-15 um wide and 0.1 to 0.2 um thick
– Reflecting mirrors are used at both sides of
the cavity – Mirrors provide strong feedback to the cavity
to have the mechanism
– The side of the cavity are formed by
roughening the edges to reduce unwantedemission
Fabry-Perot resonator cavity
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Fabry-Perot resonator cavity for a laser diode. The cleaved crystal ends function as
partially reflection mirrors. The unused end ( the rear facet) can be coated with
dielectric reflector to reduce optical loss in the cavity. Note that the light beam
emerging from the laser forms a vertical ellipse, even though the lasing spot at the
active-area facet is a horizontal ellipse.
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Two parallel light-reflecting surfaces define a Fabry –Perot cavity or etalon.
Fabry-Perot laser spectrum
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Typical spectrum from a Fabry-Perot GaAls/ GaAS aser diode
Distributed Feedback LASER (DFB)
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DFB LASERs offer better performance, less
noise and also higher cost than FP.
They are nearly monochromatic.
DFB LASERs are used for high speed digitalapplication and for most anloge application because of their faster speed, low noise andsuperior linearity.
Lasing is achieved from the Braggreflectors (gratings) along the length of thediode
DFB laser
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Structure of a distributed feedback (DFB) laser diode
Optical output vs. drive current
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Relationship between optical power and laser diode drive current. Below
the lasing threshold the optical output is a spontaneous LED-type
emission.
Vertical Cavity Surface-emitting LASER Diode
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VCSELs don’t use monitor photodiode as
they emit their light in a vertical plane perpendicular to the semiconductor wafer.
Dramatically reduce the cost of LASERs tonear those of LEDs
Available @ 850 nm
Generates interest @ 850 window
Vertical Cavity Surface-emitting LASER Diode
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1300 nm VCSELs to be ready
Currently Conventional LASERs
used @ 1300nm
VCSELs are manufactured in such a
way that they are ideal for
application requiring arrays of devices.
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Vertical Cavity Surface Emitting Laser (VCSELs)
Modulation of Laser Diodes
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The process of putting information onto a light wave is
called modulation.
Two types:
1. Direct Modulation
2. External Modulation
For Data rates of less than approximately 10 Gb/s
(Typically 2.5 Gb/s), the process of imposing information on
a laser-emitted light stream can be realized by direct
modulation.
For higher data rates external modulator is used to
temporally modify a steady optical power level emitted by
the laser.
Modulation of Laser Diodes
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The basic limitation on the direct modulation rate of laser
diodes depends on:
1. Spontaneous emission carrier lifetimes.
2. Stimulated emission carrier lifetimes.
3. Photon lifetime.
Spontaneous carrier life time is a function of the semiconductor band
structure and the carrier concentration.
Stimulated carrier life time depends on the optical density in the lasing
cavity.
The photon lifetime is the average time that the photon resides in the
lasing cavity before being lost either by absorption or by emission through
the facets.
Modulation of Laser Diodes
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Pulse modulation is carried out by modulating the
laser only in the operation region above
threshold.
In this region the carrier lifetime is now shortened
to the stimulated emission lifetime, so that the
high modulation rates are possible.
Modulation methods
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Modulation of Laser Diodes
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Directly modulated laser diode, the modulation
frequency can be no larger than the frequency of therelaxation oscillations of the laser field.
The relaxation oscillation depends on both the
spontaneous lifetime and the photon lifetime.
Spontaneous
life timePhoton
life timeThreshold
current
Total
current
Relaxation oscillation peak
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External Modulation
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Directly modulated lasers can not operate
smoothly at data rates greater than 2.5 Gbps. It
is suitable to use external modulators above this
data rate.
There are two types of external modulators
1.Electrooptical modulator
2.Electroabsorption modulator
1.Electrooptical modulator
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1.Light beam is split in half
2.A high-speed electric signal then changes the
phase of the light signal in one of the paths.
3.The constructive recombination produces a
bright signal and corresponds to a 1 pulse.
4. Destructive recombination corresponds to a
0 pulse.
The electrooptical (EO) modulator typically is
made of lithium niobate (LbNiO3).
2. Electroabsorption modulator
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The electroabsorption modulator (EAM) typically is
constructed from indium phosphide (InP).
It operates by having an electric signal change the
transmission properties of the material in the light
path to make it either transparent during a 1 pulse or opaque during a 0 pulse
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Bias Current Electronic Modulation Current
Constant Optical Output Power Modulated Optical Output Power
Pcw P (t)
Operational concept of generic external modulator
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CW
Input Light
Modulated
Input Light
Phase
Shifter
Beam
Splitter
Beam
Combiner
Operational concept of an electrooptical lithium niobate
external modulator
Temperature Effects
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Threshold current Ith (T) of laser diode also depends on the
temperature. This parameter increases with temperature in
all types of semiconductor lasers.
Adjustment in the dc-bias (varies with temperature)current level is necessary for constant optical power level.
One possible method for achieving this automatically is
an optical feedback scheme.
So, output optical power can be stabilized in two ways:
1. Controlling dc-bias current level.
2. Controlling temperature level.
Temperature Effects
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The Photodetector compares the optical power with areference level and adjusts the dc-bias current level
automatically to maintain a constant peak light output relative
to the reference.
Another standard method of stabilizing the optical output of
a laser diode is to use a miniature thermoelectric cooler.
Optical feedback can be carried out by using a
photodetector either to sense the variation in optical power
emitted from the rear facet of the laser or to tap off andmonitor a small portion of the fiber-coupled power emitted
from the front facet.
Normally both methods are used together
Temperature dependence
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Temperature-dependent behavior of the optical output power
as a function of the bias current for a particular laser diode.
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Construction of a laser transmitter that uses a rear-facet photodiode for
output monitoring and a thermoelectric cooler for temperature stabilization
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Laser diode module
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High-performance optical transmitter containing a DFB laser a fiber