This document discusses optical sources, specifically lasers. It begins with an overview of lasers and their basic concepts like stimulated emission. It then discusses semiconductor injection lasers in more detail, including their structure, characteristics like threshold current and temperature variation, and coupling lasers to optical fibers. Key aspects covered include population inversion, optical cavities, stripe geometry, and techniques for fiber coupling like butt coupling and lens systems.

1. Optical Sources
(laser)

2. contents'
Laser basic concept.
Optical emission from semiconductor.
Semiconductor injection laser.
Injection laser structure and characteristics.
Laser fiber coupling.
non semiconductor laser.
Laser modulation.

3. Main function
• Convert electrical energy into optical energy (with the condition
that light output to be effectively launched or coupled into
optical fiber)
Types
• Wideband continuous spectra (incandescent lamps)
• Monochromatic incoherent ( LEDs)
• Monochromatic coherent (LDs)

4. 1. Definition of laser
A laser is a device that generates light by a process called
STIMULATED EMISSION.
The acronym LASER stands for Light Amplification by
Stimulated Emission of Radiation
Semiconducting lasers are multilayer semiconductor devices that
generates a coherent beam of monochromatic light by laser action.
A coherent beam resulted which all of the photons are in phase.

5. Fibre Optics Communication

6. E1
E2
hυ
(a) Absorption
hυ
(b) Spontaneous emission
hυ
(c) Stimulated emission
In
hυ
Out
hυ
E2 E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

7. Background Physics
In 1917 Einstein predicted that:
 under certain circumstances a photon incident upon a material
can generate a second photon of
 Exactly the same energy (frequency)
 Phase
 Polarisation
 Direction of propagation
 In other word, a coherent beam resulted.

8. Background Physics
Consider the ‘stimulated emission’ as shown previously.
Stimulated emission is the basis of the laser action.
The two photons that have been produced can then
generate more photons, and the 4 generated can generate
16 etc… etc… which could result in a cascade of intense
monochromatic radiation.

9. E1
E2
hυ
(a) Absorption
hυ
(b) Spontaneous emission
hυ
(c) Stimulated emission
In
hυ
Out
hυ
E2 E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

10. Population Inversion
Therefore we must have a mechanism where N2> N1
This is called POPULATION INVERSION
Population inversion can be created by introducing a so call metastable centre where
electrons can piled up to achieve a situation where more N2 than N1
The process of attaining a population inversion is called pumping and the objective is to
obtain a non-thermal equilibrium.
It is not possible to achieve population inversion with a 2-state system.
If the radiation flux is made very large the probability of stimulated emission and absorption
can be made far exceed the rate of spontaneous emission.
But in 2-state system, the best we can get is N1 = N2.
To create population inversion, a 3-state system is required.
The system is pumped with radiation of energy E31 then atoms in state 3 relax to state 2 non
radiatively.
The electrons from E2 will now jump to E1 to give out radiation.

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Basic Concept (LASER)

12. Therefore in a laser….
Three key elements in a laser
•Pumping process prepares amplifying medium in suitable state
•Optical power increases on each pass through amplifying medium
•If gain exceeds loss, device will oscillate, generating a coherentoutput

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Basic Concept (LASER)
Optical feedback and laser oscillations
• Photon striking an excited atom causes emission
of a second photon which release two more photons creating an
avalanche multiplication.
• Amplification &Coherence achieved by (Febry – Perot resonator)
• Placing mirrors at either end of the amplifying medium
• Providing positive feedback
• Amplification in a single go is quite small but after multiple passes
the net gain can be large
• One mirror is partially transmitting from where useful radiation
may escape from the cavity
• Stable output occurs when optical gain is exactly matched with losses incurred (Absorption,
scattering and diffraction)

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Semi Conductor Emission
Types of material
Conductor
Insulators
Semi conductors (intrinsic & extrinsic)
N type material
Donor impurity added
Increases thermally excited electrons in the conduction band
P type material
Acceptor impurity added
Increases positive charges (holes) in the valance band
PN junction

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Semiconductor
The properties of Semiconductor lies in between conductor and insulator.
The semiconductor has valance band, conduction band and Fermi level ( the gap
between two bands ). The Fermi level is relatively small which means that small
amount of energy is sufficient to bring about electric current in semiconductors.
Conduction Band
Valance Band
Fermi Level

16. n-Type Semiconductor
a) Donor level in an n-type semiconductor.
b) The ionization of donor impurities creates an increased electron
concentration distribution.
Optical Fiber communications, 3rd
ed.,G.Keiser,McGrawHill, 2000

17. p-Type Semiconductor
a) Acceptor level in an p-type semiconductor.
b) The ionization of acceptor impurities creates an increased hole concentration distribution
Optical Fiber communications, 3rd
ed.,G.Keiser,McGrawHill, 2000

18. The pn Junction
Optical Fiber communications, 3rd
ed.,G.Keiser,McGrawHill, 2000
Electron diffusion across a pn junction
creates a barrier potential (electric field)
in the depletion region.

19. Forward-biased pn Junction
Optical Fiber communications, 3rd
ed.,G.Keiser,McGrawHill, 2000
Lowering the barrier potential with a forward bias allows majority carriers to diffuse
across the junction.

20. Reverse-biased pn Junction
Optical Fiber communications, 3rd
ed.,G.Keiser,McGrawHill, 2000
A reverse bias widens the depletion region, but allows minority carriers to move freely
with the applied field.

21. The electrical resistance of semiconductor lies in between conductors and insulators. The
increase in temperature can lower the resistance in semiconductors. Silicon is the most common
semiconductor used.
The Fermi level can be increased or decreased by adding dopants into Silicon.
If P-type material such as Aluminum, Gallium or Indium are added, which create holes and
shortage of electrons. Fermi level is increased. If N-type material such as Phosphorus, Arsenic and Boron
are added the result is excess of electrons and Fermi level is reduced.
P.N Junction
If a voltage is applied to P.N Junction ( Forward biased ). The Fermi Level on both sides of
Junction will move, so that a current will flow through the P.N Junction. The electrons will flow into the P
layer and holes are formed in N layer. The system tries to reach equilibrium by excited electrons flowing
to holes . During this process ,energy is released in the form of Photons. It is the mechanism of Light
emitting diodes. If p layer and N layer in the P.N Junction are heavily doped and strong current used a
population inversion of electrons occurs in an Optical Cavity ,a Laser can be created.

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Semiconductor Injection Laser
Stimulated emission by recombination of injected carriers
Optical cavity provided in the crystal structure
Advantages are
High radiance due to amplifying effect
Narrow line width of the order of 1nm
High modulation capabilities presently ranging in G Hz
Good spatial coherence which allows the output to be focused
by a lens into a spot to provide high coupling efficiency

23. Semiconductor Injection Laser
Adv.of the injection laser. over other semiconductor laser(LED)
1)High radiance due to the amplifying effect of stimulation emission(supply
miliwatt of o/p current)
2)Narrow line width of the order of (nm (10A)0r less ) which is useful in
minimizing the effects of material dispersion
3)Modulation capability which is present extend upto the gigahertz
4)GOOD spatial coherence which allows the o/p to be focused by a lens into
spot which has grater intensity then unfocused emission (permit coupling of
o/p power into fiber even for fibers with low NA

25. Stripe geometry
Stripe geometry to the structure to provide optical containment in
the horizontal plane
Overcome major problem associated with the broad area devices.
o/p beam diverges is typically 45 degree perpendicular to the plane
of the junction and 9 degree parallel to it

26. Stripe geometry

27. injection laser to fiber coupling
One of the major difficulties with using semiconductor laser
problem associated with the coupling of light between the
laser and the optical fiber(perticularly single mode, low NA).
Techniques of coupling of injection laser to optical fibers
1)butt coupling
2)tapered hemispherical fiber coupling
3)confocal lens system

28. 1)butt coupling
injection laser are relatively directional they have diverging o/p field
Efficiency around10%.(even good alignment and use of a fiber with a
well cleaved end)
Positioning the fiber end very close to the laser facts,
2)tapered hemispherical fiber coupling
the coupling efficiency can be substaintaily improved when the field
from the laser is matched to the output field of the fiber such
achieved by using lens
Coupling efficency 65%.

29. 3)confocal lens system
Injection laser coupling designs based on discrete lenses
have also proved fruitful
use of silicon lens within a confocal system has provide
coupling efficiencies of up to 70%

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ILD Characteristics
Threshold current Vs Temperature
Threshold current in general tends to increase with temperature
Dynamic response
Switch-on delay followed by damped oscillations known as relaxation
oscillations
Serious deterioration at data rates above 100 Mbps if td is 0.5 ns and RO
of twice this
td may be reduced by biasing the laser near threshold current
ROs damping is less straight forward

31. Temperature variation of the threshold
current
0/
)( TT
zth eITI =

32. Relaxation oscillation peak

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ILD Characteristics
Frequency Chirp
Direct current modulation of a single mode
semiconductor laser causes dynamic shift of the peak
resulting in linewidth broadening called Frequency Chirp
Combined with chromatic dispersion causes significant
performance degradation
Can be reduced by biasing the laser sufficiently above
the threshold current
Low chirp in DFB and quantum well lasers

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ILD Characteristics
Noise
Phase or frequency noise
Instabilities like kinks
Reflection of light into the device
Mode partitioning
Mode Hopping
Mode hopping to a longer wavelength as the current is
increased above threshold

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ILD Characteristics
Reliability
Major problem in ILDs
Device degradation occur
Two types of degradations
Catastrophic degradation
– Mechanical damage to mirror facet
– Results in partial or complete failure
Gradual degradation
– Defect formation in active region
– Degradation of the current confining region