Chapter3 A

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ChapterOptical Sources

3



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 areheterojunction-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



Absorption and emission of radiation

Quantum theory suggests that atoms exist only in certain discrete energy states. Theinteraction of light with matter takes place in discrete packets of energy, calledphotons.

The frequency of the absorbed or emitted radiation f is related to the difference in

energy E between the higher energy state E2 and the lower energy state E1 by the

expression:

Two level atomic system

Absorption

Spontaneous

emission

Stimulated

emission



Spontaneous & Stimulated

Emission

Spontaneous emission - the atoms returns to the lower energy state in an entirely random manner, givesincoherent radiation.

Stimulated emission

The proton produced is generally of an identicalenergy to the one which caused it and hence the lightassociated with them is of the same frequency, f.

Light associated with the simulating and simulated

photon is in phase and has the same polarization."Coherence" is the term used to describe the

in-phase property of light waves within abeam.



The Einstein Relations

At thermal equilibrium, the population of the two energy levels of such

system are described by Boltzmann statistics which give:

where N 1 and N 2 represent the density of atoms in energy levels E 1 and

E 2, respectively,with g 1 and g 2 being the corresponding degeneracies of

the levels.

The absorption transition rate

Where is the spectral density of the radiation energy at the transition

frequency f and B12 is the Einstein coefficient of absorption.

The total transition rates from level 2 to level 1 (emission rate), R21, is

the sum of thespontaneous and stimulated contributions.



where A21 is the Einstein coefficient of spontaneous emission, isequal to the reciprocal of spontaneous lifetime. The second termwhich is the rate of stimulated downward transition is similar to thatof stimulated upward transition.

For a system in thermal equilibrium, R12=R21 and gives

When g 12=g 21, then the probabilities of absorption and stimulatedemission rate are equal, and the stimulated emission rate to the

spontaneous emission rate is given by

In thermal equilibrium, spontaneous emission is the dominantmechanism.

For stimulated emission to dominate over absorption and

spontaneous emission in a two level system, both the radiationdensity and the population density of the upper energy level N 2 mustbe increased in relation to the population density of the lower energylevel N 1.



Population inversion

Boltzmann distribution for a system inthermal equilibrium, the lower energylevel E1 of the two level system containsmore atoms than upper energy level E2.

To achieve optical amplification it is

necessary to create a nonequilibriumdistribution of atoms such that thepopulation of the upper energy level isgreater than that of the lower energylevel ( N2 > N1).

This condition is known as population

inversion.

The excitation of atoms into the upper energy level is referred to as “pumping”.



Optical feedback

The basic laser structure incorporating plane mirror. The optical signal is fedback many times whilst receiving amplification (stimulated emission) as itpasses through the medium. One mirror is made partially transmitting toallow useful radiation escape from the cavity.

The structure acts as a Fabry-Perot resonator .

A stable output is obtained at saturation when the optical gain is exactly

matched by the losses incurred in the amplifying medium.The major losses results from Absorption and scattering in the amplifying medium

Absorption, scattering and diffraction at the mirrors

Nonuseful transmission through the mirrors.



Laser oscillation

Oscillations occur in the laser cavity over a small range of frequencies where thecavity gain is sufficient to overcome the various losses. Hence the device is not

a perfectly monochromatic source but emits over a narrow spectral band.

The central frequency of this spectral is

determined by the mean energy level

difference of the stimulated emission

transition.Other oscillation frequencies within the

spectral band result from the frequency

variations due to thermal motion of

atoms within the amplifying medium,

known as Doppler broadening

(inhomogeneous broadening

mechanism) and by atomic collisions

(homogeneous broadening).



Longitudinal or axial modes

Since the structure forms a resonant cavity, when sufficient population inversion

exists in the amplifying medium the radiation builds up and becomes established

as standing waves between the mirrors.

These standing waves exist only at frequencies for which the distance between

the mirrors is an integral number of half wavelengths. Thus when the optical

spacing between the mirrors is L the resonance condition along the axis of the

cavity is given by

where λ is the emission wavelength, n is the refractive index of the amplifying

medium and q is an integer.The discrete emission frequencies is where c is the velocity of light.

The different frequencies of oscillation within the laser cavity are determined by

the various integer values of q and each constitutes a resonance or mode. These

modes are separated by a frequency interval or in term of free space

wavelength, assuming



LONGITUDINAL MODES



Transverse modes

Laser oscillation may also occur in a direction which is transverse to theaxis of the cavity. This gives rise to resonant modes which are transverse tothe direction of propagation. These transverse electron magnetic modes aredesignated by TEM lm where the integers l and m indicate the number of transverse modes

Unlike the longitudinal modes which contribute only a single spot of light tothe laser output, transverse modes may give rise to a pattern of spots at theoutput. The greatest degree of coherence, together with the highest level of spectral purity is only obtained from a laser which operates in the TEM 00 (thelowest) mode as all parts of the propagating wave front are in phase.

Higher order transverse modes only occur when the width of the cavity issufficient for them oscillate. Consequently, they may be eliminated by

suitable narrowing of the laser cavity.



Threshold condition for laser oscillation

Population inversion between the energy levels providing the laser transitionis necessary for oscillation to be established but it is not alone sufficient for lasing to occur. In addition a minimum or threshold gain within theamplifying medium must be attained such that laser oscillations are initiatedand sustained. This threshold gain may be determined by considering thechange in energy of a light beam as it passes through the amplifying

medium.Consider an amplifying medium occupies a length L completely filling theregion between the two mirrors which have reflectivities r 1 and r 2. On eachround trip the beam passes through the medium twice and the fractionalloss incurred by the light beam is

Where is the loss coefficient per unit length (cm-1) for all the losses

except those due to transmission through the mirrors.The increase in beam intensity resulting from stimulated emission isexponential and the fractional round trip gain is given by

where is the gain coefficient per unit length produced by stimulatedemission.



At steady state conditions the laser oscillation are achieved when the gain

in the amplifying medium exactly balances the total losses. Hence

The threshold gain pre unit length can be written as

the second term represents the transmission loss through the mirrors.



Optical emission from semiconductors

Direct bandgap semiconductors -

• electrons and holes on either side

of the energy gap have the same

value of crystal momentum, k .

• when e-h recombination occurs k

of the electron remains virtually

constant.

• the energy released, hf ~ E g , thebandgap energy, may be emitted

as light.

Indirect bandgap semiconductors-

• the maximum and minimum

energies occur at different values

of crystal momentum.

• the e-h recombination onlypossible with the aid of a third

particle, a phonon.

• the recombination in indirect

bandgap semiconductor is

relatively slow.



Optical emission from p-n diodes

Under forward bias carriers are

injected into the empty electronstates in the conduction band of thep-type material and the empty holestates in the valence band of the n-type material.

• E-h recombination across thebandgap released energy as aphoton

The optical wavelength is

This spontaneous emission of lightfrom within the diode structure is

known as electroluminescence.The light is emitted primarily close tothe junction, recombination may takeplace throughout diode structure ascarriers diffuse away from the junction region.



Stimulated Emission in Semiconductor Diode

Population inversion may be obtained at a p-n junction by heavy doping of both the p and n type material.

Heavy doping causes the Fermi level enter the conduction band of the n-region and lowering the Fermi level into the valence band in p region.

When a forward bias ~ bandgap voltage is

applied, an active region exists near the

depletion layer that contains simultaneously

degenerate populations of electron and

holes, where population inversion is occur.

Any electromagnetic radiation of frequency

which is confined to the active region will be

amplified.

The degenerative doping distinguishes a p-n

junction which provides stimulated emissionfrom one which gives only spontaneous

emission as in the LED.

High impurity concentration within a

semiconductor causes differences in the

energy bands in comparison with an intrinsic

semiconductor



Semiconductor Injection Laser

Electrons are injected into the device from the n-

type side - a current-controlled device.Diode laser commonly takes the form of arectangular parallel piped >100mm to 1mm.

The junction is a plane within the structure.

Two of the sides perpendicular to the junctionare purposely roughened so as to reduce their reflectivity.

The other two sides are made optically flat andparallel, by either cleaving or polishing.

These two surfaces form the mirrors for thelaser cavity. The reflectivity of the air-semiconductor interface is high enough so thatno other mirrors are needed,

One of the reflecting surfaces may be coated toincrease the reflectivity and to enhance laser operation.

The thickness of the junction region is small,typically around one micrometer.Light traveling in the plane of the junction isamplified more than light perpendicular to it and

the laser emission is parallel to the plane of the junction.



Threshold Current

The rate equations for electron n, and photon density f , in the active layer of

the semiconductor laser are

where J is the injected current density ( A/m2), e is the charge on an electron,

d is the thickness of the recombination region, is the spontaneous

emission lifetime, C is a coefficient which incorporates the B coefficients, δ isthe fraction of photons produced by spontaneous emission which combine to

the energy in the lasing mode and is the photon lifetime.

The fields in the optical cavity, Φ, is build up from small initial values with

when Φ is small. By setting δ = 0 , we have

The threshold value for the electron density is

The threshold current density, J th, required to maintain n = nth in the steady

state when Φ = 0, is and the steady state photon density is



Lasing in an Injection Laser

Laser diode gives little light output in the region below thethreshold current which corresponds to spontaneousemission .

Above the threshold light output is almost entirely due tostimulated emission, where the laser is acting as an amplifier of light.

The stimulated emission minority carrier lifetime is much

shorter ( typically 10-11

s) than that due to spontaneous.The coherent emission has a linewidth of a nanometer or less, whereas incoherent spontaneous emission has alinewidth of tens of nanometers.

The photon density is proportional to the amount by which J

exceeds its threshold value.For strongly confined structures, the threshold gain coefficient

where the gain factor is a constant appropriate to specific

devices.

The threshold current density can be written as



Operational Efficiency of Injection Laser

Differential external quantum efficiency, is the ratio of the increase in

photon output rate for a given increase in the number of injected electrons

(the slope quantum efficiency)

where P e is the optical power emitted from the device, I is the current, e is

the charge on an electron, hf is the photon energy and E g is the bandgap

(eV). For a continuous wave (CW) operation laser, usually has values in the

range 40 to 60%.

is related to by the expression

(it has values in the range 50 to 100%)



Total efficiency

(external quantum efficiency)

As the power emitted P e changes linearly when the injection current I > I th, then

For high injection current (e.g. I = 5I th ), then , whereas for lower

currents (I =2I th ) the total efficiency is lower and around 15 to 25%.

External power efficiency (or device efficiency),

where P = IV is the d.c. electrical input power.

g

ee

T

IE

P

1/e

hf P

electronsinjectedof numbertotalphotonsoutputof numbertotal

/



Beam Profile from a Semiconductor Laser

The beam emitted from a semiconductor laser typically has an elliptical

spatial profile.In the direction perpendicular to the junction, the beam is confined by thenarrow junction, ~ 1mm and is spread by diffraction to an angle as large asseveral tens of degrees.

In the direction parallel to the junction, the beam is not confined sostringently and spreads less to around ten degrees.

The emission from a gallium

arsenide laser tends to be an

elliptical beam with a full

angle divergence around 20°

in the direction perpendicular

to the junction and around 5° in the direction parallel to the

junction. These angles may

vary considerably with

individual lasers.



Semiconductor Laser Materials

Most semiconductor laser materials are

fabricated from 3-5 compounds – structures formed of alloys that contain

three or four elements from columns 3

and 5 of the periodic table.

The three most important types of ternary

or quaternary compound semiconductor

laser materials.

The properties of the material varycontinuously as x and y vary (0 →1).

Material Wavelength Range (nm)

Al1 – xGaxAs 780-880

In1 – xGaxAs1 – yPy 1150-1650

In1 – xGaxAs1 – yPy 630-680

The most common of the semiconductor lasers is the Al1 – xGaxAs laser and

are used for optical disks, optical-fiber telecommunications, and laser

printers.

The quaternary alloy In1 – xGaxAs1 – yPy, are commonly used for optical-fiber

telecommunications.

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