ChapterOptical Sources

33

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

Absorption and emission of radiation

Quantum theory suggests that atoms exist only in certain discrete energy states. The interaction of light with matter takes place in discrete packets of energy, called photons.

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 systemAbsorption

Spontaneousemission

Stimulatedemission

Spontaneous & Stimulated Emission

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

Stimulated emission The proton produced is generally of an

identical energy to the one which caused it and hence the light associated 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

thein-phase property of light waves within a beam.

The Einstein Relations

At thermal equilibrium, the population of the two energy levels of such system are described by Boltzmann statistics which give:

where N1 and N2 represent the density of atoms in energy levels E1 and E2, respectively,with g1 and g2 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, is equal to the reciprocal of spontaneous lifetime. The second term which is the rate of stimulated downward transition is similar to that of stimulated upward transition.

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

When g12=g21, then the probabilities of absorption and stimulated emission rate are equal, and the stimulated emission rate to the spontaneous emission rate is given by

In thermal equilibrium, spontaneous emission is the dominant mechanism.

For stimulated emission to dominate over absorption and spontaneous emission in a two level system, both the radiation density and the population density of the upper energy level N2 must be increased in relation to the population density of the lower energy level N1.

Population inversion

Boltzmann distribution for a system in thermal equilibrium, the lower energy level E1 of the two level system contains more atoms than upper energy level E2.

To achieve optical amplification it is necessary to create a nonequilibrium distribution of atoms such that the population of the upper energy level is greater than that of the lower energy level (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 fed back many times whilst receiving amplification (stimulated emission) as it passes through the medium. One mirror is made partially transmitting to allow 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 the cavity 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 the axis of the cavity. This gives rise to resonant modes which are transverse to the direction of propagation. These transverse electron magnetic modes are designated by TEMlm where the integers l and m indicate the number of transverse modes

Unlike the longitudinal modes which contribute only a single spot of light to the laser output, transverse modes may give rise to a pattern of spots at the output. The greatest degree of coherence, together with the highest level of spectral purity is only obtained from a laser which operates in the TEM00 (the lowest) mode as all parts of the propagating wave front are in phase.

Higher order transverse modes only occur when the width of the cavity is sufficient 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 transition is necessary for oscillation to be established but it is not alone sufficient for lasing to occur. In addition a minimum or threshold gain within the amplifying medium must be attained such 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.

Consider an amplifying medium occupies a length L completely filling the region between the two mirrors which have reflectivities r1 and r2. On each round trip the beam passes through the medium twice and the fractional loss 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 is exponential and the fractional round trip gain is given by

where is the gain coefficient per unit length produced by stimulated emission.

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 ~ Eg , the bandgap 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 only possible 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 electron states in the conduction band of

the p-type material and the empty hole states in the valence band of the n-type material.

• E-h recombination across the bandgap released energy as a photon

The optical wavelength is

This spontaneous emission of light from within the diode structure is known as electroluminescence.

The light is emitted primarily close to the junction, recombination may take place throughout diode structure as carriers 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 emission from 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 a rectangular parallel piped >100mm to 1mm.

The junction is a plane within the structure. Two of the sides perpendicular to the

junction are purposely roughened so as to reduce their reflectivity.

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

These two surfaces form the mirrors for the laser cavity. The reflectivity of the air-semiconductor interface is high enough so that no other mirrors are needed,

One of the reflecting surfaces may be coated to increase 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 is amplified 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 is the 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, Jth, 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 the threshold current which corresponds to spontaneous emission .

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

The stimulated emission minority carrier lifetime is much shorter ( typically 10-11s) than that due to spontaneous.

The coherent emission has a linewidth of a nanometer or less, whereas incoherent spontaneous emission has a linewidth 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 Pe 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 Eg 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 Pe changes linearly when the injection current I > Ith, then

For high injection current (e.g. I = 5Ith), then , whereas for lower currents (I =2Ith) 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

hfP

electrons injected of number total

photons output of number total

/

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 the narrow junction, ~ 1mm and is spread by diffraction to an angle as large as several tens of degrees.

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

The emission from a gallium arsenide laser tends to be an elliptical beam with a fullangle 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 withindividual 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 vary continuously 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.