Sources for fiber communication :

• Diode laser• LED

Introduction

• Optical sources is considered to be the active component in an optical fibre communication system.

• Its fundamental function is to convert electrical energy (current) into optical energy (light/photon)

• Allows light output to be effectively launched or coupled into the optical fibre.

Types of light sources

Three main types of optical sources are available1. Wideband ‘continuous spectra’ sources

(Incandescent lamps)2. Monochromatic incoherent sources (Light-

emitting diodes (LEDs))3. Monochromatic coherent sources (lasers)

Selection of an optical sourceSelection can be done by taking the following

aspects into account1. Dimensional compatibility with the fibre core2. Fibre’s attenuation as a function of wavelength3. Must be highly coherent and monochromatic4. The purpose for which the system is used.5. Should easily be coupled to the fibre.6. Must have faster response time7. Must be highly directional

Suitable Sources

LED and semiconductor laser are sources to fulfill the major requirements for an optical fiber

1. They have the required power.2. They have high efficiency3. Dimensional characteristics are compatible

with those of the optical fiber.4. Optical power can be directly modulated by

varying the input current to the device.

Diode Laser• It is a specially fabricated p-n,junction device that

emits coherent light when it is forward biased• First semiconductor laser were made by Hall and

Coworkers and Nathan and his associates in 1962 using gallium arsenide, (GaAs) which operated at low temperature and emitted light in the near IR region.

• A laser diode, also known as an injection laser or diode laser, is a semiconductor device

• Other than optical fibre systems it can be used in compact disc (CD) players, laser printers, remote-control devices, and intrusion detection systems.

Characteristics• Small size and weight: A typical laser diode measures less than one

millimeter across and weighs a fraction of a gram, making it ideal for use in portable electronic equipment.

• Low current, voltage, and power requirements: Most laser diodes require only a few milliwatts of power. Therefore, they can operate using small battery power supplies.

• High Efficiency: It has high efficiency (of the order of 40%)• Portable: It is portable• Low intensity: A laser diode cannot be used for spectacular purposes

such as burning holes in metal, bringing down satellites, or blinding aircraft pilots. Nevertheless, its coherent output results in ease of modulation for communications and control applications.

• Wide-angle beam: A laser diode produces a "cone" rather than a "pencil" of visible light or IR, although this "cone" can be collimated using convex lenses.

Principle• During forward biasing holes of p-region and

electrons of n-region combine across the depletion region releasing energy very near the junction region.

• Photons, emitted at the moment of recombination will stimulate recombination

Construction• Diodes are extremely small in size with sides of the

order of 1mm• Junction lies in the horizontal plane (as in Fig.)• Ohmic contacts are made through top and bottom

surfaces which are metallized and currents are passed through these.

• The front and rear surfaces are roughened to prevent the lasing action in that direction.

• The other two opposite faces which are perpendicular to the plane of the junction are polished to introduce a reflecting mechanism in order to return radiation to the active region. This constitutes the Fabry-Perot resonator.

L Electrode

Current

GaAs

GaAsn+

p+

Cleaved surface mirror

Electrode

Active region(stimulated emission region)

A schematic illustration of a GaAs homojunction laserdiode. The cleaved surfaces act as reflecting mirrors.

L

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Background• Intrinsic Semiconductor has no impurities and characterised

by a valence band (Ev ) and a conduction band (EC) separated by a band gap, Eg

• In the normal state, valence band is occupied by a large number of electrons

• Occasionally, an electron may fall from conduction band to valence band and recombine with the holes by releasing energy, Eg=(Ec-Ev) in the form of a photon hν

• Electron-hole recombination is the basic mechanism involved in the emission of radiation.

• In case of silicon and germanium the energy is released in the form of heat. For gallium arsenide (GaAs), cadmium selenide (CsSe) etc energy lies in optical region and suitable materials for making laser.

Energy Band Structure of Intrinsic Semiconductor at T>0K (Background)

• At temperature above absolute zero thermal excitation raises some of electron from the valence band into the conduction band, leaving empty holes left in the valence band.

• The probability f(E) that gives the occupation of an electron in an energy level (E) at absolute temperature T

• Ef (Fermi level) is at the centre of

the band gap• K is the Boltzmann constant

1

1)(

TkEE F

eEf

Fig. 2 shows equal number of electrons and holes in Conduction and valence band, respectively

Fig. 3 Energy band Diagram of (a) n-type semiconductor (b) p-type semi conductor

•Donor level is just below the conduction band in n-type and Acceptor level is just above the valence band.•For n-type, electrons from donor impurity level goes to conduction band, creating excess of negative charge carriers, the Fermi level level is raised to a position above the centre of the band gap (Fig. 3(a))•For p-type, electrons are raised from valence band to acceptor impurity level leaving an excess of positive charge carriers in the Valence band, Fermi level is lowered below the centre of the band gap (Fig. 3(b))

Unbiased p-n junction diode (Background)• P-n junction diode is formed by joining p- and n-type

semiconductor in a single crystal (Fig.4(a))• A thin depletion region is formed at the junction through carrier

recombination establishing a potential barrier between p- and n-type which restricts the diffusion of holes and electrons

• In the absence of applied voltage, no current flows as potential barrier prevents the net flow of carriers from one region to another.

• In equilibrium state the Fermi level for the p- and n-type semiconductor is the same as shown in (Fig.4 (b))

• The width of the depletion region and thus the magnitude of the potential barrier is dependent upon the carrier concentrations (doping) in the p- and n-type regions and any external applied voltage.

Fig. 4 For p-n junction with no bias (a) impurities and charge carriers (b) the energy level diagram

Fig. 5 p-n junction with forward bias giving spontaneous emission of photons (a) with direct band

gap (b) illustration of carrier recombination

(a) (b)

P-n junction with forward bias (background)

• When forward biased, electrons from n-type region and holes from p-type region can flow more readily across the junction into the opposite type region, current flows through the device.

• The energy released by this electron-hole recombination is approximately equal to the band gap energy, Eg

• The spontaneous emission of light at the site of carrier recombination is primarily close to the junction.

• The amount of radiative, recombination and the emission area within the structure is dependent upon the semiconductor materials used and the fabrication of the device.

Working of Laser Diode

Þ A heavily doped p-n junction is usedÞ With very high doping on n-side the donor levels as well as a portion

of the conduction band are occupied by electrons and the Fermi level lies within the conduction band

Þ Similarly, on the heavily doped p-side the acceptor levels are unoccupied and holes exist in the valence band and the Fermi level lies within the valence band

Þ At thermal equilibrium, the Fermi level is uniform across the junction.

Þ When a forward bias is applied to the junction, the energy levels shift and the new distribution will be taken up.

Þ Electrons and holes are injected into the depletion region and decreases in width.

Þ When the current reaches a threshold value the carrier concentration in the depletion region will reach very high values.

Working of Laser Diode (contd…)• The depletion region contains a large concentration of electrons

within the conduction band and a large concentration of holes within the valence band.

• The upper levels in the depletion region are having high population density of electrons while the lower levels in the same region are vacant. This is the state of population inversion. The narrow region where the state of population inversion is achieved is called inversion region or active region.

• An adequate forward bias is required to develop injection carriers across a junction to initiate a population inversion between energies at EC and energies at EV

• The forward bias plays the role of pumping agent in semiconductor diode laser.

• When the current reaches a threshold value the carrier concentration in the depletion region will reach very high values.

Fig. 6 Energy band diagram of heavily doped p-n junction (a) with no bias (b) with sufficiently large

forward bias to cause population inversion and hence stimulated emission

Laser Diode (optical cavity)In addition to population inversion laser oscillation must be

sustained.Þ An optical cavity is implemented to elevate the intensity of

stimulated emission (optical resonator) providing an output of continuous coherent radiation.

Þ The ends of the crystal are cleaved to a flatness and the ends polished to provide reflection.

Þ Photons reflected from cleaved surface stimulate more photons of the same frequency and modes or resonant frequencies resonate within the cavity.

Þ The wavelength of radiation that escalates in the cavity is dependant on the length L of the cavity (resonant length) and only multiples of ½ λ exist.

Þ The number of modes that exist in the output spectrum and their magnitudes depend on the diode current.

LaserDiode=>the output spectrum of the laser diode depends upon

the nature of the optical cavity and optical gain versus wavelength.

=> lasing radiation occurs when optical gain in the medium can overcome photon losses from the cavity which requires diode current to exceed a threshold current

=> Light that exists below emits spontaneous radiation.

=> Incoherent photons are emitted randomly and device behaves like an LED.

thIthI

Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.

Laser

LaserOptical Power

Optical Power

I0

LEDOptical Power

Ith

Spontaneousemission

Stimulatedemission

Optical Power

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)