Module 5 -Light Sources and Transmitters

7/31/2019 Module 5 -Light Sources and Transmitters

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Jimma University

Jimma Institute of Technology

Department of Electrical and Computer Engineering

Communication Engineering Stream

Module 5

Optics and Optical Communication

Light Sources and Transmitters

General Source Characteristics

1. Materials

Semiconductor-based light sources are about the size of a grain of salt. This size allows efficient coupling

of their light output into the small diameters of fibers. In addition, their semiconductor structure and low-

power dissipation characteristics make them compatible with integrated-circuit electronics.

The electrons in semiconductor materials are allowed to reside in only two specific energy bands: valenceband and conduction band.

The conduction of electrons and holes in a material can be increases greatly by adding trace amounts ofimpurity atoms to a material. Hence, N-type material and P-type material are produced.

An electron sitting in the conduction band can drop down into a hole in the valence band, thereby returningan atom in its neutral state. This process is called recombination.

The recombination process releases energy in the form of a photon and is the basis by which a source emitslight.

The energy E during such recombination process is:

240.1=E

Where: = the light wavelength in micrometers

E= energy emitted in electron volts (eV)

The table below shows some LED and Laser Diode material mixtures and their characteristics:

Material range, nm Bandgap energies, eV

GaAs 900 1.4

AlGaAs 800 900 1.4 1.55

InGaAs 1000 1300 0.95 1.24

InGaAsP 900 1700 0.73 1.35 Alloys consisting of three elements are called ternary compounds whereas four-element alloys are called

quaternary compounds

2. Spectral Output Width

This is an important factor when one is choosing an optical source since signal spreading in an optical fiber

due to chromatic dispersion is directly proportional to the wavelength band over which a source emits light.

In order to send a high-speed signal (consisting of very narrow light pulses) over long distances, the source

needs to emit light within as narrow a spectral width as possible

LED has spectral widths ranging from 30 nm at a central wavelength of 850 nm to around 120 nm at a

1550-nm central wavelength

Laser diodes can have widths of a few picometers (10-3 nm) at 1550 nm

3. Modulation Speed

Direct modulation is the process of using a varying electric signal to change the optical output level of a

device.

Modulation speed refers to how fast a device can be turned on or off by an electric signal to produce a

corresponding optical output pattern.

Laser diodes can be modulated significantly faster than LEDs.

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Optical Communication Shecat 2012


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Important Requirements for an Optical Source

1. The light source should be monochromatic light source i.e. light should consists of single frequency. LED and

Laser diodes are not absolutely monochromatic but they emit over an extremely narrow band of frequencies.

2. The light sources should be capable of being amplitude modulated by the electrical signal to be transmitted

through the fiber.

3. The intensity of light should be high over a narrow band of wavelengths.

3. Optical source should be small in size, compatible with the size of the fiber, and inexpensive to manufacture.

LEDs

The LEDs used in optical communications are much smaller and emit in the infrared region, but compared

to the other telecommunication light sources used, they are much less expensive and easier to use in transmitters

designs. However, because of their relatively low output, broad emission pattern, and slow turn-on time, their use is

limited to low speed (less than 200-Mbps), short distance (up to a few kilometers) applications using multimode

fibers.

Principle of Operation

To create a supply of electrons and holes that may flow across the pn junction to recombine and thereby

emit light, one applies a voltage across the junction . This is called a bias voltage.

Nominally LEDs operate around 50- to 100-mA drive currents and require a bias voltave of around 1.5 V.

Surface Emitters

In the surface emitter a circular metal contact defines the active region in which light is generated. The contact is

nominally 50 m in diameter. To couple the light into the fiber, first a well is etched into the substrate on which the

device is fabricated.

Edge Emitters

The edge emitter consists of an active pn junction region where the photons are generated and two light-

guiding layers that function in the same manner as an optical fiber. The emission pattern is more

directional than that of a surface emitter, which allows a greater percentage of the emitted light to be

coupled into the fiber.

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Laser (Light Amplification by Stimulated Emission of Radiation) Diodes

Semiconductor-based laser diodes are the most widely used optical sources in fiber communication

systems. Key properties of these lasers include high optical output powers (greater than 1mW), narrow

linewidths (a fraction of a nanometer), and highly directional output beams for efficient coupling of light

into fiber cores.

Laser Action

(a) Spontaneous emission: In spontaneous emission atom returns to its lower energy state in random manner. In this

emission photons are radiated in arbitrary directions. Very few photons create light in the desired direction. Photons

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propagate within a wide cone thus yield widespread radiated light. Radiation of photons is independent of other

photons i.e. no phase correlation exists between different photons and total radiated light is incoherent. Spontaneous

emission occurs in LED which has very wide spectral width.

(b) Stimulated emission: When a photon having an energy equal to the energy difference between the two states (E2

E1) interacts with the atom in the upper energy state, it causes the atom to return to lower energy, state with the

emission of second photon. In stimulated emission an external photon stimulates the induced emission.

The stimulated photon propagates in the same direction as the photon that stimulated it so the stimulated light will

be well directed. The stimulated radiation is coherent as stimulated photon is in time alignment with externalphoton. All stimulated photons propagate in the same direction and contribute to output light so the sources having

stimulated emission have high current to light efficiency. LASER generally uses stimulated emission of radiation.

Optical Transmitters

Generally, a light source is part of a transmitter package. This package provides the following: a mounting

block for the light source, a holder for attaching a light-coupling fiber, a means for maintaining the temperature at

affixed value, and various control electronics. In some cases the transmitter package also contains an external

modulator for very high-speed applications.

LED Transmitters

The low power output (typically -16 dBm coupled into a 62.5-m fiber) and slow response characteristicsof an LED compared to a laser diode limit its use to short-distance, low-speed (up to 200Mbps over a few

kilometers) applications using multimode fibers.

A typical LED transmitter will contain a 1310-nm InGaAsP surface emitting LED and driver circuitry for

the source. Such a transmitter uses a single 3.3V power supply and operates over a temperature range of 0 to +70oC,

or optionally (for a more expensive device) over a -40 to +85oC range.

Laser Transmitters

Since the wavelength of a laser drifts by approximately 0.1 nm/oC and because the output efficiency

changes with temperature, a standard method of stabilizing these parameters is to use a miniature thermoelectric

cooler. This device uses a temperature sensor and an electronic controller to maintain the laser at a constant

temperature.

In addition, there normally is a photodiode in the package that monitors the optical power level emitted bythe laser, since this level can change with factors such as age or bias voltage.

The module may also contain as second photodiode for precisely monitoring the peak output wavelength.

External Modulators

Electrooptical Modulators (EO)

In this modulator, the light beam is split in half and then send through separate paths. This is done in such

a manner that when the two halves of the signal meet again at the device output, they will recombine either

constructively or destructively. The constructive recombination produces a bright signal and corresponds to a 1

pulse. On the other hand, destructive recombination results in the two signal halves canceling each other so there is

no light at the output. This of course corresponds to a 0 pulse.

Electroabsorption Modulator (EAM)

It operates by having an electric signal change the transmission properties of the material in the light path

to make either transparent during a 1pulse or opaque during a 0 pulse.

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Module 5 -Light Sources and Transmitters

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