The following very
important characteristic of this light source is small spectral linewidth of
the source. This significantly affects the dispersion magnitude is directly
proportional to the line width of the source. Causing fiber dispersion in signal
overlap and significantly reduces system capacity bandwidth.
Although there are
many different types of light sources, fiber optic systems communication
usually just use either diodes LED (light emitting diodes) or laser (LD)
because of the requirements listed above. LEDs and LDs have small size, high
efficiency and many other beneficial features.
Laser diodes (LD)
LASER means Light
Amplification by Stimulated Emission of Radiation. Highly monochromatic laser
is similar to an electronic oscillator concept. A laser consists of an active
medium which is capable of providing optical amplification and an optical
resonator that provides the required optical feedback.
The most common
laser diode is formed of a pn junction and fed by electric current injected. Is
formed by doping a very thin layer on the surface of a glass wafer. The crystal
is doped n-type to produce a p-type region, one above the other, resulting in a
pn junction.
Laser diodes are
available as laser diode modules. Some manufacturers offer a wide selection of
laser diode modules ranging from continuous wave, line generator, modular, NIR
and more.
Diode lasers
microscopic chips used gallium arsenide semiconductor or exotic to generate
coherent light in a very small package. The energy level differences between
the conduction electrons and valence band are what provide the mechanism of
action of the laser.
Diode lasers are
high power light emitting more efficient. It can also be used for the diode
laser instrumentation gives the user the ability to precisely control the laser
diode current and temperature. It can operate in continuous wave mode by
selecting a drive current modulated laser or by using a characteristic
modulation in most drivers. The laser temperature can be set to the wavelength
stabilization required.
The active element
is a solid state device is not so different from an LED. LD have some
disadvantages besides critical requirement of unity. Optical performance is
generally not equal to that of other laser types. In particular, the coherence
length and monochromicity some types are likely to be inferior.
Light Emitting Diodes (LED)
An LED is a pn
junction polarized in which EH recombination leads to the generation of optical
radiation through the process of spontaneous emission. The LED structure is
similar to that of a laser diode, except that there is no feedback cavity. The
emission of an LED is due to spontaneous recombination and an LED output
differs significantly from that of an LD (laser diode).
LEDs have many
advantages such as lower energy consumption, longer life, improved robustness,
smaller size and greater reliability. Unlike laser diode, there is no threshold
and the output power increases smoothly as a function of current. For large
currents the output power is saturated. The total power output of the LEDs can
be a few milliwatts.
Due to the random
and spontaneous emission is displayed along all directions, the output of an
LED is not directional. Output beam angles may be typically in the range of 30
° perpendicular to the junction, at about 120 ° along the junction.
LEDs are also used
in many other applications other than the optical fiber communication, such as
aviation, automotive lighting, and traffic signals, etc.