30-11-2012, 04:10 PM
PROPAGATION OF LIGHT IN UNGUIDED MEDIA
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INTRODUCTION
Free Space Optical communication (FSO) is found in a variety of telecommunication
applications. In terrestrial applications, FSO technology employs a far infrared modulated
beam through the atmosphere and particularly through its troposphere. In space
applications, laser beams are used to establish inter - satellite links (ISL), so that a cluster
of satellites forms a network in the sky [1] ; in this case, satellites comprise nodes of a
network, whereby the network may consists of geostationary (GS) or of low earth
orbiting satellites (LEOS).
Either type of application, through troposphere or in space, owns a different set of
issues [2] . In space the distance is very long and the satellite receivers may or may not
face the sun, whereas in troposphere the medium is not homogeneous or stable. A third
type of application comprises a stationary FSO node communicating with a moving
FSO node, with additional tracking issues. As such, engineering FSO requires interdisciplinary
expertise, so that the fi nal FSO network provides high data rate links with
reliability and at the expected performance under all or most environmental conditions
in a profi table manner; the laser beam and the medium it travels through are two important
entities of the FSO network.
LASER BEAM CHARACTERISTICS
Wavelength
The laser beam that is used in FSO links has a wavelength of either 800 nm, 1310 nm,
or 1550 nm. The mast popular of the three is 1550 nm for the following reasons:
• The 800 nm is generated by low cost vertical cavity surface emitting lasers
(VCSEL) laser technology but the beam has low power and therefore the beam
is modulated at very low data rates, up to 100 Mb/s and for link lengths of few
hundred meters.
• The 1310 nm used to be a popular wavelength because of the distributed feedback
(DFB) and Fabry - Perot type lasers, which support higher power than the VCSEL
and therefore higher data rate and/or longer link lengths.
• The 1550 nm has been the most popular of all because it supports higher power
levels, Gb/s data rate, longer link lengths, and also wavelength division multiplexing
(WDM) technology [3, 4] ; that is, several wavelengths in the 1520 –
1570 nm range and ITU - T standard compliant [5 – 8] ; that is, an aggregate data
rate which is the product of the number of different optical channels in the
beam times the data rate in each channel. In some WDM applications, the
1310 nm is multiplexed with the 1550 nm to provide a two - channel WDM beam;
this is acceptable in applications that do not require a large aggregate data rate;
in addition, the 1310 and 1550 nm channels have large channel separation that
turns out to be benefi cial and convenient in receiver fi lter design.
Beam Profi le and Modes
As the laser beam emerges from the device, the intensity distribution along its cross -
section is not uniform but it usually has a distribution; if the distribution is Gaussian,
the beam is also termed “ Gaussian ” .
The cross - section profi le of the beam is of importance; in general, it is supposed
to be circular with a uniform Gaussian distribution of 360 ° . Typically, lasers that emit
beams with a pure Gaussian distribution are operating on the fundamental transverse
mode , or “ TEM 00 mode ” , Figure 1.1 .
In general, the analysis of beam profi le is complex and the Hermite - Gaussian equations
are used to describe the beam modes, which are designated as “ TEM mn ” , where
m and n are polynomial indices in the x and y directions.
Near - Field and Far - Field Distribution
The beam generated by a laser device is not perfectly narrow, or cylindrical, or centered,
and the almost Gaussian distribution in the x - axis may differ from the distribution in
the y - axis. In addition, the beam intensity distribution at the “ edge ” or the output facet
of the laser device (known as the aperture of the source ) is not the same with the
intensity distribution at some distance. At a short distance from the source aperture the
intensity distribution is known as near - fi eld , and at a far distance where the intensity
distribution seems to remain almost unchanged is known as far - fi eld , Figure 1.6 .