Fiber Optic Communications Systems |
|
|
|
It's easy to understand why fiber optic transmission
systems are so desirable and have become so common. These systems have distinct
advantages over copper wire infrastructures, but also have unique components
that should be understood when selecting a system. Because the only carrier of
information in such a system is light, at a frequency many thousands of times
higher than conventional electrical signals, the fiber optic cable is virtually
immune to all forms of conventional electromagnetic interference. RF emissions,
AC power lines, arcing high voltages, and even nearby lightning strikes do not
compromise the signal in any way.
The optical cable is made of glass and plastic, so it is electrically
non-conductive, unaffected by moisture, and, in most cases (such as when used
for video transmissions), totally eliminates "hum bars" or "ground loops" caused
by unequal voltages at the transmitting and receiving ends of a link.
Furthermore, the glass and plastics employed are unaffected by most chemicals
and solvents, so the fiber optic cable can be used in all sorts of adverse
industrial environments. Even a break in a "live" fiber will not cause any shock
hazard or produce a spark in an explosive atmosphere.
Basic Transmission
Figure 1 is a simple block diagram of a typical fiber optic-based CCTV
transmission system. The camera is connected to the optical transmitter and the
monitor to the optical receiver. Video from the camera modulates a light source
in the transmitter, usually an LED or solid state laser diode.
The modulated light is then "launched" into the fiber, where it travels to the
far end of the link. Here it is detected by a photodiode in the optical
receiver, processed and amplified, and eventually exits as a reproduction of the
original signal.
While the fiber optic cable is immune to the effects of external electrical
interference, the optical transmitter and receiver are not so robust. These
components are conventional electronic devices (more or less) using "normal"
circuits, and as a result are susceptible to interference.
In fact, a fiber optic receiver usually has a very high gain, wide bandwidth
amplifier in the front-end which, if not properly shielded, can easily act as a
receiver of unwanted interfering signals. Similarly, a fiber optic transmitter
often produces high current pulses that can radiate and cause interference to
other equipment located nearby. While the fiber can usually be routed where it
is convenient, the electronic portions of the system must be either well
shielded or, at the very least, located where they will not receive or cause
unwanted interference. The light source used in most fiber optic transmitters is
usually a high speed LED or solid state laser diode. These devices operate in
the infrared portion of the spectrum and are not normally visible to the human
eye. The light that is produced by these emitters is very pure, however, and if
allowed to continuously shine directly into the eye, can be focused into a tiny
hot spot on the retina - causing temporary or even permanent damage.
Consequently, you should never stare directly into the "business end" of an
optical emitter or "live" optical fiber of any kind. This precaution is most
important when dealing with laser diodes, although there is some growing concern
with higher output LEDs as well.
Fiber Variations
There are two main types of optical fiber: multimode and single-mode. Multimode
fiber is used for short distance transmissions up to about 6.2 miles, while
single-mode fiber has been successfully used for distances beyond 62 miles.
The main difference between the two is bandwidth. Multimode fiber has a
bandwidth of 200 to 400 MHz per km of length while the bandwidth of single-mode
fiber extends well into the GHz/km region. Obviously, the longer the
transmission distance, the more important this factor is. Connections to the
optical fiber are made with connectors especially designed for the task. The
most common optical connector in use today is the ST style originally developed
and pioneered by AT&T.
This connector is a spring-loaded device that makes installing or replacing a
fiber optic cable as quick and easy as changing a coax jumper with BNCs. As a
result, this connector has become very popular and is the "connector of choice"
in most multimode applications where it performs very well.
When using single-mode fiber however, you must be very careful. The active
diameter of the light-carrying core of a multimode fiber is only about 0.0025
inches, and the tolerances of the tip of the ST connector is such that alignment
between fibers in a patch panel is not overly critical. The single-mode fiber
light-carrying core, on the other hand, is only 0.0003 to 0.0004 inches in
diameter - any significant misalignment here will cause excessive losses of
light.
Since the multimode ST was so popular, it was inevitable that a single-mode
version would also be developed. The single-mode ST connector looks exactly like
the multimode version to the unaided eye, but is manufactured with higher
tolerances so that it does align and operate properly.
Any tension or vibration of the fiber optic cable can be transmitted to the tip,
causing it to move slightly, which results in losses. Remember, the tip only has
to move 0.00003 inches for this to occur - and if it does, that can spell
disaster to a system operating near the optical attenuation limit.
The solution to this potential problem is to use a connector specifically
designed for use with single-mode fiber, such as the FCPC. This connector has
the high tolerances of the ST, but also has a threaded cap that securely locks
the connector in position. When FCPC connectors are used, the fiber optic cable
can be flexed, pulled, or vibrated at will.
Managing Multiple Transmissions
In addition to the transmission of one way (simplex) signals, there is a wide
range of products that transmit multiple signals in one or both directions
through the same optical fiber. These systems are employed in such applications
as transmitting video from a camera to a monitor and pan-tilt control signals in
the opposite direction back to the camera, transmitting video along with audio
(or stereo audio) for teleconferencing applications, and even conveying several
video, audio, and control signals for more elaborate installations.
Multiple signal transmission is accomplished by one of two methods. In the
first, as shown in Figure 2, different wavelengths of light are employed for
each signal being transmitted. Because the wavelengths are different, there is
no interference between the two channels. Such a system is often referred to as
a wavelength division multiplexed (or WDM) system.
In the second, as illustrated in Figure 3, only one
wavelength of light is used and the signals are sent as separate modulated
carriers. While both methods work well, there is a shortcoming of the WDM system
that, if not understood, can cause problems in critical applications.
The attenuation of fiber optic cable at a wavelength of 850nm can vary from
about 3 to 4 dB per kilometer of length. The attenuation for the exact same
cable at 1310nm however drops to only 1 to 2 dB per kilometer. The result is
that the 1310nm signal can travel roughly twice as far as the 850nm signal.
In a system where the attenuation of the fiber optic cable is close to the
maximum limit of the system at 850nm, video sent at 1310nm will appear to be
perfect, since it is well within its attenuation loss budget. The PTZ control at
850nm, in contrast, is marginal, and small variations in ambient temperature,
cable attenuation, or even dust in the optical connector can easily cause loss
of control or intermittent operation. What complicates this problem further is
that since the video quality is good, troubleshooting the PTZ function can be
difficult if you are not aware of the use of the two wavelengths. When one
wavelength is used however, troubleshooting is simple. Either all signals are OK
or they are not. Because both versions of such products exist in the
marketplace, the choice of a single wavelength product is always more desirable
than the choice of one employing WDM techniques.
Although the points outlined above may seem straightforward, newcomers to the
world of fiber optic transmission systems can be easily confused by the wide
range of products available. A good understanding of the positive and negative
points of the technology can help the potential designer come up with a system
that will perform properly in the specific application.
|
