The Fiber Optic Association - Tech Topics
Understanding Wavelengths In Fiber Optics
Fiber optics
is full of jargon
but it's important to understand it. One of the more confusing
terms to many is "wavelength." It sounds very scientific,
but it is simply the term used to define what we think of as the
color of light.
Light is part of the "electromagnetic spectrum" that
also includes x-rays, ultraviolet radiation, microwaves, radio,
TV, cell phones, and all the other wireless signals. They are
simply electromagnetic radiation of different wavelengths. We
refer to the range of wavelengths of electromagnetic radiation
as a spectrum.
Wavelength and frequency are related, so some radiation is identified
by its wavelength while others are referred to by their frequency.
For the radiation of shorter wavelengths, light, UV and x-rays,
for example, we generally refer to their wavelength to identify
them, while the longer wavelengths like radio, TV and microwaves,
we refer to by their frequency. 
The light we are most familiar with is, of course, the light we
can see. Our eyes are sensitive to light whose wavelength is in
the range of about 400 nanometers (billionths of a meter) to 700
nanometers, from the blue/violet to the red. If you wonder why
this is the range of colors we can see, it's because it is the
same region as the brightest output of the sun. In other words,
we developed sight in the spectral range of the output of our
local star, quite a good idea actually.
For fiber optics, we use light in the infrared region which has
wavelengths longer than visible light, typically around 850, 1300
and 1550 nm. Why do we use the infrared? Because the attenuation
of the fiber is much less there. The attenuation of glass optical
fiber is caused by two factors, absorption and scattering. Absorption
occurs in several specific wavelengths called water bands due
to the absorption by minute amounts of water vapor in the glass.
Scattering is caused by
light bouncing off atoms or molecules in the glass. It i s strongly
a function of wavelength, with longer wavelengths having much
lower scattering. Have you ever wondered why the sky is blue?
It's because the light from the sun is more strongly scattered
in the blue.
Fiber optic transmission wavelengths are determined by two factors:
longer wavelengths in the infrared for lower loss in the glass
fiber and at wavelengths which are between the absorption bands.
Thus the normal wavelengths are 850, 1300 and 1550 nm. Fortunately,
we are also able to make transmitters (lasers or LEDs) and receivers
(photodetectors) at these particular wavelengths.
If the attenuation of the fiber is less at longer wavelengths,
why don't we use even longer wavelengths? The infrared wavelengths
transition between light and heat, like you can see the dull red
glow of an electric heating element and feel the heat. At longer
wavelengths, ambient temperature becomes background noise, disturbing
signals. And there are significant water bands in the infrared.
We often refer to wavelengths in fiber optics. The wavelengths
we use for transmission must be the wavelengths we test for losses
in our cable plants. Our power meters are calibrated at those
wavelengths so we can test the networking equipment we install.
The three prime wavelengths for fiber optics, 850, 1300 and 1550
nm drive everything we design or test. NIST (the US National Institute
of Standards and Technology) provides power meter calibration
at these three wavelengths for fiber optics. Multimode fiber is
designed to operate at 850 and 1300 nm, while singlemode fiber
is optimized for 1310 and 1550 nm. The difference between 1300
nm and 1310 nm is simply a matter of convention, harking back
to the days when AT&T dictated most fiber optic jargon. Lasers
at 1310 nm and LEDs at 1300 nm were used in singlemode and multimode
fiber respectively.
Recent telecom systems use DWDM or dense wavelength-division
multiplexing.
In these systems, lasers are chosen with precise wavelengths closely
spaced - but not so close they interfere with each other - and
transmitted simultaneously on a single fiber. It's just like the
FM radio spectrum.
The final note is on safety. Look closely at the first figure.
The visible spectrum is well below the wavelengths used in fiber
optics. That means you cannot see the light in fiber systems,
so there is no reason to look into the end of a fiber. And as
we mentioned last month, some systems do have enough power to
be potentially dangerous, so you should never look at the end
of a fiber anyway.
(c) 2002, VDV Works LLC
Jim Hayes is
the founder of Fotec,
the fiber optic test equipment company and Cable U training. Find
him at www.JimHayes.com.
VDV Works
creates training
programs and technical content for fiber optics and voice-data-video
cabling.
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