MadSci Network: Physics

Re: How does intensity of light in an optic fibre depends on lengh & diameter

Date: Thu Sep 28 20:45:00 2000
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Physics
ID: 967799216.Ph


Depending on the application, optical fibers come in a
number of different forms. There are plastic fibers and
glass fibers. Glass fibers are the technology used in most
telecommunications applications and they are the type I'll

Glass optical fibers also come in a number of configurations
that depend on
1.) how long the fiber communications link is and
2) what data rate (number of light pulses per second) that
the fiber must carry.

Most fiber optic data links have more than enough light
reaching the detector, it is the DISPERSION of the light
that limits the link. Dispersion causes the data pulses to
be smeared so the information becomes undetectable, even
though there may be plenty of light. Larger diameter fibers
have greater dispersion and the fiber links with the
greatest distance and data rate have light guiding cores
with the smallest diameters, about the size of an optical
wavelength. Why is this true?

Consider a long straight optical fiber with a large
diameter. The light rays going straight down the center
arrive at the end of the fiber first. Rays traveling at
different angles reflect back and forth within the fiber
and they must travel greater distances and thus each ray
at each different angle takes a different and
longer time than the central ray to reach the end.

Since most of the light must reflect bank and forth within
the fiber, dispersion in this type of fiber is the greatest.
Depending on the length of the fiber and the dispersion, a
maximum data rate is set before the pulses become

Optical fiber manufacturers characterize their fibers by a
LENGTH times BANDWIDTH product. A high quality GRADED INDEX
optical fiber has a length-bandwidth product of one thousand
Megahertz (MHz) - kilometer. Graded index fibers have a
lens like profile for the index of refraction. Thus rays at
greater angles travel faster and try to catch up with the
central ray. That means that you could detect a thousand
megahertz signal at one km or a 100 MHz
signal at 10 km or a 10 thousand MHz signal at only 100
meters. Each of these links has the same length bandwidth

Simple STEP INDEX fiber has the poorest length bandwidth
product with the larger diameters providing the shortest
distances for a given data rate.

The longest links, with the highest data rates, are the
under sea cables that are thousands of miles/kilometers long.
These links use STEP INDEX, SINGLE MODE fiber.

Communication links less that 16 km (10 miles)long might use

Very short, low data rate links, less than a mile/km long,
might use STEP INDEX, MULTIMODE fiber.

All of the fiber types consist of a light guiding core of
glass surrounded by cladding layers of glass. For
protection, the glass fiber is usually over coated with a
thick layer of a plastic type material. Most fibers have
about the same outer diameter of glass ranging between 100
and 200 micrometers (40 to 80 micro inches), about the
diameter of thick human hair. However; the light guiding
glass cores are a only a few micrometers in diameter for the
SINGLE MODE fibers used in undersea cables while the
MULTIMODE fibers have light guiding cores about 50
micrometers (20 microinches) in diameter.

The light signals are trapped in the core glass
because the core has a higher optical index than that used
in the cladding glass. To form glass cladding layers dopant
materials such as germanium are added to the ultra pure
silica glass to reduce the optical index. Dopants are what
precisely controlled amounts of impurity materials are
called. Dopants are key to the fabrication of optical fibers
and semiconductor electronic and photonic devices. Optical
index changes of only a percent or two are all that is
required to trap light in the core
glass. The interface between the two types of glass (doped
and undoped) causes an optical effect known as TOTAL

The breakthrough in glass chemistry to enable practical
fiber optics came about in the early 1970s by purifying
quartz glass to impurity levels of parts per billion
(thousand million) rather than parts per million used in
making quartz glass before that time. In particular removing
hydrogen atoms from trapped water in the glass was the most
difficult problem to solve. Protecting optical fibers from
hydrogen atoms produced from water
molecules (H2O) diffusing into the light guiding core glass
is still a important engineering problem, especially for
under sea cables.

Depending on the bandwidth of your oscilloscope and the
highest frequency of your signal generator and modulated
laser, you might be able to measure the length- bandwidth
product for a varity of large diameter step index
optical fibers.

Best regards, Your Mad Scientist
Adrian Popa

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