|
|
Tech
Focus:
A
look at new technologies shaping the world of laser displays
CG
Tools for Laser Artists
How
I Learned to Love Fiber Optics
Fiber
Optics: Technical Primer
Evaluating
New Catweazle Scanners by MediaLas
The
Nitty Gritty of Fiber Optics:
A Technology Primer
By
Peter Mayer, Creative Laser Productions, CreativeLaser@compuserve.com
During the last few
years, fiberoptic components have become available from the fiber
industry that match the special requirements of the laser light
show industry. A Fiberoptic Remotely Operable Projector, or FROP,
is frequently found in many laser installations, with a single
laser source providing light to one or more FROPs via fiber optic
cable.
Generally, the laser
source beam is split into different paths determined by the number
of FROP projectors. A major advantage of this system is that
the final projectors can be placed in locations that are independent
of the laser system itself. Mounting FROPs is easy due to their
compact and lightweight construction. Such a fiberoptic delivery
system makes it easier and more economical to produce complex
laser light shows.
In this article, we will consider some of the technical aspects
of using fiberoptic beam delivery systems in laser light shows.
Let’s look at how a fiber delivery system would be used
in a typical laser show installation. First, the light from the
laser source must be focused onto the tip of the fiber itself,
with the tip of the fiber measuring from 50 microns to 200 microns
in diameter. To focus the beam accurately, a collimating lens
and a micropositioner are required.
The micropositioner holds the fiber input face, together with
the collimating lens, straight in line to the laser source with
micrometer accuracy. Several different configurations are available
on the market. Some models feature interchangeable collimator
modules, others have their collimating lens already integrated
with the micropositioner. The system that has individual micro
adjustments for position X/Y and angle X/Y, together with interchangeable
collimators, offers the greatest ease of coupling.
Collimating
Optics
A variety of collimating elements are available to focus the
laser light into the fiber core, including singlet lenses, GRIN
lenses, and achromats. For multiline operation over a wide spectrum
(such as a white-light laser), an achromat of the highest quality
is recommended. This will help ensure consistent coupling over
various wavelengths and beam diameters. A configuration in which
the focusing lenses are sealed in interchangeable collimator
housings gives the advantage of being able to easily reconfigure
the fiber system to use the optimum collimator module required
for different show needs. Also, using this idea, the collimator
can be kept connected to the fiber, thus preventing contamination
of the fiber tip during handling. It is very important that the
fiber tip be kept clean and scratch-free for efficient fiber
coupling!
When it comes to coupling the fiber to the collimator, a special
fiber cable connector is required. Most fibers are sold with
connectors already attached. The multimode SMA905 connector is
in common use worldwide, either with a stainless steel or a ceramic
ferrule. Special assembly techniques allow ceramic connectors
to be fitted without epoxy. This avoids outgassing effects on
the fiber assembly and results in a more rugged fiber that can
handle higher power. Creative Laser Productions offers an epoxy-free
fiber enclosed in an SMA905 connector for repeatable coupling.
When the light exits the fiber at the scanners, a far more simple
positioning and focusing device is required compared to the one
used at the fiber input face. After all, you are now aiming the
beam at scanner mirrors that are several millimeters in size,
compared to a fiber core measured in microns.
Step
Index vs. Gradient Index
One of the primary technical considerations in using a fiber
optic delivery system is the quality of the cable itself. There
are generally two types of fibers available, each with its own
advantages:
1) Gradient index fibers with a 50 micron core are often used
for laser beam delivery up to 10 watts. This type of fiber offers
very good output beam quality, good coupling efficiency, and
a moderate price.
2) Step index fibers with 15, 30, 50, and up to 200 micron cores
are mainly intended for high power applications up to 40 watts.
Their advantage is extremely low loss in coupling and delivery.
Beam output quality is moderate. Step index fibers are more highly
priced than gradient index fibers.
When estimating power
losses because of fiber optic transmission, the primary loss
is from the coupling connector. This loss occurs at the input
of the fiber and depends on input beam quality, collimation optics
and the diameter of the fiber. In practice, coupling loss is
10% to 15%.
The second area of loss is the fiber itself, with the loss remaining
constant from about 0.1% to 0.5% per meter of cable. A 30-meter
cable length of 50 micron gradient index fiber, for example will
experience 20-30% light loss with today’s commonly used
white-light lasers.
Next time you plan a laser light show or system installation,
remember to put the advantages of a fiberoptic beam delivery
to work in your application.
Peter Mayer, Creative Laser Productions, CreativeLaser@compuserve.com |
|