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PCAOM [Polychromatic Acousto-Optic Modulator]
What is it?
A PCAOM is a type of acoustooptic device that allows the selection of
discrete laser wavelengths with variable intensity. Basically, the crystal
may be considered as a tuneable electric prism. The incident laser beam is
passed through the Tellurium Dioxide crystal. Specific RF frequencies are
applied to the crystal, resulting in specific wavelengths being diffracted
into the first order. Multiple frequencies will cause multiple spectral
lines to be diffracted. The output face of the crystal is cut at a prismatic
angle, so that all lines are superimposed.
What can it do for me?
The PCAOM can replace a colour box, a coloruscan system, a three cell
traditional AOM system, and a blanking galvo, all at once. If you are
working with a "white light" system, a PCAOM will allow the
synthesis of virtually any colour. With even a "monochrome" laser
system, such as an Argon laser, the PCAOM will allow either selection of the
Argon spectral lines (514, 488, 476, 457) or modulation of all lines
Is it hard to use?
The PCAOM is by far the simplest and easiest colour/intensity control that has been devised. A trained laser technician will need about 10 minutes to install one in an optical system. The device is very stable, and requires a single adjustment to optimise it. Literally, this is a "white beam in, coloured beam out" device. It is also quite compact, under 2" in all dimensions, and requires only 3-4" on the output to separate the zero and first order beams.
Any special considerations?
The PCAOM must be used with a collimated beam, typically a laser beam. The beam must be linearly polarised, and in the correct orientation to crystal, although the crystal may be rotated to match the beam. The device can be sensitive to thermal shock,and should not be exposed to freezing temperatures. Use of multiple lasers, such as typical Argon Krypton pairs, requires precise convergence of the two beams, in both near and far field.
Is it efficient?
The PCAOM exhibits over 90% diffraction efficiency in the blue end of the
spectrum, and 80-85% in the red. A typical device will be set-up to modulate
up to 6 wavelengths simultaneously.
Can it be used with any laser?
As long as the RF driver provides the correct frequency corresponding to
the wavelength of the laser, almost any laser can be used. Standard drivers
have wavelengths at 647, 633, 514, 488, 476, and 457 nanometers. This means
it can be used with a HeNe, an Argon, an Argon/Krypton mixed gas, or an
Argon, Krypton pair of lasers. With special tuning, it can also work with a
broadband Krypton, with modulation for the 568 nM yellow,and the 520, 530 nM
What about fiber optics?
The PCAOM requires a collimated beam of less than 3 mm diameter to work properly. Use on the output of a fiber delivery system is not recommended. It is best to use the device before the fiber, prior to launching. As the output beam from the PCAOM is very stable, coupling into fiber, even 50 micron fiber, is straightforward. There is a slight deviation in the beam path, so that the input beam and the first order output beams are not coaxial. This deviation is on the order of a few degrees. Most fiber launchers will accommodate this angular deviation.
Copyright 1993 by Greg Makhov
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