|MadSci Network: Physics|
Reference: Amnon Yariv, Introduction to Optical Electronics,
Holt, Rinehart and Winston, 1971
There are numerous ways to change the index of refraction of solid crystals, liquids
and gasses and many of these techniques are used in laser based systems including
communications, radar (LIDAR), displays, copying, printing, welding, cutting and
machining. The index of refraction is controlled by placing stress on the atoms and
molecules to change their spacing in one or more dimentions. This stress changes the
optical index of refraction for light polarizations aligned with the stress vector.
Electrical, acoustic and magnetic fields are the techniques most commonly used to apply
stress to the materials to rapidly change thei index of refraction at megahertz to
gigahertz data rates.
In electrooptic (EO) materials, the index of refraction can be controlled by
changing an electric field across the material. Many materials exhibit a small EO
effect including quartz; however, the EO materials with the largest EO effect that are
commonly used in laser based systems are GaAs, KH2PO4, NH4H2PO4, CdTe,
LiNbO3, LiTaO3, and BaTiO3. Lithium Niobate (LiNbO3) has become the most
common EO material used for amplitude modulating fiber optic communications
systems operating at data rates exceeding 10 gigabits per second.
Integrated optical modulators in lithium niobate used in fiberoptic systems split the laser light
into two parallel channels about 100 micrometers apart that travel about one centimeter before
they are joined back together.
By applying a different electric field across each parallel channel
the light travels faster in one channel than in the other. The result is that the
voltage can be adjusted so that the light in the two channels are out of phase
and cancel each other when they are joined or they can be made to be in phase and add to each other.
Thus the light comming out of the modulator is amplitude modulated by the strength
of the electric field which in turn changes the refractive index of one parallel
channel with respect to the other.
In acustooptic (AO) materials the index of refraction can be controlled by passing
sound waves through the material. AO materials include air, water, alcohol, quartz,
LiNbO3, PbMO4, YAG, YIG and TeO2. AO materials are
commonly used for scanning laser beams in printers, copying machines, and in display
systems. By propagating a sound wave through a material orthogonal (at right angles)
to a laser beam the acoustic pressure wave generates a moving diffraction grating.
By changing the frequency of the sound wave, which in turn changes the grating spacing,
laser beams can be deflected rapidly scanned in angle. The sound waves used
for AO modulators and scanners are typically in the 1 to 100 megahertz range. AO
materials are used to scan and modulate large screen laser television projectors that
are very bright. However, to date these systems are very expensive to manufacture
relative to other projection technologies now in use and there application has been
limited to laser light shows.
Sound pressure waves in air (which is how we hear) can also deflect light by the
AO effect; however, the AO effect in air is very small . Testing and sampling the
power in very high energy lasers for cutting, welding and other applications has
been difficult because the light energy is so great that even very small light
absorption in the optics causes then to overheat and melt. Scientists solved this
problem by using sound waves in air across the laser beam to deflect only one
millionth of the beam power out of the main beam to monitor the high power laser
Magnetooptic materials use magnetic fields to modulate light and these materials
are now being studied for optical memory and for read/write compact optics disks.
Chemical reactions also change the optical index and researches are using this
technique to monitor very high speed chemical reactions. Now that lasers and laser
diodes have become relatively inexpensive, many new applications of optical
effects in materials including refractive changes caused by physical effects are
being studied and applied.
Best regards, Your Mad Scientist
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