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| 1.
Raw Calcite |
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Calcite is an uniaxial crystal of calcium
carbonate (CaCO3) and is the most used material for
manufacture of high quality polarizers. Relatively rare in its
optical form, its extracted from mines in a few places.
For optical component applications calcite
presents advantages of being easy to work, having a wide spectral
range, going from 260 to 1700nm, and presenting a large angular
separation between ordinary and extraordinary beams. The table
below lists refractive indices for the ordinary and extraordinary
rays for a range of wavelengths.
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| l(nm) |
no |
ne |
l(nm) |
no |
ne |
| 257 |
1.76038 |
1.53005 |
533 |
1.66277 |
1.48841 |
| 291 |
1.72774 |
1.51705 |
560 |
1.66046 |
1.48736 |
| 312 |
1.71425 |
1.51140 |
589 |
1.65835 |
1.48640 |
| 394 |
1.68374 |
1.49810 |
643 |
1.65504 |
1.48490 |
| 410 |
1.68014 |
1.49640 |
706 |
1.65207 |
1.48353 |
| 434 |
1.67552 |
1.49430 |
905 |
1.64578 |
1.48098 |
| 441 |
1.67423 |
1.49373 |
1159 |
1.64051 |
1.47910 |
| 508 |
1.66527 |
1.48956 |
1497 |
1.63457 |
1.47744 |
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| 2. Optical
Quality of Calcite Crystals |
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The optical quality of calcite
polarizers is evaluated from factors such as scattering inside the
crystal, the quantity of striae or schlieren (optically inhomogenous
regions), the polishing quality and the flatness of optical surfaces.
These in turn will influence functional factors such as spectral
range, useful aperture, extinction coefficient and resistance to
laser damage.
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| 3. Scattering |
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Light scattering in calcite polarizers
may be caused by polishing defects, or more commonly, by small inclusions,
cleavage, point defects and bubbles inside the crystal. The tolerance
in relation to these defects depends on the particular application.
In general, applications that involve lasers need much lower scattering.
The quality of our crystals is described by a system of Quality
Grades, as defined on item 9 below.
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| 4. Striae |
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Striae or schlieren are small refractive
index inhomogeneities, caused by lattice dislocations that cause
errors in the form of the wavefronts entering the crystal. They
should be avoided in applications that involve interferometry.
The quality of our crystals in relation to striae in also described
by a system of Quality Grades, defined on item
9.
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| 5. Wavelength
Range |
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The wavelength range of a calcite polarizer
depends on factors such as the crystal coloration, the way the
two prism halves are assembled together and antireflection coatings,
if any. Depending on their quality, calcite crystals present a
certain amount of red or yellowish color. The crystals used in
our polarizers are very clear, completely free of coloration.
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