Synthetic Fused Silica
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Fused silica is an ideal optical material for many applications. It is transparent over a wide spectral range, has a low coefficient of thermal expansion, and is resistant to scratching and thermal shock. Synthetic fused silica (amorphous silicon dioxide) is formed by chemical combination of silicon and oxygen. It is not to be confused with fused quartz, which is made by crushing and melting natural crystals, or by fusing silica sand, which results in a granular microstructure and bubble entrapment. Microstructure and impurities lead to local index variations and contribute, along with bubbles and opaque particles, to reduced transmission throughout the spectrum. Synthetic fused silica is far purer than fused quartz. This increased purity ensures higher ultraviolet transmission and freedom from striae or inclusions. The synthetic fused-silica materials used by Melles Griot are manufactured by flame hydrolysis to extremely high standards. The resultant material is colorless and non-crystalline, and it has an impurity content of only about one part per million. Controlling the purity of reactants and the conditions of reaction ensures the high quality of the synthetic fused silica from which our lenses are made. Synthetic fused-silica lenses offer a number of advantages over glass or fused quartz:
UV-grade synthetic fused silica (UVGSFS) is selected to offer the highest transmission (especially in the deep ultraviolet) and very low fluorescence levels (approximately 0.1% that of fused natural quartz excited at 254 nm). UV-grade synthetic fused silica does not fluoresce in response to wavelengths longer than 290 nm. In deep ultraviolet applications, UV-grade synthetic fused silica is an ideal choice. Its tight index tolerance ensures highly predictable lens specifications. The table below shows the refractive index of a typical UV-grade synthetic fused silica versus wavelength at 20ºC. To obtain the index for optical-quality synthetic fused silica, round the values off to the fourth decimal place. |
| Refractive Index of UV-Grade Synthetic Fused Silica (accuracy ±3 x 10-5) | ||||||
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Wavelength (nm) |
Index
of Refraction |
Wavelength (nm) |
Index
of Refraction |
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| 180.0 | 1.58529 | 532.0 | 1.46071 | |||
| 190.0 | 1.56572 | 546.1 | 1.46008 | |||
| 200.0 | 1.55051 | 587.6 | 1.45846 | |||
| 213.9 | 1.53431 | 589.3 | 1.45840 | |||
| 226.7 | 1.52275 | 632.8 | 1.45702 | |||
| 230.2 | 1.52008 | 643.8 | 1.45670 | |||
| 239.9 | 1.51337 | 656.3 | 1.45637 | |||
| 248.3 | 1.50840 | 694.3 | 1.45542 | |||
| 265.2 | 1.50003 | 706.5 | 1.45515 | |||
| 275.3 | 1.49591 | 786.0 | 1.45356 | |||
| 280.3 | 1.49404 | 820.0 | 1.45298 | |||
| 289.4 | 1.49099 | 830.0 | 1.45282 | |||
| 296.7 | 1.48873 | 852.1 | 1.45247 | |||
| 302.2 | 1.48719 | 904.0 | 1.45170 | |||
| 330.3 | 1.48054 | 1014.0 | 1.45024 | |||
| 340.4 | 1.47858 | 1064.0 | 1.44963 | |||
| 351.1 | 1.47671 | 1100.0 | 1.44920 | |||
| 361.1 | 1.47513 | 1200.0 | 1.44805 | |||
| 365.0 | 1.47454 | 1300.0 | 1.44692 | |||
| 404.7 | 1.46962 | 1400.0 | 1.44578 | |||
| 435.8 | 1.46669 | 1500.0 | 1.44462 | |||
| 441.6 | 1.46622 | 1550.0 | 1.44402 | |||
| 457.9 | 1.46498 | 1660.0 | 1.44267 | |||
| 476.5 | 1.46372 | 1700.0 | 1.44217 | |||
| 486.1 | 1.46313 | 1800.0 | 1.44087 | |||
| 488.0 | 1.46301 | 1900.0 | 1.43951 | |||
| 496.5 | 1.46252 | 2000.0 | 1.43809 | |||
| 514.5 | 1.46156 | 2100.0 | 1.43659 | |||
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Glass transmittances are affected by thermal history after manufacture, as well as during the manufacturing process. Depending on the manufacturer and subsequent thermal processing (coating, annealing, or tempering), it is possible for any optical glass, including BK7, to show internal transmittance reductions of several percent across the entire spectrum with external transmittance correspondingly affected. Transmittance of all glass is especially uncertain at wavelengths approaching the water absorption band at 2.7 μm. Synthetic fused silica also shows batch-to-batch transmittance variations, especially in deep ultraviolet and infrared. These variations are related to manufacture and impurity content rather than subsequent history. In the ultraviolet, these variations have been attributed to uncontrollable fluctuations in metallic impurity content at the parts per billion level. Ultraviolet transmittance is the basis for the classifications UV grade and optical quality. A specification of UV grade ensures that a specimen is represented by the broadest curve. Transmittance curves for optical quality may fall anywhere between the UVGSFS curve and the OQSFS curves shown below. |
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External transmittances for UV-grade and optical grade fused silica |
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Infrared batch-to-batch transmittance variations in synthetic fused silica
are attributable to fluctuations in the OH chemical bond content. These
variations are most pronounced at wavelengths near and beyond the water
absorption band at 2.7 μm and are normally uncontrolled because ultraviolet
transmittance is generally regarded as more important. High infrared
transmittance can be ensured by appropriate manufacturing controls, but
only at the sacrifice of ultraviolet transmittance. Visible spectrum, batch-to-batch transmittance variations in synthetic fused silica are insignificant. The high ultraviolet internal transmittance of UV-grade synthetic fused silica is correlated with a visible internal transmittance that is so high it is beyond traditional methods of measurement. It is necessary to measure optical signal attenuation in fibers drawn of the material.
When comparing the internal transmittances of UV-grade synthetic fused silica and
BK7 glass,shown in the figure below, it is evident that UV-grade synthetic
fused silica averages about two orders of magnitude less absorption loss than BK7
across the visible spectrum (see curve). In a sample thickness of 10 mm, the
internal transmittance of UV-grade synthetic fused silica differs from unity only
in the fifth decimal place. The high internal transmittance of such a material can
be exploited by maintaining the optic at Brewster’s angle for the appropriate
linear polarization, or with the assistance of high-efficiency antireflection
coatings such as HEBBAR™ or one of the laser line V-coats. With these coatings
it is possible to achieve external transmittances of 98.5% and 99.5%, respectively.
Synthetic fused silica and HEBBAR are especially well suited to each other in
visible spectrum applications. |
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Semilogarithmic comparison of internal transmittances of UV-grade fused
silica and BK7 glass |
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The internal transmittance of UV-grade synthetic fused silica shows a
pronounced dip at 950 nm, while the data for BK7 give no hint of a
corresponding feature. It should be understood that BK7 and UVGSFS are
manufactured by very different processes. One of the many differences in
these materials is that UVGSFS has a much higher content of OH chemical
bonds (hydroxyl content) than does BK7. The dip in UVGSFS transmittance
corresponds to the OH bond resonance. |
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Synthetic Fused Silica Constants |
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Abbé Constant: 67.8 ± 0.5 Change of Refractive Index with Temperature (0° to 700°C: 1.28 x 10-5/°C Homogeneity (maximum index variation over 10-cm aperture) : 2 x 10-5 Density (at 25°C): 2.20 g cm-3 Continuous Operating Temperature: Maximum 900°C Coefficient of Thermal Expansion: 5.5 x 10-7°C Specific Heat (25°C): 0.177 cal/g°C Dispersion Constants** B1 = 0.6961663 B2 = 0.4079426 B3 = 0.8974794 C1 = 0.0046791 C2 = 0.0135121 C3 = 97.9340025 ** Source: Malitson, I.H. "Interspecimen Comparison of the Refractive Index of Fused Silica," Journal of the Optical Society of America 55, no. 10 (October 1965): 1205-1209 |
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| Optics Guide Copyright 2002 Melles Griot Inc. |