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HIGH PRECISE POLISHED OPTICAL COMPONENTS OUT OF SAPPHIRE
(artificial corundum Al2O3 )
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Sapphire, due to its unique physical properties, can be used for manufacturing of optical components that work at extreme conditions – high temperature, pressure, mechanical load, aggressive environment and radiation. The quality of polished surface is of great importance for successful application of optical components.
Polishing technology that has been developed by Sapphire specialists leads to high class of roughness (Ra about 10A), but does not provide high precise surface accuracy, even for plane windows. This technology is characterized by the use of strong etching reagents at chemical-mechanical treatment. Products produced by such technology find an application in watch industry, electronics, etc., but are not good for precise optical devices.
Sapphire specialists, that produce Sapphire components for optical devices, use mechanical treatment that leads to higher surface accuracy, but , because of some technological reasons, high surface quality and roughness can not be achieved.. The main difficulty of mechanical and chemical treatment combination is in the fact that suspensions, used as etching reagents, at the same time corrode material of polishing instrument.
As a result of investigations worked up in our firm, the technology of chemical and mechanical polishing of sapphire, that provides minimum Roughness and, at the same time, high Surface Quality and Surface Accuracy, is developed.
1. Surface Roughness
Surface Roughness was tested by power-atomic microscope Solver Pro.
Fig.1-5. demonstrates profilograph pictures of polished by our technology samples at various stages of surface treatment. As it is clear from the pictures, both, roughness and a surface structure, is being changed eventually – roughness becomes smaller and a surface structure becomes more homogeneous.
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2D and 3D profiles for the time of treatment 15 minutes ( Ra=6.046 nm )
Fig. 1
2D and 3D profiles for the time of treatment 30 minutes ( Ra=4.218 íì )
Fig. 2
2D and 3D profiles for the time of treatment 60 minutes ( Ra=2.001 íì )
2D and 3D profiles for the time of treatment 90 minutes ( Ra=1.31 íì )
2D and 3D profiles for the time of treatment 240 minutes ( Ra=0.969 íì )
The achieved roughness Ra of high precise polished surface is 9À. It is equal to the value that other manufacturers get at surfaces with no Surface Accuracy requirements. Such a small roughness with no “diamond sleeks” almost exclude light scattering that is very important, especially for laser application.
According to our technology high values of surface accuracy are also achieved at the same time – see p.2
Factor that complicates precise treatment of sapphire is anisotropy of its mechanical and other physical features. It has the least influence at flat surfaces and that is why we achieved there the highest accuracy. And the most complicated among spherical surfaces are spherical domes and micro-lenses. The technology is developed for hemispherical domes and micro-lenses with sizes from 3 mm to 180 mm and with wall thickness from 0.25 mm to 6 mm, correspondingly.
According to the developed technology we manufacture windows (diameter up to 250 mm) with Surface Accuracy (TWD or Flatness) up to 0.1 lambda (lambda 633 nm) and Surface Quality up to 20/10 MIL.
Fig.6 gives the example of ZYGO GPI inteferogram of transmittance wave front for Sapphire window
110x110 mm, clear aperture 80 mm, thickness 15 mm.
TWD PV: 0.129 lambda ( lambda 633 nm)
Surface Quality 20/10 MIL
Fig. 6
The technology of polishing has been developed and domes with diameter from 10 mm to 180 mm and wall thickness from 2 mm to 5 mm are being manufactured with average parameters as follows:
Thickness variation along the whole part: 0.02 mm
Surface Accuracy: in the central zone – 0.5 lambda (lambda 633 nm), in edge zones 1-1.5 lambda (lambda 633 nm)
Surface Quality: 60/40 MIL
The examples of ZYGO interferograms for the dome with R=79.995 mm are shown at Fig.7-11.
ZYGO interferograms for the optical sapphire dome with
R=79.995 (central zone Æ50 mm)
Fig. 7
ZYGO interferograms for the optical sapphire dome with
R=79.995 (edge zone Æ50 mm)
Fig.8
ZYGO interferograms for the optical sapphire dome with
R=85.015 ( central zone Æ50 mm)
Fig.9
ZYGO interferograms for the optical sapphire dome with
R=85.015 (edge zone Æ50 mm)
 Fig.10
Photo of the optical sapphire dome , diameter 170 mm
Fig.11
The technology has been developed and Sapphire optics is being manufactured with average parameters as follows:
Diameter: from 3 mm and bigger
Central Thickness: from 0.25 mm
Surface Accuracy of spherical surface: up to 0.5 lambda (lambda 633 nm)
Surface Quality: 20/10 MIL
The example of ZYGO interferogram for lens is shown on Fig. 12 - 13.
Fig. 12
Fig 13.
Photo of micro-lenses
 Fig. 14
Anti-reflection coatings can be applied for visible and IR ranges. Fig.15 and Fig.16 represent typical spectral curves.
Fig. 15 Fig. 16
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