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Cutting and Polishing Equipment

Crystal cutting and polishing equipment in our workshop provide complete surface preparation for crystal alignment, lapping and polishing. With the help of 2-Axis Goniometer, we can cut crystal precisely along a fixed orientation determined by Laue picture. The lapping fixture will control the thickness to be polished down to the 1 micron size.

After the cutting, the cuts are frosted white and polishing allows the piece to regain its clarity and brilliance. The polishing is done on the buffer wheels by hand. Some of the lesser quality pieces are polished by immersing them in the hydro-flouric acid.

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection (still limited by the index of refraction of the material according to the Fresnel equations.) When an unpolished surface is magnified thousands of times, it usually looks like mountains and valleys. By repeated abrasion, those "mountains" are worn down until they are flat or just small "hills." The process of polishing with abrasives starts with coarse ones and graduates to fine ones.

Ion Beam Sputtering

Ion Beam Sputtering (IBS) is an advanced deposition technology for even most critical demands on laser optics, such as high power supermirrors with superior reflectivities (99.99%) and lowest losses (<100 ppm).

IBS works with RF-guns, normally used in sattelite technology. Argon-ions are accelerated by approx. 1.5kV onto a metal target. Atoms are sputtered off the target and deposit on a rotating calotte. A homogeneous coating distribution is limited to about 30 cm in diameter, unless multiplied by a planetary substrate fixture.

As with magnetron sputtering, IBS is a "cold" process, in which the endogeneous process temperature does not exceed 100°C. With the accuracy of broadband monitoring systems, fractional parts of a monolayer can be detected. As a result of highest precision in film growth and an amorphous, almost defect free microstructure, IBS is considered the most advanced coating method in the thin-film industry.

 

More information about dielectric coating

Dielectric coatings, also called thin-film coatings or interference coatings, consist of thin (typically sub-micron) layers of transparent dielectric materials, which are deposited on a substrate. Their function is essentially to modify the reflective properties of the surface by exploiting the interference of reflections from multiple optical interfaces. They can be used for highly reflecting laser mirrors or partially transmissive output couplers, for dichroic mirrors (treating different wavelengths differently), for anti-reflection coatings, for various kinds of optical filters (e.g. for attenuation of certain wavelength regions), beam splitters, heat reflectors, solar cell covers, and thin-film polarizers. While simple single-layer coatings are often used e.g. as anti-reflection coatings, dielectric mirrors normally use dozens of thin-film layers, sometimes even more than 100.

In many cases, the coating substrate is some kind of glass, with a wide transparency range and high optical quality (low bubble content), a very smooth surface (after proper polishing), and high durability. However, dielectric coatings can also be applied to crystalline materials, e.g., as anti-reflection coatings on nonlinear crystals for nonlinear frequency conversion and Pockels cells, or on semiconductor devices such as edge-emitting laser diodes, vertical cavity surface-emitting lasers, and photodiodes. A further area of increasing importance is the fabrication of dielectric coatings on polymers (plastic materials), as plastic optics are increasingly used due to their competitive properties, e.g. in terms of price and the ease of fabricating non-spherical surfaces (e.g. for aspheric lenses).

Fabrication and Choice of Materials

This section applies mostly to coatings for glasses and crystalline materials, even though some aspects also apply to polymer optics. The fabrication of dielectric mirrors is usually based on one of the following techniques:

Electron beam deposition involves the evaporation of material in a crucible by heating with an electron beam, which is generated from a hot filament and focused with a magnetic field. In the vacuum chamber, the evaporated material moves to the substrate, which can be covered with a mechanical shutter as soon as the right amount of material has been deposited. The target substrate is heated to improve the quality. For some typical coating materials, the obtained thin films tend to be somewhat porous, leading to a reduced density and refractive index. The optical properties can then exhibit a significantly increased temperature dependence, as water may fill the pores, and may be driven out of the coating at elevated temperatures. This can be a problem for some sensitive steep-edge filter designs, for example.

A similar method uses evaporation by resistive heating of the crucible.

Ion-assisted deposition (IAD) essentially works like e-beam evaporation, but involves an additional ion beam (consisting of oxygen and/or argon ions) which hits the target substrate. The comparatively high energy of the ions allows a reordering of the deposited material, leading to denser coatings, even without heating the substrate. The method works well for oxide coatings (e.g. SiO2 or TiO2), which can have a similar quality as those obtained with IBS (see below). IAD is not suitable, however, for fluoride materials, which tend to disassociate.

Ion beam sputtering (IBS) uses an ion beam which, after neutralization with a second filament, hits a metal or metal oxide target to sputter material to the substrate. The flux and energy of the ions can be controlled independently and precisely. IBS generates fairly uniform, non-porous coatings with good adhesion and very low surface roughness (possibly below 1?Å), and is well reproducible. However, it is relatively slow, requires expensive equipment and materials, and is less flexible than other methods.

Advanced plasma reactive sputtering (APRS) involves the sputtering of thin metal films, which are subsequently oxidized in a separate oxygen plasma region of the chamber. Separate magnetron sources are used for the different coating materials. APRS combines a high precision and high density of the coatings (similar to that of IBS) with a high speed (comparable to that of evaporation techniques).

In any case, one starts with some homogeneous substrate material such as BK7 glass, fused silica, or CaF2. Common coating materials are oxides such as SiO2, TiO2, Al2O3 and Ta2O5, and fluorides such as MgF2, LaF3 and AlF3. The layers obtained are usually amorphous, with a density which can (depending on the fabrication technique) deviate from that of bulk material by more than 10%. Electron-beam deposition typically generates materials with lower densities, and thus also a lower refractive index. Such porous coatings have microvoids which can fill up with water when exposed to humid air; in effect, the refractive index and thus the whole properties of the coating depend on the humidity. Ion-assisted deposition and particularly ion beam sputtering achieve a higher density and accordingly a lower dependence on humidity. The optical damage threshold can also depend on the fabrication method.

Materials with a high refractive index contrast need to be used for high reflectivity mirrors, and particularly when a large reflection bandwidth is required. However, the chosen materials should also allow for fabrication with high optical quality and should have high stability under given environmental conditions (concerning laser wavelength and intensity, operation temperature, humidity, etc.).

Important aspects for the selection of a fabrication technique are:

the suitability for given coating materials
the precision of the layer thickness values (which may be increased with automatic control involving in-situ growth monitoring)
the optical quality of the deposited layers (influencing e.g. the scattering losses)
the ability of the coatings to withstand high optical intensities
the uniformity of layer thickness values over a larger area
the consistency (reproducibility) and stability of obtained refractive indices
the required substrate temperature
the time required for the growth

For example, ion-assisted deposition produces TiO2 films which are more compact and thus more stable and homogeneous and have a higher refractive index, compared with e-beam evaporation, which however is a faster process. The reason is essentially that TiO2 has a tendency to grow in low-density nanostructures, which can be destroyed (compacted) by irradiation with high-energy ions.

In-situ growth monitoring is crucial for obtaining precisely controlled layer thickness values. One uses the fact that the optical reflection or transmission properties can be used during the process to monitor the thickness of the currently grown layer, so that the growth process for a layer can be stopped at exactly the right time. A challenge is that the growth temperature usually differs considerably from the intended operating temperature of the coating, and accurate temperature-dependent refractive index data are often not available.

Apart from the basic fabrication method, the process parameters such as the substrate temperature and growth rate can also be important for the quality. The details are often confidential proprietary information of the fabricators.

The requirements on the substrate material vary, depending on the application. For highly reflective mirrors, important technical aspects can be the surface roughness, but also the thermal expansion coefficient (which is ideally similar to that of the coating materials) and the thermal conductivity. (In high-power lasers, residual absorption in the coating can cause some bulging of the mirror surface, inducing thermal lensing.) For mirrors with partial transmission of the light (e.g. for output couplers of lasers, or dichroic mirrors), it is also important to have good transparency of the substrate in the relevant wavelength range in addition to high optical quality. The back side may have to be given an anti-reflection coating.

For anti-reflection coatings on laser crystals and particularly on nonlinear crystals with anisotropic thermal expansion, but also for substrates with small curvature radii or for optical devices exposed to some chemicals, it can be a challenge to obtain coatings which are sufficiently stable. Optical damage, often occurring at microscopic defects, can also be a problem for devices operating with high optical intensities.

Dielectric mirrors can also be made of crystalline semiconductor materials, grown e.g. with molecular beam epitaxy (MBE) or with metal-organic chemical vapor deposition (MOCVD). Such mirrors are typically parts of some larger structures, such as vertical cavity surface-emitting lasers.

Coating Types:

Anti-Reflection Coatings (AR) - include single layer MgF2 coating, V-coating, Broad-Band AR Coating.

Band Pass/Band Reject Filter Coatings - A Band Pass Filter allows only the wavelengths in the specified range to pass through the filter, while blocking wavelengths outside of the transmission region with high rejection ratio. A Band Reject Filter blocks a specific wavelength range with high rejection ratio, while transmitting wavelength regions on either side of the rejection region. More...

Bullseye® Apodizing Filters Coatings - Bullseye® Apodizing Filters Coatings are a part of the Bullseye® apodizier line of customizable gradient filters. The Bullseye® Apodizing Filters are used to eliminate undesirable intensity variations in optical systems. The custom specified density function can either decrease (dark-to-light)or increase (light-to-dark)radially from the center of the substrate.

Dichroic Filter Coatings (also known as Color Separation Filter Coatings) - Dichroic Filters are sometimes known as color separation filter coatings because their purpose is to separate incoming light into distinct wavelength regions. Commonly used to extract a narrow wavelength band for entertainment or lighting effects, they can also be used as hot mirrors or cold mirrors to separate infrared light from visible light. Dichroic coatings achieves color or wavelength separation with a much higher degree of accuracy than conventional filters.

Infrared Thin Film Coatings - Infrared (IR) Coatings effect light that extends from the edge of visible red (700nm) to the far infrared (approximately 50um for Reynard Corporation capabilities). Any type of filter (LWP, SWP, BP, etc) or mirror coating that can be produced in the visible spectrum can also be implemented to work in the infrared. The difference in design is the starting choice of substrate and coating materials used to implement the required function, as reflection and transmission characteristics of individual materials change dramatically at different wavelengths.

Laser Protection Thin Film Coatings - High performance thin film Laser Protection Coatings, are designed to withstand higher energy conditions than classical thin film designs. Improved performance comes from the use of material sets with low absorption and higher melting points as well as through process improvements, such as the use of IAD, IBS.

Longwave Pass Filter Coatings (LWP Filter Coatings) - Longwave Pass Filter Coatings (LWP) are used to create a type of wavelength selection filter operational over a specified wavelength band from the UV to the far IR. Wavelengths smaller than the filter transition point are blocked with a high rejection ratio, while wavelengths longer than the transition point are transmitted. The transition point is application specific and static as defined by the customer.

Multi-Band Thin Film Coatings - Multi-Band Thin Film Coatings are used to create a type of wavelength select filter operational over customized distinct wavelength bands from the UV to the far IR. The filters are custom designed for applications that require multiple transmission or rejection bands that are non-contiguous. A filter that transmits the 3-5um (MWIR) band as well as the 8-12um (LWIR) band, while blocking the regions in or around these transmission bands is an example of a multi-band filter.

Short-wave Pass Filter Coatings (SWP Filter Coatings) - Short-wave Pass Filter Coatings (SWP) are used to create a type of wavelength selection filter operational over a specified wavelength band from the UV to the far IR. Wavelengths smaller than the filter transition point are transmitted, while wavelengths longer than the transition point are blocked with a high rejection ratio. The transition point is application specific and static as defined by the customer.

Trichroic Filter Thin Film Coatings - Trichroic Filters are used to precisely split incoming light into three separate wavelength bands of red, green, and blue, or alternatively to recombine such bands. Applications for Trichroic Filters include multi-sensor camera systems, where the three separated bands are detected by three separate CCD arrays, and projectors, where the Trichroic Filter recombines the red, green, and blue light sources into the presentation image.

Ultraviolet (UV) Filter Thin Film Coatings - Ultraviolet (UV) Thin Film Coatings are designed to work over shorter wavelengths from the edge of the visible spectrum (400nm) down to 180nm. Any type of filter (LWP, SWP, BP, AR, etc) or mirror coating that can be produced in the visible spectrum can also be implemented to work in the UV.

The article on dielectric mirrors explains in some detail how the optical properties of dielectric coatings can be calculated, and which aspects are important for designing such structures.

Bibliography
[1] J. R. McNeil et al., “Ion-assisted deposition of optical thin films: low energy vs. high energy bombardment”, Appl. Opt. 23 (4), 552 (1984)
[2] H. Sankur and R. L. Hall, “Thin-film deposition by laser-assisted evaporation”, Appl. Opt. 24 (20), 3343 (1985)
[3] P. J. Martin, “Ion-based methods for optical thin film deposition”, J. Mat. Science 21 (1), 1 (1986)
[4] A. Thelen, Design of Optical Interference Coatings, McGraw–Hill (1989)
[5] N. Kaiser (ed.), Optical Interference Coatings, Springer, Berlin (2003)

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