Optical Window Design Characteristics - 10 step chart

Step Feature Specification Characteristics / Benefits Limitations 1 Specify the Quantity Quantity Required &#; The larger the quantity of pieces that can be used in an application, the less expensive each part becomes as material, labor and coating charges can be divided over the total number of parts.
&#; Advanced Optics has the ability to modify catalog/overrun optical windows (when possible) to reduce costs and lead times. Small number of prototypes may be more expensive due to lot charges for glass and coating. 2 Select the Material
Soda-lime Glass
&#; Commonly known as float glass. &#; Least expensive of all glass types. &#; Can be polished 1-3 waves/inch. &#; May be tempered making it 3 times stronger than non-tempered glass. &#; Transmission of uncoated material is ~ 89% average (dependent on thickness) from 400nm-700nm with poor performance in the NIR. Glass can be coated with an AR coating to increase transmission.
&#; Softer than borosilicate glass making it easily scribed and broken.
&#; Cannot be precision polished and is available in commercial grade only (1-3 waves/inch).
&#; Has the lowest thermal shock and chemical resistance of all glass materials used to fabricate optics.
&#; Not as scratch resistant as other materials used to fabricate optical windows. BOROFLOAT®33 &#; Borofloat®33 is a borosilicate glass with a low thermal expansion.
&#; Good all around general purpose mirror substrate that is moderately priced.
&#; Easier to polish than harder materials such as fused quartz, fused silica or Zerodur® and is much less costly. &#; Transmission of uncoated material ~ 92% average (dependent on thickness) from 400-700nm.
&#; May be polished down to λ10, but is not suitable for polishing down to λ/20.
&#; 2-3 times more costly than float glass (soda- lime glass).
&#; Not as thermally shock resistant as fused quartz or fused silica.
&#; Cannot be fully tempered like soda-lime glass.
&#; Not suitable for extreme high temperature conditions and will not hold its shape over 450° C for long periods of time. B 270® &#; Crown type soda-lime glass.
&#; Extremely clear and colorless.
&#; Good transmission in the visible into the IR with 90% average from 350nm-nm (dependent on thickness of glass). Available up to 10mm thick. D 263® T eco &#; Clear borosilicate glass.
&#; High chemical resistance.
&#; Ultra thin glass, wafers available up to 1.1mm.
&#; Excellent transmission over a large spectrum, ~93% or greater 350nm - nm (dependent on thickness of glass). Available up to 1.1mm thick. N-BK7® &#; Clear uniform color.
&#; Nearly free of bubbles and inclusions.
&#; High degree of purity.
&#; Very good refractive index homogeneity.
&#; Excellent transmission in the VIS to NIR spectrum with optimum transmission >95% from 350nm - nm (dependent on thickness of glass).
&#; Low absorption and uniform transmission in the visible spectrum. N-BK7 is not recommended for applications where thermal shock is a factor. Viosil &#; Viosil is a synthetic quartz glass substrate manufactured by ShinEtsu.
&#; The absence of bubbles and inclusions make it an excellent window substrate.
&#; Excellent transmission from the UV to the NIR, > 93% transmission from 200nm-nm.
&#; It offers excellent chemical resistance, mechanical strength and high heat resistance. Carry glass only up to .250" thick. Fused Silica &#; Made from a synthetically derived silicon dioxide that is extremely pure.
&#; It is a colorless, non-crystalline silica glass.
&#; The main difference between fused silica and fused quartz is that the former is composed of a non-crystalline silica glass while the latter is composed of a crystalline silica glass.
&#; Advantages of fused silica over fused quartz include; greater ultraviolet and infrared transmission, a wider thermal operating range, increased hardness and resistance to scratching and a lower CTE which provides resistance to thermal shock over a broad range of temperatures.
&#; As opposed to other less costly glasses, the surface figure (flatness) of optical windows made of fused silica are not at risk in applications that expose the material to coatings applied at high temperatures or applications that require the material to remain flat at high and/or varying temperatures.
&#; Fused silica is also chemically resistant and provides superior transmittance in the UV spectrum when compared to fused quartz.
&#; Fused silica comes in many grades with the most common being 2G. Please visit Corning&#;s Quality Grade Selection Chart for further information. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses.
&#; The homogeneity of fused silica exceeds that of crystalline fused quartz, however standard 2G (UV grade) material has a higher OH content which has dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. Quartz &#; Made from naturally occurring crystalline quartz or silica grains whereas fused silica is entirely synthetic.
&#; Fused quartz and fused silica are both extremely pure materials and have very low thermal expansion rates. However, fused quartz is more cost effective.
&#; Known for its incredible thermal shock resistance, chemical resistance and for being an excellent electrical insulator.
&#; Fused quartz has more metallic impurities and a lower OH content than standard UV grade fused silica which has dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses, but less expensive than fused silica.
&#; Fused quartz shares many of the same advantages of fused silica with the exception of metallic impurities found in the mined, natural quartz or silica sand. These impurities inhibit the materials ability to transmit well in the UV spectrum. 3 Determine the Size/Shape Round
Rectangular
Square
Custom Round provides the best opportunity for obtaining flatness/accuracy. Custom sizes and shapes available. Square, rectangular and custom shapes provide more challenges to maintaining surface flatness. 4 Refine your Mechanical Tolerances Defines the acceptable limits of both size and thickness required for an application. Specified in inches or mm and typically given a +/- value.
&#; Round: Provide tolerance for diameter.
&#; Rectangular/Square: Provide tolerance for LxW.
&#; Thickness: Provide tolerance for thickness. &#; Tighter tolerances for diameter and LxW are typically easier to hold than for thickness.
&#; Extremely tight tolerances available, but may require specialized techniques which can reduce yield leading to increased costs.
&#; Loosening your tolerances can reduce costs. 5 Establish the Correct Accuracy Commercial grade
1-3 waves/inch

Precision polished λ/4 or λ/10


Precision polished λ/10 or λ/20











Specify requirements as surface flatness or transmitted wavefront. Commercial grade mirrors are generally made from less expensive materials such as soda-lime glass or borofloat.

Working grade windows are polished either λ/4 or λ/10 and most often made of Borofloat®33 or N-BK7.

&#; Precision grade windows are polished either λ/10 or λ/20 and are typically made from harder glass materials such as quartz or fused silica.
&#; To achieve the best accuracy, optical windows are polished in a 6:1 aspect ratio (diameter to thickness). The higher the ratio, the greater probability the glass will distort during the manufacturing process. When the glass is deblocked after polishing, windows with non-standard aspect ratios may spring as they do not have the stability to hold surface flatness.
&#; Advanced Optics manufactures precision grade windows with non-standard aspect ratios.

Surface flatness is defined as the deviation between how flat the surface of an optical window is when comparted to a perfectly flat reference such as an optical flat.

Transmitted wavefront or TWE is defined as how much the light path is distorted as it passes through an optical window and is a function of surface flatness on both sides of the window, the purity and homogeneity of the material as well as the parallelism.

For a definition of the difference between transmitted wavefront and transmission see our optical terminology page. Achievable surface accuracy is dependent on choice of substrate and thickness of material. 6 Specify the Surface Quality Provide the required
Scratch and Dig 80-50: Commercial grade mirrors, suitable for non-critical applications, easily manufactured, lowest cost.

60-40 or 40-20: Working grade windows, precision quality, suitable for low to medium power lasers systems and smaller optics, moderate increase in cost.

20-10 or 10-5: Precision grade, suitable for high power laser systems and small optics. In demanding applications, even small surface defects might result in light scattering, undesired diffraction patterns, loss of contrast and stray light which can not only degrade a systems performance, but may even damage the optical window. Extremely tight tolerances available, but may require specialized techniques which can reduce yield leading to increased costs. 7 Provide Parallelism (if required) Amount of wedge or variation in thickness allowed over the surface of a part. It is defined in arc minutes (an angular measurement that is 1/16th of a degree) or arc seconds where 60 arc seconds is equal to 1 arc minute.

Advanced Optics manufactures wedged windows as well as parallel optical windows and can hold parallelism of < 2 arc seconds. Extremely tight requirements for parallelism require specialized manufacturing techniques which may reduce yield and increase manufacturing costs. 8 Define the Clear Aperture/ Edge Bevel Requirements The clear aperture is the percentage of useable area of an optical window.

An edge bevel or safety chamfer is applied around the edge of an optical window.
Normally 90% or advise requirement.


An edge bevel or safety chamfer is applied around the edge of an optical window to eliminate sharp edges and reduce edge chips caused by cutting of the glass. Typically between .010"-.040" face width at 45 degrees depending on size of part, please advise preference and tolerance. Very small edge bevels with tight tolerances will add additional costs. 9 Choose the Proper Coating Anti-reflective coatings available for the UV-VIS-NIR regions. Choices including MgFl2, V-coats and broadband coatings as well as custom coatings. &#; Provide the wavelength(s) of interest and % reflectivity required.
&#; Provide the intended AOI (angle of incidence) for the optical window. Custom coatings for a small quantity of parts may add additional expense. 10 Customization The following attributes can be added to customize your optical window. &#; Shapes: Provide drawing of custom shape.
&#; Holes and Notches: Provide location, size with tolerances.
&#; Custom Bevels: Provide location, depth and angle.
&#; Custom Coatings: Provide expected % of reflectivity over wavelength(s) of interest and AOI (angle of incidence). Additional features may add lead time and cost.

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Most optical windows are made in the form of flat plates of a transparent medium (e.g. glass, crystal or polymer). They are often used for isolating optical systems or components against detrimental influences from the environment. For example, most photodiodes and other kinds of photodetectors often contain an optical window above their light-sensitive area to protect it against dirt, corrosive influences and mechanical damage. Similarly, housings of lasers are often protected with optical windows in order to keep the housing free of any dust.

In some cases, for example for the active tubes of gas lasers like helium&#;neon lasers, there are optical windows separating the inside low-pressure gas volume from the outside atmosphere. Similarly, windows are needed for multipass gas cells as used in spectroscopy. If such windows are not rigidly connected, one may require some suitable type of seals to get a housing reliably air-tight. There are special vacuum windows built into vacuum viewports, coming together with suitable seals and mounting parts.

Some laser viewing windows are made to transmit visible light for inspection purposes, while blocking laser light (e.g. in the infrared) for laser safety reasons.

There are also strongly curved optical windows, which are called optical domes.

Common optical materials used for optical windows are glasses like fused silica and BK7 for visible or near-infrared light. For infrared optics at longer wavelengths, one also uses various types of crystalline materials such as calcium fluoride, also semiconductors like zinc selenide, silicon and germanium. Particularly for low-cost mass applications, some polymer materials are also often used, e.g. PMMA acrylic. They may be equipped with anti-scratch coatings for making them more resistant.

In some cases, an optical element such as a lens or a mirror can at the same time fulfill the function of an optical window, so that no separate part is required for that. Note, however, that a separate optical window may be advantageous in rough environments, since it is both easier and cheaper to exchange it, compared with exchanging a high quality optical element.

Optical Losses

Usually, it is important to avoid significant losses of optical radiation going through an optical window. Such losses can occur due to different effects:

• The material, even when being very pure, can exhibit some level of absorption. To avoid that, one chooses a material with a sufficiently large transparency range of wavelengths, where the absorption is minimal. There are many glasses with a transparency range well covering the whole visible region, and others which are well suited for infrared light. Crystalline materials are also often used in the infrared.
• Some level of scattering losses and/or beam distortion may occur if the medium is not optically homogeneous. For example, low-quality glasses may exhibit locally varying concentrations of certain substances, which lead to variations of refractive index.
• Furthermore, non-perfect optical surfaces can lead to scattering and also to beam profile deformations (see below). Such effects can be minimized by preparing surfaces with high optical quality, i.e., with high flatness and low roughness.
• Finally, there is usually some level of optical reflections at the surfaces. Those can be problematic not only in terms of power loss, but also when parasitic back-reflections irritate a laser device, for example, but the latter effect can be eliminated by simply avoiding near normal incidence of the laser beam on the window. For minimizing reflections, one usually uses anti-reflection coatings. These work well only in a limited wavelength range. There are broadband coatings working in a large wavelength range, but typically with lower suppression of reflections than narrowband coatings (available for many laser line applications) can achieve in a small wavelength range. Some optical windows are sold in uncoated form and are later custom-coated by the user.

Note that the power losses at an optical window can be polarization-dependent, if the incidence of light is far from normal incidence. An extreme case is that of a Brewster plate, where the angle of incident needs to be at Brewster's angle, so that the reflection losses are very small for p polarization (without using a coating), but rather high for s polarization. Such windows are called Brewster windows. They are often used for the tubes of gas lasers, for example.

Beam Distortions

It has already been mentioned above that beam distortions (wavefront errors) may be caused by optical windows of insufficient quality. That may also lead to a loss of beam quality of laser beams, or image distortions in viewing devices and cameras.

The surface quality is of high importance for some applications.

The surface quality of optical windows is often quantified with scratch&#;dig values according to the U.S. standard MIL-PRF-B, or alternatively in a more rigorous fashion based on ISO -7. In addition, there are certain tolerances for surface flatness and irregularity. The article on laser mirrors, where surface quality is of particularly vital importance, contains some more details on such issues.

Optical window of particularly high quality are also sold as interferometer flats; this marks them as being suitable for use in interferometers, where low beam distortions are often of particular importance.

Besides deficiencies of the material itself, there can be inhomogeneities induced by mechanical stress. Therefore, optical windows should be mounted such that stress effects are avoided.

Problems with Dirt

A good surface quality should not only be achieved in production, but also be maintained by carefully transporting and mounting optical windows, and by avoiding adverse effects during operation. For example, the performance can be degraded by fingerprints (when touching surfaces), deposited dust and dirt, or by scratching the surface when touching them with hard parts. In applications involving intense laser pulses, e.g. from Q-switched lasers, dust and other dirt may be burned into a surface, making it difficult afterwards to remove it.

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Cleaning optical parts can help, but only when applying appropriate techniques.

Some kinds of dirt can be removed from optical windows with appropriate cleaning procedures. For example, one may use a soft cleaning tissue and a few droplets of a suitable solvent (e.g. cleaning alcohol or acetone) to wipe the surface if it is well accessible. One should avoid wiping back and forth, only distributing dirt; instead, one should systematically wipe in one direction, getting any dirt outside the sensitive area. At the same time, care must be taken not to damage optical surfaces e.g. by touching them with any hard tools.

For applications in rough environments, one may use special holders which facilitate the quick exchange of damaged optical windows. For example, some windows are used as debris shields in laser material processing, and may have to be exchanged regularly. They are also called sacrificial windows. One may also try to protect windows to some extent e.g. with hard tube structures.

Changes of Beam Position or Direction

Most optical windows are parallel windows, having quite precisely parallel surfaces. The parallelism is often quantified, e.g. as <1 arcsec. With parallel faces, there is no change of beam direction, but only a slight beam position offset, dependent on the angle of incidence and the thickness; there is then often no need for precise alignment of the window.

The beam offset also has a slight dependence on the optical wavelength, since the wavelength-dependent refractive index leads to a wavelength-dependent beam direction within the plate. It is rare, however, that such effects cause problems.

There are also wedged windows, having a well defined angle between their surfaces. This is sometimes required for avoiding interference effects between the parasitic reflections from the two surfaces. Note that in this case there is some level of beam deflection, dependent on the orientation of the window. See also the article on wedge prisms.

Thermal Effects

For applications with very high optical powers, e.g. in laser material processing, even some small residual absorption in an optical window may cause some level of thermal lensing. (A small amount of dirt on a surface can of course strongly increase the strength of heating and its consequences.) Such thermal effects can depend on multiple properties of the material, in particular on the absorption coefficient, the thermal conductivity, the temperature dependence of the refractive index and photoelastic coefficients. Special high-quality materials may have to be chosen for such applications.

Various Relevant Properties

Various other detailed properties of optical windows may be relevant for applications. Some examples of such properties:

• cost and ease of procurement
• avoiding glass components like lead and arsenic (ecologically friendly materials)
• a suitable geometric shape for easy mounting and replacement
• a high optical damage threshold (e.g. for pulsed laser applications)
• a low thickness and density, if weight matters
• a high hardness, if resistance against mechanical influences is important
• a conductive surface, often made of ITO, used e.g. for electric shielding

Some suppliers offer custom windows with special specifications, often for special application areas such as aerospace and military.

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