Optical Windows

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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 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 of optical windows is often quantified with scratch–dig values according to the U.S. standard MIL-PRF-13830B, or alternatively in a more rigorous fashion based on ISO 10110-7. 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.

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 in 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.

More to Learn

Encyclopedia articles:

• Brewster plates
• optical domes
• anti-reflection coatings

Suppliers

The RP Photonics Buyer's Guide contains 178 suppliers for optical windows. Among them:

Knight Optical

Knight Optical offers a range of stock optical windows including standard windows manufactured from BK7, B270, borosilicate, UVFS, Gorilla Glass, and plastic. We also have UV and IR materials available including barium fluoride, calcium fluoride, magnesium fluoride, germanium, zinc selenide, sapphire, silicon, and potassium bromide. Our optical domes are also a popular component, with stock and custom options available. Knight Optical can provide fully custom windows including the addition of coatings to suit your application and with a variety of substrates available.

Shanghai Optics

Shanghai Optics Inc state of the art factory is able to manufacture both large optical windows and micro windows for nano-sized optical assemblies. A wide variety of optical substrates are available to choose from, and since production is carried on in-house our design team, will be able to match your exact specifications when carrying out your order.

Laserton

Laserton offers optical windows made of BK7 glass, fused silica or calcium flouride. They will as debris shields, for example.

Shalom EO

Hangzhou Shalom EO stands out as a leading supplier of a series of off-the-shelf and custom optical windows. Shalom EO offers:

• optical glass windows (N-BK7 and UV fused silica) of lambda/4 or lambda/10 flatness, high-precision laser windows and ultra-thin high flatness optical windows for femtosecond lasers. We are capable of achieving a minimum size down to 0.5 mm and a minimum thickness down to 0.03 mm.
• sapphire windows (standard as uncoated, c-cut or custom)
• infrared optical windows made of BaF₂, CaF₂, germanium, silicon, zinc selenide (ZnSe) and sapphire are also accessible

Through implementing in-process quality control and optical inspection using cutting-edge instruments, like Zygo interferometers and PerkinElmer Lambda 1050+ spectroscopes, Hangzhou Shalom EO is capable of providing high-precision optical windows enhanced with diverse coating options, offering reliable solutions for demanding applications.

Perkins Precision Developments

Perkins Precision Developments (PPD) manufactures multi-layer dielectric thin film coated laser windows and precision optical wedges in addition to custom uncoated substrates. Because we utilize Ion Beam Sputtering (IBS) coating technology, our AR coated parallel windows and wedge pairs are both durable and easy to clean. In addition, they exhibit low losses due to absorption and scatter, as well as high damage thresholds, making them ideal for both intra-cavity or extra-cavity laser applications.

Plane parallel windows, Brewster windows and matched wedge pairs are available in a variety of shapes and sizes from 2 mm to > 8” in diameter and with parallelism errors as low as 1 arcsecond. PPD manufactures precision plano substrates in a wide range of materials including fused silica, fused quartz, infrasil, N-BK7, YAG, silicon, SF11 and other high index glasses.

Custom low-loss anti-reflection (AR) coatings with reflectivity less than 0.1% per surface can also be deposited on customer supplied substrates up to 4 inches in diameter. Click here to see recent LDT data for an AR coating at 1064 nm.

Artifex Engineering

Artifex Engineering offers custom optical windows in almost any shape and design, including cut-outs and drillings. A wide range of optical substrates are available including N-BK7, fused silica, sapphire, Ge, ZnSe and more. AR coatings are generally applied to both surfaces of a window. In addition, Artifex offers custom coatings such as diamond-like carbon (DLC) coatings for the infrared (germanium, silicon). Visit our product page for more information. We look forward to your inquiry.

Ecoptik

Ecoptik manufacturers different types of optical windows, including custom versions. We can make different geometrical shapes, use many different materials (including some for infrared optics), and apply different optical coatings.

Kentek

Laser viewing windows made of polymer or glass with defined laser protection specifications are available in stock and custom-cut sizes. Laser safe acrylic viewing windows are durable, scratch-resistant and have excellent optical clarity. These are available in stock sizes or custom-made for large viewing ports or small instrument viewing ports. Laser protective filter glass windows are superior for infrared, ultraviolet and extreme environmental conditions.

Sinoptix

We provide optical windows down to λ/20 polishing, from 1 mm to 0.5 m diameter. All optical materials available.

OPTOMAN

OPTOMAN optical windows with IBS coatings are optimized to have excellent spectral performance, high damage threshold, temperature and humidity independent performance, and a negligible fatigue effect. OPTOMAN can coat very basic round and rectangular windows as well as wedges and prisms with extremely complex shapes. Sputtered anti-reflective coatings feature reflectance per surface down to R < 0.01%.

EKSMA OPTICS

EKSMA Optics has a wide selection of optical windows made from high quality fused silica, BK7, calcium fluoride, barium fluoride, magnesium fluoride, zinc selenide and other optical materials.

Edmund Optics

Edmund Optics’ optical windows are offered in a variety of substrates, such as germanium (Ge), silicon (Si), N-BK7, UV fused silica, zinc selenide (ZnSe), and potassium bromide (KBr). Multiple anti-reflection coating options are available for the ultraviolet (UV), visible, or Infrared (IR).

IRD Glass

IRD glass manufactures many types of optical windows – ultraviolet windows, visible windows, infrared windows, and debris shields from .100″ to 16″ in height and in thicknesses of .008″ to 4″ using a wide spectrum of materials, including nBK-7, B270, borosilicate glass, F-5, fused silica, fused quartz, high-index glass, sapphire, silicon and other IR materials.

Vortex Optical Coatings

We supply anti-reflection coated windows, 'make complete', for low and high volume applications.

UM Optics

UM Optics supplies UV-VIS-IR optical windows made of CaF₂, BaF₂, MgF₂, LiF, Si, Ge, ZnSe, BK7, UVFS, sapphire and others. These windows can be coated or uncoated with sizes from 3 mm to 350 mm. UM Optics can provide fully customized windows including round windows, rectangular windows, drilled windows and wedge windows to suit your application and with a variety of substrates available.

DayOptics

Dayoptics provides various types of optical windows, for example for projection and imaging applications, including customized versions.

Avantier

Our optical windows are designed to minimize both reflection and absorption while maximizing transmission over a target wavelength range. When choosing an optical window, you will have to keep in mind optical surface specifications, material transmission properties, and the mechanical properties required by your application.

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