How to choose your optical substrate

This is the material from which an optic is made. Although any material which is transparent at the working wavelength can be used as an optical substrate, materials such as plastic or common glass have poor reliability. Most commonly available substrate are BK7, Fused silica, Magnesium Fluoride (MgF2) and Calcium Fluoride (CaF2).

Typically, most imaging in the visible or near infra-red applications will be satisfied with BK7. This is an inexpensive material with excellent transmission between 400 nm and 2 µm. Its hardness provides a good trade-off between resistance to deformation and resistance to scratches.

For non-critical application which needs a heat resistant substrate, Pyrex® offers a cheaper alternative, but this substrate is not suitable to create a good image, and is ruled out for any laser application.

Demanding laser applications will prefer fused silica. This material can be UV grade (called “UV fused silica” to differentiate) and be used with wavelengths as low as 180 nm and up to 2 µm. Its high homogeneity allows high damage threshold and better wavefront control. It also show no fluorescence or discolouration when exposed to radiation shorter than 290 nm. Its low thermal expansion coefficient would also lower the thermal lensing effects which are common in high power lasers.

Vacuum UV (typically less than 200 nm) will require Magnesium Fluoride (MgF2). Its mechanical properties and UV transmission are ideal for this type of application. Since this is quite an expensive substrate, it is sometime replaced by Calcium Fluoride (CaF2). Both material offers good transmission in the infrared up to 8 µm, however CaF2 performs better in the infrared. MgF2 has a slight birefringence, which demands careful alignment at manufacturing. MgF2 is a rugged material which has good resistance to mechanical or thermal shock and is resistant to chemical etching: its hardness is nearly three times that of CaF2, and its thermal expansion coefficient is nearly nine times lower. Both material have no water absorption line, which makes CaF2 the material of choice for applications from 2 µm to 8 µm.

Very large optics, etalons and high accuracy interferometry mirrors (or any precision mirror requiring excellent flatness control) will generally be made using Zerodur® (Schott). Rarely used in visible transmission because of its absorption in the blue region of the visible spectrum, it is a highly homogeneous, glass ceramic whose thermal and mechanical properties are extremely stable over a wide temperature range.

Polarisation control often require birefringent material. The two most common are calcite (CaCO3) and crystal quartz (SiO2). Calcite has a very strong birefringence and can be used from 400 nm up to 2.5 µm. It is the material of choice for polarizing prisms such as the Nicol prism, the Glan-Foucault prism, and the Wollaston prism. Calcite is a very soft material and extra care must be taken while manipulating it. Crystal quartz has a higher damage threshold but lower birefringence. It is often used in high damage threshold waveplates and Brewster windows. It is also good to notice that its transmission is still acceptable below 200 nm.

Difficult environments such as high temperature, high pressure, high thermal shock environments, or optics easily exposed to scratches and humidity might choose Sapphire. Its strength (extreme hardness) being its downfall, it is a really difficult material to polish, and high quality finish is not always possible. It transmits light from 150nm up to 6µm. It is also used with PbS, PbSe, and InSb detectors to match the spectral transmittance with the spectral sensitivity of the detector.

The three materials most commonly used in mid-far infrared are Zinc Selenide (ZnSe), Germanium (Ge) and Silicon (Si). Zinc Selenide is used when a high transmission is required since it is a very soft material. It transmits well up to 20µm. Germanium, used from 2µm to 20µm, has a much lower transmission but its hardness allow the optic to be thinner. It is not suitable for high laser power. Finally Silicon will be preferred for low-cost applications up to 7µm.

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