Main parameters of optical coating substrate

With the development of optical technology, there is an increasing demand for precision optical coating, such as ultrafast laser optical components and special laser optical components, which have very high requirements for coating. Generally speaking, the price and quality of substrates depend on the material, shape, size, tolerance, and polishing quality.

Material

The first step is usually to determine the material of the substrate, which should have high transmittance for all wavelengths and almost no absorption. If it is not transmissive, low-cost materials such as Borofoam® can be used; SCHOTT AG is used for metal mirror surfaces. It is beneficial for surface shape tolerance and low thermal expansion. 、

shape

When selecting a substrate, it is necessary to consider the shape of both sides, and all combinations of flat, convex, wedge-shaped, and concave surfaces are possible. Wedge shapes (e.g. 30 39;) can be applied to every surface (flat as well as convex or concave). There are different conventions for radius symbols for curved substrates. Sometimes; +” Representing convexity,“ - ” Indicates concavity. Other users will; +” And“ - ” It is called light propagation. In this situation,“ +” Representing“ Curvature with propagation direction; ， “ - ” Representing“ Curvature relative to the propagation direction;. To avoid confusion, it is necessary to clearly indicate whether it is concave or convex.

Users need to specify the side length and diameter. Unless otherwise specified, the thickness description refers to the maximum thickness of the substrate, which is the center thickness of the flat convex substrate and the edge thickness of the flat concave substrate. Therefore, the wedge-shaped plate is generally measured on the thicker side. In order to achieve good shape tolerance, attention should also be paid to the ratio of diameter and thickness. Besides size and material, tolerance is also the most important factor for cost. Of course, optical components must be suitable for the mounting base, so the diameter should not exceed the specified value. The most common specification is -0.1mm. In most cases, the thickness is free in both directions. Haina Optics usually specifies its tolerance as± 0.1mm. There is a lot of confusion regarding the specifications for wedge shape, parallelism, and centering. Please note that the wedge and parallelism describe the angle between the optical surfaces, while the center describes the angle between the optical surface and the side surface (as shown in the figure above). Haina optical standard substrate has parallelism better than 5 arcmin. Specially designed parallel lines can have a parallelism as low as 10. The standard wedge-shaped substrate has a 0.5° Or 1° The wedge shape. According to the substrate size, larger wedge angles can be achieved. Usually, it forms a 90 degree angle with the side surface; The angle has 20 39; The precision. Centering is an additional optical process that can increase this accuracy to several arc minutes arcmin.

Surface shape tolerance

Surface shape tolerances are usually measured by interferometers and expressed in lambda; Represented as a reference wavelength. No further statement, the reference wavelength isλ =546nm. To avoid confusion, it is necessary to clearly distinguish between flatness, power, and irregularity. In the following text, flatness and irregularity will be represented for planes. Generally speaking, every real surface is more or less curved. The real surface; Peak” And“ Valley” Covered by parallel planes, the distance between these planes is called flatness. This flatness includes two contributions. The first contribution is the spherical curvature of the surface, which can be achieved by using“ Peak” And“ Valley” Describe the two best fitting spheres. The sagittal representation of this curvature relative to the ideal plane is power. This spherical curvature will not affect the quality of the reflected beam. It will only result in a limited focal length. The second contribution is the deviation of the best fit sphere, represented as irregularity. This is the most important value for evaluating the quality of optical components. The ISO 10110 standard provides an algorithm for specifying surface shape tolerances. The evaluation is based on the best comparability with the measurement results, and all values are given in terms of the number of interference fringes.

Coating stress

Thin substrates typically cannot withstand coating stress. The coating can cause spherical deformation, which means limited variations in arc height and power have occurred. In the case of circular substrates, irregularity is not affected by this issue. When the above problems are severe, the flatness deteriorates. However, the quality of the beam is not affected.