Dielectric Custom Optical Mirror

A dielectric mirror is a mirror based on multiple thin layers of (usually two) different transparent optical materials (→ dielectric coatings, thin-film coatings, interference coatings). Even if the Fresnel reflection coefficient from a single interface between two materials is small (due to a small difference in refractive indices), the reflections from many interfaces can (in a certain wavelength range) constructively interfere to result in a very high overall reflectance (reflectivity) of the device. The simplest and most common design is that of a Bragg mirror, where all optical layer thickness values are just one-quarter of the design wavelength. This design leads to the highest possible reflectance for a given number of layer pairs and given materials. It is also possible to design dichroic mirrors with controlled properties for different wavelengths.

The resonator mirrors of a laser are almost always dielectric mirrors, because such devices routinely achieve a very high reflectance of > 99.9%, and their limited reflection bandwidth can be convenient because it allows the transmission of pump light (at a shorter wavelength) through a folding mirror of the resonator (→ dichroic mirrors). Because of this use, dielectric mirrors are often called laser mirrors.

A characteristic property of dielectric mirrors is that they are optical properties depend substantially on the angle of incidence. As an example, Figure 1 shows reflectance spectra of a simple Bragg mirror for different incidence angles. The larger that angle, the more the reflection spectrum is shifted towards shorter wavelengths. This is essentially because the component of the wave vector perpendicular to the layer surfaces becomes smaller for a given wavelength, which can be compensated by reducing the wavelength.

Figure 1: The reflectance spectrum of a Bragg mirror for different incidence angles from normal incidence (red) up to 60° (blue) in steps of 10°.