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Definition: mirrors with significantly different reflection or transmission properties at two different wavelengths
German: dichroische Spiegel, dichroitische Spiegel
A dichroic mirror is a mirror with significantly different reflection or transmission properties at two different wavelengths. Dichroic mirrors are required for different purposes. Some examples:
- In a diode-pumped laser, a dichroic short-pass mirror in the resonator, placed next to the laser crystal, may be used for injecting pump light.
- In a laser with intracavity frequency doubling, a dichroic end mirror may couple out the harmonic light while fully reflecting the pump wave.
- In case of external frequency doubling, a dichroic mirror may be used as harmonic separator (see below), i.e., as a kind of wavelength-dependent beam splitter.
- In laser microscopy (fluorescence microscopy), a dichroic mirror can be used for separating the fluorescence light (containing the image information) from the pump light.
So-called harmonic separators (see Figure 1) are dichroic mirrors for separating the harmonic (frequency-doubled) beam from the pump beam, often outside the laser resonator. For that application, the two wavelengths for which the mirror needs to have very different reflectivities are so far apart from each other that it is easy to make such mirror designs.
Short-pass and Long-pass Mirrors
In electronics, the terms low-pass and high-pass filter are common, where “low” and “high” refers to the frequency. In optics, where it is more common to refer to wavelengths, one uses the terms short-pass and long-pass mirror. Here, a short-pass mirror (or shortpass mirror) is one which has a high transmission at short wavelengths and high reflectivity at longer wavelengths; it could also be called a high-pass filter.
It can be challenging to make mirrors such that the wavelength with high transmission and the wavelength with high reflectivity are close together.
Fabrication of Dielectric Mirrors
Dichroic mirrors are usually fabricated as dielectric mirrors. Depending on the case, the design of the required layer structure may be possible based on analytical considerations, possibly followed by a numerical optimization, or entirely on numerical optimization, e.g. with a Monte-Carlo method. In many cases, the design involves a compromise between the obtained optical properties, the required number of layers, and the required growth precision.
For any dielectric mirror, the reflection spectrum depends on the angle of incidence and (for non-normal incidence) also on the polarization of the input light. Only to a limited extent, mirror designs can be made such that the desired dichroic properties are achieved over some range of input angles.
As a dichroic mirror has to be transparent for at least one wavelength of interest, the quality (e.g. transmission losses) of the substrate material and possible reflections from the back side need to be considered. An anti-reflection coating on the backside can help to reduce such a reflection, and a slight wedge form of the substrate can often eliminate the effects of residual reflection.
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