Dichroic Mirrors | previous | next | feedback |
Definition: mirrors with significantly different reflection or transmission properties at two different wavelengths
A dichroic mirror is a laser mirror with significantly different reflection or transmission properties at two different wavelengths. Dichroic mirrors are required e.g. for separating or combining laser beams with different wavelengths, e.g. to introduce the pump beam into a laser resonator, or for extracting a harmonic beam in a laser with intracavity frequency doubling.
Figure 1: Reflectivity spectrum of a dichroic mirror coating, designed with the software RP Coating for high transmission (low reflectivity) around 808 nm and high reflectivity at 1064 nm.
Dichroic mirrors are usually fabricated as dielectric mirrors. Depending on the case, the design of the required layer structure may be possible based on physical 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.
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.
See also: dielectric mirrors, dielectric coatings, Bragg mirrors, anti-reflection coatings
