Chirped mirror

An ordinary dielectric mirror is made to reflect a single frequency of light.

This ability to tighten or more tightly pack a pulse of light of different wavelengths is important, because some commonly used optical elements naturally disperse a packet of light according to wavelength, a phenomenon known as chromatic dispersion.

A chirped mirror can be designed to compensate for the chromatic dispersion created by other optical elements in a system.

So if a given chirped mirror has 60 layers, light of a specific frequency interacts only with one sixth of the whole stack.

A detailed calculation (references in the external link) shows that the reflectivity of the mirror must also be chirped, which can be done by allotting the half-wavelength unequally across the high- and low-index zones.

In Ti-sapphire lasers employing Kerr-lens modelocking, chirped mirrors are often used as the sole means to compensate group delay variations.

Considering the group velocity this is enough for the 3 m air inside the cavity, for the 3 mm of Ti:sapphire crystal three more mirrors are needed, so that a simple Z-cavity can already be compensated.

In Chirped pulse amplification these mirrors are used to correct residual variations of group delay after a grating compressor is inserted into the system.

The scarab beetle species Chrysina limbata reflects close to 97% of light across the visible wavelength range.

A non-chirped dielectric mirror. This electron microscope image of a tiny circular piece of dielectric mirror being cut out from a larger substrate clearly shows the periodic layered structure of the mirror. The spacing of each layer determines the wavelength of light that is reflected by that layer. In a chirped dielectric mirror the deeper layers would be thicker than the surface layers to reflect longer wavelengths of light and create the chirped effect.