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Dispersive Mirrors

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: mirrors which provide some amount of chromatic dispersion for the reflected beam

More general term: mirrors

Categories: article belongs to category general optics general optics, article belongs to category photonic devices photonic devices

DOI: 10.61835/sxv Cite the article: BibTex plain text HTML Link to this page LinkedIn

Dispersive mirrors are mirrors – usually a kind of laser mirrors – which provide some amount of chromatic dispersion. They can be used for dispersion compensation within a laser resonator or for the compression of ultrashort pulses, for example, or for other applications in femtosecond laser optics and optical signal processing.

Usually, dispersive mirrors are made in the form of dielectric mirrors, and they can be based on different operation principles:

Some simple designs are essentially realizing a Gires–Tournois interferometer, where the chromatic dispersion essentially results from interference effects at an off-resonant optical resonator. It is possible to obtain a large amount of chromatic dispersion (many thousands of fs²) of any sign, but only with a limited optical bandwidth. The higher the amount of chromatic dispersion, the more stringent is the limit for the optical bandwidth.
A totally different operation principle is that of the chirped mirror, where the optical penetration depth is wavelength-dependent. According to a very simplified picture, the resulting chromatic dispersion can be explained as resulting from a wavelength-dependent path length. With proper optimization and precise growth of the dielectric structure, ultra broadband mirrors e.g. for octave-spanning titanium–sapphire lasers can be produced.

Apart from the amount of chromatic dispersion in the bandwidth, it is often important to achieve a high precision of the whole spectral dispersion profile. For example, one may use such mirrors for compensating even higher-order chromatic dispersion in the resonator of a mode-locked laser.

Usually, dispersive mirrors are highly reflecting. In case that there is any significant transmission, the chromatic dispersion for the transmitted light is generally totally different from the chromatic dispersion for the reflected light.

Compared with ordinary highly reflecting laser mirrors, which are generally designed as Bragg mirrors, dispersive mirrors exhibit tentatively higher parasitic losses (i.e., a somewhat lower reflectance) and the lower optical damage threshold. This results from the substantially deeper penetration depth into the dielectric structure. (See Figure 3 of the article on chirped mirrors.) The higher parasitic losses also imply a higher degree of thermal lensing on such mirrors, if they are used for high-power laser beams. With optimized high-power dispersive mirrors, however, thermal effects can be kept on the level which is quite low compared with those at other kinds of dispersive elements, e.g. prism pairs. Key aspects for such an optimization are the minimization of parasitic losses and the use of a mirror substrate material with a low thermal expansion coefficient and a high thermal conductivity.

laser beam bouncing at a pair of mirrors — Figure 1: Multiple passes of a laser beam at a pair of dispersive mirrors.

If a simple reflection on a dispersive mirror does not provide sufficiently much chromatic dispersion, one may use multiple reflections, e.g. between two dispersive mirrors, where the two mirror surfaces are parallel to each other and non-normal incidence is used (see Figure 1). Note that the angle of incidence should not be too large because the chromatic dispersion depends on that angle. Obviously, the total power losses and the total strength of thermal lensing scale with the number of reflections.

Sometimes, a matched pair of two different dispersive mirrors is used, where the dispersion errors of both mirrors are partially compensated.

Designing Dispersive Mirrors

For designing dispersive mirror coatings, a flexible simulation and design software is indispensable. The RP Coating software is an ideal tool for such work, as it is particularly flexible. For example, you can define an initial chirped mirror design with a limited number of parameters and numerically optimize only those (rather than each individual layer thickness separately), which is far more efficient. Thereafter, you may still apply a local optimization of all layer thickness values.

More to Learn

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Suppliers

The RP Photonics Buyer's Guide contains 16 suppliers for dispersive mirrors. Among them:

OPTOMAN

OPTOMAN can guide you through the features of dispersive mirror designs and manufacture optimal optics for your laser system. Dispersive mirrors come in two main types: GTI mirrors, which provide high dispersion over narrow bandwidths, and chirped mirrors, which offer broader spectral coverage with lower GDD, ideal for compensating bulk materials. OPTOMAN dispersive mirrors can be designed for any specific GDD requirement from slightly negative (−20 fs²) up to ±10000 fs².

Standard in-stock low GDD mirrors can be found in OPTOSHOP.

few-cycle

few-cycle offers state-of-the-art components (dispersive and non-dispersive mirrors, windows, wedges) that enable achieving and preserving the bandwidth-limited duration of few-cycle pulses with central wavelengths ranging from the visible to the near infrared. Our R & D projects pave the way to implementing dispersion management techniques for ultrashort pulses with average powers up to 200 W.

LASEROPTIK

LASEROPTIK designs and produces various kinds of dispersive coatings, including chirped mirrors and matched pairs of chirped mirrors, also all other kinds of coated optics for dispersion management like GTI mirrors. We use Ion Beam Sputtering (IBS) for highest quality and widely ranging dispersion measurement setups for precise characterization.

UltraFast Innovations

UltraFast Innovations (UFI®) provides a wide range of ultra-broadband compression mirror sets in various configurations. For example, mirror sets for pulse compression in chirped-pulse titanium:sapphire amplifier systems, broadband oscillators and other applications cover the wavelength range between 400–1200 nm, i.e., up to 1.5 octaves. Our customers have reached pulse durations around 3 fs after spectral broadening in hollow-core fibers and gas cells in combination with mirrors designed and manufactured by UFI. In addition, we also offer large dispersive optics with up to 200 mm diameter.

Such mirrors are not only optimized for a broad reflection bandwidth, but also for a precise chromatic dispersion profile with low group delay dispersion (GDD) fluctuations.

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