Dispersive coatings


The phase velocity of a light wave travelling through optical materials depends on the wa­velength. This phenomenon is called chroma­tic dispersion.

As a critical requirement for the generation of ultrashort pulses (e.g. in femtosecond lasers) and for controlling the pulse shape, dispersive dielectric optics are needed to compensate the group-­delay dispersion (GDD) which is introdu­ced by other components such as the laser crystal.

The GDD is defined as the derivate of the group delay with respect to the angular frequency, or the second-order derivate of the phase.

Of course all dispersive elements inside the  re­­­sonator have to be taken into account for ­op­­timizing the total GDD (see also the Group Velocity Dispersion of standard substra­tes). Otherwise the different delays of various frequency components within a short pulse result in
    pulse broadening
    generation of satellites in the time-domain and
    loss of peak power.

Mirrors may have either low (i.e. close to zero) GDD or a tailored non-zero GDD. As shown in the following sections, there are different types of coating designs. LASEROPTIK offers:

    mirrors with low or optimized GDD
    highly dispersive narrow bandwidth mirrors, so-called GTI mirrors
    chirped mirrors and matched pairs
    octave-spanning broadband chirped mirrors with moderate GDD
    partial reflectors
    coatings used with an OPO

For example, these coatings compensate the dispersion introduced by other components like the laser crystal in a laser cavity. Or they serve as compressors in chirped pulse amplification systems (CPA).

In general, various spectral requirements can be combined with a GDD value as target for coating design and production, e.g. for OPO optics or partial reflectors. The dispersion management is highly depen­ding on the coating design and a sub-nanometer manufacturing accuracy for GDD optimized optics. At LASEROPTIK, this precision is ensu­red by the sophisticated method of Ion Beam Sputtering (IBS).

With the help of an optical in-situ process control based on the calculated design, the deposition of complex, custom tailored stacks of many (n > 70) non­­­quar­terwave layers is performed. The results are verified ex-situ by in-house GDD measurements and are used for reengineering of the coating design.

LASEROPTIK uses both custom-built and commercial GDD measurement setups ranging from 500 nm to 1650 nm (see measuring methods), whereas dispersive mirrors can be produced up to 4.5 µm, optimized and characterized for GDD by retrograde analysis of the coating thickness data.