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 derivative of the group delay with respect to the angular frequency, or the second-order derivative 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 350 nm to 2400 nm (see measuring methods), whereas dispersive mirrors can be produced up to 4 µm, optimized and characterized for GDD by retrograde analysis of the coating thickness data.