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 500 nm to 1650 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.
Keep it ultrashort with
our dispersive mirrors
for ultrafast applications!