Development of high-damage threshold dispersive coatings

Whether it is to form an optical cavity, to control dispersion, or
merely to transport the laser beam, multilayer mirrors are
fundamental components of every ultrafast laser system. The
performance of current state of the art ultrafast high-power lasers
in terms of pulse energy is often restrained by optical breakdown
of multilayer coatings. One way to overcome this problem is to
increase the size of the laser beam, but this is usually
undesirable, as it rises the costs and the footprint of the laser
system. Therefore, increasing the optical resistance of multilayer
mirrors is essential to the development of cost- and
space-efficient lasers. In turn, this requires a thorough
understanding of the mechanisms behind optical damage. In this
work, we have studied the ultrafast optical breakdown of dispersive
mirrors, as well as that of other multilayer thin-films, in three
different regimes: (i) at 500 Hz repetition rate with 30 fs pulses,
at a central wavelength of 800nm; (ii) at 11:5MHz repetition rate
with 1 ps pulses, at 1030 nm; (iii) at 5 kHz repetition rate with
1:4 ps pulses at 1030 nm. The results from (ii) and (iii) have been
compared side by side. In addition, a novel technique for
dispersion measurements has been developed. In the femstosecond
regime, the samples have been: single layer coatings made of Au;
Ag; Nb2O5; SiO2;Ta2O5 and mixtures of Ta2O5 with silica in
different concentrations; and different dispersive coatings,
consisting of SiO2 as the low-index material and different
high-index materials (Nb2O5; Ta2O5; HfO2). We have also given a
suggestion as to what is the best approach to increase the damage
threshold of thin-film dielectric coatings. The ultrafast optical
breakdown of multilayer thin-films has been investigated at MHz
repetition rate and high average power. The optical breakdown
threshold of three different types of coatings has been measured.
All samples have been coated with either TiO2, Ta2O5, HfO2, or
Al2O3 as high-index material and with SiO2 as low-index material.
The same samples have been measured also at kHz repetition rate.
The results obtained in both regimes have been compared. The band
gap dependencies of damage threshold in both cases were linear.
However, the one retrieved at kHz rate was steeper than its MHz
counterpart. This is an interesting finding, which must be
investigated further. The developed method for dispersion
measurements has been based on the location of resonance peaks in a
Fabry-Perot-type of interferometer. By simultaneously processing
data obtained at different spacer thicknesses, we were able to
obtain superior resolution compared to the conventional method.