I. Angelov

High-dispersive mirrors for high power applications

We report on the development and manufacturing of two different types of high-dispersive mirrors (HDM). One of them provides a record value for the group delay dispersion (GDD) of -4000 fs2 and covers the wavelength range of 1027-1033 nm, whereas the other one provides -3000 fs2 over the wavelength range of 1020-1040 nm. Both of the fabricated mirrors exhibit a reflectance of >99.9% and are well suited for intracavity applications. Mirrors of the second type have been successfully employed in a Kerr-lens mode-locked Yb:YAG thin-disk oscillator for the generation of 200-fs pulses with multi-10-W average power.

Optical breakdown of multilayer thin-films induced by ultrashort pulses at MHz repetition rates.

Multilayer coatings composed of TiO(2), Ta(2)O(5), HfO(2), or Al(2)O(3) as high-index materials and SiO(2) as low-index material were investigated for laser-induced damage using 1 ps, 5 µJ pulses generated by a mode-locked Yb:YAG thin-disk oscillator operating at a wavelength of 1030 nm and repetition rate of 11.5 MHz. Previously reported linear band gap dependence of damage threshold at kHz repetition rates was confirmed also for the MHz regime. Additionally, we studied the effect of electric field distribution inside of the layer stack. We did not observe any significant influence of thermal effects on the laser-induced damage threshold in this regime.

Measurements of the group delay and the group delay dispersion with resonance scanning interferometer

We developed a method for group delay and group delay dispersion measurements, based on location of interference resonance peaks. Such resonance peaks can be observed in transmittance or in reflectance when two mirrors are placed parallel to each other and separated by a thin air spacer. By using a novel approach, based on simultaneous processing of the data acquired for different spacer distances we obtained reliable results with high resolution. Measurements were performed both in transmittance and reflectance layouts depending on the reflectivity of the mirror to be measured. The developed method allows dispersion measurements of ultraviolet mirrors and ultra-broadband mirrors spanning more than one optical octave to be performed.

Investigation of the laser-induced damage in dispersive coatings

Different dispersive coatings were tested in terms of laser-induced damage threshold by using a Ti:Sapphire laser yielding 1 mJ, 30 fs pulses at 500 Hz repetition rate at 790 nm central wavelength. The beam was focused down to 140 μm. Single layer coatings of Au, Ag, Nb2O5, SiO2, Ta2O5 and mixtures of Ta2O5 and silica were examined as well as different dispersive coatings. We observed a direct dependence of the damage threshold on the band gap of the materials used to produce the different samples. The damage threshold values for the dispersive coatings employing the same high index material lay within a range of 30% of each other.

Dispersive mirror technology for ultrafast lasers in the range 220-4500 nm

Abstract Nowadays, dispersive mirrors are able to cover the wavelength range of 4.5 optical octaves and can be used from 220 nm up to 4500 nm. Various design approaches to dispersive mirrors in visible and near IR are briefly discussed. We consider in more detail two dispersive mirrors representing extreme cases. The first one is a mirror working in the range of 290−360 nm and providing group delay dispersion of -75 fs2. The second one is a mirror working in the range of 2500−4500 nm and providing +500 fs2 of group delay dispersion.

Reliable optical characterization of e-beam evaporated TiO2 films deposited at different substrate temperatures

We studied e-beam evaporated TiO2 films deposited at two different substrate temperatures between 120°C and 300°C. We reliably characterized the film samples on the basis of in situ and ex situ measurements. We carried out annealing on the samples and studied the induced changes in the properties of the films. The results can be useful for further laser-induced damage threshold investigations.

Ultrafast optical breakdown of multilayer thin-films at kHz and MHz repetition rates: a direct comparison

We report on the experimental study of optical breakdown induced in multilayer thin-films by ultrashort pulses at kHz and MHz repetition rates, while keeping all other parameters similar. The investigated samples were coatings composed of TiO2, Ta2O5, HfO2, or Al2O3 as high-index material and SiO2 as low-index material. We compared the distinct band gap dependencies obtained in the two regimes.

Towards CEP stabilized pulses from a KLM Yb:YAG thin-disk oscillator

The thin-disk (TD) technology allowed reaching unprecedentedly high average powers and high energies with sub-ps pulses directly from oscillators. Such oscillators can be considered as an extremely attractive alternative to Ti:Sa oscillators, the working horses of the ultrafast community. However, two more features are missing to make TD oscillators a suitable substitute to Ti:Sa oscillators: few cycle pulses (<;10 fs) and carrier-envelope phase (CEP) stabilization. The realization of these two properties would pave the way to applications in attosecond science and XUV frequency combs.

Resonance scanning interferometer for group delay dispersion measurements

Accurate measurements of group delay (GD) and group delay dispersion (GDD) characteristics play a crucial role in achieving top performance of ultra-fast optical systems based on dispersive mirrors (DM). Currently GD and GDD measurements are typically performed with white-light interferometers (WLI). However, the resolution and accuracy of WLI is not sufficient for many present-day challenging applications, especially in cases when ultra-broadband spectrum spanning over more than an optical octave is considered, or very high levels of dispersion need to be controlled. An alternative approach for GD and GDD measurements based on inter-mirror cavity resonances was developed. The idea to use setups similar to Fabry-Pérot and Gires-Tournois interferometers for this purpose was initially proposed in. In that work a single measurement with a fixed spacer thickness was performed. In order to increase the spectral resolution, it was necessary to increase the spacer thickness. However, that caused a rapid decrease of the measured signal level and soon the resonance peaks became masked by unavoidable noise. A “golden middle” spacer thickness often was unable to provide a good balance between resolution and signal-to-noise ratio. In contrast, this paper carried out measurements at a set of different spacer thicknesses, then the acquired data was combined together and processed using a regularization theory of the solution of inverse problems.

Measurements of the group delay dispersion with resonance scanning interferometer

We developed a Resonance Scanning Interferometer for group delay dispersion measurements based on inter-mirror spacer resonances. High resolution is achieved by simultaneous processing of measurement scans obtained for different spacer thicknesses.