Ultra-short laser applications require high quality dielectric optics. The natural dispersion of light needs to be matched by dielectric components. However such dispersive components are very challenging for the deposition process and are characterized by high field intensities inside the layer stack. Such layers are expected to diminish the possible laser induced damage thresholds (LIDTs) because of their low optical gap value for suitable high refractive index materials. This paper reports about the manufacturing of amorphous nanolaminates to tune the optical gap. Such sequences are substituted into a conventional high reflective mirror to decrease the electric field of binary Tantala layers by 30 % which correlates to an improvement in LIDT of almost 16%.
The modeling of the laser-induced damage processes can be divided into thermal and electronic processes. Especially, electronic damage seems to be well understood. In corresponding models, the damage threshold is linked to the excitation of valence electrons into the conduction band, and often the damage is obtained if a critical density of free electrons is exceeded. For the modeling of the electronic excitation, rate equation models are applied which can vary in the different terms representing different excitation channels. According to the current state of the art, photoionization and avalanche ionization contribute the major part to the ionization process, and consequently the determination of laser-induced damage thresholds is based on the calculation of the respective terms. For the theoretical description of both, well established models are available. For the quantitative calculation of the photoionization, the Keldysh theory is used most frequently, and for the avalanche processes the Drude theory is often applied. Both, Drude and Keldysh theory calculations depend on the laser frequency and use a monochromatic approach. For most applications the monochromatic description matches very well with the experimental findings, but in the range of few-cycle pulses the necessary broadening of the laser emission spectrum leads to high uncertainty for the calculation. In this paper, a novel polychromatic approach is presented including photo- and avalanche ionization as well as the critical electron density. The simulation combines different ionization channels in a Monte-Carlo procedure according to the frequency distribution of the spectrum. The resulting influence on the wavelength and material dependency is discussed in detail for various pulse shapes and pulse durations. The main focus of the investigation is concentrated on the specific characteristics in the dispersion and material dependency of the laser-induced damage threshold respecting the polychromatic characteristics of the ultra-short pulse (USP) laser damage.
The present contribution is addressed to an improved method to fabricate dielectric dispersive compensating mirrors (CMs) with an increased laser induced damage threshold (LIDT) by the use of ternary composite layers. Taking advantage of a novel in-situ phase monitor system, it is possible to control the sensitive deposition process more precisely. The study is initiated by a design synthesis, to achieve optimum reflection and GDD values for a conventional high low stack (HL)n. Afterwards the field intensity is analyzed, and layers affected by highest electric field intensities are exchanged by ternary composites of Ta<sub>x</sub>Si<sub>y</sub>O<sub>z</sub>. Both designs have similar target specifications whereby one design is using ternary composites and the other one is distinguished by a (HL)<sup>n</sup>. The first layers of the stack are switched applying in-situ optical broad band monitoring in conjunction with a forward re-optimization algorithm, which also manipulates the layers remaining for deposition at each switching event. To accomplish the demanded GDD-spectra, the last layers are controlled by a novel in-situ white light interferometer operating in the infrared spectral range. Finally the CMs are measured in a 10.000 on 1 procedure according to ISO 21254 applying pulses with a duration of 130 fs at a central wavelength of 775 nm to determine the laser induced damage threshold.
New ultrashort pulse laser systems exhibit an ever increasing performance which includes shorter pulses and higher
pulse energies. Optical components used in these systems are facing increasing requirements regarding their durability,
and therefore understanding of the damage mechanism is crucial. In the ultra-short pulse regime electron ionization
processes control the damage mechanisms. For the single wavelength, single pulse regime the Keldysh  and the Drude
model  allow a quantitative description of these ionization processes. However, in this model, the electrical field is
restricted to a single wavelength, and therefore it cannot be applied in the case of irradiation with two pulses at different
wavelengths. As frequency conversion is becoming more common in ultra-short pulse applications, further research is
needed in this field to predict the damage resistance of optical components. We investigate the damage behavior of high
reflective mirrors made of different metal oxide materials under simultaneous exposure to ultra-short pulses at the
wavelengths 387.5 nm and 775 nm, respectively.
In the femtosecond regime laser damage thresholds are determined by the electric field distribution within the optical components. Especially, for radiation sources with integrated frequency conversion the simultaneous presence of photons with different frequencies introduces additional ionization channels in optical materials by cross excitation and other effects. In this work we report on the pulse delay dependency of the LIDT of HR390/780nm mirrors under simultaneous exposure to fundamental and second harmonic femtosecond radiation. We perform Son1-tests according to ISO 21254 with the addition of a second harmonic pulse at different fixed pulse energies. To determine the influence of the cross excitation between fundamental and second harmonic radiation, these tests are repeated for different time delays between the two pulses. For the 1on1, single wavelength femtosecond LIDT testing, the Keldysh theory in combination with the Drude Model has been proven to reasonably describe the time dependent electron density in the conduction band, and hence the LIDT. We extend these approaches to the determination of the LIDT for the case of simultaneous interactions of photons of two separate wavelengths.
As a consequence of the statistical nature of laser-induced damage threshold measurements in the nanosecond regime,
the evaluation method plays a vital role. Within the test procedure outlined in the corresponding ISO standard, several
steps of data reduction are required, and the resulting damage probability distribution as a function of laser fluence needs
to be fitted either based on an empirical regression function or described by models for the respective damage
Advanced optical thin film design is the key to increase laser durability significantly: either by optimizing the electric field distribution within the coating, or by multi-index or rugate designs. Both ways may be even combined. <p> </p>The electric field distribution within a thin film stack was optimized to avoid peak intensities in critical layers using refractive index engineering and/or layer thickness grading. Femtosecond laser mirrors and dichroics for 780 nm and 390 nm were designed, realized and characterized. Here we present LIDT measurements of electric field optimized mirrors and dichroics, which are almost a factor of three higher compared to standard coating designs. At 780 nm a LIDT of 1.49 J/cm<sup>2</sup> has been achieved and at 390 nm 0.58 J/cm<sup>2</sup>. With the exception of Al<sub>2</sub>O<sub>3</sub>, all investigated coating materials show a proportional dependence of the LIDT with electric field maximum, as expected by theory. For Al<sub>2</sub>O<sub>3</sub> based systems the electrical field penetrates deep into the layer stack, a high number of interfaces are involved and interface effects probably limit the achievable LIDT. A similar effect was observed for rugate designs. To exclude such interface effects from the LIDT measurement, a special AR design was developed, which is practically equal for all high index materials. Here a LIDT above substrate damage threshold of 1.7 J/cm<sup>2</sup> was achieved.
In the femtosecond regime laser damage thresholds are often determined by the electric field distribution within the
optical component. Commercially available ultra-short pulse laser systems provide ever increasing output powers in
fundamental and harmonic wavelengths. Therefore, an increasing demand for frequency conversion or multiwavelengths
optics with high damage thresholds for both, fundamental and second harmonic wavelengths is given.
These optics are under increased strain and face even more design difficulties. Also, the electric field distribution is of
higher complexity and favors multi-photon excitation of high efficiencies.
We investigate the LIDT of dichroic high reflecting mirrors under simultaneous exposure to fundamental and second
harmonic radiation. As laser source we use a Ti:Sa system delivering sub 200 fs pulses at 780nm/390nm. A delay-line
was incorporated to ensure temporal overlap of the 2 pulses in the test plane. Further, the LIDT of a single layer of Ta<sub>2</sub>O<sub>5</sub>
under irradiation with fundamental and second harmonic radiation is calculated and results are compared with our