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Characterization of the laser induced damage threshold of mirrors in ...

Characterization of the laser induced damage threshold of mirrors in ...

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List of Figures 1.1 A 40 cm x 40 cm wedged lens used at NIF.[1] . . . . . . . . . . . . . . . . 2 2.1 An example measurement for a single B4C layer on fused silica substrate. These data were acquired by positioning a photo diode close to the focus position on the sample and measuring the back scattered light as a function of the peak fluence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Pulse width dependence of threshold fluence for fused silica.[2] . . . . . . . 5 2.3 Schematic diagram of the photo excitation of an electron in an atomic potential for different values of the Keldysh parameter. The tilting of the potential is due to an external electric field in the indicated direction, caused by the incident light. From ref [3] . . . . . . . . . . . . . . . . . . 7 2.4 Schematic diagram of avalanche ionization. An initially free electron se- quentially absorbs several laser photons through free-carrier absorption, then impact ionizes another electron. From ref. [3] . . . . . . . . . . . . . . 8 2.5 Laser induced damage threshold of a T iO2 single versus the wavelength. At 680nm, a step of the LIDT towards three photon absorption is evident. From ref. [4]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.6 Qualitative comparison of typical fs damage and ns damage. Both pictures were taken with the same microscope with the same illumination. The left picture shows a damage site created with our setup on a HR Mirror, the dimensions of the image correspond to 180x135µm. The right picture shows damage from an old output coupler of an high power ns-system, the dimensions of the image correspond to 1600x1400 µm. . . . . . . . . . . . 10 2.7 An example fit of the presented model to experimental data. [5] . . . . . . 11 2.8 Damage threshold fluence as a function of the number of illumination pulses. The solid line is the result of simulations using a similar model to equations 2.9 and 2.10. The measured material was a single T a2O5 film deposited on fused silica. The repetition rate was 1kHz. [6] . . . . . . . . . 13 iv

2.9 Damage threshold fluence as a function of pulse repetition rate a)for 100 pulses with a duration of 30 fs on a T a2O5 single layer (d = 2 · λ) on fused 4 silica substrate[6] b) 1000 pulses with a duration of 130 fs on a multilayer T a2O5 / SiO2 high reflector (each layer, d = λ). [7] . . . . . . . . . . . . . 14 4 2.10 Wavelength dependence of the breakdown threshold for wide band gap materials. Solid lines show general trend as guide for the eye. [8] . . . . . . 15 2.11 Dependence of the laser ablation threshold on the beam radius measured with 1000 pulses of 30 fs at 1 kHz. Fit according to the presented model. [9] 17 2.12 a) Breakdown fluence as a function of the pulse duration for different ma- terials [10] and b) Sample parameters. The increasing damage threshold for higher bandgap materials is evident. . . . . . . . . . . . . . . . . . . . . 18 2.13 Damage threshold for fused silica from various groups. [11] . . . . . . . . . 18 3.1 Schematic layout of the relevant parts of the PFS front end. BS: beam splitter, L: lens, M: mirror, HDM : high dispersive (negatively chirped) mirrors. Abbreviated from [12]. . . . . . . . . . . . . . . . . . . . . . . . . 19 3.2 Normalized spectrum of the pulses delivered by the PFS front end, mea- sured with the beam focused into an Avantes CC-VIS/NIR cosine corrector. 20 3.3 FROG-trace taken at the position where the beam enters the damage threshold setup, corresponding to a pulse duration 59 fs. Done with a commercial Swamp Optics GRENOUILLE. . . . . . . . . . . . . . . . . . . 21 3.4 Schematic diagram of the undamaged (a) and damaged (b) case. The change of the surface topology in the second case leads to a strong increase in the backscattered light. The thick solid black line indicates the incoming and reflected laser beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.5 Schematic layout of the damage threshold setup. BS: beam splitter, F: variable neutral density filter wheel, L: focusing lens, PD1: power diode, PD2: scattered light detection diode, TS: translation stage with sample mount. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.6 Example focus characterization measurement. The two curves indicate two perpendicular axes of the beam showing a clearly different focus position and a different minimal beamwidth. The position is given relative to the translation stage, the beam profiling camera is mounted on. The error bars show the standard deviation of the average. . . . . . . . . . . . . . . . . . 24 3.7 a) Two-dimensional picture of the approximately round focus, the two lines are indicating the two main axes. b) Three-dimensional plot of the same focus. Both pictures were taken with a WinCamD-UCD23 beam profiling camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 v

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