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

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

Chapter 2 Femtosecond

Chapter 2 Femtosecond Damage Threshold 2.1 Definition of optical damage, and damage threshold In the common definition, optical damage is a permanent impairment of the optical properties of person or property after irradiation.[22, 23] Permanent means in this context that the lifetime of the modification is longer then a reasonable observation period, which is usually in the order of hours. Figure 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. Now for the detection of optical damage the optical parameters of a given material have to be monitored. As it is a technical challenge to do pulse resolved measurements of spectral reflectivity, spectral transmission, spectral- and angular resolved scattering behavior at once, more practical damage definitions have been used for the determination of the damage threshold. Starting from monitoring surface modifications 4

observable under a visible light microscope [24, 8], various techniques have been applied to identify damage. To give a short overview: Difference interference contrast microscopy 1 [27, 2], plasma emission from the focal region [28], evaluation of the ablation crater geometry [9, 7], evaluation of the ablated volume [29] and finally a change in the scattering behavior.[23, 30, 31, 32, 33] Figure 2.1 shows that the transition between the undamaged and the damaged state can be quite sharp when using backscattering detection, which makes it easy to define the actual damage event. The next question is which physical quantity should be used to identify the threshold. As discussed later, the responsible mechanisms are correlated with the intensity of the beam on the sample [34, 35, 3], thus it would be straight forward to state a damage threshold intensity. Strangely enough it is common in literature to publish threshold fluences. This may have historical reasons, as already the very first papers about laser damage on dielectric surfaces stated damage fluences[24]. Anyhow, neither the peak intensity nor the peak fluence contains enough information about the incident pulse to fully characterize it with respect to the damage behavior. The temporal shape of the pulse is quite important as can be seen on figure 2.2 and given a temporal Gaussian pulse shape and a known pulse duration it’s arbitrary to state peak fluence or peak intensity. As for our case of sub 100 fs pulses the dependence of the damage fluence on the pulse width becomes small [2] and in order to have results comparable to literature data, we use the peak fluence convention for our measurements. Figure 2.2: Pulse width dependence of threshold fluence for fused silica.[2] The international standard (ISO11254 [36]) defines conditions, procedure and detection 1 for a short description of DIC, also known as Nomarski microscopy, see [25, 26] 5

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