Abstract
The nanosecond laser-induced damage growth phenomenon on the exit surface of fused silica grating is investigated at 1064 nm and 355 nm separately and also simultaneously. Experiments are first carried out on damage sites on a plane fused silica sample showing two different morphologies, and a damage type is selected for ensuring the repeatability of the subsequent tests. Comparing the mono-wavelength growth results on a grating and a plane fused silica sample, the periodic surface structure is found to be an aggravating factor for damage growth. This is highly supported by calculations of the enhancement of the optical electric field intensity thanks to Finite-Difference Time-Domain simulations. Finally, the mono-wavelength results enable us to quantify a coupling occurring in the multi-wavelength configuration, which could originate from the heating of the plasma (more likely produced in the ultraviolet) preferentially by the infrared pulse. This study provides interesting results about the involvement of the surface topography in damage growth, and paves the way towards the comprehension of this phenomenon at high-energy nanosecond laser facilities where fused silica gratings are simultaneously irradiated at several wavelengths.
Highlights
Laser-induced damage (LID) phenomenon is almost as old as the invention of the laser[1], efforts are still required for fully understanding this issue
Prior to determine the growth behaviour on grating in the mono- and the multi-wavelength configurations, we evaluate the impact of the initial damage morphology on the growth results on the exit surface of a plane fused silica sample
Since the employed pulses show multiple longitudinal modes and strong temporal modulations, the morphology of the damage sites initiated at 1ω on the exit surface of fused silica systematically exhibits a typical ring structure Fig. 1(a), originating from the expansion of a plasma driven by the intensity spikes[26]
Summary
Laser-induced damage (LID) phenomenon is almost as old as the invention of the laser[1], efforts are still required for fully understanding this issue. In the particular case of fused silica optics, LID sites initiated by parallel beams are preferentially located on the exit surface of the components[5,6], and their size may exponentially increase after successive irradiations[7]. This latter phenomenon known as “LID growth” mainly originates from the absorption of the laser flux by subsurface cracks under a mechanically modified material that may reignite the damage process[8], leading to an expansion of these cracks as well as a material ejection induced by shock waves[9]. The physical mechanisms that may be involved in the experiments are discussed
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