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Optical Coatings for High Power Lasers
Published in Marvin J. Weber, and TECHNOLOGY, 2020
Mark R. Kozlowski, Robert Chow, Ian M. Thomas
In general the damage threshold is defined as the lowest fluence at which damage will occur. The damage threshold can be based on the mean fluence between the lowest damaging fluence and the next lowest nondamaging fluence. The difference in fluence between the two levels determining the threshold should not exceed the experimental accuracy range of the damage measurement, typically ±15%. In many cases there is not a sharp transition between damaging and nondamaging fluences. In such cases a more statistical determination of the threshold may be preferred. In one such method, several sites (typically 10) are exposed to the same fluence and the percentage that damage at a given fluence is noted. The sample is tested at enough fluences for a plot of damage probability versus fluence to be made. The threshold is defined as the extrapolation, generally by a linear fit, to zero damage probability.29
Damage theory
Published in Roger M Wood, Laser-Induced Damage of Optical Materials, 2003
Laser-induced damage may arise from single mechanisms or from a number of the above mechanisms acting in concert. There is no one value for the damage threshold for any particular material irrespective of the wavelength, pulse duration, beam size or shape. It should also be recognized that at the micrometre level all samples of material may be slightly different and that these differences may affect the measured LIDTs. Damage thresholds are therefore component specific and it is not enough to state that the damage threshold is x MW cm−2 (or any other unit) without specifying the wavelength, pulse duration and the test spot dimensions (Wood 1997).
Colourful 3D anti-counterfeiting label using nanoscale additive manufacturing
Published in Virtual and Physical Prototyping, 2023
Sida Peng, Shengzhi Sun, Yi Zhu, Jianrong Qiu, Huayong Yang
The printed microstructures were characterised by an optical microscope (VHX-5000, Keyence) and a field emission scanning electron microscopy (SEM, HITACHI SU-70) at an accelerating voltage of 3.0 kV. The absorption spectra of photoresists were obtained with an ultraviolet spectrophotometer (Lambda 950, PerkinElmer). Micro-region fluorescent spectra were explored by an optical microscope (Hangzhou SPL Photonics Co., Ltd) under excitation with a 366 nm semiconductor CW laser source. The fluorescence was collected by a 20× objective lens and coupled to a fibre optic spectrometer (QEPro, Ocean Optics). During the measurement, the ultraviolet pump intensity was set below the damage threshold of the printed samples. The internal quantum efficiency was measured by UV–NIR absolute PL quantum yield spectrometer (Quantaurus-QY Plus, Hamamatsu Photonics, Japan). The fluorescent images of 2D patterns were taken by a commercial microscope (DM 2500, Leica) with an objective lens (100×, NA = 1.30, Leica) under excitation with UV light. 3D fluorescent patterns were taken by a commercial inverted microscope (LSM880 AxioObserver, Carl Zeiss) with an oil-immersion objective (63×, NA = 1.40, Carl Zeiss). The depth of focus is around 0.40 μm for each z-layer. The samples were excited by semiconductor CW lasers with different wavelengths (405 nm for the blue channel, 488 nm for the green channel and 561 nm for the red channel).
Numerical study of length and angle gauging for subsurface-inclined cracks based on reflected surface waves with laser ultrasonic technique
Published in Nondestructive Testing and Evaluation, 2023
Chuanyong Wang, Daxing Yang, Keqing Lu, Wen Wang, Zhanfeng Chen, Wule Zhu, Bing-Feng Ju
There are two main mechanisms to generate ultrasonic waves using pulsed lasers, the thermalelastic mechanism and ablation mechanism, depending on whether it causes damage to the material surface [24]. When the low power pulse laser irradiates the material surface, the material absorbs the laser energy and converts it into heat energy, resulting in a temperature gradient and rapid thermal expansion in the local area to excite ultrasonic waves. This is the thermoelastic mechanism, which will not cause damage to the material surface. When the pulsed laser power exceeds the material damage threshold, it will cause material surface melting, gasification, plasma and other phenomena. The ablation and sputtering of the material surface will cause a recoil force on the material surface to excite ultrasonic waves, which is the ablation mechanism. The ablation mechanism will cause some damage to the material surface, which is not a real sense of non-destructive testing. The study in the paper is based on non-destructive testing; therefore, the energy density of the pulsed laser needs to be controlled to ensure that the laser ultrasound is excited by the thermoelastic mechanism. The laser energy density can be written as:
Hot carrier effects on Brillouin amplification in AIIIBV and AIIBVI semiconductors
Published in Journal of Modern Optics, 2022
Optical fibres have been extensively used as a nonlinear medium for Brillouin amplification to explore the optical parameters in a wide variety of experiments [7,12]. Important parameters affecting the selection of a Brillouin medium for a given application include the pumping conditions, the amplification coefficient, the transmitted intensity, transmitted wavelength, the optical damage threshold, size constraints, etc. Although Brillouin media in the same state of matter have similar characteristics in many cases, the nonlinear optical properties of a given material under different circumstances can vary widely [13]. Therefore, it is important to select the appropriate Brillouin medium according to specific requirements. Depending on the different types of materials adopted, different designs for the Brillouin cell are desirable. Hence, in addition to optical fibres, the Brillouin medium can also be a bulk crystal, a cell with a gas or liquid medium, or a waveguide in a photonic integrated circuit.