A modified photothermal deformation technique is used to measure the absorptance behaviors of optical multilayered dielectric coatings for a high-power laser system. The surface thermal-lensing modification uses an enlarged probe beam to facilitate alignment of the laser beam and data acquisition. The coatings, both reflective and transmissive types, are made by a physical vapor-deposition process. Coating absorptances are observed to depend on the laser's exposure time and power density. Time-dependent absorptance defect models are proposed. Also, micrometer-sized sites of high absorptance and an area with physical damage can be found during the spatial scans. It is proposed that absorptance values reported for coatings in high-repetition-rate or cw-laser systems include time- and power-dependent behaviors in addition to other relevant irradiation parameters.
The majority of the layers of a multilayer optical coating typically have optical thicknesses equal to one quarter of the appropriate wavelength of radiation. Replacing this constraint with the stipulation that a pair of adjacent layers should have a total optical thickness of one half of a wavelength introduces a significant new component of design flexibility while having a minimal impact upon the desired optical properties of the film. Taking a matrix approach, we derive a general expression for the reflectance of a periodic thin-film structure that is based on layers of two different materials of arbitrary thickness. This result is applied to highly reflective coatings at normal incidence and to off-normal polarizing coatings. Specific results involving HfO(2)/SiO(2) films and TiO(2)/SiO(2) films are displayed. We discuss how the thickness of the high-index layers may be reduced to increase damage thresholds. We also show a mirror design that is effective not only at lambda = 1.06 mum but also at the frequency-doubled wavelength.
Laser Doppler vibrometer can measure the displacement, velocity, acceleration and other parameters of vibration target. It has the characteristics of non-contact, high precision and long distance. So, it has a great advantage for the vibration measurement in a special working environment, where the target is light and thin, hard to contact, hard to approach. Laser heterodyne interferometry is an important means of detecting the micro vibration. With the development of micro vibration application, the sensitivity of phase measurement is highly required. Traditionally, there are several ways of improving the measurement sensitivity, such as optimizing the heterodyne interference scheme, improving the phase reconstruction algorithm and reducing the noise of key devices and so on. However, based on the analysis of the influence of stray light in the system, it is found that the controllable multi-beam interference can greatly improve the detection capability of the system. Therefore, a phase enhancement technique of multi-beam hybrid interference is proposed to meet the needs of high sensitivity detection of micro vibration. In this paper the physical mechanism and boundary conditions of phase enhancement are investigated in detail, and the quantitative relationship between the boundary conditions and phase enhancement is also analyzed thereby providing a technical reference for the enhancement detection of micro vibration targets. Through the numerical simulation and experimental verification, the following boundary conditions are obtained: the initial phase difference between the correction light and the signal light is π rad and the closer the power values of the two beams, the greater the enhancement effect of the demodulation phase is. The power difference between the two beams designed in the experiment is 1%, which means that detection capability is enhanced by 146 times. It has great application value in the high sensitivity measurement of micro vibration objects. This technology can also enhance the detection capability of heterodyne interference measurement system without changing the existing device index or phase demodulation algorithm.
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