This paper presents the optical and lens design for alignment to meet the challenging position specifications. Fabrication
of the prototypes and testing results and analysis are also presented. The system components as well as their interaction
with each other were simulated with Zemax software and tested in an experimental setup in order to conduct tolerance
study and provide specifications for the mechanical fixtures used in the system. The epoxy is used to affix the parts
together in a cost effective manner for prototyping. The position accuracy of ±3 μm compared to the golden unit has
been achieved.
KEYWORDS: Laser beam diagnostics, Diffraction, Sensors, Near field diffraction, Refractive index, Physics, Pulsed laser operation, Near field optics, Near field, Temperature metrology
Based on the mechanisms of sequential two-photon absorption in biological molecules, a theoretical model is proposed for the absorption of light in melanin. To test this model, melanin samples were studied using the collinear photothermal deflection spectroscopy (PDS), with an He-Ne laser and an Argon-Krypton ion laser to provide two simultaneously exciting wavelengths. It is found that melanin samples have a strong decay in amplitude of the photothermal signal, in contrast to a reference India ink whose photothermal signal is stable. The power dependence of the amplitude of the photothermal signal is also found to exhibit a departure from linearity at higher power densities for a melanin sample. Simultaneous exposure of the same sample to He-Ne and Argon-Krypton ion laser irradiation produces a signal which is significantly less than that expected from the sum of the signals obtained from using each laser alone. These results are explained with a modification of the sequential two-photon absorption process, which involves a conversion of the photon energy to a non- thermal form such as photochemical energy.
Multilayer optical thin film coatings are typically based on quarterwave design using alternating high- and low-index materials. The standing wave within these multilayer coatings introduces peaks of electrical field at the interfaces between the high- and low-index materials, which makes the interfaces potentially weaker parts in terms of laser damage resistance. The possible interfacial absorption and thermal resistance will further weaken these interfaces and make them prone to laser induced damage. In this paper a numerical model is developed for analyzing the interfacial effects on the transient temperature rise of multilayer coatings induced by a short pulse laser irradiation. The parameters studied include coating design, thermal physical properties of thin films, and interfacial absorption and thermal resistance. It is found that by choosing an appropriate coating design and eliminating the interfacial effects, the laser induced peak temperature rise of multilayer coatings can be drastically reduced. Examples are presented for both quarterwave and non-quarterwave HfO2/SiO2 high reflectivity coatings.
The surface thermal lensing technique (STL) successfully resolved and measured the absorptance of transmissive optical components: near-normal angle-of-incidence anti-reflectors and beam splitters. The STL system uses an Ar ion laser to pump the components at 514.5 nm. The absorptance-induced surface deformation diffracts the HeNe probe beam into a photo- detector. The signal intensity was calibrated with a sample of known absorptance. The optical components were designed to function in a copper vapor laser (CVL) transport system, and were previously tested for absorptance with a high power CVL system at 511 nm. To assure proper absorptance data from the STL system, the pump laser power densities were set at the operational level of the coatings, absorptance time trends were monitored, and absorptance area scans were made. Both types of transmissive optics are more stable than the CVL high reflectors that were measured in another study. Parameter studies based on Fresnel diffraction theory were also performed to optimize experimental condition. The STL system was assessed to have 10 ppb sensitivity for absorption measurement given 2 W of pump power.
KEYWORDS: Diffraction, Near field, Spectroscopy, Laser beam diagnostics, Laser optics, Laser applications, Absorption, Geometrical optics, Near field optics, Near field diffraction
Laser-induced thermal waves are widely used for materials research and other applications. For low absorption optical materials the thermal waves are typically detected by collinear photothermal deflection spectroscopy (CPDS). The available theory of CPDS is based on the approximation of geometric optics, which, while being straightforward and having been extensively used, has obvious shortcomings. For example, to calculate the signal of CPDS, the probe beam size is assumed to be infinitely small. In real applications, however, the probe beam always has a finite dimension, which results in a diffraction effect and leads to unexpected experimental error. In order to overcome such shortcomings, there is a need to develop a more sophisticated theoretical model that can encompass both photothermal deflection and diffraction effects, and give precise predictions of the nature of both effects. In this paper we present a comprehensive diffraction theory suitable for treating the effect of pulsed CPDS. The work is an extension of the theoretical model previously developed for the mirage effect. It provides a detailed theoretical analysis of the laser-induced optical diffraction effect and helps to optimize the experimental parameters. It is a useful tool not only for quantitatively describing CPDS but also for the traditional thermal lens spectroscopy. With the application of the Fresnel diffraction theory, both techniques can be treated in a unified model. A summary of the theoretical modeling and experimental studies will be presented, with an emphasis on the advantages of using a near-field detection scheme for achieving the best sensitivity and stability.
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