Gabriele Emiliani

Optical coatings with variable reflectance for laser mirrors.

Design procedures and fabrication techniques of variable reflectance dielectric mirrors for laser output couplers are proposed. The design criteria, based on the use of only one shaped layer in a multilayer structure, yield any peak value and shape of the reflectance profile. The deposition technique of the variable thickness layer consists of a sputtering process with a suitable mask interposed between the target and substrate. Results of a simple model that correlates the mirror parameters to mask diameter and position are presented. A few devices for a Nd:YAG laser have been proposed, characterized, and successfully tested in a laser resonator.

Laser mirrors with variable reflected intensity and uniform phase shift: design process.

Optical coatings with circularly symmetric graded reflectance are used as laser mirrors in unstable resonators. A proper design of such coatings permits any maximum central reflectance to be obtained along with a null external reflectance. Different design approaches are discussed, and an optimized design that gives negligible distortion of the reflected and transmitted wave front is proposed. Coatings with super-Gaussian and step reflectance profiles are examined as two different solutions for improving laser beam quality.

Characterization and calibration of a variable-angle absolute reflectometer

The instrument described is a newly designed reflectometer. Measurements are made with a polarized light beam as a function of wavelength and angle of incidence. Calibration tests have outlined the possibility of using the instrument for reflectance measurements on mirrors with good reproducibility (0.2%) and accuracy (better than 1%). This has been obtained by the alignment system of the sample and the absolute method used for the determination of its reflectance.

All-oxide broadband antireflection coatings by reactive ion plating deposition

Coating experiments were made with reactive low-voltage ion plating to achieve the computer designed performance of all-oxide broadband antireflection systems on BK-7 glass. Reasonable accordance between calculated design and coating experiment as well as reliable production have been demonstrated.

Thin Film Refractive Index Determination By Different Techniques

The refractive index of some dielectric thin films has been determined by three different techniques. Spectrophotometry, ellipsometry and a technique based on the Abeles method are chosen for the analysis of the prepared samples in the visible spectrum. The first technique is the most widely used for obtaining a quick evaluation of the refractive index but, in order to get accurate results, a complex data processing is necessary. The second one is the most accurate but the experimental apparatus is rather complex and a skilled operator is generally required. The third technique is simpler than ellipsometry and a comparable accuracy on the refractive index determination can be obtained but only for transparent films. The relative advantages and limits of these techniques make each of them more or less convenient depending on the sample properties. Results are compared in areas where all three methods are successfully used.

Optical coatings for improving laser beam quality

Thin film coatings having a radially variable reflectance with a circular symmetry are used for obtaining diffraction limited laser beams from unstable resonators. Different approaches are used for the design of such coatings with a consequent different final coating structure that contains one or more profiled thickness layers. Some design methods are discussed with reference to reflected intensity and phase profiles and the fabrication techniques for coatings at different wavelengths are illustrated.

Recent improvements in PDS technique for low-absorption measurements

Photothermal Deflection Spectroscopy (PDS) is a recently developed technique that is finding a useful application in the measurement of low optical absorptance of thin films. Among the noise sources affecting the PDS measurement, probe beam pointing instability and mechanical vibration play a considerable role. In this work an optoelectronic system for the reduction of their influence is described. Moreover, PDS measurements are typically performed keeping the sample immersed in a deflecting liquid; thus measured values of absorptance must be corrected when other surrounding media, as air, are considered. This correction is an easy task for single film coatings. Here the general case of an unknown multiplayer coating is analysed; a range of values containing the true absorptance in air is obtained by theoretical analysis and a practical method to evaluate the absorptance in air is discussed. Finally, deflecting liquids alternative to the commonly used CCI4 have been examined. Useful optical range, thermal diffusivity and “relative deflecting power” of CCI4, CS2, Iso-octane and Aceton are reported.

Laser optical coatings with different reflectance profiles

Graded reflectance coatings are typically identified with optical coatings having a gaussian reflectance profile along the surface with circular symmetry. Indeed the first use of such coatings was as output couplers of gaussian laser resonators, then also super-gaussian reflectance profiles were considered. The variation of reflectance along the surface is obtained in most cases by a coating thickness variation, with an appropriate choice of both coating structure and thickness profile it is possible to obtain any maximum reflectance and any spatial behavior of the reflectance itself. New reflectance profiles that seem to be of interest are: graded reflectances with a central minimum, that is high transmittance at the coating center, and non- monotonic radial reflectance profiles as for example high reflectance on an annular area of the optical component. In addition, coatings with non-circular symmetry appear useful for particular laser applications. The methods for obtaining such reflectance profiles are discussed.

Thin film design program based on the flip-flop method with a random search

A method for the design of optical coatings is proposed in which the starting multilayers are made of very thin high and low index layers as the starting design of the flip-flop method. Many sequences of high and low index layers are generated randomly and a number of them are selected before starting the convergence process that is based on the times that each layer shows a high or low index within those sequences. Owing to the initial random choice, the dependence of the final result on the starting coating structure is avoided. Some typical problems are solved to test this method.