Ferran Laguarta

Optical glass polishing by controlled laser surface-heat treatment.

It is shown that optical surfaces traditionally ground in conventional glasses with high coefficients of thermal expansion may be polished by irradiation with a space- and time-controlled uniform CO(2) laser beam. Comparisons of a theoretical simulation model of the laser-driven heating process with the experimental results allow us to determine the conditions for successful and reliable use of this technique. The technique can be applied indiscriminately to preheated samples made of different glasses, with any topography, and, of any size in a limited range that depends only on the available laser power.

Laser application for optical glass polishing

Rapid generation of large area polished optical surfaces by high-power CO2 laser irradiation is shown. Results focus on glasses with high expansion coefficients (a>10 25 /°C) conventionally used in the op- tical industry. The technique involves active beam integration to obtain an intensity irradiation profile with a good uniformity over large spot sizes and is applied to preheated glass samples with initial rms roughness up to 500 nm. To find out the conditions for successful and reliable use of the proposed laser polishing method, the laser-driven heating process was monitored by means of the surface and depth temperature distribu- tions. Whereas the former was determined in situ from the IR radiation emitted by the glass surface, the latter was obtained by comparison of the IR radiation emitted by the bulk sample with a theoretical model. Laser polishing of 5000 mm 2 glass surfaces is reported and processes involved in the modification of the surface texture of the irradiated samples are described and discussed.

Dual-technology optical sensor head for 3D surface shape measurements on the micro- and nanoscales

New material applications and novel manufacturing processes are driving a systematic rise in market demands concerning surface inspection methods and the performance of non-contact profilers. However, analysis of the specifications and application notes of commercial optical profilers shows that no single system is able to offer all the features a general purpose user would like simultaneously. Whereas white light interferometers can achieve very fast measurements on the micro and nano-scale without any range limitation, they can not easily deal with steep smooth surfaces or structured samples containing dissimilar materials. PSI techniques allow the user to perform shape and texture measurements even below the 0.1 nm scale, but they have an extremely short measurement range. Imaging confocal profilers overcome most of these difficulties. They provide the best lateral resolution achievable with an optical profiler, but they have a resolution limit, which is dependent on the NA and cannot achieve the 0.1 nm vertical resolution. In this paper we introduce a new dual-technology (confocal & interferometer) illumination hardware setup. With this new sensor head it is possible to choose between standard microscope imaging, confocal imaging, confocal profiling, PSI and white light interferometry, by simply placing the right objective on the revolving nosepiece.

Design of an interferometric system for the measurement of phasing errors in segmented mirrors

One of the new problems that has to be solved for segmented mirrors is related to periodic phasing, because for such mirrors to exhibit diffraction-limited performance the segments have to be positioned with an accuracy of a fraction of a wavelength. We describe the optical design of an instrument that measures the phasing errors (i.e., tip, tilt, and piston) between two segments under daylight conditions. Its design is based on a high-aperture white-light Michelson interferometer. It was developed at the Center for Sensors, Instruments and Systems Development (CD6) of the Technical University of Catalunya, Spain, and its final testing was carried out on the Gran Telescopio Canarias test workbench.

Three-dimensional micromeasurements on smooth and rough surfaces with a new confocal optical profiler

One of the objectives of surface metrology is to obtain a better and faster assessment of the micro- or nanogeometry of component surfaces. In this way the innovative concept of the profiler is changing towards non-contact modular computer- controlled systems for measuring and analyzing shape and texture of a surface. In this paper we present a new instrument which is based on the concept of confocal microscopy. In this instrument (which may be used for measurements on smooth and rough surfaces) a pattern of slits is imaged by a very high numerical aperture optical system on the surface of the sample to be measured. The reflected or diffused light is observed with a CCD array and analyzed with different digital image processing algorithms. In addition to the replacement of the existing stylus systems there are also important new potential applications for this type of instrument. We present the results obtained in micro- or nanomeasurements of high precision optical surfaces, texture assessment of non-homogeneous liquid depositions and metrology of microstructures such as master gratings and certified calibration standards. The obtained results show that the confocal profiler is robust enough to provide a surface topography with spatial resolution lower than 0.5 micrometer and uncertainty of about 10 nm.

Development of confocal-based techniques for shape measurements on structured surfaces containing dissimilar materials

One of the applications, which is considered to be very difficult to carry out with most optical imaging profilers, is the shape and texture measurements of structured surfaces obtained from the superposition of various micro or sub-micrometric layers of dissimilar materials. Typical examples are the architectures of microelectronics samples made up of Si, SiO2, Si3N4, photoresists and metal layers. Because of the very different values of the index of refraction of the involved materials, visible light is reflected in the various interfaces. As a result, some reflected wavefronts are superposed giving rise to interference patterns, which are difficult to understand in terms of surface topography and layer thickness. In this paper we introduce a new method based on non-contact confocal techniques to measure the shape of structured samples. The method is based on the comparison of the axial responses obtained in areas of the surface where there is a layer and in other areas where there is just the substrate. To our knowledge, this approach enables the confocal profilers to measure the thickness of layers on the sub-micrometric scale for the first time.

Two-faceted mirror for active integration of coherent high-power laser beams.

A new integration method suited for spatially coherent high-power laser beams is demonstrated. The integrator system is based on a mirror with two facets, one of which can vibrate under the action of a piezoelectric translator. After reflection in the faceted mirror, the beam intensity distribution is modified to obtain greater uniformity. However, because of the coherence of the reflected beamlets, this distribution is affected by an interference pattern. The active integration consists of a periodic displacement of the moving facet that causes the interference pattern to vibrate, and its contribution to the intensity profile therefore averages out (fringe visibility within a 5% range). The combination of a faceted mirror and a simple imaging system results in an intensity profile with good uniformity over large spot sizes. Both simulated and experimental results are presented, the latter showing that a final uniformity within a 10% range can be achieved and it is limited mainly by diffraction at the edges of the facets.

Non-contact measurement of aspherical and freeform optics with a new confocal tracking profiler

In this paper we introduce a new optical technique for the measurement of aspheric and free-form optics and moulds. This technique, called confocal tracking, consists on tracking the focus on the sample while it is moved along the horizontal XY axes. Unlike all single-point based techniques, confocal tracking images the surface, which makes it possible to determine the best in focus position within every field of view and to correct the residual tracking errors for each measured point. As a result, confocal tracking provides shape measurements with nm-level accuracy and acquisition speeds of 1 mm/s typically. Depending on sample geometry, high NA objectives can be used, with which it is possible to measure slopes as high as 65°. In addition, because confocal tracking is not a single-point but an imaging technique, it is possible to center the surface to be measured with a very quick procedure that can be automated easily. This step may be particularly relevant for optics with symmetry of revolution. The confocal tracking profiler is a proprietary technology of UPC and Sensofar and can be considered the optical equivalent of a high-accuracy contact profiler.

New interferometric technique for piston measurement in segmented mirrors

Present trends in the design of ground-based telescopes point towards the use of segmented primary mirrors. A major problem in this type of mirror is the achievement of proper segment positioning, as they have to be aligned with an accuracy of the order of a fraction of a wavelength for near-diffraction-limit telescope performance in the infrared. In this paper we present a new interferometric technique applied to the measurement of segment vertical misalignment (piston error) in segmented mirrors. The instrument is based on a high-aperture Michelson interferometer using a broadband light spectrum. The main innovation introduced in this instrument is the use of a novel optical fibre illumination technique that allows the system to measure piston error during the daytime with an uncertainty of 5 nm in a 30 µm range. A detailed description of the light spectrum, expected interferograms and piston extraction algorithms is presented here.

Surface dynamics during laser polishing of glass

Laser heating of glass samples is a simple and versatile method for obtaining polished surfaces of optical quality. Since laser beam intensity non-uniformity can translate into significant variations in the induced surface temperature, the success of the laser surface-polishing process strongly depends on obtaining uniform intensity profiles or flat-top distributions at the sample plane. In this paper we present a comparison between large-area CO2 laser-polishing experiments carried out in optical glass substrates following two different approaches: (1) A reshaped beam obtained by an active integration method is swept over the glass surface. (2) A static beam reshaped by means of both a multifaceted mirror and a square pipe light guide is applied.