Daniel Aguirre-Aguirre

Phase shifting interferometry using a spatial light modulator to measure optical thin films

This work describes a process for measuring thin film steps, using phase shifting interferometry (PSI). The phase shifts are applied only in the region where the thin film steps are located. The phase shift is achieved by displaying different gray levels on a spatial light modulator (SLM Holoeye LC2012) placed in one arm of a Twyman-Green (T-G) interferometer. Before measuring the thin film steps, it was necessary to quantify the phase shifts achieved with this SLM by measuring the fringe shifts in experimental interferograms. The phase shifts observed in the interference patterns were produced by displaying the different gray levels on the SLM one by one, from 0 to 255. The experimental interferograms and the thicknesses of the thin film steps were successfully quantified, proving that this method can be used to measure thin films by applying the PSI method only on the region occupied by them.

Algorithm for Ronchigram recovery with random aberrations coefficients

Abstract. We present the validation for Ronchigram recovery with the random aberrations coefficients (ReRRCA) algorithm. This algorithm was proposed to obtain the wavefront aberrations of synthetic Ronchigrams, using only one Ronchigram without the need for polynomial fits or trapezoidal integrations. The validation is performed by simulating different types of Ronchigrams for on-axis and off-axis surfaces. In order to validate the proposed analysis, the polynomial aberration coefficients that were used to generate the simulated Ronchigrams were retrieved. Therefore, it was verified that the coefficients correspond to the retrieved ones by the algorithm. The results show that the ReRRCA algorithm retrieves the aberration coefficients from the analyzed Ronchigram with a maximum error of 9%.

Substructured Ronchi gratings from the linear combination of classical gratings

Abstract. Substructured Ronchi gratings are used to sharpen and increase the number of fringes in Ronchigrams, thereby increasing their spatial resolution and allowing greater accuracy in the evaluation of a surface under test. This work presents a simple method for generating substructured Ronchi gratings and for calculating the intensity pattern produced by this type of grating. For this, we propose the generation of this kind of grating from the linear combination of classical gratings; the pattern of irradiance produced by these Ronchi gratings will be a linear combination of the intensity patterns produced by each combined classical grating. A comparison between theoretical and experimental Ronchigrams was obtained by analyzing a parabolic mirror.

Optical Testing and Finite Element Analysis Applied in Stressed Mirror Polishing

In this work an off-axis conical mirror is fabricated by stressed mirror polishing; the fabrication process is monitoring by using the interferometrical developed theory and the deformation accuracy by means of finite element analysis software.

Off-axis mirror fabrication from spherical surfaces under mechanical stress

The preliminary results in the fabrication of off-axis optical surfaces are presented. The propose using the conventional polishing method and with the surface under mechanical stress at its edges. It starts fabricating a spherical surface using ZERODUR® optical glass with the conventional polishing method, the surface is deformed by applying tension and/or compression at the surface edges using a specially designed mechanical mount. To know the necessary deformation, the interferogram of the deformed surface is analyzed in real time with a ZYGO® Mark II Fizeau type interferometer, the mechanical stress is applied until obtain the inverse interferogram associated to the off-axis surface that we need to fabricate. Polishing process is carried out again until obtain a spherical surface, then mechanical stress in the edges are removed and compares the actual interferogram with the theoretical associated to the off-axis surface. To analyze the resulting interferograms of the surface we used the phase shifting analysis method by using a piezoelectric phase-shifter and Durango® interferometry software from Diffraction International™.

Characterization of a transmissive spatial light modulator using four different wavelengths

The present work shows the characterization of a transmissive spatial light modulator (SLM) trademark CRL-Opto and model XGA2L11. This modulator will be used as a variable retarder to generate different polarization states which are produced when the different gray levels from 0 to 250 are displayed in the SLM. So, it is important to determine its response and linear range of work, these parameters were determined experimentally. For this, the SLM was tested with four different wavelengths: red (633 nm), orange (612 nm), violet (405 nm), and green (550 nm). For doing this, the SLM was placed between two linear polarizers aligned first in parallel and then in perpendicular form. The gray levels were varying uniformly from 0 to 250 with increments of 10 in gray levels, and the gray levels were displayed over all the pixels of the SLM. The different intensities for each gray level were measured with a Thorlabs detector model PM100A. Finally, we show the results obtained for each wavelength where we found out that the best linear response was for the green laser with a wavelength of 550 nm.

Characterization of the microlithography and mercury drop methods to generate PDI's

In this work, we compare two techniques to make point-diffraction interferometers (PDI): microlithography and the mercury drop method to know with which of these the best results can be obtained. For the comparison, we used the wavefront generated by a commercial reference surface of λ/20 analyzing the interference pattern generated by the PDIs, we obtained information from the wavefront generated by the pinhole. Several PDIs were created and analyzed to have a statistical error information of both techniques.

Obtaining the topography of human corneas with the null-screen testing method

In this work, the topography of human corneas is evaluated with a conical corneal topographer based on the null-screen method. Geometrical parameters such as the radius of curvature and the conic constant, are obtained. Additionally, elevation, sagittal curvatures and meridional curvature maps can be calculated with the proposed method. Here, it is assumed that the shape of the cornea surface is an aspherical surface. To validate our proposal, we compare the results with those obtained by a commercial corneal topographer.

Fast conical surface evaluation via randomized algorithm in the null-screen test

This work shows a method to recover the shape of the surface via randomized algorithms when the null-screen test is used, instead of the integration process that is commonly performed. This, because the majority of the errors are added during the reconstruction of the surface (or the integration process). This kind of large surfaces are widely used in the aerospace sector and industry in general, and a big problem exists when these surfaces have to be tested. The null-screen method is a low-cost test, and a complete surface analysis can be done by using this method. In this paper, we show the simulations done for the analysis of fast conic surfaces, where it was proved that the quality and shape of a surface under study can be recovered with a percentage error < 2.

MEGARA Optics: stain removal in PBM2Y prisms

MEGARA is the new integral-field and multi-object optical spectrograph for the GTC. For medium and high resolution, the dispersive elements are volume phase holographic gratings, sandwiched between two flat windows and two prisms of high optical precision. The prisms are made of Ohara PBM2Y optical glass. After the prisms polishing process, some stains appeared on the surfaces. For this, in this work is shown the comparative study of five different products (muriatic acid, paint remover, sodium hydroxide, aqua regia and rare earth liquid polish) used for trying to eliminate the stains of the HR MEGARA prisms. It was found that by polishing with the hands the affected area, and using a towel like a kind of pad, and polish during five minutes using rare earth, the stains disappear completely affecting only a 5% the rms of the surface quality. Not so the use of the other products that did not show any apparent result.