Eugenio Garbusi

Interferometer for precise and flexible asphere testing

A novel non-null interferometer for the precise measurement of aspheric surfaces is presented. In contrast to a classical interferometer, where only one test wavefront is used, the proposed system makes use of multiple test beams propagating under different angles through the interferometer. This allows the measurement of aspheric surfaces in a very short time, even for strong aspheres with deviations from the best-fit sphere of up to 900 microm. The non-null test configuration implies that any additional aberrations introduced by the interferometer have to be well characterized to precisely measure the asphere. Experimental measurements of a calibrated non-null interferometer on an aspheric element with 900 microm SAG deviation are presented.

Perturbation methods in optics: application to the interferometric measurement of surfaces

Manufacturing and misalignment errors are present in every optical system. Usually these errors lead to intolerable wavefront deviations and system inaccuracies if they are not characterized and taken into consideration. In the interferometric measurement of surfaces, the characterization of the interferometer aberrations plays a central role, since unknown phase contributions lead to an erroneous assessment of the test surface and therefore an incorrect estimation of the performance of an optical system. In this work, we present a method for the interferometric characterization of surfaces based on the principles of Hamiltons characteristic functions and perturbation theory. The application of the proposed method to an interferometer for the measurement of aspherical surfaces is shown.

Optical metrology: from the laboratory to the real world

Optical metrology has shown to be a versatile tool for the solution of many inspection problems. The main advantages of optical methods are the noncontact nature, the non-destructive and fieldwise working principle, the fast response, high sensitivity, resolution and accuracy. Consequently, optical principles are increasingly being considered in all steps of the evolution of modern products. However, the step out of the laboratory into the harsh environment of the factory floor was and is a big challenge for optical metrology. The advantages mentioned above must be paid often with strict requirements concerning the measurement conditions and the object under test. For instance, the request for interferometric precision in general needs an environment where high stability is guaranteed. If this cannot be satisfied to a great extent special measures have to be taken or compromises have to be accepted. But the rapid technological development of the components that are used for creating modern optical measurement systems, the unrestrained growth of the computing power and the implementation of new measurement and inspection strategies give cause for optimism and show that the high potential of optical metrology is far from being fully utilized. In this article current challenges to optical metrology are discussed and new technical improvements that help to overcome existing restrictions are treated. On example of selected applications the progress in bringing optical metrology to the real world is shown.

Single frame interferogram evaluation

We present a simple and novel algorithm for the phase extraction from a single interferogram based on the spatial processing of interference patterns. This new evaluation procedure is suitable for application in environments where the presence of vibrations impedes the use of a classical phase-shifting interferometry scheme with multiple exposures. The algorithm does not require the introduction of a linear carrier as required in Fourier transform techniques. The addition of a carrier can be a significant drawback, e.g. in the case of wavefronts with strong aberrations where the minimum required linear carrier is not even resolved by the detector. The basic idea relies on the spatial application of a temporal phase-shifting algorithm and an iterative correction process to obtain an accurate reconstruction of the wavefront. The validity and performance of the proposed method is shown with numerical and experimental results.

Automated surface positioning for a non-null test interferometer

An automatic method for the positioning of the test surface in a non-null interferometer is presented. A major task in the interferometric testing of surfaces is to avoid the introduction of surface aberrations due to an incorrect placement of the test object in the interferometer cavity. In the case of plane and spherical surfaces, adjustment errors can usually be distinguished from surface figure errors and therefore removed, but in the case of aspherical surfaces this task becomes nontrivial. In this work, the effect on the measured phase due to lateral and axial displacements of the aspherical surface is calculated, and an adjustment method for the positioning of the surface at a predefined measurement location presented. Experimental results showing the feasibility of the proposed procedure are shown.

Modified Michelson interferometer with electrooptic phase control

A modified Michelson interferometer with external electrooptic phase control is presented. The device is a polarization interferometer, in which the reference and test arms are parts of the same collimated beam. The reference wave undergoes an odd number of reflections while the test wave is reflected an even number of times. As a consequence, the elliptical state of polarization of the reference wave will have opposite handedness in comparison with the state of polarization of the wave traveling through the test object. The phase shift between the two waves is externally controlled with a Pockels cell. This allows to make a very fast phase modulation.

Phase-shifting (Sagnac) interferometer with external phase control

A novel ring configuration for phase-shifting interferometry with external phase-shifting control is presented. The device is a polarization (ring) interferometer, in which the reference and test arms are parts of the same collimated beam. The key point is to manage the polarization of the light such that orthogonal linear polarizations describe counterpropagating paths in the ring interferometer. The phase shift between the two waves is externally controlled with a Pockels cell, which permits fast phase modulation without the need for moving parts inside the interferometer.

Phase modulation by polarization recording in bacteriorhodopsin: application to phase-shifting interferometry

A novel phase-control method with application to phase-shifting interferometry is presented. The linear polarization state of an external (green) light beam is recorded on a bacteriorhodopsin film, and this polarization state is read by a circular polarized (red) laser beam. By reading the bacteriorhodopsin film, the original (red) wave reverses its circularity and becomes phase shifted by an amount that is dependent on the polarization of the external (green) beam. This method of phase control can be applied in a two-beam interferometer in which the test and reference waves are orthogonally polarized, which allows one to obtain phase modulation without moving parts inside the interferometer.

New technique for flexible and rapid measurement of precision aspheres

A new interferometric technique for the measurement of aspheric elements based on multiple test beams is presented. By means of an array of sources (Point Source Array) an aspheric surface is illuminated under different angles which allow the measurement of the zones where the local gradient of the test piece is compensated. One of the main advantages of the system is that the measurement process is performed in parallel (many sources are used at the same time) thus requiring extremely short measurement time in comparison with other available subaperture testing techniques. Another important aspect is that the asphere stays in the same position during the whole process; there are no mechanical movements of the test part involved. The technique allows the measurement of strong aspheric elements with departures from the best fit sphere up to ±10°. The method was developed to obtain accuracies of up to λ/30 and better. Simulations and first experimental results are presented.

Harmonic suppression and defect enhancement using Schlieren processing

The Schlieren technique is a well-known coherent processing method that is usually applied to the visualization of phase objects. In this paper, we demonstrate that, when the Schlieren processing is applied to a light wave modulated in amplitude and possessing some periodicity, the harmonic contents of the resultant image decreases (i.e., the higher harmonics are suppressed). Also, we show that, when the amplitude-modulated (periodic) light wave possesses faults, the Schlieren processing produces an enhancement of the faults relative to the periodic carrier. This technique can be applied to defect detection in periodic structures such as photomasks used for LCD panels, integrated-circuit masks, or semiconductor wafers.