Santiago Royo

Adaptive self-mixing vibrometer based on a liquid lens

A self-mixing laser diode vibrometer including an adaptive optical element in the form of a liquid lens (LL) has been implemented and its benefits demonstrated. The LL arrangement is able to control the feedback level of the self-mixing phenomenon, keeping it in the moderate feedback regime, particularly suitable for displacement measurements. This control capability has enabled a remarkable increase in the sensor-to-target distance range where measurements are feasible. Target vibration signal reconstructions present a maximum error of lambda radical16 as compared with a commercial sensor, thus providing an improved working range of 6.5 cm to 265 cm.

Comparison of cubic B-spline and Zernike-fitting techniques in complex wavefront reconstruction

We analyze an alternative to classical Zernike fitting based on the cubic B-spline model, and compare the strengths and weaknesses of each representation over a set of different wavefronts that cover a wide range of shape complexity. The results obtained show that a Zernike low-degree polynomial expansion or a cubic B-spline with a low number of breakpoints are the best choices for fitting simple wavefronts, whereas the cubic B-spline approach performs much better when more complex wavefronts are involved. The effect of noise level in the fit quality for the different wavefronts is also studied.

Shack-Hartmann sensor based on a cylindrical microlens array

We present a Shack-Hartmann wavefront sensor (SHWS) based on a cylindrical microlens array as a device for measuring highly aberrated wavefronts. Instead of the typical spot pattern created by a conventional SHWS, two orthogonal line patterns are detected on a CCD and are superimposed. A processing algorithm uses the continuity of the focal line to extend the dynamic range of measurement by localizing the line, even if it leaves the CCD area confined by the corresponding microcylinder. The measurement of a wavefront from a progressive addition lens with an 80 lambda peak-to-valley value reveals the capabilities of the sensor.

Dealing With Speckle Effects in Self-Mixing Interferometry Measurements

An analysis of speckle effects and the techniques to overcome them in self-mixing interferometry signals is presented. We characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and then propose two simple experimental approaches to overcome the amplitude fading induced by the speckle effect. Unlike the techniques proposed until now, our first approach uses an adaptive optical element in the form of a voltage-programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.

A Nanometric Displacement Measurement System Using Differential Optical Feedback Interferometry

We propose differential optical feedback interferometry, a technique able to measure nanometer-size amplitude displacements by comparing the optical power of two lasers subject to optical feedback. In this letter, the principles of the technique are explained in detail, and its limits are explored by simulation. Theoretical results are presented showing that the technique can measure nanometer scale displacements with resolution within the angstrom scale. An experimental setup for validation has been built, and a series of experimental tests were performed using a capacitive sensor as a reference. Results show good agreement between theory and experiment with a reasonable reduction in performance due to mechanical coupling and signal noise. The proposed technique, thus, provides measurements of a very high resolution using an extremely simple and robust experimental setup.

Shaft Trajectory Analysis in a Partially Demagnetized Permanent-Magnet Synchronous Motor

Demagnetization faults have a negative impact on the behavior of permanent-magnet synchronous machines, thus reducing their efficiency, generating torque ripple, mechanical vibrations, and acoustic noise, among others. In this paper, the displacement of the shaft trajectory induced by demagnetization faults is studied. It is proved that such faults may increase considerably the amplitude of the rotor displacement. The direct measure of the shaft trajectory is performed by means of a noncontact self-mixing interferometric sensor. In addition, the new harmonics in the back electromotive force (EMF) and the stator current spectrum arising from the shaft displacement are analyzed by means of finite-element method (FEM) simulations and experimental tests. Since conventional finite-element electromagnetic models are unable to predict the harmonics arising from the shaft trajectory displacement, an improved finite-element model which takes into account the measured trajectory has been developed. It is shown that this improved model allows obtaining more accurate back EMF and stator current spectra than those obtained by means of conventional models. This work presents a comprehensive analysis of the effects generated by demagnetization faults, which may be useful to develop improved fault diagnosis schemes.

Runout Tracking in Electric Motors Using Self-Mixing Interferometry

In this paper, a self-mixing interferometry sensor has been used as a proximity probe to measure possible runout in permanent magnet synchronous motors, for fault diagnosis. A general procedure for the measurement of the 2-D trajectory of the motor shaft is described in detail, including procedures for the characterization of the uncertainty due to the shape of the shaft, and the management of speckle noise. The performance of the proposed sensor has been compared to that of a commercial Polytec laser vibrometer, for validation purposes. Results show inaccuracies in the order of ±6 μm, which agree well with the measured uncertainty introduced by shaft surface imperfections.

A micrometric non-contact profiler for optical quality surfaces

A non-contact technique for obtaining accurate profiles of optical quality surfaces with micrometric accuracy has been developed. The technique is based on the Ronchi test principle, that is, on the study of the interaction of a wavefront reflected on the surface to be profiled with a square-wave transmittance ruling. From the resultant fringe pattern and some basic geometrical optics principles it is possible to measure the local normal to the surface being tested at a set of given points. This local normal map may then be integrated, yielding the surface profile. By use of a theoretically expected surface shape, the main parameters of the surface may then be determined by surface fitting of the measured data to that expected surface shape. Results of the profilometric measurements both of a spherical and of a toroidal surface are presented. The measured profiles are validated by comparison of the radii of curvature obtained using a high precision radioscope with the ones obtained by surface fitting of the measured profiles to their expected surface shapes. Additionally, subtracting the best-fit theoretical surface from the measured profile allows the observation of surface deviations from the theoretical shape to within some tenths of a nanometres.

Analysis and control of speckle effects in self-mixing interferometry

An analysis of speckle effects in self-mixing interferometry signals has been performed. We will characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and we will propose two simple experimental approaches to overcome the amplitude fading induced by speckle effect. Differently to the techniques proposed up to the moment, our first approach uses an adaptive optical element in the form of a voltage programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.

Current Developments on Optical Feedback Interferometry as an All-Optical Sensor for Biomedical Applications

Optical feedback interferometry (OFI) sensors are experiencing a consistent increase in their applications to biosensing due to their contactless nature, low cost and compactness, features that fit very well with current biophotonics research and market trends. The present paper is a review of the work in progress at UPC-CD6 and LAAS-CNRS related to the application of OFI to different aspects of biosensing, both in vivo and ex vivo. This work is intended to present the variety of opportunities and potential applications related to OFI that are available in the field. The activities presented are divided into two main sensing strategies: The measurement of optical path changes and the monitoring of flows, which correspond to sensing strategies linked to the reconstruction of changes of amplitude from the interferometric signal, and to classical Doppler frequency measurements, respectively. For optical path change measurements, measurements of transient pulses, usual in biosensing, together with the measurement of large displacements applied to designing palliative care instrumentation for Parkinson disease are discussed. Regarding the Doppler-based approach, progress in flow-related signal processing and applications in real-time monitoring of non-steady flows, human blood flow monitoring and OFI pressure myograph sensing will be presented. In all cases, experimental setups are discussed and results presented, showing the versatility of the technique. The described applications show the wide capabilities in biosensing of the OFI sensor, showing it as an enabler of low-cost, all-optical, high accuracy biomedical applications.