Mette Owner-Petersen

Decorrelation and fringe visibility: on the limiting behavior of various electronic speckle-pattern correlation interferometers

I discuss the behavior of fringe formation in image-plane electronic speckle-pattern correlation interferometers as the limit of total decorrelation is approached. The interferometers are assumed to operate in the difference mode. The effect of decorrelation will be a decrease in fringe visibility until the limit of total decorrelation, when no fringes will be formed, is reached. A quantitative evaluation of the partially decorrelated fringe pattern is presented for the case of decorrelation due to both tilt and in-plane translation of an object surface element. It is shown that the fringe pattern depends on whether only one or both of the wave fronts forming the primary interferograms suffer from decorrelation. The results are discussed in relation to both in-plane and out-of-plane displacement-sensitive interferometers and to the out-of-plane gradient-sensitive interferometer.

Digital speckle pattern shearing interferometry: limitations and prospects.

Since shearing interferometry, also called shearography, does not depict fringes caused by object tilt, it is a tool well suited for either nondestructive testing of objects under load or for quantitative evaluation of flexural strains. In traditional shearing interferometry, observation of fringes requires optical processing of doubleexposed interferograms. Hence the technique is not in real time. This paper explores the possibilities and limitations for real time shearing fringe observation using the electronic speckle pattern interferometry technique. Prospects for quantitative determination of local tilt and flexural strain using the phase shifting technique are commented on, and some preliminary results are shown.

Extreme attenuation of total internal reflection used for determination of optical properties of metals

It is shown that attenuation of total internal reflection, when measured under conditions in which the reflectivity is minimized with respect to the thickness of the intermediate layer for a three-layer structure, might be well suited for determining the dielectric function of metals. Experimental results for several metals in the optical range of the spectrum are presented.

Dielectric function of metals determined by surface-plasmon excitation: ambiguities caused by ultrathin oxide layers

Because of the small penetration depth of the electromagnetic field, the deposition of a thin surface layer may lead to large errors in the measurement of the dielectric properties of highly reflective materials (e.g., metals). It is shown that unless experimental conditions are chosen carefully, it is not possible to distinguish unambiguously between the effect of an ultrathin surface layer and that of a change in the dielectric function of the material when probing the latter by surface-plasmon excitation.

25-m Live Optics Telescope

A 25 m four mirror live optics telescope is studied. M1 is spherical with 141 segments and f/0.96. M1 is re-imaged onto M4, also with 141 segments. Image FWHM is less than 0.10 arc sec over greater than 20 arc min. A horseshoe solution with a simple azimuth platform is applied. M1 segments are supported by a fine meniscus form truss structure, tied to the horseshoes by a coarser mesh. A FEM with 104 dof was developed and applied. Live optics control M1 and M4 segments (the latter with potentially high bandwidth), M1/M4 segment balancing and servos. Correction signals in tilt, coma and defocus are traced. A correlation tracker and a laser guide star system are included. Low and high wind speed regimes are studied. An end-to-end simulation model is developed, based on modal representation of our FEM. Image quality dependence on wind load is studied from segment piston and tilt deflections. Eigenmodes are recorded. Using wind time series, we study dynamic effects and image quality resulting from the 141 segment spots. Automatic segment control at a bandwidth of only 1 Hz gives excellent image quality. We foresee to reach a bandwidth greater than 50 Hz, securing a system partly adaptive, with effects of atmospheric wave front tilt removed through M4 segment tilting at high frequency. Further progress include optimization of mechanical design and end-to-end simulation model, wind tunnel testing and studies of wave front sensor, correlation tracker and instruments. A fully adaptive system is tentatively studied as is coherent operation at IR wavelengths.

Phase-map unwrapping: a comparison of some traditional methods and a presentation of a new approach

This paper deals with the problems encountered when unwrapping a noisy phase map obtained in connection with phase shifting speckle interferometry. Due to decorrelation of the speckle patterns obtained before and after loading the object, these phase maps are very noisy and unwrapping requires smoothing. Traditional procedures based upon averaging or median filtering in a window have some disadvantages. Averaging tends to smooth the jumps to be identified in the phase map and median filtering is very slow. A new fast-edge preserving approach is presented and compared to some traditional methods by means of evaluating their performance when filtering and unwrapping a computer-generated noisy sawtooth pattern.

Optical design and performance analysis of a 25-m class telescope with a segmented spherical primary

The basic design and an analysis of the performance possibilities of a 25 m class optical telescope are presented here. The configuration consists of a 28 m segmented spherical primary M1 followed by three highly aspherical corrective mirrors M2, M3 and M4 which also deviates from Cartesian shape. The construction is axially folded. The combination M1- M2 forms a focus close to a coupling aperture in M4, and the combination M3-M4 relays this focus to the final focus behind M1 and M3. The combination M2-M3 images M1 on the segmented M4 to be used for correction of wavefront errors induced by M1 form errors arising from gravitational sag and windbuffeting. Several types of aspherical figuring of M2, M3 and M4 all resulting in a field performance better than characterized by a rms spot radius smaller than 0.1 arcseconds within a full FOV of 21 arcminutes are presented.

Accelerometer design based on attenuated total reflection

It is shown that the effect of attenuated total internal reflection might be suitable for acceleration measurements. A prototype design and preliminary measurements are presented. The performance limitations of the device are discussed.

Vibrational amplitude and phase retrieval from pulsed interferograms

A possible scheme for performing vibrational phase and amplitude mapping using pulsed interferometry is presented. Both the case of electronic speckle pattern interferometry (ESPI) and of conventional holographic recording (HI) are treated. The proposed methods utilize the possibility of performing optical phase shifting, and the number of required interferograms are kept at a minimum.

ESPI Used Qualitatively For Vibration Analysis And Quantitatively For Deformation Analysis

For quite a number of years Electronic Specklepattern Interferometry (ESPI) has been an attractive alternative to Holographic Interferometry in industrial surroundings. As for conventional holographic interferometry, the information which is primarily obtained, is an image of the object superposed by a fringe pattern related to object motion. For harmonic loading integration over many periods leads to Besselfringes whereas subtraction mode used for deformation analysis leads to cosinefringes. In both cases a qualitative evaluation can be performed by visual inspection of the fringe pattern whereas quantitative analysis requires digital image processing, the latter becomning increasingly attractive with the access to more efficient PCs. A very prominent feature of ESPI is the possibility of aquiring one hologram per 1/25 sec. This makes it possible to obtain information of both vibrational amplitudes and phases within reasonable timeframes. This paper presents results of qualitative ESPI measurements on two transducers (an electretmicrophone and an accelerometer) identifying critical resonances. In both cases the measurements resulted in improved transducerdesign. Further presented are results of quantitative deformation analysis by phaseshifted ESPI followed by digital imageprocessing using a very simple phaseshift algorithm.