R. Dändliker

I Heterodyne Holographic Interferometry

Publisher Summary Interferometry is an old and very powerful technique to measure the deviation between two wavefields with a sensitivity of a fraction of a wavelength. In holographic interferometry, at least one of the wavefields to be compared interferometrically is stored in a hologram. The hologram is usually recorded experimentally with the help of a reference wave. However, computer-generated holograms constructed theoretically may also be used to supply special, previously not existing wavefields. In most cases, the hologram itself also acts as the beamsplitter for the superposition of the two wavefields. Holographic interferometry has some unique properties, which makes it superior to classical interferometry; wavefields from the same object, but under different conditions and at different times, can be compared. This is an essential prerequisite to compare interferometrically different states of solid objects with opaque, diffusely scattering surfaces. Transient wavefields can be frozen instantaneously by short exposure times. Time averaged wavefields can be recorded and compared afterward. This results in reduction of noise and increase of accuracy and stability.

High resolution hologram interferometry by electronic phase measurement

Abstract The described dual frequency method interpolates the fringe pattern of any kind of hologram interfermetry down to better than 1 100 of a fringe, independent of intensity variations. Experiments with double-exposure holography demonstrate a resolution of 6 × 10−4 fringes at any point of the object. Spatial derivatives of deformations can be measured accurately for mechanical strain analysis.

Hybrid optical and electronic image processing for strain measurements by speckle photography

A Youngs fringe processor for speckle photographs is described. It automatically measures fringe separation and orientation with an accuracy of 1% and 1 degrees , respectively. The fringe evaluation is fast and computer controlled. Fringe densities of one to twenty fringes within the diffraction pattern can be treated with the same resolution. Application to displacement and strain measurements by focused image speckle photography is reported. Local strains can be determined with an accuracy of ~E = 10(-5) for displacements between 10 and 100 microm and a spatial resolution of 10 mm.

Determination of coherence length from speckle contrast on a rough surface

ConclusionsThe speckle pattern in the image of a diffusely scattering plane surface illuminated by two mutually inclined quasi plane waves split from a common laser source gives a direct display of the coherence properties of that light source. This can be used as a simple device to check the coherence of a laser source for holographic or interferometric work. The speckle contrast is a linear function of the square modulus of the degree of coherence. This relation has been proved experimentally for coherent and for incoherent laser radiation. The main difference of the intensity distributions for coherent and incoherent illumination occurs at low intensities, which have maximum probability in the coherent and minimum probability in the incoherent case. The intensity probability distributions have been determined experimentally for two limiting cases. Within the experimental limitations they show very good agreement with the theoretical predictions.

Direction sensitive laser Doppler velocimeter with polarized beams.

The described system measures the flow velocity without directional ambiguity. It does not need an optical frequency offset. Polarized illumination beams and two optical detection channels with appropriate polarizing elements are used to determine the flow direction. The Doppler signals from the two channels are identical but have a phase offset that changes sign if the flow direction is reversed. The method has been verified experimentally for low and high concentrations of scattering particles.

Nonlinear cross-talk in two-reference-beam holographic interferometry

Abstract Nonlinear hologram recording leads to cross-talk in two-reference-beam holographic interferometry. This cross-talk generates spurious interference fringes and introduces an error in heterodyne holographic interferometry. It is shown that the cross-talk depends essentially on the recorded scence between the two exposures and can be eliminated by clearing the scene from any visible parts which remain unchanged in position.

Holographic strain analysis: extension of fringe-vector method to include perspective; erratum

R. Dändliker Brown Boveri Research Center, CH-5401 Baden, Switzerland. Received 1 May 1976. The authors of the paper, Holographic strain analysis: extension of fringe-vector method to include perspective, quote as Ref. 2 our contribution to the Conference on the Engineering Uses of Coherent Optics, Glasgow. This reference is not complete, and the page number is not correct. The correct reference should read:

Relaxation of the Coherence Requirements in Holography

Spatial and temporal coherence limitations in holography can be partially overcome with a simple diffuser box (similar to an integrating sphere). Experimental results show that the scene depth can be extended to at least 10 times the coherence length of the light source. Even holographic intprferometry is possible. An analysis in the time and frequency domain is presented. It shows the important role of the speckling and explains the observed reduction of the SNR.

Heterodyne Holographic Interferometry : a review

Heterodyne holographic interferometry is a combination of holographic infometry and opto-electronic fringe evaluation with the following outstanding properties (1,2): - fringe interpolation to better than 10-3 of a fringe (+ 0.3° for the interference phase); - measurement with the same accuracy at any desired position in the image, therefore high spatial resolution (> 100x100 points); - independent of brightness variations across the image; - inherently direction sensitive, i.e. increase and decrease of interference phase can be distinguished; - computer readable output both for position and phase easily obtained (allows on-line data processing); - inherently less sensitive to speckle noise than fringe intensity measurements.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.Heterodyne holographic interferometry is a combination of holographic infometry and opto-electronic fringe evaluation with the following outstanding properties (1,2): - fringe interpolation to better than 10-3 of a fringe (+ 0.3° for the interference phase); - measurement with the same accuracy at any desired position in the image, therefore high spatial resolution (> 100x100 points); - independent of brightness variations across the image; - inherently direction sensitive, i.e. increase and decrease of interference phase can be distinguished; - computer readable output both for position and phase easily obtained (allows on-line data processing); - inherently less sensitive to speckle noise than fringe intensity measurements.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.