Laser Speckle Field Research Articles

Analysis of Influences of Micro-Rough Surface Parameters on Laser Speckle Field

Analysis of Influences of Micro-Rough Surface Parameters on Laser Speckle Field

Calibration and Improved Speckle Statistics of IM-CW Lidar for Atmospheric CO2 Measurements

An intensity modulated, continuous-wave (IM-CW) integrated path differential absorption (IPDA) fiber-based lidar is developed herein for measuring atmospheric carbon dioxide (CO2). There are two main challenges in improving measurement accuracy, which have not been given enough attention in the previous research: one is that temperature sensitivity in optical components causes biases, due to the drift of component characteristic, and the other is that speckle noise deteriorates the signal-to-noise ratio. With the components thermally controlled, a target calibration accuracy of 0.003 dB is realized, corresponding to a CO2 concentration precision of better than 1 ppm for a 1 km path. A moving diffuser can reduce speckle noise by time averaging. In this paper, movement of the diffuser is substituted by the perturbation of the emitted laser beam by using a vibrating motor mounted on the optical antenna. Selecting on and off wavelengths with a small wavelength separation can improve the correlation between two laser speckle fields. These improvements result in the improved accuracy of the IPDA lidar system. Finally, the lidar performance was analyzed after the improvements described above were implemented. The diurnal variations of the atmospheric CO2 concentration using a topographic target were performed, and the results showed good agreement with the data measured by an in situ sensor. The root mean square (rms) of the deviation between the IPDA lidar and the in situ sensor was less than 1.4%.

Sharp Small-Scale Perturbations of Fringes at the Interference of Dispersed Broad-Spectrum Laser Beams

The interference of two dispersed laser speckle-fields with a great spectrum width has been studied by numerical modeling. Local small-scale perturbations of interference fringes with an abrupt drop of contrast in the interference picture have been revealed. It has been shown that these perturbations occur in the region of a large phase gradient and a small module of the mutual coherence function of coinciding interfering fields. The analytical description of small-scale perturbations is given, and the probability of their appearance is estimated.

Order statistics of high-intensity speckles in stimulated Brillouin scattering and plasma-induced laser beam smoothing

We have studied plasma-induced smoothing due to stimulated Brillouin scattering (SBS) under the aspect of the extremal statistics of smoothed laser beams. As pointed out in the work by Rose and DuBois (1994 Phys. Rev. Lett.72 2883), scattered light can be subject to uncontrolled (or even ‘explosive’) behaviour, associated with a critical gain value for SBS. In this work we show how this critical behaviour can be predicted on the basis of the order statistics of laser speckle fields, and we analyse the transition to uncontrolled behaviour of the laser beam due to the dominance of high intensity speckles.

Vibration-based specklegram fiber sensor for measurement of properties of liquids

A specklegram fiber sensor for the measurement of properties of liquids is proposed. The laser speckle field formed at the output of a multimode fiber is modulated by a vibrating microbending transducer. Once the transducer is placed in vessel filled by the sample, the frequency response of the specklegrams can be correlated to the properties of the liquid. The system was applied on the measurement of the mass and viscosity of water and ethanol samples, by processing the inner product of speckle patterns with artificial neural networks. The sensor provided measurements with absolute error lower than 0.5g and 0.1mPas on the prediction of mass and viscosity, respectively, with potential application on the determination of the concentration of liquid mixtures.

Laser speckle field as a multiple particle trap

We demonstrate that a speckle pattern in the spatially coherent laser field transmitted by adiffuser forms a multitude of three-dimensional intensity micro-pockets acting asparticle traps for air-borne light absorbing particles. Confinement of up to a fewthousand particles in air with a unidirectional single beam has been achieved.Theoretical analysis of the speckle defined trapping volume is in a good agreement withexperimental results on capturing of aggregates of carbon nanoparticles in air.

Polarization Singularities in 2D and 3D Speckle Fields

The 3D structure of randomly polarized light fields is exemplified by its polarization singularities: lines along which the polarization is purely circular (C lines) and surfaces on which the polarization is linear (L surfaces). We visualize these polarization singularities experimentally in vector laser speckle fields, and in numerical simulations of random wave superpositions. Our results confirm previous analytical predictions [M. R. Dennis, Opt. Commun. 213, 201 (2002)] regarding the statistical distribution of types of C points and relate their 2D properties to their 3D structure.

Comparison of two methods for measuring the polymerization characteristics of flowable resin composites

Objectives The purpose of this study was to compare two methods for assessing the polymerization characteristics of flowable resin composites. Methods Two different flowable resin composites and a hybrid resin composite control were investigated. In order to measure the volumetric shrinkage (Δ V), each material was placed into a mould and extruded into a water-filled dilatometer. The specimens were then light irradiated for 30 s using a curing unit with the power density adjusted to either 100 or 600 mW/cm 2. For the speckle contrast measurement, each resin composite was condensed into a glass tube and irradiated. The laser–speckle field was recorded in a digital frame. The calculated values were obtained for each pair of adjacent patterns and the changes in speckle contrast as a function of time were obtained. Results The average Δ V values of the resins after 180 s ranged from ∼3.3 to 4.4% for the flowable composites and from ∼1.8 to 2.3% for the hybrid composite control. The overall magnitude of the speckle contrasts decreased soon after the initial light exposure commenced and gradually increased thereafter. The speckle contrast measurements revealed changes in the pastes due to the polymerization of the flowable resins that were greater than those obtained with the water-filled dilatometer. Significance These data suggest that the polymerization characteristics of flowable resins can be measured successfully using two different methods. Moreover, our findings are of clinical relevance, as the data obtained under laboratory conditions might give an indication of the suitability of flowable resin composites for specific clinical applications.

Experimental Investigation of Local Properties and Statistics of Optical Vortices in Random Wave Fields

We present the first direct experimental evidence of the local properties of optical vortices in a random laser speckle field. We have observed the Berry anisotropy ellipse describing the anisotropic squeezing of phase lines close to vortex cores and quantitatively verified the Dennis angular momentum rule for its phase. Some statistics associated with vortices, such as density, anisotropy ellipse eccentricity, and its relation to zero crossings of real and imaginary parts of the random field, are also investigated by experiments.

Phase object data obtained by pulsed TV holography and defocused laser speckle displacement.

Transient events in optically transparent media occur in many engineering applications. Using pulsed TV holography to capture a laser speckle field propagated through an optical disturbance makes it possible to obtain both the position and the phase gradients of the disturbance. The technique depends on the fact that speckles transmitted through an optical disturbance will be displaced by an amount that depends on the relation of the defocus to the object. First the speckle field is captured holographically, without and with disturbance present. Then the recorded fields are numerically refocused in a computer to a number of different focal planes. With a cross-correlation technique a number of speckle displacement fields are obtained, and from them the data about the disturbance are obtained. So far the technique has been shown to work for thin objects.

Phase object data obtained from defocused laser speckle displacement.

An optical technique that is based on defocused digital speckle photography is proposed for the evaluation of phase objects. Phase objects are different kinds of transparent or semi-transparent media that allow light to be transmitted. A phase object inserted in a laser speckle field introduces speckle displacement, from which information about the object may be extracted. It is shown that one may use speckle displacements to determine both the phase gradients and the positions of phase objects. As an illustration the positions and focal lengths of two weak lenses have been derived from defocused laser speckle displacement.

Digital laser speckle spectral correlation within the framework of the fresnel approximation of the scalar kirchhoff theory and its application in surface roughness measurement

Correlation of laser speckle fields generated by light of two different wavelengths, which illuminates successively a randomly rough surface, is solved theoretically within the framework of the scalar Kirchhoff theory of wave scattering from random rough surfaces. The Fresnel approximation is used in description of the scattered wave. The solution obtained is a contribution to the scalar Kirchhoff theory of electromagnetic wave scattering from randomly rough surfaces. Its utilization for surface roughness measurement by means of the digital processing of corresponding specklegrams is presented.

Degree of polarization in laser speckles from turbid media: implications in tissue optics.

The degree of polarization (DOP) of laser-speckle fields, where the speckles were generated by a polarized laser beam incident upon two kinds of samples: ground glass and wax, was investigated within a single coherence area as well as over multiple coherence areas. For the surface-scattering ground glass, the incident polarization state was preserved in the speckle field, and hence the DOP remained at unity regardless of the area of detection. For the volume-scattering wax, the polarization states varied with positions in the field, and consequently the DOP depended on the area of detection: the DOP decreased with an increasing area of detection, and only when the area was much smaller than the coherence area would the DOP approach unity. A numerical simulation explained the experimental observation. These results are important for the understanding of polarization phenomena in turbid media such as biological tissue.

Laser speckle decorrelation in NDT

The random noise of the laser speckle field which develops at the focusing plane of an imaging system, is, by now, efficiently used in several interferometric techniques as an information carrier of the macroscopic wavefront distortion induced by the surface displacement field of the object under investigation. The actual noise in this kind of techniques is represented by the speckle decorrelation at the image plane — i.e. the destruction of the carrier — which may be caused by the modification of the texture surface (e.g. by yielding under a severe stress state), but it is inherently produced by the same displacement field under measurement. In the paper the phenomenon of laser speckle decorrelation is numerically simulated and experimentally investigated with the aim of estimating its sensitivity to local deformation and assessing a possible field of application. Satisfactory results in the field of NDT of multilayer fiber-reinforced composites were obtained by reducing the diaphragm of the lens to increase the sensitivity of the imaging system to speckle decorrelation induced by local deformation; unfortunately this simple approach requires a considerable amount of laser power for illuminating the object. Different aperture shapes were therefore numerically simulated which provided improved efficiency and sensitivity and whereby a semi-quantitative analysis of the displacement field could be experimented.

Strain field measurements with dual-beam digital speckle photography

A non-contact strain field measurement technique called dual-beam digital speckle photography is presented. The system facilitates strain measurements in rough environments and at elevated temperatures and can be used on materials where instrumentation of standard strain gauges are difficult. The system uses two symmetrically oriented laser illumination beams and an imaging system oriented normal to the object surface. The laser speckle field for each illumination beam is recorded simultaneously before and after object deformation. From these data sets it is possible to extract one strain component, e.g. ε xx , without the need of numerical differentiation. It is shown that the most important criteria is normal viewing but also that deviation from perfect symmetry in the illumination will affect the results. A dynamic strain field, acquired at 15 Hz, illustrates the system performance. The effect of oblique viewing and unstable lasers are also shown.

Digital speckle techniques for measuring light deflection profiles of inhomogeneous phase objects

Digital speckle pattern photography is applied to the measurement of light deflection profiles of inhomogeneous transparent objects by transmission of a He-Ne laser speckle field through the object. The speckles are recorded with a CCD camera, and the local displacements are evaluated by comparison of the deflected speckle field with the undeflected field by a digital cross-correlation technique. In this way a complete two-dimensional deflection field can be obtained from a single pair of CCD records. The basic technique and two experimental setups suitable for objects with a size larger than that of the CCD chip are described. These novel techniques are applied to determine profiles of the refractive index of a free-burning arc in air. In addition, a cinematographic setup is described, which allows the investigation of transient objects by means of a video tape recorder.

Shot-noise-corrected measurement of the g^(2)(0) of the speckle backscattered from a rotating randomly rough metal surface

The zero time-delay degree of second-order coherence g(2)(0) is investigated in the laser speckle field backscattered from a continuously rotating randomly rough aluminum metal surface. The s-polarized laser beam is incident with an angle of −30° to the surface, and the angular distributions of the mean intensity and g(2)(0) are measured in the incidence plane. In obtaining g(2)(0) from the experimental data, the contribution of shot noise is subtracted from the photoelectric signal. It is found that the s and p components of the scattered light are close to those of the Gaussian light and that the measured g(2)(0) is consistent with the theoretical value.

Coherent image synthesis from wave-front sensor measurements of a nonimaged laser speckle field: a laboratory demonstrations

We report what are to our knowledge the first coherent images recovered in the laboratory from measurements made with a Shack-Hartmann wave-front sensor of the phase and amplitude of a laser speckle wave front. We discuss the design of our wave-front sensor, which can obtain the phase and amplitude of an optical field with a single intensity measurement, and we point out a particular type of phase jump that cannot be detected by the Shack-Hartmann sensor. We also discuss implementations of this technique that may permit near-diffraction-limited imaging through turbulent media.

Screw dislocations of laser speckle fields in interferograms with a circular line structure

Experimental results are used to show that circular interferograms are of interest in studies of screw dislocations of speckle-distorted laser beams because of a close correspondence between the symmetry of these interferograms and defects. The presence of dislocations transforms a system of the usual rings into a split network. Typical structure elements of such interferograms are right- and left-handed spirals connecting singular points of dislocations. Spiral fragments can be used in the diagnostics of dislocations regarded as independent defects. A method is suggested for the formation of highly directed optical beams with a low level of the usual aberrations, but with a complex phase surface topology. Screw dislocations are shown to be a natural analog of helical waves known in optics and capable of deliberate generation.

Image synthesis from wave-front sensor measurements of a coherent diffraction field

We demonstrate that images of laser-illuminated objects can be formed from measurements of the wave-front slope (gradient) associated with the backscattered, coherent laser-speckle field. A digital wave-front recovery and image synthesis procedure is described, and the results of computer-simulation experiments are presented in which coherent images are reconstructed from digitally simulated Fourier-plane laser-speckle measurements. Images are recovered from noisy wave-front-difference data to illustrate the effect that measurement noise has on recovered image quality.