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Off-axis digital holographic particle positioning based on polarization-sensitive wavefront curvature estimation
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. (Experimentell mekanik)ORCID iD: 0000-0003-0398-1919
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics. (Experimentell mekanik)ORCID iD: 0000-0003-4879-8261
Number of Authors: 22016 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, no 27, p. 7503-7510Article in journal (Refereed) Published
Abstract [en]

Poor axial resolution in holographic particle imaging applications makes particle positioning in 3D space morecomplex since the positions are not directly obtained. In this paper we estimate the axial position of micrometerparticles by finding the location where the wavefront curvature from the scattered light becomes zero. By record-ing scattered light at 90°using off-axis holography, the complex amplitude of the light is obtained. Byreconstruction of the imaged scene, a complex valued volume is produced. From this volume, phase gradientsare calculated for each particle and used to estimate the wavefront curvature. From simulations it is found that thewavefront curvature became zero at the true axial position of the particle. We applied this metric to track an axialtranslation experimentally using a telecentric off-axis holographic imaging system with a lateral magnification ofM1.33. A silicon cube with molded particles inside was used as sample. Holographic recordings are performedboth before and after a 100μm axial translation. From the estimated positions, it was found that the mean dis-placement of particles between recordings was 105.0μm with a standard deviation of 25.3μm.

Place, publisher, year, edition, pages
Optical Society of America, 2016. Vol. 55, no 27, p. 7503-7510
Keywords [en]
Digital holography, Scattering, Particles
National Category
Applied Mechanics
Research subject
Experimental Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-59662DOI: 10.1364/AO.55.007503ISI: 000383996900008Scopus ID: 2-s2.0-84988878500OAI: oai:DiVA.org:ltu-59662DiVA, id: diva2:1034170
Funder
Swedish Research Council, 621-2014-4906
Note

Validerad; 2016; Nivå 2; 2016-10-25 (andbra)

Available from: 2016-10-11 Created: 2016-10-11 Last updated: 2020-04-16Bibliographically approved
In thesis
1. 3D localization in digital holography from scattered light from micrometer-sized particles
Open this publication in new window or tab >>3D localization in digital holography from scattered light from micrometer-sized particles
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

When a particle is illuminated by a beam of light it will scatter and redistribute the light in all directions. How it scatters depends on the size, shape and refractive index of the particle. Additionally, it depends on the wavelength and polarization of the illuminating beam. The direction and distance to the observer relative the particle also needs to be considered.  A digital holographic imaging system is used to collect parts of the scattered light from micrometer-sized particles. By utilizing digital holography a three-dimensional reconstruction of the imaged scene is possible. Traditionally, particles are localized based on the intensity in the holographic reconstructions. In this licentiate thesis, the phase response of the scattered light is investigated and utilized. An alternative method for locating spherical particles is presented. The method locate particles based on a simple feature of a propagating wave, namely the fact that the wavefront curvature changes from converging to diverging at the axial location of the particle. The wavefront curvature is estimated using two different methods. The first method estimates the lateral phase-gradients using a finite-difference method. The second method uses a three-dimensional parametric model based on a Chebyshev polynomial expansion. The methods are demonstrated using both simulations and experimental measurements. The simulations are based on the Lorenz-Mie scattering theory for spherical particles and are combined with an imaging system model. Experiments are performed using an off-axis polarization sensitive digital holographic system with a coherent Nd:YAG laser. Measurements of stationary particles are made to validate and evaluate the proposed method. It is found that these methods estimate the true axial position and does not have the offset that is associated with intensity-based methods. Additionally, it is possible to exclude noise that shows up as false particles since noise does not have the same phase response as a real particle. The second method, that uses a parametric model, also improves the standard deviation in the positioning.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
Digital Holography, Polarization, Particle Scattering, Metrology
National Category
Applied Mechanics
Research subject
Experimental Mechanics
Identifiers
urn:nbn:se:ltu:diva-68374 (URN)978-91-7790-114-3 (ISBN)978-91-7790-115-0 (ISBN)
Presentation
2018-06-14, E243, Luleå Tekniska Universitet, Luleå, 09:00 (Swedish)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2014-4906
Available from: 2018-04-17 Created: 2018-04-16 Last updated: 2018-06-12Bibliographically approved
2. Polarization-Resolved Particle Holography
Open this publication in new window or tab >>Polarization-Resolved Particle Holography
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Polarisationsupplöst Partikelholografi
Abstract [en]

In this thesis, measurement of particle fields using digital holography is the main subject. The questions investigated consider positioning, identification and sizing of nanometer and micrometer particles. The thesis explore these topics using both simulations and experimental measurements. Measuring particles is inherently a three-dimensional problem. Digital holography is, therefore, chosen as the measurement technique since it can record three-dimensional information in the interference pattern.

Two main digital holographic setups are considered in this work, one single-view and one dual-view, both with off-axis configuration for the reference wave. Methods for positioning along the optical axis is the central question for the single-view system. This work presents a new method for axial positioning based on the wavefront curvature of the scattered light. In the reconstructed volume, along the optical axis, the scattered wave changes from converging to diverging around its location. This assumption is verified using simulations. An estimation of the position where this change occurs, hence, is an estimation of the actual axial position. Two different methods for quantification of the wavefront curvature is presented. The first uses the finite difference method of the reconstructed phase. The second uses a Chebyshev model for the phase-response. The difference between the two methods is that the one based on the Chebyshev model is more robust and less sensitive to noise.

The dual-view system is an extension of the single-view setup where an identical system is placed perpendicular with the first system. The sample is illuminated from below, making the angle between the illumination and the two systems $90^\circ$. The concept of polarization-resolved registration is also incorporated in the detection. This detection is made possible by using two reference waves with linear and mutually orthogonal polarization at different off-axis tilts. One hologram can, therefore, be reconstructed into two complex amplitudes, one for each polarization component. The measurements using this system focus on how particle properties influence the polarization-response. The transition from single to dual-view polarization-resolved detection increases the complexity in the reconstruction. There is a need for accurate calibrations for this type of setup. The thesis contains details on calibrations of both the spatial mapping and polarization detection.

The first application of the dual-view polarization-resolved setup is for identification and size estimation of nanometer-sized particles. The T-matrix method is used to establish a model for the sizing of spherical and spheroid particles. It is possible to estimate the size unambiguously up to approximately 200 nm for smooth particles. The sizing is limited to this lower size range.

The second application investigates how the detected polarization varies for different kinds of microplastics. The measurements show that the microplastics have a complex polarization-response, indicating an irregular and non-spherical shape. In general, for both sizing and identification, the problem becomes more complex as the size increases and the particle shape is less smooth.

This thesis shows that it is possible to estimate particle information from the polarization-response. However, the method is constricted both by the size and complexity of the particles.

Place, publisher, year, edition, pages
Luleå University of Technology, 2020
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Applied Mechanics
Research subject
Experimental Mechanics
Identifiers
urn:nbn:se:ltu:diva-78513 (URN)978-91-7790-581-3 (ISBN)978-91-7790-582-0 (ISBN)
Public defence
2020-06-11, E632, Luleå tekniska universitet, 97187 Luleå, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2020-04-16 Created: 2020-04-16 Last updated: 2020-05-15Bibliographically approved

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Öhman, JohanSjödahl, Mikael

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