Improvements in Digital Holographic Microscopy

The Ph.D. dissertation consists of developing a series of innovative computational methods for improving digital holographic microscopy (DHM). DHM systems are widely used in quantitative phase imaging for studying micrometer-size biological and non-biological samples. As any imaging technique, DHM systems have limitations that reduce their applicability. Current limitations in DHM systems are: i) the number of holograms (more than three holograms) required in slightly off-axis DHM systems to reconstruct the object phase information without applying complex computational algorithms; ii) the lack of an automatic and robust computation algorithm to compensate for the interference angle and reconstruct the object phase information without phase distortions in off-axis DHM systems operating in telecentric and image plane conditions; iii) the necessity of an automatic computational algorithm to simultaneously compensate for the interference angle and numerically focus out-of-focus holograms on reconstructing the object phase information without phase distortions in off-axis DHM systems operating in telecentric regime; iv) the deficiency of reconstructing phase images without phase distortions at video-rate speed in off-axis DHM operating in telecentric regime, and image plane conditions; v) the lack of an open-source library for any DHM optical configuration; and, finally, vi) the tradeoff between speckle contrast and spatial resolution existing in current computational strategies to reduce the speckle contrast. This Ph.D. dissertation is motivated to overcome or at least reduce the six limitations mentioned above. Each chapter of this dissertation presents and discusses a novel computational method from the theoretical and experimental point of view to address each of these limitations

A New Versatile Electronic Speckle Pattern Interferometer For Vibration Measurements

Electronic speckle pattern interferometry (ESPI) has been widely used for vibration amplitude and phase measurements. Conventional ESPI systems are bulk and expensive and need careful alignment of all the optical components which is a time consuming task. To overcome these problems alternative compact ESPI systems were developed using fibre-optical components or holographic optical elements (HOEs). The fibre-optic based ESPI systems suffer from random phase fluctuations induced by environmental temperature changes. Hence HOEs can be used as more powerful alternative optical elements to design ESPI systems. The time average ESPI method is widely used for vibration studies. The time average method combined with phase stepping can be used for automatic vibration measurements. Using this technique higher vibration amplitudes cannot be measured because fringe patterns follow Bessel function intensity distribution. To overcome this problem an alternative technique can be used by modulating the phase of the reference beam in an unbalanced interferometer. This thesis reports a novel ESPI system for vibration measurements by combining use of holographic optical elements (HOEs) and optical path length modulation (reference beam phase modulation). The optical path length modulation is implemented using laser diode wavelength (frequency) modulation. Different HOE based ESPI systems are reported in this thesis using either a single HOE or dual HOE. This thesis examines performance of different HOE based ESPI systems that are sensitive to out-of-plane displacement components using laser diodes operating either in the near infrared or visible electromagnetic spectrum. Vibration modes of a circular metal plate clamped at the edges of a loud speaker and a circular metal plate driven by a piezoelectric actuator (PZT) were studied using a single RHOE based ESPI system and a hybrid (transmission HOE with a partially reflecting mirror) HOE based ESPI system respectively using a near infrared laser diode (763nm). Optical path length modulation technique was implemented using a laser diode operating in visible electromagnetic spectrum (658nm). Vibration mode patterns of a circular metal plate driven by a PZT actuator were obtained using both single RHOE and dual HOE based ESPI systems. Using optical path length modulation technique in a dual HOE based ESPI system detailed phase and amplitude maps of a circular metal plate driven by a PZT actuator are obtained. The dual HOE based ESPI system was also used for measuring roations of a circular metal plate mounted on a mirror mount. In conclusion we have developed a compact HOE based ESPI system to conduct vibration measurements. A few potential future developments are also suggested at the end of the thesis

Multisource Holography

Holographic displays promise several benefits including high quality 3D imagery, accurate accommodation cues, and compact form-factors. However, holography relies on coherent illumination which can create undesirable speckle noise in the final image. Although smooth phase holograms can be speckle-free, their non-uniform eyebox makes them impractical, and speckle mitigation with partially coherent sources also reduces resolution. Averaging sequential frames for speckle reduction requires high speed modulators and consumes temporal bandwidth that may be needed elsewhere in the system. In this work, we propose multisource holography, a novel architecture that uses an array of sources to suppress speckle in a single frame without sacrificing resolution. By using two spatial light modulators, arranged sequentially, each source in the array can be controlled almost independently to create a version of the target content with different speckle. Speckle is then suppressed when the contributions from the multiple sources are averaged at the image plane. We introduce an algorithm to calculate multisource holograms, analyze the design space, and demonstrate up to a 10 dB increase in peak signal-to-noise ratio compared to an equivalent single source system. Finally, we validate the concept with a benchtop experimental prototype by producing both 2D images and focal stacks with natural defocus cues.Comment: 14 pages, 9 figures, to be published in SIGGRAPH Asia 202

High-sensitivity interferometry

High-sensitivity interferometric techniques are considered for non-destructive testing applications. The methods enable quantitative measurement of optical path variations, resulting from dynamic changes within the test object. [Continues.

A Simple and Inexpensive Optical Technique to Help Students Visualize Mode Shapes

An imaging technique is introduced that is suitable for visualizing the mode shapes of vibrating structures in an educational setting. The method produces images similar to those obtained using electronic speckle pattern interferometry (ESPI) but it can be implemented for less than 1=10 the cost of a commercial ESPI system, and the apparatus is simple enough that it can be constructed by undergraduate students. This technique allows for real-time visualization of the normal modes and detection shapes of harmonically vibrating structures, including those with shapes that make generating Chladni patterns with sand or powder impossible. The theory of operation and construction details are discussed

Toward commercial realisation of whole field interferometric analysis

The objective of this work was to produce an instrument which could undertake wholefield inspection and displacement measurement utilising a non-contacting technology. The instrument has been designed to permit operation by engineers not necessarily familiar with the underlying technology and produce results in a meaningful form. Of the possible techniques considered Holographic Interferometry was originally identified as meeting these objectives. Experimental work undertaken 'provides' data which confirms the potential of the technique for solving problems but also highlights some difficulties. In order to perform a complete three dimensional displacement analysis a number of holographic views must be recorded. Considerable effort is required to extract quantitative data from the holograms. Error analysis of the experimental arrangement has highlighted a number of practical restrictions which lead to data uncertainties. Qualitative analysis of engineering components using Holographic Interferometry has been successfully undertaken and results in useful analytical data which is used in three different engineering design programmes. Unfortunately, attempts to quantify the data to provide strain values relies upon double differentiation of the fringe field, a process that is highly sensitive to fringe position errors. In spite of this, these experiments provided the confidence that optical interferometry is able to produce data of suitable displacement sensitivity, with results acceptable to other engineers.....

Roadmap on digital holography [Invited]

This Roadmap article on digital holography provides an overview of a vast array of research activities in the field of digital holography. The paper consists of a series of 25 sections from the prominent experts in digital holography presenting various aspects of the field on sensing, 3D imaging and displays, virtual and augmented reality, microscopy, cell identification, tomography, label-free live cell imaging, and other applications. Each section represents the vision of its author to describe the significant progress, potential impact, important developments, and challenging issues in the field of digital holography

Extending the measurement range of high-speed speckle pattern interferometry

A digital speckle pattern interferometer based on a complementary metal-oxide semiconductor (CMOS) camera was developed. The temporal evolution of dynamic deformation was measured using inter-frame phase stepping. A numerical and analytical investigation showed that the maximum surface velocity that can be reliably measured with inter-frame phase stepping corresponds to ±0.3 times the surface velocity at which the interferogram is sampled at the Nyquist limit (vNyq). The flexibility of the CMOS detector readout was used to identify regions of interest with full-field time-averaged measurements and then to interrogate those regions with time-resolved measurements sampled at up to 70 kHz. To increase the surface velocity measurement range, spatial phase stepping was introduced to the high-speed CMOS system. A pair of binary phase gratings introduced double-channel sensing with a fixed phase step between the two channels. The maximum surface velocity was increased to ±1.0vNyq. Sub-Nyquist theory was implemented for the dynamic measurements and the measurement range with a continuous-wave laser illumination was increased by an order of magnitude with respect to inter-frame phase stepping to ±3.0vNyq. A numerical and analytical investigation showed that with a reduced exposure, for example from pulsed laser illumination, the maximum surface velocity that can be reliably measured is ±15.9vNyq and the surface acceleration is ±253.3vNyq with the current set-up. Due to spatial variations in speckle intensity, some low-modulating and saturated pixels within the small regions of interest interrogated at up to 70 kHz could not be analysed. The nonlinear LinlogTM response of the CMOS camera was used to increase the valid measurement area on the object surface by incorporating pixels that would be below a modulation threshold or saturated if recorded with a linear CCD detector

Measurement of 3D Ultrasonic Wavefield Using Pulsed Laser Holographic Microscopy for Ultrasonic Nondestructive Evaluation

In ultrasonic array imaging, 3D ultrasonic wavefields are normally recorded by an ultrasonic piezo array transducer. Its performance is limited by the configuration and size of the array transducer. In this paper, a method based on digital holographic interferometry is proposed to record the 3D ultrasonic wavefields instead of the array transducer, and the measurement system consisting of a pulsed laser, ultrasonic excitation, and synchronization and control circuit is designed. A consecutive sequence of holograms of ultrasonic wavefields are recorded by the system. The interferograms are calculated from the recorded holograms at different time sequence. The amplitudes and phases of the transient ultrasonic wavefields are recovered from the interferograms by phase unwrapping. The consecutive sequence of transient ultrasonic wavefields are stacked together to generate 3D ultrasonic wavefields. Simulation and experiments are carried out to verify the proposed technique, and preliminary results are presented