KEYWORDS: Terahertz radiation, Signal to noise ratio, Diffraction, Reconstruction algorithms, Phase retrieval, Phase distribution, High dynamic range imaging, Wavefront reconstruction, Data acquisition, Wavefronts
In the present work, we investigate the multiple plane phase retrieval (MPPR) algorithm which addresses the issue of missing center in diffraction patterns, which results from saturation due to the use of a detection system with a limited dynamic range. Using an extrapolation technique in saturated areas, we algorithmically overcome this hardware constraint associated with the restricted dynamic range of the detection system. The validation of the proposed phase retrieval method is conducted on a dataset captured in the terahertz (THz) region, which contains overexposed areas. We compared the reconstructed amplitude and phase distributions obtained from incomplete data with reference images obtained using a conventional algorithm when processing a high dynamic range (HDR) dataset. The algorithm’s convergence was examined when processing datasets with different levels of noise and saturation. Phase retrieval is shown to be possible even if 90−94% of the total energy of the beam is compromised by saturation in recordings, and at a low signal-to-noise ratio (SNR) near 6. Our proposed method effectively addresses the limitation associated with the insufficient dynamic range of the receiver, significantly simplifying data acquisition compared to HDR registration. For the first time, we demonstrate the application of MPPR to address the issue of data saturation and show the feasibility of retrieving the wavefront using only information from the high-frequency diffraction components while extrapolating the missing center information.
In this paper, we present a review of the latest results obtained in the scope of ghost imaging using auxiliary multiplex channels. The proposed methods make it possible to increase the speed of image acquisition, as well as eliminate distortions in the signal of a bucket detector. One of the most serious limitations of ghost imaging is the large number of measurements required. During the data acquisition, the optical environment where the observation object is located may change its parameters, which will negatively affect the quality of the reconstructed image. To solve these problems, it is possible to use fast spatial light modulators to set structured patterns and fast detectors. However, such equipment is not available in some spectral ranges and is not able to suppress distortions in the detected signal. In this regard, our team has proposed a number of techniques to solve the problems of a large number of measurements and non-stationarity of the optical medium in ghost imaging systems. First of all, an auxiliary polarization channel method was presented in a ghost imaging system to eliminate distortion in the signal of a bucket detector. Secondly, wavelength multiplexing methods have been proposed in the visible and terahertz wavelength ranges. These methods will make ghost imaging systems more applicable to the tasks of remote sensing and mapping of areas in various spectral ranges.
Recently, data mining and neural networks are increasingly used for wavefront recognition from interferograms. In this case, there is considerable freedom in choosing the structure of the reference beam. In this work, a comparative study of the effectiveness of using neural networks for solving the problem of recognizing wavefront aberrations based on linear (flat reference beam) and conical (conical reference wavefront) interferograms is carried out. The effectiveness of recognition of types and levels of aberrations by conical interferograms based on the use of neural networks is shown: the average absolute error is reduced by 3 times, compared with linear interferograms. This effect is related to the rotational invariance of the introduced aberrations.
We introduced several approaches of terahertz wavefront phase retrieval from intensity measured in a volumetric grid. Our developments include several experimental solutions for the registration of multiple intensity distributions spaced along the optical axis for two types of terahertz sources, namely Gunn diode with frequency multiplication chain and quantum cascade laser. We implemented several measurement modes: (i) sequential raster scanning by single Schottky diode with two lock-in amplifiers, complimentary tuned to different sensitivities for high dynamic range recording; (ii) step by step registration on matrix photodetectors, with averaging over several images for every measurement plane; (iii) continuous measurement during the displacement of the motorized translation stage. The high dynamic range data acquisition allowed us to successfully implement single-beam terahertz surface profilometry in the reflection, while the on-the-go recording ensures the shortest measurement times. In addition, we experimentally appraised two matrix detectors (INO and I2S) and applied several phase retrieval algorithms which proved their effectiveness in various experimental conditions, namely for the intensity registration in various diffraction zones and axial measurement plane allocations.
In this contribution, we discuss the features of design and polarimetric inspection of terahertz achromatic waveplates. The design of the crystalline quartz half-wave plate was performed taking into account the ellipticity and the introduced phase difference between the orthogonal components of the output radiation polarization vector. The designed waveplate are relatively thin, work equally efficiently in the frequency range from 0.4 to 1.4 THz, and, most importantly, are cheap to manufacture. The modification of the experimental terahertz time-domain spectroscopy polarimetric setup involving direct waveform detection is proposed. The proposed polarimetry THz time-domain spectrometer with electro-optic detection shows parasitical signals absence and easier measurement procedure.
In this contribution, we investigate the features of the two-beam interference of a set of wave trains, in which one of the beams contains an orbital angular momentum caused by the presence of inline topologically charged vortices in each spectral component. The infrared pump beam that generates terahertz radiation is first doubled in Mach-Zehnder interferometer with a specific time delay value to form the wave train with quasidiscrete temporal spectrum. Then, the two delayed terahertz pulses are split into two arms. One of the arms contains a delay line and geometric phase shaping elements for broadband uniformly topologically charged beam formation. The resulting structures are combined with a beamsplitter and detected with a terahertz holographic system upon their propagation. We analyze the features of the resulting spatio-frequency structures and discuss the possibility to implement the information encoding without spectral decomposition.
We present validation of a tilt angle illumination technique for height maps’ reconstruction using digital holography and propose a few solutions for improvement of the inclined probe wave front approach. The general algorithm of tilt angle digital holography approach is described and validated in experimental monitoring of a coin relief. Several approaches of height map quality improvement concerning tilt angle estimation, relation between measurement accuracy and dynamic range, and selective averaging algorithm are proposed and discussed.
Terahertz pulse time-domain holography (THz PTDH) is an ultimate technique both for the measurement of object properties in the THz range and broadband wavefront sensing. In this proceeding, we reveal the key principles of the technique, including the layout solutions for recording a collimated THz wavefront in the form of spatio-temporal profiles. The possibilities to investigate ultrashort THz field propagation dynamics based on the data measured in one transverse plane is discussed. The evolution for both transverse and longitudinal components of the electromagnetic field thus can be estimated. We illustrate these possibilities on the example of Bessel-Gaussian pulsed THz beam propagation formed by an on axicon lens.
Terahertz pulse time-domain holography (THz PTDH) is an ultimate technique both for the measurement of object optical properties and broadband wavefront sensing. However, THz PTDH has valuable restriction connected with low signal-to-noise ratio which becomes a serious issue in coherent measurements. This noise problem could be solved by filtering with use of modern block-matching algorithms based on nonlocal similarity of small patches of images existing in investigated objects. Here we present the study on the use of denoising algorithms applied for hyperspectral THz data in the spatio-temporal and spatial-spectral domain. We provide a numerical simulation of denoising in case of broadband uniform topologically charged (BUTCH) beam of pulsed THz radiation.
Complex wavefront manipulation is a promising technique for many applications of optics and photonics. In this report we will present our results on development of DMD wavefront correction experimental setup and discussion of its performance for various parameters of binary fringe pattern and 1st diffraction order filtration aperture. It was shown that trade-off between spatial resolution and discretisation of the desired amplitude and phase distribution should be achieved. Decrease of the binarized interference fringes width results in higher spatial resolution of the modulated complex wave but increase discretisation of amplitude and phase distributions as well. The correction of wavefront aberrations using digital micromirror device was performed. We observed significant reduction in wavefront phase error by conduction of Zernike polynomials decomposition.
This work further develops a recently proposed time-resolved inline digital holography (TRIDH) [Petrov, N. V. et al. Opt. Lett. 43, 3481 (2018)] for studying degenerate phase modulation induced by an inclined collimated pump beam in the glass substrate with the quantum dots at the surface. Similar to many techniques for measuring nonlinear properties of materials, it is based on a comparison of the prediction obtained by the mathematical model of the phenomenon with experimental data. We have extended the mathematical model for the case of interaction of two femtosecond laser pulses in the double-layered sample. The impact of the ratio between nonlinear refractive indexes of two layers and their thicknesses on induced phase modulation is analyzed.
We investigate an approach to short and medium-range wireless communications based on the use of terahertz beams possessing an orbital angular momentum (OAM) that allows for noise-resistant broadband carrier. A the- oretical model of the proposed beams generation is developed and numerical predictions are given for propagation and visualization of complex-structured THz beams, including ones carrying a unit topological charge on a large number of spectral components of broadband terahertz radiation. The assessment method which in our case is terahertz pulse time-domain holography allows for analyzing spatio-temporal and spatio-spectral evolution of arbitrary shaped THz wave trains during their propagation in free space and interaction with obstacles.
Pulse holographic imaging along with time-domain spectroscopy scan and tomographic techniques are of great interest. Since the advantages of holography are the lack of focusing optics and high spatial resolution, and, comparing with tomography, less computation cost for numerical reconstruction, this technique is preferable for the analysis of thin histological samples. In this work we have created the experimental scheme that involves measurement of diffraction pattern of the collimated THz pulse field spatial distribution at some distance behind the object in the time-domain mode, thus allowing reconstruction of amplitude and phase distribution at the object plane by numerical backpropagation of the wavefront in the spectral domain. In our experiment, we used a breast biopsy sample containing cancer tissues, we also performed numerical simulations accounting for experimental conditions to confirm the conceptual applicability of the reconstruction method.
This article considers the use of holographic interferometer to overwrite the holograms for distortion correction. Each optical system contains some deviations of the beam path, called aberrations of the optical system. They are considered in the resulting interference figure as a distortion of the bands. While increasing the sensitivity of the interference pattern, new aberrations caused by re-registration of the installation in addition to the aberrations already presented on the interferogram caused by the initial record, also multiplied by N times, are introduced N times. In this experiment we decided to use a modified setup with spatially combined interferograms with use of reflective SLM (spatial light modulator) LETO and digital image handling of the interferograms recorded by CCD or CMOS camera.
The article considers the use of holographic interferometer to overwrite the holograms for distortion correction. Each optical system contains some deviations of the beam path, called aberrations of the optical system. They are considered in the resulting interference figure as a distortion of the bands. While increasing the sensitivity of the interference pattern, new aberrations caused by re-registration of the installation in addition to the aberrations already presented on the interferogram caused by the initial record, also multiplied by N times, are introduced N times. In this experiment we decided to use a modified setup with spatially combined interferograms with use of reflective SLM (spatial light modulator) LETO and digital image handling of the interferograms recorded by CCD or CMOS camera.
In this work we demonstrate wavefront complex modulation of semiconductor light sources via digital micromirror device (DMD). Proposed holographic configuration allows to correct the aberrations caused by the imperfections of the DMD and the optical elements. We consider the blazing effect of the DMD surface and configured the optimal condition of the optical setup for certain DMD and the wavelength of the radiation source. The technique was approved with the experiment of obtaining different kinds of wavefront distributions from the semiconductor laser with high M2 factor.
We propose an algorithm for absolute phase retrieval from multiwavelength noisy phase coded diffraction patterns. A lensless optical system is considered with a set of successive single wavelength experiments (wavelength-division setup). The phase masks are applied for modulation of the multiwavelength object wavefronts. The algorithm uses the forward/backward propagation for coherent light beams and sparsely encoding wavefronts, which leads to the complex-domain block-matching three-dimensional filtering. The key-element of the algorithm is an original aggregation of the multiwavelength object wavefronts for high-dynamic-range absolute phase reconstruction. Simulation tests demonstrate that the developed approach leads to the effective solutions explicitly using the sparsity for noise suppression and high-accuracy object absolute phase reconstruction from noisy data.
One of the problems of interferometric methods is the difficulty of measuring surface shape with sharp boundaries due to the wavelength-limited dynamic range of the measurement. To circumvent this limitation multiwavelength methods or techniques based on hologram capturing at the different tilt of the illumination beam are applied. In this work we examine the performance of the digital holographic interferometry with multi-inclination illumi- nation in the numerical and real experiments. Lensless implementation of the technique implies the wavefront propagation by numerical algorithms. In this regard the speckle scattering in the Fresnel diffraction area caused by surface roughness and the impact of distance from the object to the registration plane are analyzed. Since shape measurement is based on the calculation of phase difference for the wavefronts recorded with tilt of the object illuminating beam, the requirements to preciseness of measurements of the angle of incidence of this beam are considered. The algorithm of the inclination angle determination are developed. The performance of noise suppression techniques, namely sine-cosine and BM3D methods are considered for high noisy conditions, when the phase distributions are formed by reflecting object with a great roughness and height differences.
In this proceeding, we present the description of the numerical approach for resolution enhancement, field of view widening and noise reduction in pulse time-domain holography. The approach comprises iterative procedure of the recorded hologram self-extrapolation into wider spatial area, and consecutive ‘self-healing’ of an object. The concept has been proofed on a synthetically generated pulse time-domain holograms. The proposed method is sought after, especially in THz range, where the distance between the object and the hologram lies in the order of several tens of wavelengths, and the detector sizes are usually limited, and with minor modifications can be applied for other THz holographic approaches.
We propose a new algorithm for absolute phase retrieval from multiwavelength noisy phase coded diffraction patterns in the task of surface contouring. A lensless optical setup is considered with a set of successive single wavelength experiments. The phase masks are applied for modulation of the multiwavelength object wavefronts. The algorithm uses the forward and backward propagation for coherent light beams and sparsely encoding wavefronts which leads to the complex-domain block-matching 3D filtering. The key-element of the algorithm is an original aggregation of the multiwavelength object wavefronts for high-dynamic-range profile measurement. Numerical experiments demonstrate that the developed approach leads to the effective solutions explicitly using the sparsity for noise suppression and high-accuracy object profile reconstruction.
In this proceeding, we discuss the method that allows for field of view and reconstruction quality enhancement of pulsed THz holograms recorded by matrix detectors that do not exceed the the object transverse dimensions, at distances, that are comparable with the object size. The method comprises the use of random phase mask situated between the object and the hologram, at the hologram registration process. The introduced phase variation levels out the input from closer and further (to the hologram pixel) points of the object, and thus improves overall reconstruction quality. Here, we study numerically this approach and demonstrate 4 times increase of the properly reconstructed object area, if compared to the undisturbed hologram recording, and consecutive increase of the correlation between the reconstructed and actual object from 0.34 to 0.82.
The method of amplification of hologram was applied to the so-called Rozhdestvenskiy hooks, that were obtained in the Rozhdestvenskiy interferometer (Michelson interferometer, combined with a grating spectrograph). In such a device the absorption lines reveal themselves as specific “hooks”, whose curvature provides the information about the atomic oscillator force. The holographic amplification “smoothes” the hooks and thus makes their analysis much simpler.
Measurements of average phase shifts introduced by living HeLa cells to probe wave front were carried out. Variations of this value were monitored in the course of morphological changes caused by photodynamic treatment at various irradiation doses. Observations of changes in living cells were also performed by means of far field optical microscopy and confocal fluorescent microscopy. Quantitative analysis of the data obtained shows that average phase shift introduced by the cells may either increase or decrease depending upon major parameters of the treatment.
In-line lensless holography is considered with a random phase modulation at the object plane. The forward wavefront propagation is modelled using the Fourier transform with the angular spectrum transfer function. The multiple intensities (holograms) recorded by the sensor are random due to the random phase modulation and noisy with Poissonian noise distribution. It is shown by computational experiments that high-accuracy reconstructions can be achieved with resolution going up to the two thirds of the wavelength. With respect to the sensor pixel size it is a super-resolution with a factor of 32. The algorithm designed for optimal superresolution phase/amplitude reconstruction from Poissonian data is based on the general methodology developed for phase retrieval with a pixel-wise resolution in V. Katkovnik, ”Phase retrieval from noisy data based on sparse approximation of object phase and amplitude”, http://www.cs.tut.fi/~lasip/DDT/index3.html.
In this paper we propose an investigation of digital optical phase conjugation (DOPC) method applicability and efficiency for the problem of three-dimensional (3D) holographic imaging. We validate the basic properties of developed DOPC-based method for different cases of imaging objects, from simple two-dimensional (2D) case to 3D figure composed of 2D polygons. We implement the method of adaptive optimization of the wavefront (AOWF) for auxiliary image formation. Since the simplicity and universality of AOWF method, this approach is useful for the fast basic 3D holographic image formation.
In this paper we propose a technique of the distribution of transparent particles suspended in a volume investigation.
We use a method that implies division of the volume into a plurality of layers containing particles therein.
An inline hologram of this volume is reconstructed in two adjacent layers, which are compared using correlation
function. We have derived dependencies of correlation on particle parameters. We perform an experimental
validation of this study. This technique is useful for applications that require instant assessment of the particle
distribution.
The paper presents results on singlet oxygen detection in aqueous solutions of a photosensitizer based on the reconstruction of 3D temperature gradients resulting from nonradiative deactivation of excited oxygen molecules. 3D temperature distributions were reconstructed by means of the inverse Abel transformation from a single digital hologram in the case of cylindrically symmetric distribution of the temperature gradient and using holographic tomography algorithm with filtered back projection in the case of nonsymmetrical distribution. Major features of the applied techniques are discussed and results obtained by the two methods are compared.
KEYWORDS: Particles, Image segmentation, Image processing, Statistical analysis, 3D image reconstruction, Signal to noise ratio, Digital holography, Holograms, Wave propagation, Radio propagation
We give a broad discussion of existing typical works devoted to particle image processing. We propose the approach based on the postprocessing of coherent images of the particles at various planes of the volume. These images can be obtained both by reconstruction of inline digital hologram and by means of defocussing of the lens with high numerical aperture. Processing of the reconstructed holograms or recorded images is carried out using the proposed image analysis approach based on the edge-point linking and thresholding technique, which is considered to be simple to implement and reliable. After the review of existing methods and approaches, we noted that, in general, only cases of low concentrations are considered and, therefore, we investigated the performance of our proposed approach for characterization of particles of high density in a volume of optical medium. In this study of the method, we increase the concentration of particles until we ensure that every volume element comprises many particle images, yet these images do not create a speckle pattern, and look for the concentrations at which normalized density distributions of the particles can be constructed with an acceptable error for us. It is shown that the proposed approach exhibits good results of recognition and allows investigation of high concentrations.
Digital holography is widely used nowadays for interferometric studies of various objects and processes. However, peculiarities of objects under study often imply difficulties in holograms recording, reconstruction and processing. One of the major factors is a typically large number of singular points at phase distributions caused by either low signal to noise ratio at the recorded holograms or sample inhomogeneities. The basic operations applied for absolute phase extracting from digital holograms are noise filtration, phase unwrapping and subtraction of phase distributions. In this paper we demonstrate that the sequence of these operations may drastically affect the resulting image quality and the data obtained. An optimized algorithm suitable for studies of dynamic processes in biological media on microscopic level has been developed. The algorithm was applied for monitoring of nonradiative deactivation processes occurring in onion cell specimens at photosensitized generation of singlet oxygen.
Modern structural elements are often made of laminated polymer materials or composites on the base of polymer matrices. The proper functioning of these elements may be of vital importance especially in automotive and aerospace industries, in gas and oil transportation. The major problem in their performance is a possibility of a sudden and irreversible delamination caused by various factors. We propose and study a NDT approach aimed to detect delamination areas in adhesively bonded layered structural elements made of different materials. The proposed approach is evaluated by use of holographic detection and monitoring of the evolution of bulk strain solitons generated in such structures.
The paper presents a novel combined approach aimed to detect and monitor singlet oxygen molecules in biological specimens by means of the simultaneous recording and monitoring of their deactivation dynamics in the two complementary channels: radiative and nonradiative. The approach involves both the direct registration of phosphorescence at the wavelength of about 1270 nm caused by radiative relaxation of excited singlet oxygen molecules and holographic recording of thermal disturbances in the medium produced by their nonradiative relaxation. The data provides a complete set of information on singlet oxygen location and dynamics in the medium. The approach was validated in the case study of photosensitized generation of singlet oxygen in onion cell structures.
The topic of sparse representations (SR) of images has attracted tremendous interest from the research community in the last ten years. This interest stems from the fundamental role that the low dimensional models play in many signal and image processing areas, i.e., real world images can be well approximated by a linear combination of a small number of atoms (i.e., patches of images) taken from a large frame, often termed dictionary. The principal point is that these large dictionaries as well as the elements of these dictionaries taken for approximation are not known in advance and should be taken from given noisy observations. The sparse phase and amplitude reconstruction (SPAR) algorithm has been developed for monochromatic coherent wave field reconstruction, for phase-shifting interferometry and holography. In this paper the SPAR technique is extended to off-axis holography. Pragmatically, SPAR representations are result in design of efficient data-adaptive filters. We develop and study the algorithm where these filters are applied for denoising of phase and amplitude in object and sensor planes. This algorithm is iterative and developed as a maximum likelihood optimal solution provided that the noise in intensity measurements is Gaussian. The multiple simulation and real data experiments demonstrate the advance performance of the new technique.
The study of particles that are transparent to the probing radiation but introduce a phase delay appears to be relevant especially in the fields of biology and medicine. In this paper, we propose an approach to the study of the distribution of transparent particles suspended in a volume of optical medium, which combines the method of digital holography and the concept of singular optics. For the numerical study of the particles, we use a method, based on the obtainment and analysis of zerograms that correspond to the spatial distributions of amplitude zeros of the complex amplitude of the field. We explore the features of the application of analysis of the amplitude zero distributions in solving the problem of studying transparent particles suspended in a volume of the optical medium. We investigate the effect of various particle parameters on the recorded hologram and the structure and distribution of the amplitude zeros of the electromagnetic field. We demonstrate the use of histograms of distributions of local densities of amplitude zeros for the characterization of a higher number of transparent particles in volume. Numerical experiments on the use of analysis of the amplitude zeros of the field have shown that there are a number of particular qualities in the distribution of the zeros of amplitude, which can be subsequently used to develop more accurate and efficient method of characterization of transparent particles.
In this paper we consider using the terahertz (THz) time domain spectroscopy (TDS) for non destructive testing and determining the chemical composition of the vanes and rotor-blade spars. A versatile terahertz spectrometer for reflection and transmission has been used for experiments. We consider the features of measured terahertz signal in temporal and spectral domains during propagation through and reflecting from various defects in investigated objects, such as voids and foliation. We discuss requirements are applicable to the setup and are necessary to produce an image of these defects, such as signal-to-noise ratio and a method for registration THz radiation. Obtained results indicated the prospects of the THz TDS method for the inspection of defects and determination of the particularities of chemical composition of aircraft parts.
An experimental comparison of four methods of wavefront reconstruction is presented. We considered two iterative and two holographic methods with differences in mathematical models and reconstruction algorithms. The first two of these methods do not use the reference wave in the recording scheme that reduces the need of setup stability. A set of spatial intensity measurements of a volume scattered field plays the main role in phase retrieval in such methods. The obtained data are sequentially used for iterative wavefront reconstruction. Iterative approach involves numerical wavefront propagation between various planes of the volume scattered fiels. Throughout this procedure the phase information of the wavefront is retained while the calculated amplitudes is replaced by the square root of the intensity distributions measured in corresponding planes. In the first compared phase retrieval method (FRIM), a two-dimensional Fresnel transform and iterative calculation in the object plane are used as a mathematical model. In the second method (SBMIR), the angular spectrum is used for numerical wavefront propagation, and iterative calculation is made only between closely spaced planes for data registration. Two methods of digital holography, which we compared, differ from each other in algorithm of a waverfont reconstruction. The first holographic method (CWR-DH) uses the conception of spatial phase steps for complex wave retrieval, and the second method (FT-DH) is a widespread Fourier transformation method. All methods provide satisfactory capacity for image reconstruction. The results of the comparison showed that FRIM produces better quality of reconstruction, but a diffraction artifacts takes place at the boundaries of the reconstructed image. Taking this into account we can conclude that the CWR-DH method is the best among considered.
In this paper, the problems of recognition of individual particle images in the volume of optical medium conditioned upon their coherent superposition are described. To evaluate the efficiency of methods of particle image recognition, a typical problem of analysis of the intensity distributions formed by laser radiation scattered on suspended particles in a volume of an optical medium is considered. As a result of applying of the method of statistical accounting of particle images, based on the edge-point linking and thresholding technique, normalized density distributions of particles in the image plane are obtained. Evaluation of the performance of applied recognition method for individual particle images is conducted using correlation analysis to assess the quality of obtained images.
We study interaction of structured illumination with randomly scattering media. Based on the equations of
scalar diffraction theory we perform a numerical simulations of the propagation of a structured illumination
through various scattering media. Applying the continuous sequential algorithm of adaptive phase optimization
for focusing a scattered wavefront to a target, we explore the resulting phase distributions at the input and
output of the scattering media. We demonstrate that corrected wavefront takes a spherical shape, which can be
removed for imaging through the scattering sample. According to the obtained results the using scalar diffraction
theory extends the limits of applicability of the sequential adaptive phase optimization algorithm beyond the
focusing, making they as broad as in transmission matrix method.
In this paper, we present review and latest results obtained in the scope of terahertz holographic and other
methods for phase retrieval in terahertz imaging. Not only accurate change of amplitude, but also rigorous
phase retrieval is essential for precise calculation of optical parameters of the samples in terahertz range. Pulse
terahertz holography introduced some years ago shows itself as perfect method for overall-object phase retrieval
technique, but in the same time it allows measurement with low signal to noise that leads to less precise derivation
of sample optical parameters. And certainly just point-by-point terahertz time-domain spectroscopy provides the
most precise information of sample phase, but it is rather time consuming and has low spatial resolution as well.
The other possible way assumes, in contrary to pulse terahertz holography and spectroscopy, using narrow-band
continuous terahertz source, which tunability might also make the measurement process easier. And diffraction
patterns registered with microbolometer array or any other terahertz intensity sensor placed at several different
distances from the object and/or taken for several different terahertz frequencies are used for phase retrieval in
this case. We present both numerical predictions and experimental results for the proposed methods, estimate
the achievable spatial and other limits of the techniques and compare them to the others used in different spectral
ranges.
In this paper, the results of using the method of angular spectrum of plane waves as a mathematical model
of the iterative algorithm for phase retrieval based on the intensity distributions formed by the radiation of
THz frequency range are presented. This mathematical model is designed to calculate the wave field propagation
in the near-field diffraction, and in comparison with the Fresnel transform is more effective when working
with the THz radiation. The criterion for selection of registration planes which provides the best quality of
wave field reconstruction was determined as a result of studying the characteristics of the longitudinal intensity
distributions.
Using the infrared matrix of pyroelectric or other photodetectors along with THz band pass filters with pulsed
or CW sources one can record the 2D intensity distribution of THz radiation with a high degree of monochromatization.
This allows one to use various approaches to solving the phase problem which were developed for
the visible frequencies. In this contribution we present the results of the numerical investigation of the wavefront
reconstruction using THz radiation at several wavelengths and taking the intensity distribution at various
distances.
An iterative wavefront retrieval method based on intensity measurements formed by several wavelengths is investigated
in the present contribution. This multiwavelength technique is extended to use the intensity distributions
recorded in various planes of the volume speckle field. The ability to retrieve the wavefront using speckle patterns
is demonstrated in experiment. Two different experimental techniques have been used. The first proposed
method allows one to record three different intensity distributions corresponding to the three CCD RGB channels
at single exposure. This gives the advantage in the analysis of fast processes, e.g. phase microscopy of moving
biological cell-like objects investigation. The second technique involves using a large number of wavelengths of
supercontinuum radiation formed by photonic-crystal fiber. This approach provides faster and more accurate
convergence of the proposed method, has simple and rugged recording scheme with fiber optic elements.
We present a wavefront retrieval method for radiation comprising several wavelengths. Both numerical models
and experimental results are presented. Numerical modeling implies iterative phase retrieval procedure for all
wavelengths in spectrum. For reconstruction we can use two different algorithms, one inherits from one proposed
by Osten, Pedrini and Almoro, the second one implies expansion by Hermite-Gauss or Laguerre-Gauss basis set
which allows to decrease calculation time consumption. In experiment, speckle patterns can be formed either
by spectral supercontinuum radiation from photonic-crystal fiber (PCF) or by Stokes components of stimulated
Raman scattering (SRS) from second harmonic of pulse Nd:YAG laser radiation in barium nitrate crystal.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.