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.
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.
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 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.
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.
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.
The optical properties of normal fibroblasts and fibroblasts cultured with cancer cells were studied in the frequency range of 0.2 - 1.0 THz. The results show the possibility to distinguish healthy cells from corrupted ones using their optical parameters.
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.
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