Terahertz deconvolution (original) (raw)

Terahertz time-gated spectral imaging for content extraction through layered structures

Nature Communications, 2016

Spatial resolution, spectral contrast and occlusion are three major bottlenecks for non-invasive inspection of complex samples with current imaging technologies. We exploit the sub-picosecond time resolution along with spectral resolution provided by terahertz timedomain spectroscopy to computationally extract occluding content from layers whose thicknesses are wavelength comparable. The method uses the statistics of the reflected terahertz electric field at subwavelength gaps to lock into each layer position and then uses a time-gated spectral kurtosis to tune to highest spectral contrast of the content on that specific layer. To demonstrate, occluding textual content was successfully extracted from a packed stack of paper pages down to nine pages without human supervision. The method provides over an order of magnitude enhancement in the signal contrast and can impact inspection of structural defects in wooden objects, plastic components, composites, drugs and especially cultural artefacts with subwavelength or wavelength comparable layers.

Nondestructive Extraction of Parameters of Multilayered Media Using Terahertz Pulse Technique

Frequenz, 2019

In this work, an efficient non-invasive terahertz pulse technique is proposed and investigated to determine the thickness and refractive index of each layer in an optically thick stratified media. A closed form formulations are derived for simultaneous extraction of the thickness and complex refractive index of each layer with the help of primary reflected signals from the multilayered structure. The proposed technique is numerically tested using a full wave electromagnetic simulator and is experimental verified in the millimeter wave frequency range by utilizing the power peaks corresponding to the primary reflected signals. The numerical and measured results of multilayered samples under test are in good agreement with the reference data. The proposed terahertz pulse technique can be used for non-destructive testing of the multilayered system existing in various industries.

Terahertz spectroscopy and imaging for material analysis in conservation science

2010 10th IEEE International Conference on Solid Dielectrics, 2010

Terahertz (THz) waves can penetrate opaque materials and fingerprint spectra appear as those in infrared bands. The technique is expected to be used as a new non-invasive analysis method for various materials. Time domain reflection imaging, in particular, uses THz pulses that propagate in specimens, and in this technique, pulses reflected from the internal boundaries of the specimen indicate the internal structure. We have developed a spectral database of painting materials, and applied THz time domain imaging for analysis of artworks. Experimental results, including first ever non-invasive cross section image of a tempera masterpiece, proved that THz wave can observe layer structure of the artwork from the wood support, gesso preparation layers, and the painting layer. This technique should also be useful to detect internal defect of various types of opaque dielectric materials such as multi layer insulations.

Self-Referenced Method for Geometrical Distortion Removal in THz Time-Domain Reflection Imaging

IEEE Transactions on Terahertz Science and Technology

In this article, we develop a method for removing the phase drift induced by physically distorted object in terahertz timedomain reflection imaging (THz-TDRI). The proposed approach is defined as self-referenced, as it does not rely on any numerical parameter optimization nor extra instrumental components, and it is based on the unique manipulation of time-domain imaging data. In fact, we demonstrate that the problem can be solved assuming a linear contribution of the temporal shift induced by surface curvature. We illustrate how the self-referenced method is modeled and implemented, and we report the results obtained on two objects with different characteristics: a tilted and highly reflective surface, and a warped and heterogeneous surface. The proposed method demonstrates how to successfully remove the phase alterations induced on the reflected electric field, and how to repair the heavily corrupted images in the frequency-domain. Index Terms-Image restoration, phase variation problem, reflection geometry, terahertz time-domain reflection imaging (THz-TDS). I. INTRODUCTION T HE use of electromagnetic radiation at terahertz frequencies (from 0.1 to 10 THz) is becoming highly popular, mostly thanks to the increased availability of stable devices. A valuable characteristic of THz radiation is that it can penetrate a wide range of nonconducting materials, including plastics, polymers, and ceramics [1], [2], [3]. It is strongly absorbed by polar molecules, such as water [4], reflected by metals [5], and at the same time, it is not ionizing and harmless for biological tissues [6]. For these reasons, THz-waves are broadly applicable in many disciplines, such as bio-medicine, agriculture, and security and communication services [7], [8], [9]. Terahertz time-domain spectroscopy (THz-TDS) is appreciated especially in the field of material studies because it enables

Deconvolution: Imaging the unturned page

2011 International Conference on Infrared, Millimeter, and Terahertz Waves, 2011

The ability to retrieve information from different layers within a stratified sample using terahertz pulsed reflection imaging and spectroscopy has traditionally been resolution limited by the pulse width available. In this paper, a deconvolution algorithm is presented which circumvents this resolution limit, enabling deep sub-wavelength and sub-pulse width depth resolution. The algorithm is explained through theoretical investigation, and demonstrated by reconstructing signals reflected from boundaries in stratified materials that cannot be resolved directly from the unprocessed timedomain reflection signal. Furthermore, the deconvolution technique has been used to recreate subsurface images from a stratified sample: imaging the reverse side of a piece of paper.

Terahertz subsurface imaging system

A subsurface imaging system based on a terahertz time-domain spectrometer (THz-TDS) is described in this paper. The system performance has been simulated in terms of spatial resolution, penetration capabilities and SNR. Moreover, a commercial THz-TDS has been used to perform the proof-of-concept of the described system.

Advanced analysis concepts for terahertz time domain imaging

Optics Communications, 2007

Imaging based on ultrashort terahertz (THz) pulses (100-3000 lm) is investigated. The measured pulses are analyzed and the resulting amplitude and time delay information are compared. An algorithm for discrimination of multiple pulses is presented, which can distinguish several layers inside an object. A new measurement concept is presented, which accelerates the measurement of samples with small optical path differences about two orders of magnitudes. Exemplarily different applications from the field of quality management are shown.

Pre- and post-processing for tomographic reconstruction of terahertz time-domain spectroscopy

Optics Express, 2013

Reflection-type terahertz tomography is obtained using timedomain spectroscopy. Due to different velocities of the terahertz ray in free space and inside a sample, the tomographic transverse plane is not obtained by a simple reconstruction using time index. A pre-processing method is proposed to compensate for the different velocities of the terahertz ray for tomographic reconstruction. Maximum intensity projection, averaging, and short-time Fourier transform are proposed as post-processing methods along the depth direction for the terahertz tomography. Log-scale display is also suggested for a better visualization. Some experimental results with the pre-and post-processing are demonstrated.

Improved sample characterization in terahertz reflection imaging and spectroscopy: erratum

Optics Express, 2011

For imaging applications involving biological subjects, the strong attenuation of terahertz radiation by water means that terahertz pulsed imaging is most likely to be successfully implemented in a reflection geometry. Many terahertz reflection geometry systems have a window onto which the sample is placed -this window may introduce unwanted reflections which interfere with the reflection of interest from the sample. In this paper we derive a new approach to account for the effects of these reflections and illustrate its success with improved calculations of sample optical properties.