Time reversal and object reconstruction with single-cycle pulses
TL;DR: The reconstruction of one- and two-dimensional objects is demonstrated by numerically backpropagating measured scattered terahertz transients by determining the spatial resolution determined by the Sparrow criterion.
Abstract: We demonstrate the reconstruction of one- and two-dimensional objects by numerically backpropagating measured scattered terahertz transients. The spatial resolution determined by the Sparrow criterion is found to correspond to approximately 30% of the peak wavelength and 85% of the mean wavelength of the power spectrum of the single-cycle waveform.
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TL;DR: A comprehensive review of the various techniques used for terahertz image formation can be found in this paper, as well as numerous examples which illustrate the many exciting potential uses for these emerging technologies.
Abstract: Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.
962 citations
Cites methods from "Time reversal and object reconstruc..."
...Ruffin and colleagues measured the diffracted field transmitted through a patterned two-dimensional aperture at many locations after the aperture, and then back-propagated these measured fields using the Kirchhoff diffraction integral to reconstruct the aperture pattern [108, 109]....
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TL;DR: A tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional structures in the far-infrared, analogous to conventional CT techniques such as x-ray CT.
Abstract: We demonstrate a tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional (3D) structures in the far-infrared. This THz-wave computed tomography (T-ray CT) system provides sectional images of objects in a manner analogous to conventional CT techniques such as x-ray CT. The transmitted amplitude and phase of broadband pulses of THz radiation are measured at multiple projection angles. The filtered backprojection algorithm is then used to reconstruct the target object, including both its 3D structure and its frequency-dependent far-infrared optical properties.
322 citations
TL;DR: The uniqueness and limitations of T-ray imaging are discussed, the major challenges impeding T-rays imaging are identified, and solutions and opportunities are proposed.
Abstract: Terahertz (THz) science will profoundly impact biotechnology It has tremendous potential for applications in imaging, medical diagnosis, health monitoring, environmental control and chemical and biological identification THz research will become one of the most promising research areas in the 21st century for transformational advances in imaging, as well as in other interdisciplinary fields However, terahertz wave (T-ray) imaging is still in its infancy This paper discusses the uniqueness and limitations of T-ray imaging, identifies the major challenges impeding T-ray imaging and proposes solutions and opportunities in this field It also concentrates on the generation, propagation and detection of T-rays by the use of femtosecond optics
303 citations
Additional excerpts
...In addition, a biostatic THz imaging system consisting of THz receivers at multiple angles relative to the illuminating antenna has been used to image cylindrical reflecting structures [32] and irregular apertures [30]....
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TL;DR: In this paper, the phase and amplitude of a terahertz time-domain spectroscopy (THz-TDS) pulse at each frequency can be determined by using a digital holography method.
Abstract: Terahertz time-domain spectroscopy (THz-TDS) is a coherent measurement technology. Using THz-TDS, the phase and amplitude of the THz pulse at each frequency can be determined. Like radar, THz-TDS also provides time information that allows us to develop various three-dimensional THz tomographic imaging modalities. The three-dimensional THz tomographic imagings we investigated are: terahertz diffraction tomography (THz DT), terahertz computed tomography (THz CT), THz binary lens tomography and THz digital holography. THz DT uses the THz wave as a probe beam to interact with a target, and then reconstructs the three-dimensional image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired by x-ray CT. THz binary lens tomography uses the frequency dependent focal length property of binary lenses to obtain tomographic images of an object. THz three-dimensional holography combines radar and conventional holography technology. By separating the multiple scattered THz waves of different scattering orders, we used a digital holography method to reconstruct the sparsely distributed scattering centres. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, three-dimensional THz imaging is a very useful tool to inspect or characterize dielectric and semiconductor objects. For example, three-dimensional THz imaging can be used to detect and identify the defects inside a space shuttle insulation tile.
257 citations
TL;DR: In this paper, the principles of tomography for terahertz Computed tomography (CT), tomosynthesis (TS), synthetic aperture radar (SAR), and time-of-flight (TOF) tomography are established.
Abstract: Terahertz and millimeter waves penetrate various dielectric materials, including plastics, ceramics, crystals, and concrete, allowing terahertz transmission and reflection images to be considered as a new imaging tool complementary to X-Ray or Infrared. Terahertz imaging is a well-established technique in various laboratory and industrial applications. However, these images are often two-dimensional. Three-dimensional, transmission-mode imaging is limited to thin samples, due to the absorption of the sample accumulated in the propagation direction. A tomographic imaging procedure can be used to acquire and to render three-dimensional images in the terahertz frequency range, as in the optical, infrared or X-ray regions of the electromagnetic spectrum. In this paper, after a brief introduction to two dimensional millimeter waves and terahertz imaging we establish the principles of tomography for Terahertz Computed tomography (CT), tomosynthesis (TS), synthetic aperture radar (SAR) and time-of-flight (TOF) terahertz tomography. For each technique, we present advantages, drawbacks and limitations for imaging the internal structure of an object.
215 citations
References
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01 Oct 1999
TL;DR: In this article, the authors discuss various topics about optics, such as geometrical theories, image forming instruments, and optics of metals and crystals, including interference, interferometers, and diffraction.
Abstract: The book is comprised of 15 chapters that discuss various topics about optics, such as geometrical theories, image forming instruments, and optics of metals and crystals. The text covers the elements of the theories of interference, interferometers, and diffraction. The book tackles several behaviors of light, including its diffraction when exposed to ultrasonic waves.
19,503 citations
TL;DR: The latest results in tomographic imaging, in which waveforms reflected from an object can be used to form a three-dimensional representation, are described and substantial improvements in the spatial resolution are demonstrated.
Abstract: We review recent progress in the field of terahertz “T-ray” imaging. This relatively new imaging technique, based on terahertz time-domain spectroscopy, has the potential to be the first portable far-infrared imaging spectrometer. We give several examples which illustrate the possible applications of this technology, using both the amplitude and phase information contained in the THz waveforms. We describe the latest results in tomographic imaging, in which waveforms reflected from an object can be used to form a three-dimensional representation. Advanced signal processing tools are exploited for the purposes of extracting tomographic results, including spectroscopic information about each reflecting layer of a sample. We also describe the application of optical near-field techniques to the THz imaging system. Substantial improvements in the spatial resolution are demonstrated.
731 citations
IBM1
TL;DR: In this article, the performance of an optoelectronic terahertz (THz) beam system was described, where the transmitter operation was based on the repetitive, sub-picosecond laser excitation of a Hertzian dipole antenna embedded in a charged coplanar line.
Abstract: The performance of an optoelectronic terahertz (THz) beam system is described. The transmitter operation is based on the repetitive, subpicosecond laser excitation of a Hertzian dipole antenna embedded in a charged coplanar line. With this transmitter electromagnetic beams of 1/2 cycle THz pulses at a repetition rate of 100 MHz are produced. The associated optoelectronic receiver is gated in synchronism with the excitation of the transmitter by subpicosecond pulses from the same laser source. With this receiver, the 10-nW beams of THz pulses were observed with a signal-to-noise ratio greater than 10000:1. Several sources contributing to the noise of the receiver are discussed, together with ways to reduce them. With an integration time of 125 ms, a signal-to-noise ratio of 1 is obtained for a THz beam with an average power of 10/sup -16/ W. The receiver operates in the sampling mode and has a time resolution of 0.5 ps. >
488 citations
TL;DR: In this paper, the authors present time domain impulse scattering measurements of freely propagating terahertz radiation measured with sub-picosecond resolution, which corresponds to a usable bandwidth of over 1 THz.
Abstract: We present time domain impulse scattering measurements of freely propagating terahertz radiation measured with subpicosecond resolution. This fast time response corresponds to a usable bandwidth of over 1 THz. Measured scattered fields from thin wire targets agree well with the calculated scattering for the early and late time response in both the time and frequency domains. Realistic ranging from scale model aircraft is demonstrated.
129 citations
23 May 2000
TL;DR: In this article, a fiber-pigtailed terahertz imaging system is presented, which uses an integrated amplifier to obtain a 1000:1 S/N with only 1 mW of power on both the transmitter and receiver with a one second integration time.
Abstract: Terahertz imaging has been shown to be a powerful tool for analyzing a variety of materials. From the amount of water in a leaf over time to looking at the spectroscopic species in a flame, this technique shows great potential for commercial applications. However, in order to work in a commercial environment, the present free-space optical systems must be abandoned in favor of fiber-optic delivery. To this end, we have developed a compact, fiber-pigtailed terahertz imaging system that utilizes a hermetically sealed, photoconductive, transmitter and receiver. The receiver uses an integrated amplifier to obtain a 1000:1 S/N with only 1 mW of power on both the transmitter and receiver and with a one second integration time. This system has usable energy extending from 0.04 to 2 THz and has both a rapid (20 Hz) scanner for short, 40-ps, scans as well as a long rail for scans up to 1 ns. The system hardware is contained in a 1.5 cu. ft. box with fibers feeding both the transceiver units. These units can be configured into either a transmission or reflection mode depending on the user's application. An advanced software system controls the hardware, collects the data, and does image processing.
67 citations