THz Imaging System Research Articles

Identification of biological tissue using chirped probe THz imaging

We consider the application of pulsed THz imaging systems in biomedical diagnostics and mail/packaging inspection. The sub-millimetre spectroscopic measurements obtained from T-ray systems contain a wealth of information about the sample under test. We demonstrate that different types of tissue can be classified based on their terahertz response measured with the chirped probe pulse technique. We demonstrate the performance of a quadratic classifier using linear filter models for feature extraction in the discrimination between different tissues.Modern THz systems are hindered by the slow acquisition speed. The chirped probe pulse technique offers a significant improvement in this context. We present the terahertz responses of biological samples measured using a chirped probe pulse, and discuss the problem of data processing and extracting sample characteristics.

Development of a THz spectroscopic imaging system

We have developed a real-time THz imaging system based on the two-dimensional (2D) electro-optic (EO) sampling technique. Employing the 2D EO-sampling technique, we can obtain THz images using a CCD camera at a video rate of up to 30 frames per second. A spatial resolution of 1.4 mm was achieved. This resolution was reasonably close to the theoretical limit determined by diffraction. We observed not only static objects but also moving ones. To acquire spectroscopic information, time-domain images were collected. By processing these images on a computer, we can obtain spectroscopic images. Spectroscopy for silicon wafers was demonstrated.

All-optoelectronic continuous wave THz imaging for biomedical applications

We present an all-optoelectronic THz imaging system for ex vivo biomedical applications based on photomixing of two continuous-wave laser beams using photoconductive antennas. The application of hyperboloidal lenses is discussed. They allow for f-numbers less than 1/2 permitting better focusing and higher spatial resolution compared to off-axis paraboloidal mirrors whose f-numbers for practical reasons must be larger than 1/2. For a specific histological sample, an analysis of image noise is discussed.

Medical THz imaging: an investigation of histo-pathological samples

We present a THz investigation of histo-pathological samples including the larynx of a pig and a human liver with metastasis. Our measurements show that different types of tissue can be clearly distinguished in THz transmission images, either within a single image or by a comparison of images obtained for different frequency windows. This leads to the problem that images obtained for different frequencies inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical operation may contain artefacts. We discuss measures to deal with this problem. Furthermore, we investigate the possibility of improving the spatial resolution of THz images. Finally, we present a cw THz imaging system based on a photomixer and an external cavity semiconductor laser that allows for simultaneous two-mode operation. The cw system is less expensive and more compact than conventional time-domain imaging systems.

Continuous-wave all-optoelectronic terahertz imaging

We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.

Angle-dependent THz tomography – characterization of thin ceramic oxide films for fuel cell applications

A time-resolved THz tomography system for the incidence-angle-dependent three-dimensional characterization of layered structures is presented. The capabilities of the developed system are demonstrated on multi-layer ceramic samples used for solid oxide fuel cells (SOFC). Appropriate methods for determining unknown refractive indices are discussed. It is shown how the angle of incidence of a THz imaging system has a significant influence on measured signals. This fact can be exploited especially in Brewster-angle configurations to enhance the capabilities of any THz tomography system. Data evaluation algorithms are presented.

2D measurement and spatio-temporal coupling of few-cycle THz pulses

The propagation of few-cycle THz pulses are measured by the improved CCD based 2D THz imaging system. There exists strong spatio-temporal shaping effect, leading to dramatically time dependent beam diameter and wave front. Theoretical simulation is given.

Recent advances in terahertz imaging

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.