TeTechS Blog-Robust, compact and versatile terahertz systems are realized by utilizing the fiber optics

Robust, compact and versatile terahertz systems are realized by utilizing the fiber optics

Posted by Alireza Zandieh on Wednesday, 09 April 2014 in Terahertz

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THz technology has become increasingly significant in a diverse range of applications such as spectroscopy, imaging, and communication as a consequence of a plethora of significant advances in this field. Photonic based THz systems where the THz signal is generated by illumination of the photoconductive materials in the presence of a strong static electric field are one the promising areas for employing the THz technology in industrial application. A compact, robust, stable and versatile THz system can be applied in industrial environments. In the research laboratories, a THz module with the aforementioned features would be helpful in making measurements more reliable and less time consuming. The main challenge in using photoconductive antennas is the difficulty in the excitation of the THz antenna in a free space optical setting. This problem is usually addressed by directly coupling the light on the antenna using optical fibers.

In CW THz systems where two optical laser beam with frequency difference in THz range should be combined and focused on the photoconductive antenna, applying the fiber optics realizes a compact and less complicated system due to the fact that all the required fiber optics components have been developed for both 800nm and 1550nm wavelength. However, utilizing fiber optics in THz pulse systems is more challenging as the dispersion and non-linear effects can change the properties of the high power femtosecond pulse propagating in optical fiber. To address this problem and retrieve the optical pulse after propagating in the fiber, usually a dispersion compensation technique should be applied.  One approach is to control optical pulse shape by tuning the laser structure. However, to realize a modular system which is independent of lasers module, a dispersion compensation unit should be integrated to the system. In 1550nm THz system, an all fiber compensation unit can be realized by using Dispersion Compensation Fibers (DCF). On the other hand, the compensation technique usually is implemented in free beam setup by using a pair of grating structures in 800nm THz systems due to the large amount of dispersion at this wavelength.

In addition to the optical beam propagation in fiber optics, THz fiber-coupled transceiver sensors have been developed to focus the optical beam on the THz antenna structure. In the current developed sensors, either the fiber optics is directly attached to the antenna or the output beam of the fiber is focused on the antenna by a small lens in a compact enclosure. THz systems with fiber coupled sensors (see Rigel 1550) can be robust and configurable and easy to mount which are the key requirements for industrial applications.

TeTechS blog-Terahertz imaging applications in cancer detection and dental imaging

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Terahertz imaging applications in cancer detection and dental imaging

Posted by Alireza Zandieh on Monday, 13 January 2014 in Terahertz

As a consequence of extensive research over the last decade, the Terahertz(THz) technology has advanced considerably. Significant amount of research is conducting on application of THz imaging in biological problems as unique properties of THz technology can address many unresolved issues in this area. Compared to X-ray imaging, THz wave can realize a non-invasive and non-ionizing imaging for biological tissues due to its low photon energy. The scattering effect is also reduced considerably as the size of the scattering particles in biological tissues is much smaller than THz wavelength range (3-100um). This effect along with the inherent small wavelength of THz waves makes the spatial resolution of 250um laterally and 20um axially easily achievable. Additionally, coherent detection technique (which measures the amplitude and the phase of THz signal) in THz imaging provides precise information on the material properties such as refractive index and absorption coefficient over the THz range frequency. Since these properties depend on the chemical content of the biological cells, monitoring them can be used for medical diagnosis and examine the condition of biological cells.

Two main applications of THz imaging are dental carries detection and cancer detection. Early erosion in tooth through radiography is hard to diagnose, but THz imaging can be applied to detect the early carries through characterizing the properties of different tooth layers (enamel and dentine). The indication of early carries is the change in refractive index of enamel which is resulted from mineral loss in this layer. THz imaging has shown a great potential to be an alternative to x-ray for dental imaging. However, as a consequence of substantial attenuation of THz wave in an actual tooth, most of the researches have been conducted on characterization of a thin slice or on the surface of the tooth. This issue originates from irregular shape of the tooth, thick tooth layers (few millimeters in total), and high absorption coefficient of composed materials (varies from 2dB/mm to 4dB/mm). Therefore, to develop a THz dental imaging system, a high power, compact and affordable THz source is essential.

Skin cancer and breast cancer detection can be considered as other applications of THz imaging. Since skin is a superficial layer, it can be a suitable target for THz imaging in a reflection measurement configuration. In skin structure, the basal layer lies against the dermis, innermost layer. Studies have shown that too much UV exposure to this layer can transform it to cancerous cells. Compared to the normal cells, the cancerous cells show an increase in the interstitial water or a change in the vibrational modes of their water molecules; therefore, THz imaging can be utilized to evaluate the boundary and infiltration depth of skin cancer. Breast cancer can also be a suitable target for THz imaging as the breast is mostly composed of low water content fat tissues.  In breast cancer cases, there is an essential need to identify accurately tumor margin to avoid second breast-conserving surgery. The available technologies are incapable of accurate detection and up to 55% of the cases need the second operation. The high sensitivity of THz absorption to water density can be exploited for breast cancer detection as the cancerous cells contain more water compared with the normal fat tissues.

Although THz medical imaging has demonstrated a great potential and some commercial THz systems have been developed for medical applications, the THz medical imaging is still in its early stages and more research should be conducted to enable this technology to compete with other alternative imaging techniques.

TeTech blog-Terahertz Standoff Detection Systems for Security Applications

Terahertz Standoff Detection Systems for Security Applications

Posted by Alireza Zandieh on Tuesday, 12 November 2013 in Terahertz

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Benefiting from unique characteristics, THz technology has demonstrated a great potential to address many of the challenges facing modern security and surveillance systems. Therefore, THz technology has drawn a lot of attentions and received huge investments.  Some major factors are contributing in capability of THz technology to address the existing problems in security applications. The ability of “seeing through” concealing barriers might be the most fascinating aspect of THz technology as the THz waves can readily transmit through the most conventionally opaque materials such as cloth, paper, wood, and plastic. As opposed to the x-ray package inspection systems which raise health concerns, THz radiation is safe and non-invasive due to its low energy photons. THz imaging systems therefor has been applied in different locations such as airports to provide standoff accurate detection for packaged contraband. To this end, the systems usually work in single point reflection mode measurements where the high power THz signal is focused on the object under inspection, and the reflected signal which depends on the characteristics of the object, is detected. The whole image of object can be created through raster scanning. Recently, the New York police department has also employed the THz technology for detecting firearms concealed under cloth.

Additionally, many chemical materials of interest can be identified using THz spectroscopy due to their distinctive finger prints in THz range of frequency. Most of the explosives (TNT, RDX, HMX...) molecules show unique spectral feature in THz range as a consequence of being organics compounds. Other existing explosives detection technologies such as X-ray imaging, nuclear detection and infrared imaging cannot identify the type of explosives. They are also either harmful for human or blind to the opaque packages. THz technology is also able to detect much greater amount (in range of milligrams) of energetic materials. The reflection scheme is applied for explosives inspection too where the received signal is compared with the library of the fingerprints to identify the explosive type.

Through improvement of THz radiation efficiency and also development of the THz scanners in array configuration (THz camera), a high speed and high performance THz security system can be realized. The system would be an excellent candidate to eliminate the aforementioned problems in other technologies and be widely used for security purposes.