A novel nested anti-resonant hollow core fiber (NAHF), based on Topas, with low loss and flattened dispersion is proposed for efficient transmission of terahertz wave. Finite element method with an ideally matched layer (PML) boundary condition is used to investigate its guiding properties. A cladding structure of nested anti-resonant elliptical rings is introduced to reduce mode power leakage. The NAHF shows a low confinement loss (< 0.29 cm-1 ) and a small effective material loss (< 0.019 cm-1 ) in the frequency range of 0.9-1.5 THz. An ultra-flatted near zero dispersion profile of ±0.029 ps/THz/cm is obtained within a broad frequency range of 0.6-1.5 THz. Furthermore, optimizing the structure parameters in NAHF, higher core power fraction over 80 %, higher effective mode area of ~10-6 μm2 and the bending loss of 3.05×10-5 cm-1 at the bending radius of 10 cm are also achieved.
Region of interest segmentation is essential for computer aided application of THz imaging. However, THz images is severely degraded by motion blur, poor resolution and noise. A robust, accurate and time-saving algorithm is in dire need for the ROI segmentation of THz images. Recently, ROI segmentation of THz-TDS images and passive THz images has been widely studied. While the ROI segmentation of THz continuous wave (CW) image is still in its infancy. In this paper, we proposed a hybrid ROI segmentation method for THz CW images. The hybrid method combines block match 3D denoising, fuzzy c-means clustering, morphology operation and canny edge detection. The hybrid method is implemented to two images acquired with a THz CW reflection imaging system. To evaluate the performance of our algorithm, we calculated the accuracy, sensitivity and specificity. As the result indicates, this hybrid ROI segmentation method performs well for THz images.
We have demonstrated a high-energy and broadly tunable monochromatic terahertz (THz) source via difference frequency generation (DFG) in DAST crystal. The THz frequency is tuned randomly in the range of 0.3-19.6 THz, which is much wider than the THz source based on the inorganic crystal and the photoconductive antenna. The highest energy of 2.53μJ/pulse is obtained at 18.9 THz corresponding to the optical-to-optical conversion efficiency of 1.31×10-4. The THz output spectroscopy is theoretically and experimentally explained by DFG process and Raman spectroscopy. Meanwhile, a phenomenon of blue light from the KTP-OPO with tunable and multiple wavelengths was firstly observed and explained. Based on our THz source, an ultra-wideband THz frequency domain system (THz-FDS) with transmission mode is realized to measure the ultra-wideband THz spectroscopies of typical materials in solid and liquid states, such as Si, SiC, White PE, water, isopropyl myristate, simethicone, atonlein and oleic acid, etc.. Furthermore, we have studied the THz spectral characteristic of biomedical tissue in the ultra-wideband THz frequency range of 0.3-15THz to study the biomedical response in the entire THz frequency range, which contains more abundant spectral information and was rarely focused with the limit of the THz source.
A gain-boosted terahertz-wave parametric generator (TPG) in high frequency tuning range based on MgO-doped nearstoichiometric LiNbO3 (MgO:SLN) crystal has been demonstrated with 1064 nm nanosecond pulsed laser pumping. The pulse-seed is provided by nanosecond singly resonant near-degenerated KTP optical parametric oscillator with the wavelength range of 1068.08 nm to 1084.76 nm. The terahertz tuning range of 0.97 THz to 4.07 THz was achieved. The maximum THz wave output signal was 4285mV at 1.82 THz under the pump energy of 180 mJ and pulse-seed energy of 20.2 mJ. During the frequency range of 1.25 THz to 3.43 THz, the THz output energies were larger than 2000mV. Compared with the maximum THz output energy, the THz energy attenuation factors of 0.55 dB, 1.71 dB and 3.31 dB were realized in pulse-seeded TPG at 2.5 THz, 3.0THz and 3.5THz, respectively. The significantly increasing of THz gain in high frequency range (<2.5 THz) was achieved.
Recently, terahertz (THz) computed tomography (CT) has emerged as a possible effective technique for 3D structural information detection. However, THz-CT is difficult to be applied to high refractive index object, due to the severe refraction phenomenon occurred during the acquisition of raw data. We propose a novel experimental procedure to solve this problem. Including the use of a sink filled with liquid whose refractive index is close to the sample, and a correction algorithm to eliminate the noise of liquid. The proposed method is applied to the high-density polyethylene samples of different shapes.
We demonstrated that different degrees of experimental traumatic brain injury (TBI) can be differentiated clearly in fresh slices of rat brain tissues using transmission-type terahertz (THz) imaging system. The high absorption region in THz images corresponded well with the injured area in visible images and magnetic resonance imaging results. The THz image and absorption characteristics of dehydrated paraffin-embedded brain slices and the hematoxylin and eosin (H&E)-stained microscopic images were investigated to account for the intrinsic differences in the THz images for the brain tissues suffered from different degrees of TBI and normal tissue aside from water. The THz absorption coefficients of rat brain tissues showed an increase in the aggravation of brain damage, particularly in the high-frequency range, whereas the cell density decreased as the order of mild, moderate, and severe TBI tissues compared with the normal tissue. Our results indicated that the different degrees of TBI were distinguishable owing to the different water contents and probable hematoma components distribution rather than intrinsic cell intensity. These promising results suggest that THz imaging has great potential as an alternative method for the fast diagnosis of TBI.
A surface-emitted ring-cavity terahertz (THz) wave parametric oscillator has been demonstrated for high-energy THz output and fast frequency tuning. Through the special optical design with a Galvano optical scanner and four-mirror ring-cavity structure, a maximum THz output of 12.9 μJ/pulse is achieved at 1.359 THz under the pump pulse energy of 172.8 mJ with the repetition rate of 10 Hz. A further research on the performance of the SE ring-cavity TPO has done to explore more characteristics of THz output. The THz pulse instability and the influence of cavity loss has analyzed. Moreover, the pump depletion rate of the ring-cavity configuration is much lower than the conventional surface-emitted terahertz wave parametric oscillator at the same experimental conditions.
A widely tunable, high-energy terahertz wave parametric oscillator based on 1 mol. % MgO-doped near-stoichiometric LiNbO3 crystal has been proposed with 1064 nm nanosecond pulsed laser pumping. The tunable range of 1.16 to 4.64 THz was obtained. Under the pump energy of 150 mJ/pulse, the maximum THz wave output energy of 12.56 μJ was achieved at 1.88 THz, corresponding to the THz wave conversion efficiency of 7.61×10-5 and the photon conversion efficiency of 1.14%, respectively. Moreover, the THz half maximum (FWHM) beam diameters of MgO:SLN TPO measured at 4 cm from the output surface were 7.42 mm and 6.06 mm in the vertical and horizontal directions, respectively.
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