After entering the near space, a layer of plasma sheath is formed outside the hypersonic vehicles due to the hightemperature and high-pressure environment. The plasma sheath, which characteristic frequency is similar to microwave, will cause serious impediment to communication signal. This phenomenon is known as the blackout problem. With the rapid development of aerospace industry, plasma sheath blackout has become an urgent problem to be solved. Current research shows that increasing the frequency of electromagnetic wave higher than the plasma characteristic frequency can effectively reduce the shielding effect of plasma. The frequency of terahertz (THz) wave is much higher than microwave, it can propagate through plasma sheath, which provides an effective method to solve the problem of plasma sheath. In this paper, a theoretical model of plasma is established, and the transmission properties of THz wave in plasma is simulated using scattering matrix method. Then a kind of plasma jet is produced in laboratory environment according to dielectric barrier discharge. And the experiments of a broadband THz source and THz time-domain spectrum transmission in this kind of plasma and a 2.52 THz wave reflection imaging of target under plasma shelter are carried out respectively. The transmittance increases with frequency under 0.5 THz and becomes stable at 100% over 0.5 THz, and the result of experiments and simulation are in good agreement. Both theory and experiments show that THz wave has good penetration in plasma jet and can detect targets behind plasma, and this study will lay a theoretical foundation for solving the plasma blackout problem of hypersonic vehicle in near space.
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.
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.
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.
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.
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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