Considering the overmoded structures of high-power Terahertz(THz) sources are often electrically large, it’s difficult to compute the radiation of THz antennas on a personal computer due to over long time and prohibitive computation resources. A parallelized finite-difference time domain (FDTD) algorithm based on MPI platform and virtual topology structure, combined with theory of guided waves, is presented for analysis of the radiation of the large THz conical horn excited by mixed-mode souce. Cartesian virtual topology structure is firstly defined by MPI_CART_CREATE( ) function based on MPI platform. And MPI_CART_SHIFT() function is used to define the position relations of the subdomains. Then FDTD method is used in each subdomain. The absorbing boundary of the whole FDTD domain is uniaxial perfectly matchedlayer (UPML), and that of the waveguide is convolutional PML(CPML). Synchronous communication mode is used in parallelized FDTD between the adjacent subdomains. The coefficient of field components for each mode source can be got based on the given power of each mode. Thus the mixed-mode excitation source can be set by the coefficient and each mode’s initial phase. Examples of an electrically large THz horn with 4 or 6 modes mixed excited are given in this paper. Considering the universal characteristic of FDTD method, the method shown in this paper can be used to simulate the radiation of other kinds of THz antennas with mixed-mode exicitation source. And it’s useful for the design of those structures.
This paper has successfully setup a calibration system for terahertz devices at frequencies from 0.11 THz to 0.17 THz in waveguide WR6.5. The system consists of a terahertz wave generator, a power meter, a digital oscilloscope and a computer, and is able to accomplish calibrations for some devices automatically. Calibrations of a 10 dB attenuator, a 20 dB directional coupler, and a Schottky diode detector are carried out by the system and a detailed discussion is given to the results. The results obtained have shown that the characteristics of the devices under calibrated remained essentially the same throughout the experiment and met rather good agreement to the factory data. Due to the use of GPIB, it is proved that the system can also improve the efficiency of work greatly. The calibration approach described in this paper can be a necessary supplement to the other calibration techniques and play an important role in terahertz measurement especially at lower frequencies.
The transmission characteristics of cylinder and rectangular metal overmoded waveguides frequently used in 0.14 THz frequency band are numerically studied in this paper. At first, skin depths and losses of overmoded cylinder waveguides for different waveguide materials and terahertz wave modes are theoretically calculated and validated by electromagnetic finite difference time domain (FDTD) method. Loss as high as 5 dB/m is obtained for stainless steel and TM01 mode, and it increases with mode orders and decreases with waveguide radius. Also the mode conversion is observed in the straight overmoded cylinder waveguide with finite conductivity. Then transmission characteristics of overmoded rectangular metal waveguide including bend waveguides, straight waveguides, and transition waveguides are researched. The significant mode conversions in the overmoded E and H bend waveguides are found even ignoring the effect of finite conductivity. Only a few of the TE10 mode within frequency band of 0.13-0.16 THz is successfully transported through the standard bend waveguides in Ka band. The transmission characteristics of overmoded straight rectangle waveguide are almost the same as the cylinder one’s. Non-cutoff modes for the two waveguides connected by transition waveguide can transfer through it without losses, while the rest modes can’t.
Interaction between the high temperature (high-T<sub>c</sub>) Josephson junctions and 0.14 THz nanosecond pulse has been
numerically investigated in this paper. A general equivalent circuit based on resistively-shunted junction (RSJ) model
was applied to simulate a typical high-T<sub>c</sub> Josephson junction under the radiation of 0.14 THz narrow-band pulse with
pulse duration of 2 ns. The varying ratio of phase difference of electron wave functions between the two sides of the
junction, the current-voltage characteristics, and the voltage responses were determined at several specific times during
the interaction. The Shapiro steps were clearly observed but distorted, and then the irradiation frequency was derived,
coinciding with the simulated frequency. Also discussed were the effects of some parameters, including the pulse power,
the normal resistance and the critical current of junction, on the current-voltage characteristics and the voltage responses.
All the results showed that the high-T<sub>c</sub> Josephson junctions probably could be used for the direct frequency
measurements of narrow-band terahertz pulses under some specific conditions.
A high beam quality foilless diode for high power 0.14 Terahertz backward wave oscillator (BWO) is presented in this
paper. Limited by the wavelength in terahertz region, the diode is so small that its cathode radius is only 2.5 mm, and its
anode radius is 9 mm. Based on an empirical formula for foilless diode, the initial structural parameters are estimated.
Then the static electric field simulations with Superfish code and particle simulations using a 2.5 dimensional UNIPIC
code are performed to optimize these parameters. The simulation results show that the beam parameters satisfy our
design requirements. Finally, the experiments of the diode are carried out. The copper targets by beam bombardment are
measured to prove that the electron beam is azimuthally uniform with an inner radius of 2.0 mm and outer radius of 2.54
mm. From the experimental results, the diode can provide a high quality beam over a wide range of diode voltage (160
kV<U<220 kV) and current (600 A<I<1.5 kA). Meanwhile, the diode resistance versus the distance between the cathode
and anode is compared with numerical simulations. This foilless diode is being used in the experiment of high power
terahertz wave generation.