An integrated microstrip meander line (MML) slow-wave structure (SWS) comprised of an MML SWS and compact wideband metamaterial absorber (MMA) is proposed and demonstrated theoretically and experimentally in this paper. The MMA consists of metallic rectangular resonators with a metallic ground plane and a separating dielectric layer. Simulation results show that the MMA has multidistinctive absorption peaks; the bandwidth at 56-GHz peak frequency is reach up to 30 GHz. The integrated MML SWS can provide flatter gain response of output power than that made by the MML SWS with a traditional resistive coating attenuator. Experimental results are in close agreement with simulation results. A traveling wave tube (TWT) is simulated based on this integrated MML SWS. The output power of this TWT is 48.21 W at 33 GHz corresponding to a maximum gain of 30.3 dB; the output power gain is greater than 25 dB across the entire working frequency range.

A novel electromagnetic bandgap meander-line (EBG-ML) slow-wave structure (SWS) is proposed for improvement of the interaction impedance and reduction of circuit attenuation by in-phase reflection of EBG for traveling-wave tube applications. With high on-axis interaction impedance, the EBG-ML SWS can interact with a low-current-density electron beam. The backward-wave mode of EBG-ML SWS can be suppressed by the transmission stopband of EBG. Particle-in-cell simulations are carried out for the proposed structure with a sheet beam source. The simulation results show that a 50% improved interaction impedance and a 20% reduced circuit attenuation of EBG-ML SWS can be achieved, in comparison with those of traditional ML SWS. With a small input signal, the RF efficiency of EBG-ML is improved by 137.5% over that of the ML SWS.

A defect photonic crystal waveguide (DPCW) is presented as a slow-wave structure (SWS) for terahertz (THz) traveling-wave tube (TWT). Due to the characteristics of DPCW, the mode competition and backward wave are greatly depressed, improving the stability of TWT. The DPCW SWS has flat dispersion characterization and high interaction impedance at 220 GHz. Simulation results show that saturated output power of 12.1 W, corresponding to a gain of 24.3 dB, and the 3-dB gain ripple within 20 GHz can be achieved at the center frequency of 220 GHz. The DPCW-TWT has a low working voltage at 9.52 kV compared to that of folded waveguide (FWG) SWS in this frequency band. The proposed DPCW for the THz regime is easy to fabricate and assemble using the modern fabrication processing technology.

The impact of thermal state on backward-wave oscillation (BWO) in a helix traveling-wave tube (TWT) under operation is explored, considering total thermal losses induced by high-frequency electromagnetic field power dissipation on helix, output port, carbon-coating attenuators, support rods, inner wall of the barrel, and the electron beam collision on the helix. By numerically solving the Maxwell equations, the electron motion equation, the heat conduction equation, and the thermal expansion equation, we analyze integrally the beam-wave interaction process and the resulting thermal distribution as well as thermal deformation of both slow wave structure and the output port. The simulation results for a broadband (8-18 GHz) helix TWT show the temperature on the last 20 turns of the helix is the highest, with a peak value of 347 °C at the end of helix. The greatest thermal deformation (0.0151 mm) is located on the inner conductor of the output port. The experimental result for the saturated output power of the TWT prototype shows a BWO power hole at 11 GHz, which is not found in simulation without considering thermal deformation. We find also that, considering the thermal deformation of helix and output port, BWO occurs at 22.4 GHz with the amplitude of 0.0708 V, which is over 5 times of that without considering thermal deformation, and it induces the same power hole as the experimental one at 11 GHz. Thermal deformation is one of the contributing factors for BWO in the helix TWT.

A low voltage standing wave ratio (VSWR) broad bandwidth pillbox window with metamaterial is proposed for W-band traveling-wave tubes (TWTs) in this paper. The metamaterial structure is designed on a dielectric surface, which decreases the VSWR and broadens the bandwidth of the pillbox window. The cold-test results of the window with metamaterial are presented, which showed that the metamaterial pillbox window (MPW) achieves a bandwidth of 31.5 GHz with VSWR below 1.2, and a bandwidth of 18 GHz with VSWR below 1.1. The ghost mode and the power capacity of the window were also investigated. The results showed that the proposed metamaterial pillbox window is suitable for medium power TWTs. A Ka-band MPW was fabricated and the test results agreed well with the simulation results.

We demonstrate and optimize a tri-layer vertical coupler for a silicon nitride (Si₃N₄) multilayer platform operating at a 2 µm band. The large spacing between the topmost and bottommost layers of a gradient structure enables ultra-low crossing loss and interlayer crosstalk without affecting the efficiency interlayer transition. We achieve a 0.31 dB transition loss, ultra-low multi-layer crosstalk of -59.3 dB at a crossing angle of 90° with an interlayer gap of 2300 nm at 1950nm. With width optimization of this structure, the fabrication tolerances toward lateral misalignment of two stages in this coupler have increased 61% and 56%, respectively. We also propose a vertical coupler, based on this design, with mode selectivity and achieve an extinction ratio of < 15 dB for wavelengths in the 1910-1990 range. Meanwhile, a multi-layer interlaced AWGs centered at 1950nm and based on vertical coupler has been demonstrated. The proposed vertical couplers exhibit potential for application in large-scale photonic-integrated circuits and broadly in photonic devices.