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A novel type of two-section folded wave- guide (TSFW) slow wave structure for the development of a subterahertz (sub-THz) extended interaction oscillator (EIO) is proposed in this paper. We focus on the development of sub-THz EIO, in which the key parts, including the electron optics system, the high frequency structure for beam–wave interaction, and output window, are fabricated and tested. Based on the key parts, the prototype of EIO is developed. The experimental results show that over 12-W output power at the frequency of 107.9 GHz at 2% duty cycle is achieved. This paper demonstrates that the feasibility of a high power THz oscillator can be obtained by the use of a prebunching electron beam produced by TSFW.

Predicting mode-dependent bending loss is important for large-mode-area (LMA) fibers to strip out unwanted modes. In order to seek a reasonable calculation method, a new bending loss formula that allows for both mode field distortion and weaker mode confinement was derived in detail. Based on the formula, a semi-analytical method was introduced to calculate the mode bending loss for LMA fibers. By combining the applications of the finite element method, discrete Fourier transforms and numerical integral, the method is valid for LMA fibers with arbitrary refractive index profile. The obtained loss characteristics are basically consistent with the previous results. This accordance demonstrates the feasibility and generality of the semi-analytical method.

In order to evaluate a fiber amplifier’s output beam quality, we have explored the beam quality factor calculation. A theoretical beam quality factor calculation method is presented in this paper. The method bases on modal power decomposition, laser rate equations and Fourier diffraction theory. Through the rate equations, each modal power weight factor at the fiber output facet is obtained. By the modal power decomposition, the optical field at the fiber output facet is established. Finally, running Fourier transformation, the beam diverging state in free space and corresponding M2 parameter are obtained. The calculation process is verified with applying it to working out the beam M2 parameters of large-mode-area (LMA) fiber amplifier system under different conditions. The numerical results help us well know and predict the out beam property. Some obtained results are also accord with other reporting outcomes.

In large mode area fiber amplifier, it is difficult to avoid the bend induced mode distortion. With the existence of mode distortion taken into account, a previous transverse mode competition model has been improved. Based on this model and the finite element difference-beam propagation method, the behaviors of bent mode competition are demonstrated and compared with the straight mode competition where the mode distortion is neglected, the comparison and quantitative results show that serious mode distortion can change the appearance of transverse mode competition in bent fiber amplifiers and even drastically degrade the amplifier performance.

In this paper, an interesting type of a two-section folded wave-guide (TSFW) slow wave structure (SWS) for the development of sub-Terahertz (sub-THz) extended interaction oscillator (EIO) is proposed. In this sub-THz device, the prebunching electron beam is produced by the TSFW SWS, which results in the enhancement of the output power. To verify this concept, the TSFW for sub-THz EIO is developed, which includes the design, simulation, and some fabrications. A small size of electron optics system (EOS), the TSFW SWS for beam-wave interactions, and the output structure are studied with simulations. Through the codes Egun and Superfish, the EOS is designed and optimized. With a help of CST studio and 3D particle-in-cell (PIC) simulation CHIPIC, the characteristics of beam-wave interaction generated by the TSFW are studied. The results of PIC simulation show that the output power is remarkably enhanced by a factor of 3, which exceeds 200 W at the frequency of 108 GHz. Based on the optimum parameters, the TSF...

A novel two-beam folded waveguide (FW) oscillator is presented for the purpose of gaining higher power with a small-size circuit compared with the normal FW oscillator. The high-frequency characteristics of the two-beam FW, including dispersion and interaction impedance, were investigated by the numerical simulation and compared with the one-beam FW. The radio-frequency loss of the two-beam FW was also analyzed. A 3-D particle-in-cell code CHIPIC was applied to analyze and optimize the performance of a G-band two-beam FW oscillator. The influences of the distance between the two beam tunnels, beam voltage, the number of periods, magnetic field, radius of beam tunnel, and the packing ratio on the circuit performance are investigated in detail. Compared with a one-beam circuit, a larger output power of the two-beam circuit with the same beam power was observed by the simulation. Moreover, the start-oscillation current of two-beam circuit is much lower than the one-beam circuit with better performance. It will favor the miniaturized design of the high-power terahertz oscillator.

The modal weight factors and their changing properties in large-mode-area fibers are investigated when the optical beam propagates from one fiber into another one with a different cross-sectional structure. Based on a weighted mode power superposition theory, a novel scheme of inserting a filter fiber to change the power allocation among the co-existing modes in active fibers is proposed. In order to verify the effectiveness of this method in practical implementations, the mode power propagation of a fiber laser and a fiber amplifier with an inserted filter fiber are calculated, respectively. Compared with the cases without a filter fiber, the numerical results show an obvious suppression effect of high-order modes.

A model based on propagation-rate equations with consideration of transverse gain distribution is built up to describe the transverse mode competition in strongly pumped multimode fiber lasers and amplifiers. An approximate practical numerical algorithm by multilayer method is presented. Based on the model and the numerical algorithm, the behaviors of multitransverse mode competition are demonstrated and individual transverse modes power distributions of output are simulated numerically for both fiber lasers and amplifiers under various conditions.