We present the design of a porous-core PCF with elliptical holes in the core that achieved low loss, high birefringence, and flattened dispersion for guiding terahertz waves. The finite element method is used to study the properties of the designed waveguide in detail: effective material loss, birefringence, confinement losses, and dispersion. Simulation results show that the proposed structure exhibits simultaneously high modal birefringence of 1.32 × 10 −2 and a flattened dispersion over a broadband of 1.28 THz. Then, polarization splitters, based on both symmetric and asymmetric porous-core PCF structures, are designed and evaluated at 1 THz. We show that this kind of device exhibits a strong polarization-dependent coupling behavior. Numerical results show that the configuration based on dual-core waveguide with asymmetric cores can achieve a 10.9 cm long splitter with a broadband of 0.306 THz for x-polarization and 0.23 THz for y-polarization. Finally, this paper offers an effective method to design an ultrawideband polarization beam splitter to operate in the THz region, which might be relevant for future applications in technical areas, such as spectroscopy, sensing, and high-speed data transmission.
A compact tunable mode converter device based on the thermo-optically characteristics of liquid crystals (LCs) is proposed and numerically analyzed herein. The proposed mode converter consists of an asymmetric dual-core photonic crystal fiber (PCF) with a highly thermo-responsive LC core. The verification of the proposed mode converter was ensured through an accurate PCF analysis based on the vector finite element method. With an appropriate choice of the design parameters associated with the LC core, phase matching at a single wavelength is available in the important O-band wavelength region. The simulation results showed that high conversion efficiencies between LP01 and LP11 mode are readily achieved over a broad wavelength range from 1278 nm to 1317 nm. Likewise, the tunable capability of the proposed mode converter was evaluated when it was submitted to thermal changes; thus, we evidence the strong thermo-responsive dependence of the operating wavelength, mode conversion efficiency and full-width at the half maximum (FWHM) bandwidth. Finally, the fabrication tolerances of the devices were also investigated. Therefore, the thermo-responsive characteristics of this novel PCF mode converter can be of fundamental importance in the future space division multiplexing technology.
A novel design of a single-mode photonic crystal fiber for applications in the terahertz region is proposed and numerically analyzed. The geometrical and optical parameters of the structure are estimated by using algebraic expressions while the guidance properties are calculated by using the full-vector finite element method. The porous-core of the fiber is suspended by eight equally spaced bridges connected to an outer cladding. The obtained results show that by adjusting the structural parameters appropriately, low-loss and flat dispersion properties could be easily achieved. The numerical results show that our best configuration, with a 60% filling fraction, exhibits a low effective material loss, below 0.2 dB/cm, and a flattened dispersion behavior in the frequency range from 0.79 THz to 1.5 THz.
Double-clad fibers where the second cladding has a lower refractive index than the first cladding, prove to be ideal structures for potentiating and tuning the sensitivity in longperiod fiber gratings (LPFGs) operating in mode transition. When a thin film is deposited on the optical fiber, the second cladding performs acts as a barrier that initially prevents the transition to guidance in the thin film of one of the modes guided in the first cladding. Finally, the transition to guidance occurs with a sensitivity increase, in analogy to the tunnel effect observed in semiconductors. This improvement has been demonstrated both as a function of the thin film thickness and the surrounding medium refractive index, with enhancement factors of 4 and 2, respectively. This idea reinforces the performance of LPFGs, adding a new degree of freedom to the mode transition and the dispersion turning point phenomena. Moreover, the control of the variation of the effective index of cladding modes could be applied in other structures, such as tilted-fiber gratings or evanescent wave sensors.
In this work, a novel and compact mode selective coupler based on an asymmetric dual core photonic crystal fiber with a thermo-responsive core is proposed and numerically studied. Simulation results show that high conversion efficiencies are achieved between LP01 and LP11 modes in the O-band with a wide bandwidth of 24.55 nm and a mode conversion efficiency over 60% when it has a total length of 3.15 mm and the operating wavelength is 1310 nm. In addition, the thermally tuneability was evaluated when this device was submitted to thermal changes from 15°C to 35°C. The results evidence that thermal effect allows to control the operating wavelength and the mode coupling efficiency of the mode converter when it is used to couple energy from the LP01 mode to the LP11 mode. Therefore, the proposed device could be useful in high-bandwidth mode division multiplexed communication systems.
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