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.

The traditional numerical analysis in waveguide design can be time-consuming and inefficient. This is even more prominent in the THz region and with complex shapes and materials. As an alternative to overcome these drawbacks, we propose a machine learning (ML) approach to design porous-core photonic crystal fibers (PCFs) for the THz band. This method is based on an artificial neural network (ANN) model trained to predict key parameters such as the effective refractive index, effective area, dispersion, and loss values with accuracy and speed. In that sense, the network was trained to perform multiple-output regression of the above parameters. The training data for this model comes from numerical calculations that use the finite element method (FEM) to simulate and evaluate analytical expressions. Our results demonstrate the ML model’s ability to capture the complex and nonlinear relationships between the input and output parameters and accurately predict the behavior of the THz PCF. Moreover, the proposed model has an inference time of ∼0.03584 s for a batch of 32 data sets, which substantially outperforms typical calculation times needed in FEM simulations for THz waveguide design. These results show that this approach is efficient and effective and has the potential to significantly accelerate the design process of PCFs for THz applications.

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 long-period 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.

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.

We propose a compact all-fiber Mach-Zehnder interferometer based on a dual core transversally chirped microstructured optical fiber to measure glucose concentration. The interrogation of the sensing device has been performed monitoring the wavelength shifts in the transmission spectrum in the infrared region. The response of the proposed device was evaluated when the glucose concentration changes from 70 g/L to 526 g/L at room temperature. The numerical results show that a high glucose concentration sensitivity of 3470 nm/RIU could be achieved with this configuration. Size, operational ranges and sensibilities of the proposed sensor allow it to be used in medical and biological applications.