Flexible low-loss THz and ultra high-power IR waveguiding using PCFs

Photonic Crystal Fibers (PCFs) have attracted considerable attention of research communities and industries for peculiar propagation characteristics and prove to have wide range of applications, such as security, sensing, high power fiber laser, and communications, in wide range of frequency regions from visible, infrared, to THz frequency. The optical properties of materials in PCF can be controlled by proper design of the fiber cross-section. Among wide area of PCF applications, the PhD activity mainly focus on two interesting applications of PCF: high power fiber laser in IR region and low-loss waveguiding in THz region. The applications will be presented in relation with the kind of PCF designs. Large Pitch PCF, short called by Large Pitch Fiber (LPF), a hexagonal PCF structure, enable the large mode area and achieves the delocalization property of higher order modes, granted high output power and good beam quality. The properties are maintained even by scaling the whole structure to obtain the large mode area. In the mid-infrared range of 2 µm - 3 µm, the doublecladding silica based PCF with 80 µm-core and reduced cladding symmetry has been investigated, aiming to obtain robust single mode guiding even at high power load of fiber lasers. Single mode propagation with effective area larger than 2500 µm(2) has been demonstrated at heat load of 340 W/m. For THz region, the PCF obtained by arranging N dielectric tubes made by Zeonex plastic material in a circular pattern has been designed and optimized for flexible low-loss waveguiding of terahertz application. The fibers guide through inhibited coupling. Using the FEM, the scaling laws of confinement loss and absorption loss of the fibers have been numerically investigated with the purpose to obtaine design guidelines for small and flexible low-loss THz waveguides.By choosing the a total reference value of 10 dB/m, a remarkable transmission band of about an octave, ranging from 0.7 THz to 1.3 THz, is obtained with number of tubes 16 and tube thickness 100 µm. By reducing the reference loss value as low as 1 dB/m the maximum band of 280 GHz is obtained with number of tubes 14, ranging from 0.94 THz to 1.22 THz. The bandwidth can be further increased around one and half larger by reducing tube thickness to 90 µm and applying touchless cladding tubes.