The Terahertz frequency range of the electromagnetic spectrum has attracted much research attention as a result of their impactful applications in sensing, imaging, security and communication. Despite recent technological advances, a majority of the commercial THz systems are still based on free space propagation. These systems suffer from drawbacks such as absorption losses, difficulties in integration with other devices, and high sensitivity to the surrounding environment. Alternative THz waveguides have thus been proposed, however, they are all based on metallic wires and subwavelength dielectric fibers. On the other hand, photonic crystal fibers have been successfully deployed in the optical regime due to their performance advantages. For effective photonic crystal fiber practical applications, characteristics such as loss, bandwidth, dispersion and birefringence must be considered. In particular, high birefringence is desired. Unfortunately, the polarization maintaining ability of birefringent photonic crystal fibers remains affected by polarization cross talk and polarization mode dispersion.
To this effect, University of Technology Sydney scientists from the Global Big Data Technologies Centre: Tianyu Yang (PhD candidate), Dr. Can Ding, Professor Richard W. Ziolkowski and Professor Y. Jay Guo have developed a novel photonic crystal fiber design that would yield very high birefringence. Specifically, high index contrast between the propagating X- and Y-polarized modes was attained without breaking the symmetry of the overall geometrical structure, but with breaking the material symmetry by loading only certain of its circular air holes with materials having different refractive indexes. Their work is currently published in the research journal, Optics Express.
In brief, the research method employed commenced with the introduction and thorough study of two complementary photonic crystal fiber configurations and their design parameters. Next, the researchers selected one model based on its performance characteristics and optimized it for the THz regime. The optimized THz photonic crystal fiber model was then scaled to the optical regime. They then recalculated the performance characteristics of both the THz and optical photonic crystal fibers with realistic epsilon-near-zero (ENZ) materials. Lastly, potential methods to realize the reported epsilon-near-zero -based THz and optical photonic crystal fiber designs were evaluated.
The authors observed a birefringence above 0.1 and a loss below 0.01 cm−1 over a wide band of frequencies. These properties were achieved based on extensive simulation results of the symmetric geometry and asymmetric material distribution in the lower THz range. Additionally, they noted that the optimized THz photonic crystal fiber exhibited near zero dispersion at 0.75 THz for both the X- and Y-polarization modes and a birefringence equal to 0.28. These results were obtained with ideal ENZ materials. With currently available ENZ materials this THz photonic crystal fiber design still exhibits a high birefringence, but with slightly larger losses. Anticipated future developments of ENZ materials with lower loss properties will mitigate this practical issue.
In summary, the study by the University of Technology Sydney researchers presented a photonic crystal fiber design that was able to achieve very high birefringence, low loss, and near zero dispersion characteristics in both the THz and optical regimes. While its configuration was essentially a simple circular hole, triangular lattice, the enhanced performance characteristics were seen to be facilitated by the introduction of localized asymmetries in its material distributions. Altogether, significantly enhanced birefringence was achieved in a novel photonic crystal fiber by filling selected air holes in its cladding with an epsilon-near-zero material. Its development presents potential for future applications.
Tianyu Yang, Can Ding, Richard W. Ziolkowski, Y. Jay Guo. Circular hole ENZ photonic crystal fibers exhibit high birefringence. Volume 26, Number 13 | 2018 | Optics Express 17264Go To Optics Express