Abstract
This paper presents a novel approach to attain a single-polarization-single-mode (SPSM) photonic crystal fiber (PCF) in the terahertz (THz) regime. An initial circular hole PCF design is modified by introducing asymmetry in the first ring of six air holes in the cladding, i.e., epsilon-near-zero (ENZ) material is introduced into only four of those air holes and the other two remain air-filled but have different diameters. The resulting fundamental X-polarized (XP) and Y-polarized (YP) modes have distinctly different electric field distributions. The asymmetry is arranged so that the YP mode has a much larger amount of the field distributed in the ENZ material than the XP mode. Since the ENZ material is very lossy, the YP mode suffers a much higher loss than the XP mode. Consequently, after a short propagation distance, the loss difference (LD) between the XP and YP modes will be large enough that only the XP mode still realistically exists in the PCF. To further enhance the outcome, gain material is introduced into the core area to increase the LDs between the wanted XP mode and any unwanted higher order (HO) modes, as well as to compensate for the XP mode loss without affecting the LD between the XP and YP modes. The optimized PCF exhibits LDs between the desired XP mode and all other modes greater than 8.0 dB/cm across a wide frequency range of 0.312 THz. Consequently, the reported PCF only needs a length of 2.5 cm to attain an SPSM result, with the unwanted modes being more than 20 dB smaller than the wanted mode over the entire operational band.
Highlights
Polarization-maintaining photonic crystal fibers (PM PCFs) have been widely used in various applications, such as precision optical instruments, optical communication systems, and sensors [1].There are basically two main types of PM PCFs, i.e., highly birefringent (HB) PCFs and single-polarization-single-mode (SPSM) PCFs
Previous studies have shown that different epsilon-near-zero (ENZ) materials [15] or the liquid toluene [16] can be inserted into selected air holes of a PCF to manipulate the local field distributions in it and to introduce asymmetries that facilitate eliminating the degeneracy of its two fundamental modes to achieve high birefringence properties
Perfectly matched layer (PML) boundary conditions were adopted for adaptive meshing provided by the software, the minimum mesh edge length was 0.12 μm, which is the simulations to absorb any numerical reflections arising from its outer boundaries
Summary
Polarization-maintaining photonic crystal fibers (PM PCFs) have been widely used in various applications, such as precision optical instruments, optical communication systems, and sensors [1]. The TL is usually dominated by the EML in the THz regime This fact makes it difficult to produce a large enough MLD between the two propagating polarization modes with conventional methods that focus on manipulating the CLs. For example, a THz SPSM PCF based on index-matching techniques was reported in [13]. Previous studies have shown that different epsilon-near-zero (ENZ) materials [15] or the liquid toluene [16] can be inserted into selected air holes of a PCF to manipulate the local field distributions in it and to introduce asymmetries that facilitate eliminating the degeneracy of its two fundamental modes to achieve high birefringence properties These studies warned of the fact that using ENZ materials, which are themselves usually very lossy [17], could yield PCFs with nontrivial propagation losses. 20 dB smaller than the wanted mode over the entire operational band
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