Objective Orbital angular momentum (OAM) beams have recently spurred great interest because they are widely applicable to optical manipulation, quantum information, imaging, and communications. The four states of the OAMl,mmode family have been already demonstrated in multiple-input multiple-output (MIMO)-free transmission. When using the four states of a LPl,m mode (l ≥1) in an optical fiber with circular symmetry, 4×4 MIMO processing is necessary. As N×N MIMO complexity scales with N2, OAM modes are preferred in communication systems for mobile device management. However, the effective refractive index difference Δneff between the HE(l+1)m and EH(l-1)m modes should be below 10 -4 to minimize coupling between the OAM modes. Ring-core geometry, usually with a high-index contrast, is well adaptable to the guidance of OAM modes because it can eliminate the undesirable modes (m=2) and reaches a sufficiently large intra-mode-group OAM Δneff value. Moreover, avoiding the higher-order OAM modes (m≥2) simplifies the multiplexing-demultiplexing of the modes because all OAM modes present an annular-shaped intensity.
Methods Photonic crystal fibers (PCFs) can be flexibly designed to provide unique fiber properties such as endlessly single mode, controllable nonlinearity and confinement loss, and tailorable chromatic dispersion. The PCF structure that realizes OAM-mode transmission is potentially applicable to fiber communications. Although air-silica PCFs achieving high index contrast, OAM-dedicated PCFs are not yet realized, and an OAM with l=1 guidance in a PCF has been reported only in twisted PCFs. This study evaluates the potential of a new ring-core PCF designed for guiding OAM modes with low mode coupling. Such a geometry appears to be easily manufactured by the stack-and-draw method.
Results and Discussions In the proposed structure [Fig. 1(a)], the solid ring core is installed in a central hole surrounded by a circular ring of air holes in the cladding structure. This structure is prolongated by several rings of air holes with hexagonal symmetry. The intensity profiles of the first 10 vector modes in the PCF structure are theoretically studied using the finite element method, and are displayed in Fig. 1(b). The minimum effective refractive index (ERI) difference between the HE and EH modes corresponding to the same order OAM mode exceeds 3.04 × 10 -3 within the wavelength range 1.3--2.0 μm. Here, the diameter of the hollow core is 5.6 μm, the diameters of the first-ring, second-ring, and third-ring air holes in the cladding region are 2.67, 2.87, and 2.87 μm, respectively, and the pitch of the air holes in the two outermost rings is set to 3.09 μm (Figs. 3 and 4). The maximum ERI difference between the HE and EH modes reaches 1.55×10 -2(Fig. 5). Such a large ERI difference has not been previously achieved in a 10-mode OAM guiding fiber. Bending is known to induce modal birefringence (Fig. 7). When the bending radius is only 18 mm, the birefringence is less than 5×10 -5 RIU, implying that the odd and even modes are not easily combined into an OAM.
Conclusions We have designed an annular, hollow-core photonic crystal fiber for vortex light transmission that exploits the flexible characteristics of PCF. The structural parameters were simulated in a finite element analysis, method and the transmission characteristics of the orbital angular momentum modes in the PCF structure were analyzed. In this new design, the degeneracy lifting between the even and odd modes was limited to less than 3.5×10 -7 by the size of the air holes. The neff values of the HE and EH modes are well separated by 1.55 μm. In a theoretical analysis of the transmission characteristics of the first 10 vector modes, the number of allowable high-order vector modes was increased by adjusting the structural parameter (air-hole period) of the fiber. This strategy typified the early design of PCFs. More significantly, the proposed structure presented excellent bending-loss resistance. The simulations demonstrated that hollow-core PCFs ensure the stable and efficient transmission of multiple OAM modes when the bending radius is low (18 mm or less). The proposed PCF structure can potentially enhance the capacity of future optical communication systems.
