Hollow Ring-Core Photonic Crystal Fiber With >500 OAM Modes Over 360-nm Communications Bandwidth

We propose and design a hollow As
₂
S
₃
ring-core photonic crystal fiber (PCF) with 514 radially fundamental orbital angular momentum (OAM) modes over 360 nm communications bandwidth across all the O, E, S, C, and L bands. The designed PCF with 40
$\mu \text{m}$
-radius air core and 150 nm-width As
₂
S
₃
ring can support eigenmodes up to HE
_130,1
and EH
_128,1
. The numerical analysis shows that the designed ring PCF has large effective refractive index contrast, and can transmit up to 874 OAM modes near 1.55
$\mu \text{m}$
. Simulation results show that in the C and L bands, the PCF with a hollow-core radius of 40
$\mu \text{m}$
and a ring width of 0.15
$\mu \text{m}$
can retain an
$2.5\times 10 ^{-3}$
effective refractive index difference between the two highest order OAM modes, which achieves effective mode separation, thereby achieving stable OAM mode transmission. The
$n_{eff}$
difference between the even and odd fiber eigenmodes and the intra-mode walk-off are also carefully studied under different bending radii. The results show that higher-order OAM modes has better tolerance to the fiber bending, compared with the lower-order modes. The fiber has the potential to support ultra-high capacity OAM mode division multiplexing in the optical fiber communication systems.