Design, fabrication and validation of an OAM fiber supporting 36 states

Abstract

We present an optical fiber supporting 36 information bearing orbital angular momentum (OAM) states spanning 9 OAM orders. We introduce design techniques to maximize the number of OAM modes supported in the fiber; while avoiding LP mode excitation. We fabricate such a fiber with an air core and an annular index profile using the MCVD process. We introduce a new technique for shaping OAM beams in free-space to obtain better coupling efficiency with fiber with annular index profiles. We excite 9 orders of OAM in the fiber, using interferometry to verify the OAM state on exiting the fiber. Using polarization multiplexing and both signs for the topological charge, we confirm support of 36 states, exploiting to our knowledge the highest number of OAM modes ever transmitted in optical fiber.

Orbital-Angular-Momentum Polarization Mode Dispersion in Optical Fibers

“…According to (10), this scaling factor would also be transferred to g θ (z). As a result, the modal birefringence correlation length is dependent on the topological charge l: (12) where L C 0 is the birefringence correlation length of the fundamental HE 11 mode. The superscript "±" over the term L C accounts for the CP-OAM handedness, the same as that forˆl s  .…”

“…Assuming that the modal birefringence originates only from the core ellipticity, we can then find out the beat lengths of the higher-order OAM modes, L B ±|l| , using the simulation results shown in Fig. 13, and also find out the higher-order birefringence correlation, L C ±|l| , using the scaling rule as shown by (12). Finally, using (14) again, <τ ±|l| > for all the higher-order OAM modes can be calculated.…”

“…A variety of OAM fibers have been proposed with either solid [8]- [10] or hollow cores [11], [12]. Most of them feature a ring-shaped refractive index profile and a large index contrast between the core and cladding.…”

The Orbital Angular Momentum Modes Supporting Fibers Based on the Photonic Crystal Fiber Structure

“…In order to hold the ring-shaped OAM modes, rationally, the OAM fibers should have a circular or ring-shaped structure, in which the ring core area is with higher refractive index and inner hole, and the outer cladding is with lower indices. The ring fiber with step-index profile is a rational good design which can support 34 OAM modes [20]. The circular-shaped PCF is another rational design whose structure has a ring area (higher index area) surrounding several air-hole arrays (lower index area in average).…”

“…The distinction between odd and even HE modes is π/2 azimuthal rotation in polarization, and is the same for odd and even EH modes; j presents a π/2 phase shift. The two OAM ± 0,m modes composed by the fundamental HE 1,m modes with left or right circular polarization cannot be considered as OAM mode for they carry no OAM [20]. Moreover, the two OAM ∓ ±1,m modes possessing opposite directions of polarization and wavefront rotation are actually composed by azimuthally polarized TE 0,m and radially polarized TM 0,m , which can also not be viewed as OAM modes because they have different propagation constants and are unstable in the fiber [14].…”