Octave-spanning supercontinuum generation of vortices in an As2S3 ring photonic crystal fiber

Design and optimization of GI-PCF supporting the orbital angular momentum modes based on negative curvature structure

Based on the negative curvature structure, we design a graded-index photonic crystal fiber (GI-PCF) supporting the orbital angular momentum (OAM) mode transmission and discuss its optimization strategy. The core of the designed GI-PCF is sandwiched by three-layer inner air-hole arrays with gradually decreasing air-hole radii and a single outer air-hole array, where the inner side of the annular core forms a graded refractive index distribution. All these structures are clad with negative-curvature tubes. By optimizing characteristic structural parameters, including the air-filling fraction of the outer array, the air-hole radii of the inner arrays, and the thickness of the tubes, the GI-PCF can support 42 OAM modes and most of them have a purity greater than 85%. Compared with conventional structures, the present design of GI-PCF has better properties on an overall level, which can stably transmit multiple OAM modes with high mode purity. These results inject new interest in the flexible design of PCF and have potential applications in various fields, including but not limited to the mode division multiplexing system and terabit data transmission.

GeO2-doped ring-core photonic crystal fiber for supporting robust orbital angular momentum modes

Orbital angular momentum (OAM)-mode-supported photonic crystal fibers (PCFs) have inspired intensive research in modern fiber optics due to the robust propagation and theoretically unlimited signal-carried channels. In this paper, a dual-cladding GeO₂-doped ring-core PCF is designed, and a strategy for optimizing OAM mode properties is analyzed by structure parameters and GeO₂-doping concentration. Numeric results show that high structural degrees of freedom are available to improve the effective refractive index separation (within the vector modes), chromatic dispersion, effective mode field area, nonlinear coefficient, and OAM mode purity in terms of inner cladding, outer cladding, and ring-core. In particular, the effective refractive index separation and chromatic dispersion can exhibit a high order magnitude of 10^-3 and a low value in the broad band from 1.3 µm to 1.7 µm, respectively. In addition to structural optimization, doping high index material into the ring-core is another way to regulate the fiber performance by controlling the doping concentration. A systematic investigation shows that as the doping concentration increases, the effective refractive index separation and mode purity increase obviously, while the dispersion and mode field area gradually decrease. This flexible manipulation offers a method for customizing the optical properties of OAM-supported PCFs in communication and sensor systems.

Over-Two-Octave Supercontinuum Generation of Light-Carrying Orbital Angular Momentum in Germania-Doped Ring-Core Fiber

In this paper, we design a silica-cladded Germania-doped ring-core fiber (RCF) that supports orbital angular momentum (OAM) modes. By optimizing the fiber structure parameters, the RCF possesses a near-zero flat dispersion with a total variation of <±30 ps/nm/km over 1770 nm bandwidth from 1040 to 2810 nm for the OAM1,1 mode. A beyond-two-octave supercontinuum spectrum of the OAM1,1 mode is generated numerically by launching a 40 fs 120 kW pulse train centered at 1400 nm into a 12 cm long designed 50 mol% Ge-doped fiber, which covers 2130 nm bandwidth from 630 nm to 2760 nm at −40 dB of power level. This design can serve as an efficient way to extend the spectral coverage of beams carrying OAM modes for various applications.

Design of PCF Supporting 86 OAM Modes with High Mode Quality and Low Nonlinear Coefficient

A unique photonic crystal fiber with square and circular air holes (SC-PCF) is designed in this research. Three layers of circular air holes and two levels of square air holes make up the fiber cladding. The finite element approach is used to simulate the fiber construction, and numerical calculations are used to examine the transmission properties in the S+C+L band. The results reveal that the SC-PCF can sustain 86 Orbital Angular Momentum (OAM) modes in the wavelength range of 1400 nm to 1700 nm (300 nm), with an effective refractive index difference (ERID) of 5.88 × 10−3 between them, thus avoiding mode coupling. The mode purity of all modes is greater than 96% at 1550 nm, and the lowest dispersion and dispersion change are 4.939 ps/nm/km and 0.956 ps/nm/km, respectively. The confinement loss (CL) of all modes is lower than 10−9 dB/m, and the nonlinear coefficient (NC) is lower than 1.5 W−1·km−1 in the whole band. The proposed SC-PCF has important value in long-distance and large-capacity communication systems.

Vortex Polymer Optical Fiber with 64 Stable OAM States

This research introduces a numerical design of an air-core vortex polymer optical fiber in cyclic transparent optical polymer (CYTOP) that propagates 32 orbital angular momentum (OAM) modes, i.e., it may support up to 64 stable OAM-states considering left- and right-handed circular polarizations. This fiber seeks to be an alternative to increase the capacity of short-range optical communication systems multiplexed by modes, in agreement with the high demand of low-cost, insensitive-to-bending and easy-to-handle fibers similar to others optical fibers fabricated in polymers. This novel fiber possesses unique characteristics: a diameter of 50 µm that would allow a high mechanical compatibility with commercially available polymer optical fibers, a difference of effective index between neighbor OAM modes of around 10-4 over a bandwidth from 1 to 1.6 µm, propagation losses of approximately 15 × 10-3 dB/m for all OAM modes, and a very low dispersion for OAM higher order modes (±l = 16) of up to +2.5 ps/km-nm compared with OAM lower order modes at a telecom wavelength of 1.3 µm, in which the CYTOP exhibits a minimal attenuation. The spectra of mutual coupling coefficients between modes are computed considering small bends of up to 3 cm of radius and slight ellipticity in the ring of up to 5%. Results show lower-charge weights for higher order OAM modes.

Transmission of Orbital Angular Momentum and Cylindrical Vector Beams in a Large-Bandwidth Annular Core Photonic Crystal Fiber

The stable propagation of orbital angular momentum and cylindrical vector beams in a newly designed annular core photonic crystal fiber (AC-PCF) tailored for the broadband single-radial order beam transmission (within the so-called “endlessly mono-radial” guiding regime) is demonstrated for the first time. It is shown that the vector-vortex beams can maintain high mode purities above 18 dB after propagation in the fiber under test over all of the wavelength range from 805 to 845 nm (over 17 THz bandwidth) investigated with the help of a tunable laser and an S-plate for the generation of singular beams in free space. Our results confirm that the AC-PCF is a promising design for the broadband transmission of vector-vortex beams that have potential applications in space-division multiplexing, quantum communications, optical sensing and trapping.

Analysis of the Transmission Characteristic and Stress-Induced Birefringence of Hollow-Core Circular Photonic Crystal Fiber

Orbital angular momentum modes in optical fibers have polarization mode dispersion. The relationship between polarization mode dispersion and the birefringence vector can be deduced using an optical fiber dynamic equation. First, a mathematical model was established to formulate mode dispersion caused by stress-induced birefringence. Second, in the stress-induced birefringence simulation model, the finite element method was used to analyze the transmission characteristics of the hollow-core circular photonic crystal fiber. Finally, mode dispersion caused by stress-induced birefringence was obtained using theoretical derivation and simulation analyses. The results showed that the new fiber type has good transmission characteristics and strong stress sensitivity, which provide key theoretical support for optimizing the structural parameters of optical fiber and designing stress sensors.

Endlessly mono-radial annular core photonic crystal fiber for the broadband transmission and supercontinuum generation of vortex beams

We demonstrate a new guiding regime termed endlessly mono-radial, in the proposed annular core photonic crystal fiber (AC-PCF), whereby only modes of the fundamental radial order are supported by the fiber at all input wavelengths. This attribute is of high interest for applications that require the stable and broadband guiding of mono-radial (i.e. doughnut shaped) cylindrical vector beams and vortex beams carrying orbital angular momentum. We further show that one can significantly tailor the chromatic dispersion and optical nonlinearities of the waveguide through proper optimization of the photonic crystal microstructured cladding. The analytical investigation of the remarkable modal properties of the AC-PCF is validated by full-vector simulations. As an example, we performed simulations of the nonlinear fiber propagation of short femtosecond pulses at 835 nm center wavelength and kilowatt-level peak power, which indicate that the AC-PCF represents a promising avenue to investigate the supercontinuum generation of optical vortex light. The proposed fiber design has potential applications in space-division multiplexing, optical sensing and super-resolution microscopy.

Design tool for circular photonic crystal fibers supporting orbital angular momentum modes

We propose a design tool for the family of circular photonic crystal fiber (C-PCF) supporting orbital angular momentum modes. The calibrated normalized parameters for the C-PCF family are presented. The information about the cutoff condition of modes, the number of modes supported in fibers, and the effective index difference between adjacent modes can be obtained by using the design tool. Also, the mode properties, such as confinement loss, effective mode area (A_eff), and spin-orbital coupling are analyzed using the design tool. At the end of paper, we also give some fiber design examples by using the design tool.

Photonic crystal fiber for supporting 26 orbital angular momentum modes

We propose and numerically investigate a photonic crystal fiber (PCF) based on As₂S₃ for supporting the orbital angular momentum (OAM) modes up to 26. The designed PCF is composed of four well-ordered air hole rings in the cladding and an air hole at the center. The OAM modes can be well separated due to the large effective index difference of above 10^-4 between the eigenmodes and maintain single-mode condition radially. In addition, the dispersions of the modes increase slowly with wavelengths, while the confinement loss keeps as low as 10^-9 dB/m. The proposed PCF increases the supported OAM modes which could have some potential applications in short-distance, high-capacity transmission.

On the scalability of ring fiber designs for OAM multiplexing

The promise of the infinite-dimensionality of orbital angular momentum (OAM) and its application to free-space and fiber communications has attracted immense attention in recent years. In order to facilitate OAM-guidance, novel fibers have been proposed and developed, including a class of so-called ring-fibers. In these fibers, the wave-guiding region is a high-index annulus instead of a conventional circular core, which for reasons related to polarization-dependent differential phase shifts for light at waveguide boundaries, leads to enhanced stability for OAM modes. We review the theory and implementation of this nascent class of waveguides, and discuss the opportunities and limitations they present for OAM scalability.

Few-mode fiber with inverse-parabolic graded-index profile for transmission of OAM-carrying modes

A novel type of few-mode fiber, characterized by an inverse-parabolic graded-index profile, is proposed for the robust transmission of cylindrical vector modes as well as modes carrying quantized orbital angular momentum (OAM). Large effective index separations between vector modes (>2.1 × 10(-4)) are numerically calculated and experimentally confirmed in this fiber over the whole C-band, enabling transmission of OAM(+/-1,1) modes for distances up to 1.1 km. Simple design rules are provided for the optimization of the fiber parameters.

Fabrication and characterization of a hybrid four-hole AsSe₂-As₂S₅ microstructured optical fiber with a large refractive index difference

A hybrid four-hole AsSe2-As2S5 microstructured optical fiber (MOF) with a large refractive index difference is fabricated by the rod-in-tube drawing technique. The core and the cladding are made from the AsSe2 glass and As2S5 glass, respectively. The propagation loss is ~1.8 dB/m and the nonlinear coefficient is ~2.03 × 10(4) km(-1)W(-1) at 2000 nm. Raman scattering is observed in the normal dispersion regime when the fiber is pumped by a 2 μm mode-locked picosecond fiber laser. Additionally, soliton is generated in the anomalous dispersion regime when the fiber is pumped by an optical parametric oscillator (OPO) at the pump wavelength of ~3000 nm.

A compact trench-assisted multi-orbital-angular-momentum multi-ring fiber for ultrahigh-density space-division multiplexing (19 rings × 22 modes)

We present a compact (130 μm cladding diameter) trench-assisted multi-orbital-angular-momentum (OAM) multi-ring fiber with 19 rings each supporting 22 modes with 18 OAM ones. Using the high-contrast-index ring and trench designs, the trench-assisted multi-OAM multi-ring fiber (TA-MOMRF) features both low-level inter-mode crosstalk and inter-ring crosstalk within a wide wavelength range (1520 to 1630 nm), which can potentially enable Pbit/s total transmission capacity and hundreds bit/s/Hz spectral efficiency in a single TA-MOMRF. Moreover, the effective refractive index difference of even and odd fiber eigenmodes induced by the ellipticity of ring and fiber bending and their impacts on the purity of OAM mode and mode coupling/crosstalk are analyzed. It is found that high-order OAM modes show preferable tolerance to the ring ellipticity and fiber bending. The designed fiber offers favorable tolerance to both small ellipticity of ring (<−22 dB crosstalk under an ellipticity of 0.5%) and small bend radius (<−20 dB crosstalk under a bend radius of 2 cm).

Mid-infrared supercontinuum generation in a suspended-core As2S3 chalcogenide microstructured optical fiber

We demonstrate the supercontinuum (SC) generation in a suspended-core As(2)S(3) chalcogenide microstructured optical fiber (MOF). The variation of SC is investigated by changing the fiber length, pump peak power and pump wavelength. In the case of long fibers (20 and 40 cm), the SC ranges are discontinuous and stop at the wavelengths shorter than 3500 nm, due to the absorption of fiber. In the case of short fibers (1.3 and 2.4 cm), the SC ranges are continuous and can extend to the wavelengths longer than 4 μm. The SC broadening is observed when the pump peak power increases from 0.24 to 1.32 kW at 2500 nm. The SC range increases with the pump wavelength changing from 2200 to 2600 nm, corresponding to the dispersion of As(2)S(3) MOF from the normal to anomalous region. The SC generation is simulated by the generalized nonlinear Schrödinger equation. The simulation includes the SC difference between 1.3 and 2.4 cm long fiber by 2500 nm pumping, the variation of SC with pump peak power in 2.4 cm long fiber, and the variation of SC with pump wavelength in 1.3 cm long fiber. The simulation agrees well with the experiment.

Fiber structure to convert a Gaussian beam to higher-order optical orbital angular momentum modes

We propose a fiber structure of a square core and ring refractive index profile that converts an input circular polarized Gaussian mode into optical orbital angular momentum (OAM) modes. By breaking the circular symmetry of the waveguide, the input circularly polarized fundamental mode in the square core can be coupled into the ring region to generate higher-order OAM modes, corresponding to the transference of spin angular momentum and orbital angular momentum. We show, by using simulation, the generation of OAM modes with a topological charge l up to 9 using <10 mm long fiber. The mode purity is above 96.4% and the extinction ratio can be 30 dB.