Ring-core Fiber Research Articles

Mode conversion between fibers with different refraction index distribution based on adiabatically tapered structures.

Different fibers generally have different mode characteristics so their connections in many practical applications often require mode conversion. The feasibility of mode conversion between fibers with different refractive index distributions based on adiabatically tapered structures is theoretically analyzed. The first kind of mode conversion is between ring core fiber and convex core fiber; the second kind is between multicore fiber and single-core fiber. Three common tapered structures are investigated: tapered core, diffused core, and tapered cladding. The analysis results show that mode conversion by a tapered structure is not suitable for all the modes for a ring core fiber and a convex core fiber; however, it can be accomplished for multicore fiber and single-core fiber.

High sensitivity all-fiber bend sensor based on modal interferences in a ring core fiber

We proposed and experimentally demonstrated an all-fiber sensor for measuring bend with high sensitivity based on a ring core fiber (RCF) modal interferometer. The sensor was fabricated by splicing a segment of RCF between two pieces of multimode fiber (MMF) and single-mode fiber (SMF) at the ends of the MMF as lead-in and lead-out. Due to the first segment of the MMF, the transmitted light is coupled into the ring core, silica center, and cladding of the RCF, exciting multiple modes in the RCF. By the modal interferences in the structure, bending sensing can be realized by interrogating the intensity of the interference dip. Experimental results show a high bending sensitivity of -25.63 dB/m-1 in the range of 1.0954 m-1 to 1.4696 m-1. In addition, the advantages of the bend sensor, such as small size, low temperature sensitivity, and simple fabrication process, can be used for curvature measurement in building health monitoring.

Accurate Mode Decomposition for Ring Core Fibers Based on Deep Learning

Accurate Mode Decomposition for Ring Core Fibers Based on Deep Learning

Accurate and noise-robust mode decomposition by analyzing the intensity distribution of a few-mode ring core fiber

Accurate and noise-robust mode decomposition by analyzing the intensity distribution of a few-mode ring core fiber

Polymer negative curvature ring-core fiber for OAM modes guidance.

In this paper, a novel, to the best of our knowledge, polymer-based negative curvature ring-core fiber (NC-RCF) is proposed and investigated. The hollow-core NC-RCF is composed of TOPAS as background material. The inner and outer negative curvature structure layers are connected to the annular area, and the orbital angular momentum (OAM) modes can propagate in the annular core. In the frequency region of 1.0-1.5THz, the designed NC-RCF can stably transmit 82 OAM modes. Investigation results indicate that the effective refractive index differences between the corresponding HE and EH modes are above 10-4. The confinement losses of EH or HE modes are smaller than 10-8 d B/m, and the dispersion variations are lower than 0.31ps/THz/cm. Effective mode areas are larger than 5.14m m 2. Additionally, the highest mode purity of all vector modes is 99.78%. In addition, modal birefringence, also known as the walk-off length, has also been discussed. All these operation performances indicate that the designed NC-RCF make contributions to the optical communication systems.

Heterogeneous optical network incorporating low-loss ring-core fiber and single-mode fibers seamlessly connected by all-fiber vortex (de)multiplexer

We propose and experimentally demonstrate heterogeneous optical network to make the space-division multiplexing (SDM) network with ring-core fiber (RCF) compatible with the legacy optical networks with conventional single-mode fiber (SMF). Two homemade all-fiber mode selective couplers are fabricated by fusing pre-tapered conventional SMF with multi-mode fiber (Type: Corning OM3), serving as orbital angular momentum (OAM) (de)multiplexer to enable seamless connection between heterogeneous optical network incorporating low-loss RCF (loss: 0.26 dB/km) based SDM network and ultra-low-loss SMFs (loss: 0.154 dB/km) based networks. We transmit 30-Gbit/s 8-ary quadrature amplitude modulation signals over two OAM modes (OAM1,1 and OAM2,1) in different mode groups without multiple-input multiple-output equalization techniques through 100-km heterogeneous RCF and SMF network (25-km ultra-low-loss SMF + 50-km low-loss RCF + 25-km ultra-low-loss SMF). The measured optical signal-to-noise ratio penalties for multiplexing transmission are less than 5.73 dB at a bit-error rate of 3.8 × 10−3, indicating that the demonstrated OAM-assisted heterogeneous optical network has a favorable performance.

Mode manipulation in a ring–core fiber for OAM monitoring and conversion

Abstract The monitoring and conversion of photonic orbital angular momentum (OAM) play fundamental and important roles for both classic and quantum technologies, especially in low-loss transmission media such as ring-core fibers (RCFs), which make many OAM applications practical or vastly more flexible. However, in a RCF, the modes associated with different OAM states are highly overlapping due to the circular refractive index distribution structure, which makes it difficult to distinguish and monitor the OAM modes and in turn limits its inline conversion. Here, we report the first experimental realization of mode monitoring in a RCF using mode filters (MFs), which takes advantage of the difference in the mode adiabatic evolution and the higher-order mode cutoff conditions in tapered RCFs. Different-order OAM can be filtered using MFs with different geometric parameters, as demonstrated by the linearly polarized mode intensity. Combined the mode manipulations in RCF and single-mode fiber, the fundamental mode coupling efficiency can reach 90%, the RCF mode conversion monitoring through inline transmission spectrum evolution can be realized, and the inline fabrication of RCF grating, which couples one mode to a desired mode, can be demonstrated by the fabricating process of three long-period fiber gratings. The mode conversion efficiency between 0-order and 1, 2- or 3-order OAM modes exceeds 96%. Our work provides an efficient approach to monitor and convert OAM modes in higher-order mode supporting RCFs and even other special fibers and further promotes the improvement of the capacity of OAM transmission in RCFs.

Generation, Transmission, and Amplification of OAM Modes in the PbSe-Doped Ring-Core Fiber Carrying 3D Printed Spiral Phase Plate

Vortex beams carrying orbital angular momentum (OAM) have increasingly attracted attention in the field of optical communication. However, transmission is still an issue due to transmission loss, especially in optical fibers. In this work, we proposed, designed, and fabricated micro spiral phase plates (SPPs) directly on an end facet of a piece of PbSe-doped ring-core fiber (RCF) through two-photon polymerization, realizing the integration of OAM beam generation, transmission, and amplification. The prepared RCF comprises a double-clad structure with a core-clad refractive index difference of 2.2% and the fluorescence range is 1150 nm–1700 nm. The intensity distribution of the OAM beam and the spiral interference fringes were obtained, which indicated that the OAM mode (|l|=1, 2, 3, 4) was generated and transmitted directly within the fiber. The small-signal amplification of four OAM modes was accomplished at 1550 nm under a pump power of 634 mW. The on–off gain is >13.2 dB for all modes and the differential mode gain (DMG) is <1.7 dB. The SPP-carrying RCF structure demonstrates the integration of generation, transmission, and amplification of higher-order OAM modes in all-fiber systems.

Broadband vector mode converters based on cascaded long-period fiber gratings written in slightly elliptical ring-core fiber

A broadband vector mode converter is proposed based on cascaded long-period fiber gratings(LPFGs) written in slightly elliptical ring-core fiber. In order to realize the phase shift between the cascaded LPFGs, an unirradiated optical fiber is introduced. The mode converter can directly convert fundamental mode into first-order vector modes (TE01, HE21even, HE21odd and TM01 modes) with high efficiency without using assistive devices. The effect of cascaded grating period and the unirradiated optical fiber length on the performance of the mode converter is investigated. Through optimizing the structure parameters, the designed broadband vector mode converters provide a 20-dB bandwidth of 104 nm, 97.5 nm, 101 nm and 97 nm for respectively generating TE01, HE21even, HE21odd and TM01 modes covering C+L band, which could be applied in broadband mode division multiplexing (MDM) transmission systems based on vector modes.

Design and Characteristics of Diamond-Assisted Ring-Core Fiber for Space Division Multiplexing

In this paper, a novel diamond-assisted ring-core fiber (DRF) is proposed. With the introduction of a low-refractive-index diamond-shaped region located in the center of the core, the proposed fiber effectively eliminates spatial degeneracy of the LPmn mode groups and maintains a low level of birefringence. Under the fiber structure parameters proposed in this paper, the effective refractive index difference (Δneff) between the spatial modes supported by the fiber in the entire C-band is greater than 2.25 × 10−4, and the Δneff between adjacent modes falls within the scope of (2.11~9.41) × 10−4. The degree of degenerate separation between the two polarization modes of all modes is very low, which is 2~3 orders of magnitude lower than that of the spatial mode. By discussing the mode characteristics of DRF and several other center-assisted ring-core fibers, the method that can be used to manipulate the spatial mode degenerate separation with structural symmetry is obtained, which can be applied to provide guidance for similar fiber designs. The proposed fiber structure is a promising candidate in space division multiplexing systems.

High-order orbital angular momentum mode conversion based on a chiral long period fiber grating inscribed in a ring core fiber.

A chiral long period fiber grating (CLPFG) was designed according to the phase-matching condition and conservation law of angular momentum, and was inscribed in a ring core fiber (RCF). This CLPFG was used to directly excite the ±2nd- and ±3rd-order orbital angular momentum (OAM) modes. The coupling efficiency of the OAM mode is up to 98.7% and the insertion loss is within 0.5 dB. The uniformity of the annular mode intensity distribution, polarization characteristics, and the mode purity of coupled OAM modes were investigated in detail. Results show that the coupled high-order OAM modes possess a relative uniform annular intensity distribution, its mode purity is up to 93.2%, and the helical phase modulation is independent on the polarization state of incident light. These results indicate that the RCF-based CLPFG is an ideal OAM mode converter for future high-capacity optical fiber communication systems.

Weakly guiding graded-index ring-core fiber supporting 16-channel long distance mode division multiplexing systems based on OAM modes with low MIMO-DSP complexity.

We propose a weakly guiding graded-index ring-core fiber (RCF) with trench-assisted structure. The simulation analysis indicates that such a special fiber design is able to support 8 orbital angular momentum (OAM) mode groups (MGs) with low inter-group crosstalk (< -25 dB/km) and low intra-group differential mode delay (DMD) (< 125 ps/km) for higher order OAM MGs with topological charge |l| = 4, 5, 6, 7. The designed RCF also shows favorable tolerance characteristics to ellipticity and bending. Moreover, stable and distinguished broadband performance of proposed RCF is verified over the whole C band ranging from 1530 nm to 1565 nm. This kind of fiber design could be employed in small-scale multiple-input multiple-output digital signal processing (MIMO-DSP) intra-group modes multiplexing transmission combined with MIMO-free inter-group mode multiplexing transmission. The simulated results of the designed RCF show its great potential of the 16-channel long-distance mode division multiplexing (MDM) transmission with low MIMO-DSP complexity.

Broadband dispersion compensating ring-core fiber for orbital angular momentum modes.

A well designed ring-core fiber can theoretically support numerous orbital angular momentum (OAM) modes with low crosstalk for space-division-multiplexing (SDM) data transmission, which is considered as a promising solution for overcoming the capacity crunch in optical communication network. However, the accumulated chromatic dispersion in OAM-fiber could limit the data speed and transmission distance of communication systems. A potential solution is to insert a dispersion compensation ring-core fiber with opposite-sign of dispersion in the transmission fiber along the fiber link. In this work, we propose a triple ring-core fiber with broadband negative dispersion. A highest negative dispersion of -24.47 ps/(nm·km) at 1550 nm and an average dispersion slope in the C band from -0.182 ps/(nm2·km) to 0.065 ps/(nm2·km) can be achieved to compensate multi-order dispersion. The effects of Ge-doping concentration fluctuation in the high-index ring core and fabrication errors on fiber geometric structures are also investigated. Furthermore, the effective mode area decreases as the widths of high-index rings increase due to the enhanced confinement ability. The designed triple ring-core fiber could offer potential for compensating OAM fiber links with positive dispersions.

Discriminative strain and temperature sensing using a ring-hyperbolic tangent fiber sensor.

Brillouin fiber sensors have demonstrated strong capability in discriminative and high-sensitivity multiparameter measurements. In this study, we proposed and numerically investigated novel ring core fiber-based stimulated Brillouin scattering for the simultaneous measurement of temperature and strain. The novel fiber, referred to as ring hyperbolic tangent (R-HTAN) fiber, is characterized by a shape parameter (α) that controls the optical refractive index and longitudinal acoustic velocity profiles. Numerical modal simulations indicated that the Brillouin gain spectrum contained multiple widely spaced and high-gain peaks, which were attributed to the strong interaction between the optical linearly polarized mode (i.e., LP0,1 as a pump wave) and multiple high-order longitudinal acoustic modes. The designed R-HTAN fiber enabled the discriminative sensing of temperature and strain with levels that clearly surpassed values recently reported in the literature. In case of straight R-HTAN fiber (α = 0), the maximum C(α=0)T and C(α=0)ε are 1.928 MHz/ ∘C and 0.087 MHz, respectively. In case of graded R-HTAN fiber (α = 1), the maximum C(α=1)T and C(α=1)ε are 1.872 MHz/ ∘C and 0.0842 MHz/μɛ, respectively. The errors associated with temperature measurements (maximum δT(α=0) = 0.0846 ∘C and maximum δT(α=1) = 7.4184 ∘C) and strain measurements (maximum δɛ(α=0) = 0.7250 μɛ and maximum δɛ(α=1) = 7.4184 μɛ) demonstrated that the proposed fiber could be a promising candidate for next-generation Brillouin sensing systems for enabling temperature and strain discrimination.

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 and characteristics of ellipse-assisted ring-core fiber for space division multiplexing

• Elliptical-assisted ring-core fiber with central low-refractive-index elliptical region. • 9 effectively separated spatial modes have uniform Δn eff with values greater than 3.14 × 10 −4 at 1550 nm. • The level of polarization separation is lower than 3.33 × 10 −6 . • Good scalability and fabrication error tolerance. • This fiber design could be promising candidates toward high-capacity SDM systems. We propose a novel ellipse-assisted ring-core fiber characterized by an elliptical region of low refractive index in the core. The proposed fiber proves effective in eliminating the spatial degeneracy of the LP mn mode groups and keeping the birefringence properties at a low level. This fiber, with optimized structural parameters, can guide 9 modes over the entire C-band. The simulation results show that the effective refractive index difference (Δn eff ) between adjacent modes at 1530–1565 nm falls within the scope of (3.1 ∼ 6.64) × 10 −4 . At the same time, the polarization separation level of each mode is lower than 3.23 × 10 −6 , which is 2 orders of magnitude lower than the degree of spatial mode degeneracy separation. This fiber design has good application prospects in space division multiplexing systems.

Quantum orbital angular momentum in fibers: A review

The field of quantum communication is thriving as a complement to conventional telecommunication with its distinctive feature of absolute security. As the core technology for delivering quantum information, substantial advances in quantum communication have already been demonstrated on various platforms, including photonic systems. Among all of them, the orbital angular momentum (OAM) of photons with its infinite Hilbert space has attracted much attention and has been widely employed in both classical and quantum regimes. In particular, many types of fiber have been designed and fabricated to allow transmitting OAM of photons. Here, we review recent progress in transmitting OAM quantum states through different types of fiber, including few-mode fibers, multi-mode fibers, ring-core fibers, and single-mode fibers. We also discuss the challenges and prospects of quantum OAM in fibers.

Few-mode fiber design for multiple-input-multiple-output-less mode division multiplexing by machine learning

Neural networks (NNs), random forests (RF), and extreme gradient boosting (XGBoost) are applied separately to inversely design the ring-core few-mode fiber (FMF) with a desired weakly coupled performance. We demonstrate the procedure of inverse designing of FMF via machine learning (ML) algorithms and evaluate the prediction accuracy of the above ML algorithms. Compared with RF and XGBoost, the NN performs the highest prediction accuracy. For the NN, RF, and XGBoost, the correlation coefficients of the mode effective index difference are 0.99993, 0.99857, and 0.99937, respectively. Subsequently, by utilizing the method of permuting feature importance ranking, we obtain the high-correlation fiber structural features with the mode effective index difference. Moreover, we analyze the effect of the minimum index difference between two adjacent modes ( Δ n e f f , m i n ) on the structural parameters and get consistent feature attribution via permuting feature importance ranking above. Finally, we design a weakly coupled ring-core fiber that supports four modes ( H E 11 , T E 01 , H E 21 , T M 01 ) based on the NN algorithm, which could be successfully applied in vector mode division multiplexing.

Efficient mode controlling and dispersion tuning in low-loss chalcogenide glass ring-core fiber

The efficiency of mode controlling and dispersion tunning in chalcogenide glass fiber is vital to improve the transmission capacity and spectral spanning efficiency in the mid-infrared. A solid-state ring-core fiber (RCF) is designed for mode controlling and dispersion tuning. The simulation results show that LP01-mode is well confined in the high-index cladding-ring, and the confinement loss (CL) analysis of other modes is performed as well. RCF can be regarded as an LP02-mode fiber in the center, which is different from LP01-mode in the step-index fiber when the wavelength is larger than 5.6 μm. Moreover, OAM0,1 can be stably transmitted in the ring, since LP01-mode and OAM0,1 have identical ring-shaped profiles. For LP02 in the core, the zero-dispersion wavelength (ZDW) can be blue-shifted to 4.4 µm from 6.7 µm. For the first time, we have successfully fabricated a novel mid-infrared RCF based on Ge9As22Se69-Ge9As22.5Se68.5-Ge10As22Se68 glasses. The lowest loss is 0.28 dB/m at 8.1 μm. The supercontinuum (SC) spectra can be effective broadening (1–14.4 µm) in this RCF via pumping the ZDW around 4.5 µm. In all, this RCF has great advantages in effective mode controlling and dispersion tuning, and has great potentials in developing broad SC and vortex mode controlling.

Low-loss and helical-phase-dependent selective excitation of high-order orbital angular momentum modes in a twisted ring-core fiber

We propose a method for the selective generation of high-order orbital angular momentum (OAM) modes by twisting a ring-core fiber (RCF). Theoretically, 22 OAM modes can be generated in the twisted RCF with effective mode separation. We experimentally demonstrate the excitation of OAM modes up to the fourth order. The positive or negative topological charge is determined by the clockwise or counterclockwise twist direction, indicating that the helical phase of the OAM can be controlled by the twist orientation. Compared with other fiber-based methods, the high-order OAM modes generated by our approach have the characteristics of low loss (minimum 0.3 dB), high mode purity (maximum 98%), and simple fabrication. This has potential applications in high-resolution imaging, high-capacity fiber-optics communication, vortex-optics sensing, and other fields.