Elliptical-core Few-mode Fiber Research Articles

An Elliptical-Core Few-Mode Fiber with Low Loss and Low Crosstalk for the MIMO-FREE Applications

In this invited paper, a novel elliptical-core few-mode fiber (EFMF) with low loss and low Crosstalk is proposed for multiple input multiple output free (MIMO-FREE) applications. The EFMF, which adopts pure silica elliptical core with a step refractive index profile and trench refractive index, can be used in MIMO-FREE operation. The result using a full-vector finite element method shows that the proposed EFMF breaks the mode degeneracy and achieves modes maintaining function. The EFMF performs five non-degenerate modes operations with the modes LP01, LP11a, LP11b, LP21b, and LP21a for the MIMO-FREE applications. The mode crosstalk of the EFMF is effectively reduced using the optimized ellipticity and the large effective refractive index difference between the modes. If the data rate or capacity is conventionally set to a multiple of 2, the transmission system can be upgraded using the EFMF with four modes operations where the effective refractive index difference between any two modes is more than 2 × 10−3. A pure silica core is employed to effectively reduce intrinsic loss. The low bending loss is realized by using the trench refractive index at the cladding. The EFMF can be applied to the good transmission of mode division multiplexing in MIMO-FREE applications while eliminating the difficulty and complexity of the MIMO digital signal processing.

Mode Selective Conversion Enabled by the Long-Period Gratings Inscribed in Elliptical Core Few-Mode Fiber

We demonstrate the fabrication of long-period fiber gratings (LPFGs) in an elliptical core few-mode fiber (e-FMF) by CO2 laser. The mode conversion from fundamental mode to high-order modes (LP11a, LP11b, and LP21a mode) can be realized by the LPFGs with high efficiency of more than 95%. The effect of bending, torsion, temperature, and strain on the resonance wavelength of the e-FMF based LPFG mode converters are experimentally investigated. The temperature and strain sensitivities of the LPFGs are characterized to be −36 pm/°C and −4.9 pm/μϵ, and the LPFGs are found to be insensitive to the bending and torsion. Due to the existence of trench in the e-FMF, the higher-order LP02 and LP31a mode can also be stimulated by the e-FMF based LPFGs with a grating period of 515 μm and 400 μm, respectively. The proposed e-FMF based LPFGs could have a promising prospect in the application of fiber sensing and mode division multiplexing optical communication system.

Design of elliptical-core five-mode group selective photonic lantern over the C-band.

We report the design of an elliptical-core five-mode group selective photonic lantern (EC-F-MGS-PL) supporting ten spatial modes over the C-band, whose output fiber is an elliptical-core few mode fiber (EC-FMF). Initially, we fix the cladding diameters of all input fibers to form an elliptical structure. With the help of beam propagation methods (BPM), we carry out comprehensive geometrical optimization of various input fiber bundle, and identify the optimal setting of input fiber bundle in order to achieve the maximum mode selectivity, low insertion loss (IL), and high mode conversion efficiency (MCE) for all five mode groups. Next, we optimize each fiber core diameter and propose a two-step tapering process, for the ease of performance enhancement for the EC-F-MGS-PL. Finally, the mode field evolution and the wavelength dependent operation of the proposed EC-F-MGS-PL is numerically investigated. A mode selectivity of 9∼17 dB and IL of 0.1∼0.38 dB for all five mode groups can be achieved over the C-band, while MCE keeps between 83.3%-95.8%. By taking into account of the fabrication process, we believe that both the design strategy and optimization procedure of the EC-F-MGS-PL are helpful for the implementation of multi-input-multi-output (MIMO)-less mode division multiplexing (MDM) transmission.

All-Fiber Flexible Generation of the Generalized Cylindrical Vector Beam (CVB) Over the C-Band

We report all-fiber flexible generation of the generalized cylindrical vector beam (CVB) over the C-band, by the rotation manipulation of either polarization orientation or mode profile orientation for two linearly polarized Hermite-Gaussian (HG) modes arising in different elliptical-core few-mode fibers (E-FMFs) before their spatial superposition. By this means, the generalized CVB with arbitrary polarization rotation angle from its radial direction can be conveniently obtained. Such all-fiber flexible CVB generation is experimentally enabled by the E-FMF-based polarization controller and fiber rotator, because the effective refractive index difference between adjacent spatial modes arising in the E-FMF is enhanced. Our experimental results verify that there is almost no performance penalty over the C-band between two all-fiber rotation manipulation methods.

Multi-parameter distributed fiber sensing with higher-order optical and acoustic modes.

We propose a novel multi-parameter sensing technique based on a Brillouin optical time domain reflectometry in the elliptical-core few-mode fiber, using higher-order optical and acoustic modes. Multiple Brillouin peaks are observed for the backscattering of both the LP01 mode and LP11 mode. We characterize the temperature and strain coefficients for various optical-acoustic mode pairs. By selecting the proper combination of modes pairs, the performance of multi-parameter sensing can be optimized. Distributed sensing of temperature and strain is demonstrated over a 0.5-km elliptical-core few-mode fiber, with the discriminative uncertainty of 0.28°C and 5.81 με for temperature and strain, respectively.

Panda type elliptical core few-mode fiber

Mode division multiplexing together with multiple-input multiple-output digital signal processing (MIMO-DSP) has the potential for fulfilling future ever-increasing transmission capacity demand. However, for the mitigation of severe mode coupling, the cost and computation complexity of MIMO-DSP become prohibitive with the growing numbers of guided modes arising in the traditional circular core few-mode fiber. We design, fabricate, and characterize panda type elliptical core few-mode fiber (PE-FMF) supporting six eigenmodes of LP01x, LP01y, LP11ax, LP11ay, LP11bx, and LP11by independent transmission. Such PE-FMF, with both low refractive index difference of 0.006 between the core and the cladding and low ellipticity of 1.449, can alleviate the trade-off between the eigenmode numbers and the desirable mode spacing occurring in traditional elliptical-core FMF design. The numerical results indicate that the minimal mode spacing among six eigenmodes is above 1.95 × 10−4 over the whole C + L band. The maintaining capability of both mode intensity profile and polarization is experimentally verified, when the PE-FMF is under a twisting rate range from 0 to 1.57 rad/m. During the fiber twisting, the mode intensity profile can be maintained and the polarization extinction ratio of individual eigenmode is above 20 dB over 50 m PE-FMF.

Demonstration of 6 <inline-formula> <tex-math notation="LaTeX">$\times$ </tex-math> </inline-formula> 10-Gb/s MIMO-Free Polarization- and Mode-Multiplexed Transmission

$6\times 10$ Gb/s polarization- and mode-multiplexed transmission, without MIMO equalization, is experimentally demonstrated over a 200-m elliptical core few-mode fiber with two PANDA stress rods. This fiber simultaneously breaks the degeneracies in space and polarization in a mode group. A polarization extinction ratio of <−16 dB and loss <0.63 dB/km were achieved.

Stability of Ince-Gaussian beams in elliptical core few-mode fibers.

A comparative stability analysis of Ince-Gaussian and Hermite-Gaussian modes in elliptical core few-mode fibers is provided to inform the design of spatial division multiplexing systems. The correlation method is used to construct crosstalk matrices that characterize the spatial modes of the fiber. Up to six low-order modes are shown to exhibit about -20 dB crosstalk. The crosstalk performance of each mode set is found to be similar. However, a direct comparison between modes of equal Gouy phase shift, a parameter that ensures identical beam quality, and phase at the detector, demonstrates better relative power transmission for Ince-Gaussian beams. This result is consistent with the natural modes supported by a 100m elliptical core fiber for which a mode ellipticity of ϵ=2 was found to be optimal. The relative power difference is expected to be magnified over longer fiber lengths in favor of Ince-Gaussian modes.

Design of elliptical-core mode-selective photonic lanterns with six modes for MIMO-free mode division multiplexing systems.

Elliptical-core few mode fiber (EC-FMF) is used in a mode division multiplexing (MDM) transmission system to release multiple-input-multiple-output (MIMO) digital-signal-processing, which reduces the cost and the complexity of the receiver. However, EC-FMF does not match with conventional multiplexers/de-multiplexers (MUXs/DeMUXs) such as a photonic lantern, leading to extra mode coupling loss and crosstalk. We design elliptical-core mode-selective photonic lanterns (EC-MSPLs) with six modes, which can match well with EC-FMF in MIMO-free MDM systems. Simulation of the EC-MSPL using the beam propagation method was demonstrated employing a combination of either step-index or graded-index fibers with six different sizes of cores, and the taper transition length of 8cm or 4cm. Through numerical simulations and optimizations, both types of photonic lanterns can realize low loss transmission and low crosstalk of below -20.0 dB for all modes.

MDM transmission of CAP-16 signals over 1.1- km anti-bending trench-assisted elliptical-core few-mode fiber in passive optical networks.

Mode-division multiplexing passive optical network (MDM-PON) is a promising scheme for next-generation access networks to further increase the transmission capacity and number of end-customers. In this paper, we propose and experimentally demonstrate the implementation of MDM-PON architecture with the carrier-less amplitude/phase (CAP) modulation signals and two effectively separated spatial modes multiplexing transmission in an elliptical-core few-mode fiber (EC-FMF). The trench-assisted EC-FMF features favorable anti-bending performance with negligible power variations and stable mode intensity profiles under extreme bending conditions. Two spatial modes carrying CAP-16 signals for four users with net data rate of 5 Gb/s per user in the downstream transmission over 1.1-km EC-FMF is demonstrated. The measured mode crosstalk including mode (de)multiplexing and 1.1-km EC-FMF transmission is less than -18 dB and multiple input multiple output (MIMO) technique is not used in the experiment for simplicity. The measured optical signal-to-noise ratio (OSNR) penalties for the downstream transmission without or with crosstalk at a bit-error rate (BER) of 3.8x10-3 (7% forward error correction (FEC) threshold) are less than 1.8 dB and 3.1 dB, respectively. Larger transmission capacity and more users are expected when further employing higher dimension CAP signals and EC-FMF supporting more separable spatial modes.

All-fiber polarization manipulation for high-order LP modes with mode profile maintenance.

We propose and experimentally demonstrate the polarization manipulation with mode profile maintenance for LP01, LP11a, and LP11b modes, with the help of elliptical-core few-mode fiber (e-FMF) based traditional polarization controller (PC) configuration. By manually adjusting the PC, we can successfully realize polarization conversion between horizontal and vertical linear polarizations, while individual mode profiles can be maintained without distortion. More than 30 dB of polarization extinction ratio (PER) can be achieved over the C-band, with little insertion loss of 0.2dB. The proposed all-fiber configuration is helpful to manage the polarization of high-order LP modes.

Temperature-insensitive fiber twist sensor based on elliptical-core few-mode fiber.

A highly sensitive fiber optics twist sensor is proposed and experimentally demonstrated using elliptical-core few-mode fiber (e-FMF). With the help of a fixed-phase plate for conversion from LP11 to LP01 mode, the twist angle can be linearly mapped to the spatial rotation of LP11 mode profile and consequently discriminated by monitoring the optical power variation at the standard single-mode fiber (SSMF) output. When the optical power at the e-FMF output is 10 mW, the twist angle within the range of ±39° can be successfully detected with a sensitivity of more than 20 μW/°. Meanwhile, the twist direction can be identified simultaneously. In particular, when the twist angle ranges from -12.5° to 20.1°, the sensitivity is higher than 100 μW/°. Both temperature-insensitive operation from 20°C to 150°C and e-FMF length insensitive operation are experimentally verified.

2 × 2 MIMO OFDM/OQAM radio signals over an elliptical core few-mode fiber.

We experimentally demonstrate a 4.46 Gb/s2×2 multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM)/OQAM radio signal over a 2 km elliptical core 3-mode fiber, together with 0.4 m wireless transmission. Meanwhile, to cope with differential channel delay (DCD) among involved MIMO channels, we propose a time-offset crosstalk cancellation algorithm to extend the DCD tolerance from 10 to 60 ns without using a circle prefix (CP), leading to an 18.7% improvement of spectral efficiency. For the purpose of comparison, we also examine the transmission performance of CP-OFDM signals with different lengths of CPs, under the same system configuration. The proposed algorithm is also effective for the DCD compensation of a radio signal over a 2 km 7-core fiber. These results not only demonstrate the feasibility of space division multiplexing for RoF application but also validate that the elliptical core few-mode fiber can provide the same independent channels as the multicore fiber.

Design and fabrication of elliptical-core few-mode fiber for MIMO-less data transmission.

We propose a design strategy of elliptical core few-mode fiber (e-FMF) that supports three spatial modes with enhanced mode spacing between LP11a and LP11b, to suppress intra-mode coupling during mode-division multiplexing (MDM) transmission. Our theoretical investigations show that there exist two optimization regimes for the e-FMF, as a comparison with traditional circular core FMF(c-FMF). At the regime of three-mode operation, there occurs a trade-off between mode spacing and bending-induced loss. Meanwhile, in terms of five-mode regime, a trade-off between mode spacing and high-order mode crosstalk happens. Finally, we fabricate 7.94km e-FMF with the optimal parameters, based on the commercial fiber manufacture facility. The primary characterizations at 1550nm show that three spatial modes of e-FMF can be transmitted with a loss less than 0.3dB/km. Meanwhile, -22.44 dB crosstalk between LP11a and LP11b is observed, even when the 2km e-FMF is under stress-induced strong perturbation.

Amplification sharing of non-degenerate modes in an elliptical-core few-mode erbium-doped fiber.

We report on the study of a possible first step integration of mode division multiplexed optical component for single-mode fiber networks. State-of-the-art on few-mode erbium-doped fiber amplifiers is used to integrate the amplification function in a single component, which is expected to save energy in comparison to parallelized active components. So as to limit the impact of modal cross-talk, an elliptical-core few-mode erbium-doped fiber has been used to assemble an amplifier sharing setup for different single mode fibers, using non-degenerate modes. With this simple setup, we show the level of performances that can be reached for cross-talk, gain, differential modal gain and losses and discuss the ways to improve them for a possible integration in a real network.

SDM transmission of real-time 10GbE traffic using commercial SFP + transceivers over 0.5km elliptical-core few-mode fiber.

We experimentally demonstrate the first few-mode space division multiplexed (SDM) transmission of real-time 10Gb/s Ethernet (10GbE) traffic using commercial small form-factor pluggable SFP + transceivers without coherent detection or multiple input multiple output digital signal processing (MIMO-DSP) over 0.5km elliptical-core few-mode-fiber, achieving <-26dB crosstalk between LP(11e) and LP(11o) modes at 1.3μm.

Nonlinear effects in elliptical-core few-mode optical fibers

We examine the nonlinear interaction between the linearly polarized modes on a few-mode elliptical-core optical fiber. Three modes are involved in this process: the fundamental (LP01) mode and the two second-order symmetry modes, LP11 (even) and LP11 (odd), which have slightly different propagation constants. In practical situations the large δβ = β01 − β11(β is the propagation constant) precludes any significant coupling between the fundamental and either of the LP11 modes; however, coupling between the two LP11 symmetry modes may occur for sufficient pump intensities. Exact and approximate analytic expressions are given.

Selective mode injection and observation for few-mode fiber optics

By etching the cladding along several millimeters of circular- or elliptical-core few-mode fiber optics, we gain access to the core and can inject extremely pure modes. The same etching technique allows one to measure the modal purity at the far end of the fiber. Modal purities of -26 dB have been obtained. We also demonstrate a holographic technique that allows all the light of each mode to be focused to an independent spot.