Mode crosstalk matrix measurement of a 1 km elliptical core few-mode optical fiber

Utilizing adaptive optics to mitigate intra-modal-group power coupling of graded-index few-mode fiber in a 200-Gbit/s mode-division-multiplexed link

We experimentally demonstrate the utilization of adaptive optics (AO) to mitigate intra-group power coupling among linearly polarized (LP) modes in a graded-index few-mode fiber (GI FMF). Generally, in this fiber, the coupling between degenerate modes inside a modal group tends to be stronger than between modes belonging to different groups. In our approach, the coupling inside the LP₁₁ group can be represented by a combination of orbital-angular-momentum (OAM) modes, such that reducing power coupling in OAM set tends to indicate the capability to reduce the coupling inside the LP₁₁ group. We employ two output OAM modes l=+1 and l=-1 as resultant linear combinations of degenerate LP_11a and LP_11b modes inside the LP₁₁ group of a ∼0.6-km GI FMF. The power coupling is mitigated by shaping the amplitude and phase of the distorted OAM modes. Each OAM mode carries an independent 20-, 40-, or 100-Gbit/s quadrature-phase-shift-keying data stream. We measure the transmission matrix (TM) in the OAM basis within LP₁₁ group, which is a subset of the full LP TM of the FMF-based system. An inverse TM is subsequently implemented before the receiver by a spatial light modulator to mitigate the intra-modal-group power coupling. With AO mitigation, the experimental results for l=+1 and l=-1 modes show, respectively, that (i) intra-modal-group crosstalk is reduced by >5.8dB and >5.6dB and (ii) near-error-free bit-error-rate performance is achieved with a penalty of ∼0.6dB and ∼3.8dB, respectively.

Simulation of multimode optical fibers using the angular spectrum algorithm and a Fourier analysis

Multimode optical fibers are gaining increased interest for higher data rate transmissions. In this work, we examine the propagation of beams in multimode optical fibers using the angular spectrum method for several values for the propagation constant V. We prepare a sequence of 4096 images over 200,000 steps, each representing a wavelength within the fiber. We perform a one-dimensional Fourier transform of these data and obtain the propagation constant for each transmitted mode. We then obtain the electric field profile for each transmitted mode. We find excellent agreement with weakly guided mode theory. This work represents another step in the use of this algorithm for analyzing more complicated optical fiber structures.

Design trade-offs and optimization of radially single-mode and azimuthally multimode ring-core fibers for mode-division multiplexing systems

This paper investigates the design trade-offs of radially single-mode and azimuthally multimode (RSMAM) ring-core fibers (RCFs), thereby revealing a more efficient use of the weakly coupled linearly polarized (LP) modes in mode-division multiplexing (MDM) systems. The influences of the increasing number of LP modes on the main propagation properties (i.e., effective index difference, effective area Aeff, and macro-bending sensitivity at a wavelength of 1550 nm) and the key limiting factors for such an increase are numerically described. Based on 1) the design criteria of weakly coupled few-mode fibers described in [P. Sillard, J. Lightw. Technol.32, 2824 (2014)] and 2) an assumption that the refractive index contrast is ≤1% (for facilitating the fiber manufacturing), we point out that the step-index 4-LP-mode RSMAM RCF appears feasible, while RSMAM RCFs with a higher number of LP modes are still primarily limited by the oversized Aeff and undesired macro-bending sensitivities. Finally, in order to provide a better compatibility with weakly coupled MDM systems, we present improved designs for the 3- and 4-LP-mode RSMAM RCFs given in [M. Kasahara, J. Lightw. Technol.32, 1337 (2014)Y. Jung, J. Lightw. Technol.35, 1363 (2017)] by taking into account the spatial information densities and macro-bending sensitivities.

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 100 m 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.

Discrimination of Temperature and Strain in Brillouin Optical Time Domain Analysis Using a Multicore Optical Fiber

Brillouin optical time domain analysis is the sensing of temperature and strain changes along an optical fiber by measuring the frequency shift changes of Brillouin backscattering. Because frequency shift changes are a linear combination of temperature and strain changes, their discrimination is a challenge. Here, a multicore optical fiber that has two cores is fabricated. The differences between the cores' temperature and strain coefficients are such that temperature (strain) changes can be discriminated with error amplification factors of 4.57 °C/MHz (69.11 μ ϵ /MHz), which is 2.63 (3.67) times lower than previously demonstrated. As proof of principle, using the multicore optical fiber and a commercial Brillouin optical time domain analyzer, the temperature (strain) changes of a thermally expanding metal cylinder are discriminated with an error of 0.24% (3.7%).

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 8 cm or 4 cm. 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.

Orbital angular momentum mode groups multiplexing transmission over 2.6-km conventional multi-mode fiber

Twisted light carrying orbital angular momentum (OAM) is a special kind of structured light that has a helical phase front, a phase singularity, and a doughnut intensity profile. Beyond widespread developments in manipulation, microscopy, metrology, astronomy, nonlinear and quantum optics, OAM-carrying twisted light has seen emerging application of optical communications in free space and specially designed fibers. Instead of specialty fibers, here we show the direct use of a conventional graded-index multi-mode fiber (MMF) for OAM communications. By exploiting fiber-compatible mode exciting and filtering elements, we excite the first four OAM mode groups in an MMF. We demonstrate 2.6-km MMF transmission using four data-carrying OAM mode groups (OAM_0,1, OAM_+1,1/OAM_-1,1, OAM_+2,1, OAM_+3,1). Moreover, we demonstrate two data-carrying OAM mode groups multiplexing transmission over the 2.6-km MMF with low-level crosstalk free of multiple-input multiple-output digital signal processing (MIMO-DSP). The demonstrations may open up new perspectives to fiber-based OAM communication/non-communication applications using already existing conventional fibers.

Comparing mode-crosstalk and mode-dependent loss of laterally displaced orbital angular momentum and Hermite-Gaussian modes for free-space optical communication

There is interest in using orbital angular momentum (OAM) modes to increase the data speed of free-space optical communication. A prevalent challenge is the mitigation of mode-crosstalk and mode-dependent loss that is caused by the modes' lateral displacement at the data receiver. Here, the mode-crosstalk and mode-dependent loss of laterally displaced OAM modes (LG_0,+1, LG_0,-1) are experimentally compared to that of a Hermite-Gaussian (HG) mode subset (HG_0,1, HG_1,0). It is shown, for an aperture larger than the modes' waist sizes, some of the HG modes can experience less mode-crosstalk and mode-dependent loss when laterally displaced along a symmetry axis. It is also shown, over a normal distribution of lateral displacements whose standard deviation is 2× the modes' waist sizes, on average, the HG modes experience 66% less mode-crosstalk and 17% less mode-dependent loss.

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-optical mode-group multiplexed transmission over a graded-index ring-core fiber with single radial mode

We present a design of graded-index ring-core fiber (GI-RCF) supporting 3 linearly polarized (LP) mode-groups (i.e. LP₀₁, LP₁₁ and LP₂₁) with a single radial index of one for mode-division multiplexed (MDM) transmission. Reconfigurable spatial light modulator (SLM) based spatial (mode) (de)multiplexers are used to systematically characterize spatial/temporal modal properties of the GI-RCF. We also demonstrate all-optical mode-group multiplexed transmissions over a 360m fabricated GI-RCF without using multiple-input multiple-output digital signal processing (MIMO DSP).

Linearly polarized vector modes: enabling MIMO-free mode-division multiplexing

We experimentally investigate mode-division multiplexing in an elliptical ring core fiber (ERCF) that supports linearly polarized vector modes (LPV). Characterization show that the ERCF exhibits good polarization maintaining properties over eight LPV modes with effective index difference larger than 1 × 10^-4. The ERCF further displays stable mode power and polarization extinction ratio when subjected to external perturbations. Crosstalk between the LPV modes, after propagating through 0.9 km ERCF, is below -14 dB. By using six LPV modes as independent data channels, we achieved the transmission of 32 Gbaud QPSK over 0.9 km ERCF without any multiple-input-multiple-output (MIMO) or polarization-division multiplexing (PDM) signal processing.

Hybrid generation and analysis of vector vortex beams

A method is described for generating optical vector vortex beams carrying superpositions of orbital angular momentum states by using a tandem application of a spatial light modulator with a vortex retarder. The vortex component has a spatially inhomogeneous phase front that can carry orbital angular momentum, and the vector nature is a spatially inhomogeneous state of polarization in the laser beam profile. The vector vortex beams are characterized experimentally by imaging the beams at points across the focal plane in an astigmatic system using a tilted lens. Mathematical analysis of the Gouy phase shows good agreement with the phase structure obtained in the experimental images. The polarization structure of the vector beam and the orbital angular momentum of the vortex beam are shown to be preserved.

Recent advances in high-capacity free-space optical and radio-frequency communications using orbital angular momentum multiplexing

There is a continuing growth in the demand for data bandwidth, and the multiplexing of multiple independent data streams has the potential to provide the needed data capacity. One technique uses the spatial domain of an electromagnetic (EM) wave, and space division multiplexing (SDM) has become increasingly important for increased transmission capacity and spectral efficiency of a communication system. A subset of SDM is mode division multiplexing (MDM), in which multiple orthogonal beams each on a different mode can be multiplexed. A potential modal basis set to achieve MDM is to use orbital angular momentum (OAM) of EM waves. In such a system, multiple OAM beams each carrying an independent data stream are multiplexed at the transmitter, propagate through a common medium and are demultiplexed at the receiver. As a result, the total capacity and spectral efficiency of the communication system can be multiplied by a factor equal to the number of transmitted OAM modes. Over the past few years, progress has been made in understanding the advantages and limitations of using multiplexed OAM beams for communication systems. In this review paper, we highlight recent advances in the use of OAM multiplexing for high-capacity free-space optical and millimetre-wave communications. We discuss different technical challenges (e.g. atmospheric turbulence and crosstalk) as well as potential techniques to mitigate such degrading effects.This article is part of the themed issue 'Optical orbital angular momentum'.

Ultra-broadband mode multiplexers based on three-dimensional asymmetric waveguide branches

We propose a three-dimensional waveguide mode multiplexer structure that operates on the principle of adiabatic mode transition along multilayer asymmetric waveguide branches. Using this structure, we designed and fabricated a three-mode and a four-mode multiplexer with polymer material that can operate over the C+L band and beyond with small modal crosstalk (<-10  dB) and negligible polarization dependence and be directly connected to fibers with low loss. These multiplexers could be used as mode-selective devices for ultra-broadband mode-division multiplexing based on few-mode fibers.

Mode-selective coupling between few-mode fibers and buried channel waveguides

With intensive computation, we analyze in detail butt-coupling between few-mode fibers and buried channel waveguides and discuss, in particular, the conditions to achieve mode-selective coupling for the first 6 spatial modes. In the case of coupling a buried rectangular-core waveguide to a step-index or a parabolic-index circular-core fiber, a modal crosstalk smaller than -20 dB for all the 6 spatial modes can be achieved, when the shape of the waveguide core is sufficiently close to a square (within ± 0.7% variation in its aspect ratio) and the waveguide and the fiber have close mode volumes. In the case of coupling a buried rectangular-core waveguide to a step-index elliptical-core fiber, it is possible to achieve a modal crosstalk smaller than -20 dB with many combinations of waveguide and fiber parameters. The aspect ratios of the elliptical core and the best matched rectangular core can be very different (e.g., 0.85 for the elliptical core and 1.05 for the rectangular core) and an elliptical core that has a moderate ellipticity and an area not close to the upper limit allowed for supporting 6 spatial modes is preferred. The use of a parabolic-index profile in the elliptical core can further improve the mode selectivity with greatly relaxed tolerances on both the aspect ratio and the area of the rectangular core required. In general, when the fiber and the waveguide core are matched for low-crosstalk performance, the butt-coupling losses for all the 6 spatial modes as well as the mode-dependent loss are also small (typically well below 1 dB). Our results are useful for the design of mode-selective waveguide devices, such as mode (de)multiplexers, mode converters, and mode switches, for fiber-based mode-division multiplexing applications.

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.

Encoding/decoding using superpositions of spatial modes for image transfer in km-scale few-mode fiber

Space domain is regarded as the only known physical dimension left of lightwave to exploit in optical communications. Recently, lots of research efforts have been devoted to using spatial modes of fibers to increase data transmission capacity in optical fiber communications. In this paper, we propose and demonstrate a different approach to exploiting the space dimension, i.e. transferring image by space dimension encoding/decoding using superpositions of spatial modes in km-scale few-mode fiber. Three grayscale images are successfully transmitted through a 1.1-km few-mode fiber by employing either 4 modes, i.e. three linearly polarized (LP) modes of LP₀₁, LP_11a, LP_11b and one orbital angular momentum (OAM) mode of OAM_-1, or 2 modes (OAM₊₁, OAM_-1). The bit-error rate is evaluated and zero error among all received data is achieved, showing favorable fiber link communication performance using the spatial modes of fiber for encoding/decoding. Moreover, we also demonstrate the 4 modes (LP₀₁, LP_11a, LP_11b and OAM_-1) encoding/decoding for image transfer in a 10-km few-mode fiber in the experiment.