Characteristics of homogeneous multi-core fibers for SDM transmission

We describe optical data transmission systems using homogeneous, single-mode, multi-core fibers (MCFs). We first briefly discuss space-division multiplexing (SDM) fibers, observing that no individual SDM fiber offers overwhelming advantages over bundles of single-mode fiber (SMF) across all transmission regimes. We note that for early adoption of SDM fibers, uncoupled or weakly coupled fibers which are compatible with existing SDM infrastructure have a practical advantage. Yet, to be more attractive than parallel SMF, it is also necessary to demonstrate benefits beyond improved spatial spectral efficiency. It is hoped that the lower spread of propagation delays (skew) between spatial channels in some fibers can be exploited for improved performance and greater efficiency from hardware sharing and joint processing. However, whether these benefits can be practically harnessed and outweigh impairments or effort to mitigate cross talk between spatial channels is not yet clear. Hence, focusing on homogeneous MCFs, we first describe measurements and simulations on the impact of inter-core cross talk in such fibers before reporting experimental investigation into the spatial channel skew variation with a series of the experimental results including a comparison with SMF in varying environmental conditions. Finally, we present some system and transmission experiments using parallel recirculating loops that enable demonstration of both multi-dimensional modulation and joint digital processing techniques across three MCF cores. Both techniques lead to increased transmission reach but highlight the need for further experimental analysis to properly characterize the potential benefits of correlated propagation delays in such fibers

Space Division Multiplexing in Optical Fibres

Optical communications technology has made enormous and steady progress for several decades, providing the key resource in our increasingly information-driven society and economy. Much of this progress has been in finding innovative ways to increase the data carrying capacity of a single optical fibre. In this search, researchers have explored (and close to maximally exploited) every available degree of freedom, and even commercial systems now utilize multiplexing in time, wavelength, polarization, and phase to speed more information through the fibre infrastructure. Conspicuously, one potentially enormous source of improvement has however been left untapped in these systems: fibres can easily support hundreds of spatial modes, but today's commercial systems (single-mode or multi-mode) make no attempt to use these as parallel channels for independent signals.Comment: to appear in Nature Photonic

High capacity transmission with few-mode fibers

We experimentally investigate high-capacity few-mode fiber transmission for short and medium-haul optical links. In separate experiments, we demonstrate C + L band transmission of 283 Tbit/s over a single 30 km span and recirculating loop transmission of 159 Tbit/s over 1045 km graded-index three mode fiber. The first experiment reached a data-rate per fiber mode within 90% of the record data-rates reported in the same transmission bands for single-mode fibers. The second experiment demonstrated the feasibility of reaching high data-rates over long distance few-mode fiber transmission, despite strong impairments due to mode-dependent loss and differential mode delay

Rate Optimized Probabilistic Shaping-Based Transmission Over Field Deployed Coupled-Core 4-Core-Fiber

Multi-core fiber (MCF) transmission is a promising solution to support ever-increasing future traffic demands. Compared with uncoupled-core MCFs [1], the induced strong coupling in coupled-core (CC)-MCFs reduces the nonlinearity impact [2]. Transmission in these fibers leverages both spatial and wavelength division multiplexing and it has been experimentally tested mainly considering uniform quadrature amplitude modulation (QAM) formats [3]. Spectral efficiency can be further optimized by employing probabilistic shaping (PS) but the joint use of CC-MCF and PS has been rarely investigated [4]. In this paper, we present a transmission of PS signals through an infrastructure based on a CC-four core fiber (CC-4CF) deployed in the city of L'Aquila, Italy [5]. We ran experiments comparing the performance of standard polarization multiplexed 16QAM and PS-32QAM signals at a symbol rate of 30 GBaud: 800 Gbps net rate considering the spatial super-channel over four cores. We used the generalized mutual information (GMI) as performance metric and averaged over the 8 polarizations concidering the central channel. A realistic threshold (GMIth) of 3.6 bits/symbol (per spatial mode and polarization) has been set as a target: it is a typical value that guarantees post-FEC error-free transmission for most realistic SD-FEC

Controlling Cherenkov angles with resonance transition radiation

Cherenkov radiation provides a valuable way to identify high energy particles in a wide momentum range, through the relation between the particle velocity and the Cherenkov angle. However, since the Cherenkov angle depends only on material's permittivity, the material unavoidably sets a fundamental limit to the momentum coverage and sensitivity of Cherenkov detectors. For example, Ring Imaging Cherenkov detectors must employ materials transparent to the frequency of interest as well as possessing permittivities close to unity to identify particles in the multi GeV range, and thus are often limited to large gas chambers. It would be extremely important albeit challenging to lift this fundamental limit and control Cherenkov angles as preferred. Here we propose a new mechanism that uses constructive interference of resonance transition radiation from photonic crystals to generate both forward and backward Cherenkov radiation. This mechanism can control Cherenkov angles in a flexible way with high sensitivity to any desired range of velocities. Photonic crystals thus overcome the severe material limit for Cherenkov detectors, enabling the use of transparent materials with arbitrary values of permittivity, and provide a promising option suited for identification of particles at high energy with enhanced sensitivity.Comment: There are 16 pages and 4 figures for the manuscript. Supplementary information with 18 pages and 5 figures, appended at the end of the file with the manuscript. Source files in Word format converted to PDF. Submitted to Nature Physic

Wavelength-Selective Switch with Direct Few Mode Fiber Integration

The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described. The freespace optics FMF-WSS dynamically steers spectral information-bearing beams containing three spatial modes from an input port to one of nine output ports using a phase spatial light modulator. Sources of mode dependent losses (MDL) are identified, analytically analyzed and experimentally confirmed on account of different modal sensitivities to fiber coupling in imperfect imaging and at spectral channel edges due to mode clipping. These performance impacting effects can be reduced by adhering to provided design guidelines, which scale in support of higher spatial mode counts. The effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail

Lowloss mode coupler for mode-multiplexed transmission

Abstract: We present a novel low-loss 3-spot mode coupler to selectively address 6 spatial and polarization modes of a few-mode fiber. The coupler is used in a 6 × 6 MIMO-transmission experiment over a 154-km hybrid span consisting of 129-km depressed-cladding and 25-km graded-index few-mode fiber

Transmission of 273.6 Tb/s Over 1001 km of 15-Mode Multi-Mode Fiber Using C-Band Only 16-QAM Signals

In recent years, space-division multiplexing (SDM) has been proposed and investigated as a technique to increase the per-fiber capacity in order to cope with the ever-increasing demand for capacity in optical transmission networks. Considering the various SDM architectures proposed, multi-mode fibers potentially allow for the highest spatial channel density, but current demonstrations have been limited to mostly short-distance high-mode count or long-distance low-mode count transmission. In this work, we transmit 15 modes × 184 wavelength channels × 24.5 GBd PDM-16-QAM signals, spanning the full C-band, over 1001 km of 15-mode multi-mode fiber. The resulting net data rate of 273.6 Tb/s is the highest reported data rate in long-distance multi-mode fiber transmission and results in a record capacity-distance product of 273.9 Pb/s · km for multi-mode transmission. This was achieved by using mode multiplexers with low mode-dependent loss (MDL) and insertion loss, as well as a 15-mode fiber optimized for a low differential mode delay (DMD) transmission regime

Heterogeneous space-division multiplexing and joint wavelength switching demonstration

We demonstrate a six spatial-mode, wavelength-routing network interoperable with few-mode, coupled-multi-core, and single-mode fiber spans using a custom 57-port wavelength-selective switch configured for joint-switching of spatial-superchannels