Homogeneous Multicore Fibers Research Articles

Impulse response of weakly-coupled multicore fiber measured by using high-dynamic-range linear optical sampling

Abstract We report the impulse response measurement of weakly-coupled homogeneous multicore fiber accomplished by coherence-recovered linear optical sampling, which simultaneously realizes a picosecond-level time resolution and a dynamic range of over 70 dB. The observed core-by-core impulse responses include a directly transmitted pulse and plateaus that are yielded by inter-core couplings. A consideration of the observed results suggests that the group velocities are perturbed, and the core with the slowest group velocity alters along the fiber. We also found that the polarization states of the plateaus alter in the time axis of the impulse response chart, with a correlation width as short as the pulse width: we presume that this phenomenon is observed because the polarization change that occurs during propagation is transferred to the coupled signals with the shrinkage of the time axis. Next, we selected a 100 m-long piece of fiber, and measured the group delay of each core. We found that the variation of the core-by-core group delays was not similar to that of the long fiber, and it may change considerably with bending induced in the fiber. These results also suggest a variation in the local group delays over the length of the fiber.

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.

Long distance crosstalk-supported transmission using homogeneous multicore fibers and SDM-MIMO demultiplexing.

We propose and evaluate the use of spatial-division multiplexing (SDM) multiple input multiple output (MIMO) systems to support long distance transmission using single-mode homogeneous multicore fibers. We show that on a uniform link with per-span inter-core skew compensation, the required SDM-MIMO memory length corresponds to the largest inter-core skew per span on the link. Furthermore, we show that with inter-core skew compensation, the required memory length of the SDM-MIMO is nearly constant with the transmission distance for accumulated crosstalk below -11 dB. We experimentally demonstrate the use of SDM-MIMO with a memory length of 20 ns on a long distance transmission link using 20 GBaud PDM-QPSK signals. We achieve a reach of 9780 km, which corresponds to a 9% improvement over the case without SDM-MIMO. We also show that the use of SDM-MIMO is applicable to the transmission of signals with higher modulation order, achieving transmission reach improvements of 14% for 20 GBaud PDM-16QAM and PDM-64QAM signals.

Impact of Longitudinal Variation of the Coupling Coefficient Due to Bending and Twisting on Inter-Core Crosstalk in Weakly-Coupled MCFs

A novel analytical discrete changes model (DCM) of inter-core crosstalk (ICXT) in weakly-coupled multi-core fibers (MCFs) that considers the inter-core coupling coefficient (ICCC) variation, induced by bending and twisting effects along the longitudinal direction of propagation, is proposed. An approximated closed-form expression of the ICCC is proposed for step-index MCFs and validated by comparison with the rigorous ICCC expression, solved numerically. The linearization of the ICCC, in a logarithmic scale, around the intrinsic ICCC, is proposed to simplify the derivation of the novel analytical DCM. The novel DCM is validated by comparison of the mean ICXT power estimates obtained analytically, which are very quick to estimate, with the ones obtained by solving the coupled-mode equations numerically, which are much more computationally demanding. It is shown that, in the range of tested MCF parameters, the proposed model allows for an improvement of the mean ICXT power estimates reaching 4.5 dB, compared to the estimates obtained without considering the longitudinal variation of the ICCC, depending on the fiber parameters. The improvement of the mean ICXT power estimates was assessed for MCFs between 100 m and 10 km long, core pitches between 30 $\mu$ m and 50 $\mu$ m, and normalized difference between the refractive index of the cores between $-0.028\%$ and $0.028\%$ , typical of weakly-coupled real homogeneous MCFs. The largest improvements of mean ICXT power estimates tend to occur for larger core pitches and for larger differences between the refractive index of the cores. Accurate mean ICXT power estimates obtained with the novel DCM for MCFs with typical fiber birefringence parameters are also reported.

IIR Microwave Photonic Filters Based on Homogeneous Multicore Fibers

We propose and experimentally demonstrate two types of infinite impulse response (IIR) microwave photonic filters (MPFs) with different configurations, named cascaded configuration and parallel configuration. Both configurations are based on the homogeneous multicore fiber (MCF) in which the intercore power coupling is enabled by the tapering technique. The quality factors (Q factors) of cascaded and parallel IIR-MPFs are 143 and 136, respectively, which is much higher than that of one single IIR filter. The geometric structure of the tapered MCF and the number of spatial cores open new dimensions to manipulate the performance of MPFs. Theoretically, higher Q factor and rejection ratio can be obtained by using more cores of MCF to build more cascaded or parallel filters. These two types of IIR-MPFs show good reconfigurability and tunability by adjusting optical power and time delay through simulations. We anticipate that microwave photonics systems will benefit from these two types of IIR-MPFs in terms of compactness, reconfigurability, tunability, and performance stability.

Space-Division Multiplexing in Data Center Networks: On Multi-Core Fiber Solutions and Crosstalk-Suppressed Resource Allocation

The rapid growth of traffic inside data centers caused by the increasing adoption of cloud services necessitates a scalable and cost-efficient networking infrastructure. Space-division multiplexing (SDM) is considered as a promising solution to overcome the optical network capacity crunch and support cost-effective network capacity scaling. Multi-core fiber (MCF) is regarded as the most feasible and efficient way to realize SDM networks, and its deployment inside data centers seems very likely as the issue of inter-core crosstalk (XT) is not severe over short link spans (<1 km) compared to that in long-haul transmission. However, XT can still have a considerable effect in MCF over short distances,which can limit the transmission reach and in turn the data center’s size. XT can be further reduced by bi-directional transmission of optical signals in adjacent MCF cores. This paper evaluates the benefits of MCF-based SDM solutions in terms of maximizing the capacity and spatial efficiency of data center networks. To this end, we present an analytical model for XT in bi-directional normal step-index and trench-assisted MCFs and propose corresponding XT-aware core prioritization schemes. We further develop XT-aware spectrum resource allocation strategies aimed at relieving the complexity of online XT computation. These strategies divide the available spectrum into disjoint bands and incrementally add them to the pool of accessible resources based on the network conditions. Several combinations of core mapping and spectrum resource allocation algorithms are investigated for eight types of homogeneous MCFs comprising 7–61 cores, three different multiplexing schemes, and three data center network topologies with two traffic scenarios. Extensive simulation results showthat combining bi-directional transmission in dense core fibers with tailored resource allocation schemes significantly increases the network capacity. Moreover, a multiplexing scheme that combines SDM and WDM can achieve up to 33 times higher link spatial efficiency and up to 300 times greater capacity compared to a WDM solution.

Optimal design for crosstalk analysis in 12-core 5-LP mode homogeneous multicore fiber for different lattice structure

Abstract 12-Core 5-LP mode homogeneous multicore fibers have been proposed for analysis of inter-core crosstalk and dispersion, with four different lattice structures (circular, 2-ring, square lattice, and triangular lattice) having cladding diameter of 200 µm and a fixed cladding thickness of 35 µm. The core-to-core crosstalk impact has been studied numerically with respect to bending radius, core pitch, transmission distance, wavelength, and core diameter for all 5-LP modes. In anticipation of further reduction in crosstalk levels, the trench-assisted cores have been incorporated for all respective designs. Ultra-low crosstalk (-138 dB/100 km) has been achieved through the triangular lattice arrangement, with trench depth Δ2 = −1.40% for fundamental (LP01) mode. It has been noted that the impact of mode polarization on crosstalk behavior is minor, with difference in crosstalk levels between two polarized spatial modes as ≤0.2 dB. Moreover, the optimized cladding diameter has been obtained for all 5-LP modes for a target value of crosstalk of −50 dB/100 km, with all the core arrangements. The dispersion characteristic has also been analyzed with respect to wavelength, which is nearly 2.5 ps/nm km at operating wavelength 1550 nm. The relative core multiplicity factor (RCMF) for the proposed design is obtained as 64.

Intercore crosstalk in direct-detection homogeneous multicore fiber systems impaired by laser phase noise

The impact of the laser phase noise on the photodetected intercore crosstalk and performance of direct-detection orthogonal frequency division multiplexing multicore fiber systems is experimentally investigated. A new solution to overcome the performance fluctuations over time induced by the combined effect of laser phaser noise and intercore crosstalk is proposed. The solution uses adaptive modulation with extended time memory to estimate the bit loading scheme of each subcarrier from the mean and maximum error vector magnitude evaluated over the last ten blocks of transmitted training symbols. During measurements of up to 90 hours, intercore crosstalk power variation induced by fast laser phase noise variations exceeded 20 dB in both time and frequency, and error vector magnitude fluctuations of 4 dB were observed on a sub-second timescale. It is shown that direct-detection orthogonal frequency division multiplexing multicore fiber based systems employing a typical adaptive modulation solution, in which the bit loading scheme is evaluated from a single set of training symbols, suffer from unacceptable outage probabilities and are unable to counteract the fast power variations of intercore crosstalk and phase noise induced impairments. By extending the system memory used to estimate the bit loading scheme employed by the adaptive technique, an outage probability reduction by one order of magnitude is achieved. This reduction is attained by using the mean of the error vector magnitude evaluated over the last ten blocks of training symbols to estimate the bit loading scheme of subcarriers. Further reduction of the outage probability by four orders of magnitude is also demonstrated using a more conservative approach to estimate the bit loading scheme of the subcarriers. However, this conservative approach, based on the maximum error vector magnitude, may lead to additional loss of the average throughput.

Intercore Crosstalk in Homogeneous Multicore Fibers: Theoretical Characterization of Stochastic Time Evolution

A time-varying phase shift associated with each phase matching point of homogeneous weakly coupled multicore fibers is proposed to describe the stochastic time evolution of the intercore crosstalk (ICXT) observed experimentally. This model for the stochastic ICXT time evolution represents a generalization of current ICXT theoretical models and potentiates the development of new time-adaptive ICXT mitigation techniques. The model is first proposed considering a single polarization scheme, and is then generalized to a dual polarization scheme and to account for the polarization coupling between cores. Comparison between spectrograms of the crosstalk transfer function (XTTF) amplitude evaluated using the single polarization model and measured experimentally shows excellent agreement for short (few minutes) decorrelation times. For large decorrelation times (above 1 h), some differences in the behavior of the time evolution of the XTTF amplitude are observed. Nevertheless, excellent match between the mean and variance of the XTTF amplitude evaluated from the model and from analytical expressions proposed in the literature is observed for short and large decorrelation times. It is also shown that the decorrelation time of the short-term average crosstalk remains unchanged when the dual polarization scheme is considered. Similar spectrograms of the XTTF obtained with the single and dual polarization ICXT models are also shown.

Dual Polarization Discrete Changes Model of Inter-Core Crosstalk in Multi-Core Fibers

The discrete changes model (DCM) of inter-core crosstalk (ICXT) in weakly-coupled homogeneous multi-core fibers (MCFs) is generalized to a dual polarization (DP) scheme. This model provides theoretical expressions for the two polarization fields of the ICXT at the MCF output. Therefore, it may be of particular interest in the design of direct-detection MCF systems, where the photodetected ICXT results mainly from the beating between the ICXT field at the MCF output and the carrier of the interfered core. The DP-DCM is validated by comparison of the mean ICXT power and ICXT field amplitude estimates with the ones obtained with a rigorous, yet much more computationally demanding, model based on the coupled local mode theory (CLMT). Good agreement between the mean ICXT power estimates obtained with the DP-DCM and CLMT is observed when the inter-core coupling coefficient variation along the MCF is small. Good agreement is also observed when comparing the probability density functions of the ICXT field amplitude.

Crosstalk dynamics in multi-core fibers.

Inter-core crosstalk is a potential limitation on the achievable data-rates in optical fiber transmission systems using multi-core fibers. Crosstalk arises from unwanted coupling between cores of a homogenous multi-core fiber and it's average power has been observed to vary over time by 10s of decibels, potentially requiring an additional performance margin to achieve acceptable outage probability. Most investigations of crosstalk have so far only considered continuous wave laser light or amplified spontaneous emission as sources of crosstalk. In this paper, we theoretically and experimentally investigate the time-dependence of inter-core crosstalk in a homogeneous multi-core fiber when considering signals with various modulation formats and symbol rates. We find that crosstalk power fluctuations depend on the symbol rate, modulation and skew between cores. For carrier-free signals, such as quadrature amplitude modulation, the crosstalk power is nearly constant for expected conditions of multi-core transmission systems. However, carrier-supported signals, such as OOK, always induce time-varying crosstalk powers.

High Capacity Transmission Systems Using Homogeneous Multi-Core Fibers

We discuss optical transmission using homogeneous, single-mode, multicore fibers. We first describe the key fiber properties that can affect transmission, such as intercore crosstalk and the variation of propagation delay between cores, and the consequences of these properties on spatial super-channel transmission. We then describe a series of experiments and demonstrations of system features including self-homodyne detection and joint spatial superchannel modulation before discussing their suitability to high-capacity transmission.

Spatial Division Multiplexed Microwave Signal processing by selective grating inscription in homogeneous multicore fibers

The use of Spatial Division Multiplexing for Microwave Photonics signal processing is proposed and experimentally demonstrated, for the first time to our knowledge, based on the selective inscription of Bragg gratings in homogeneous multicore fibers. The fabricated devices behave as sampled true time delay elements for radiofrequency signals offering a wide range of operation possibilities within the same optical fiber. The key to processing flexibility comes from the implementation of novel multi-cavity configurations by inscribing a variety of different fiber Bragg gratings along the different cores of a 7-core fiber. This entails the development of the first fabrication method to inscribe high-quality gratings characterized by arbitrary frequency spectra and located in arbitrary longitudinal positions along the individual cores of a multicore fiber. Our work opens the way towards the development of unique compact fiber-based solutions that enable the implementation of a wide variety of 2D (spatial and wavelength diversity) signal processing functionalities that will be key in future fiber-wireless communications scenarios. We envisage that Microwave Photonics systems and networks will benefit from this technology in terms of compactness, operation versatility and performance stability.

Discrete Changes Model of Inter-core Crosstalk of Real Homogeneous Multi-core Fibers

An analytical discrete changes model of the inter-core crosstalk (ICXT) in real homogeneous weakly-coupled multi-core fibers (MCF) (cores with similar but different parameters) with bending and twisting perturbations is proposed for operation in the phase-matching region, where the bending radii are smaller than the critical bending radius. We show that this model is a generalization of the ICXT model proposed by other authors for perfectly homogeneous MCF. We compare simulation results of the coupled mode equation for a weakly-coupled step-index MCF with seven cores with theoretical predictions from the proposed model, and much faster and accurate mean ICXT power estimates are shown in the whole range of bending radii and difference of intrinsic effective refractive indexes corresponding to operation in the phase-matching region. We show that differences of mean ICXT power exceeding 4 dB can occur between the estimates provided by the model considering perfectly homogeneous MCF and the proposed model.

Dispersion Impact on the Crosstalk Amplitude Response of Homogeneous Multi-Core Fibers

A novel analytical model of the small-signal inter-core crosstalk transfer function (XTTF) in a homogeneous multi-core fiber (MCF) taking into account the group velocity dispersion (GVD) of different cores and the walkoff between cores is presented. Empirical expressions for the dependence of the mean and variance of the XTTF amplitude on the GVD and walkoff between cores are estimated from simulation results and shown to provide good estimates for dispersion parameters typical of homogeneous MCFs. Results show that GVD may cause large fluctuations and pronounced dips in the mean and variance of the XTTF amplitude. The critical walkoff, below which the GVD impact on the XTTF amplitude is observable, is derived and shown to be in qualitative agreement with experimental results.

Wavelength-Dependence of Inter-Core Crosstalk in Homogeneous Multi-Core Fibers

The wavelength dependence of inter-core crosstalk in homogeneous multi-core fibers (MCFs) is investigated, and the corresponding analytical expressions are derived. The derived analytical expressions can be used to determine the crosstalk at any wavelength necessary for designing future MCF wavelength-division multiplexing (MCF-WDM) networks and transmission systems.

On the Suitability of Multicore Fiber for LTE–Advanced MIMO Optical Fronthaul Systems

This paper proposes and evaluates analytically and experimentally the suitability of spatial multiplexing in multicore fiber (MCF) for multiple-input multiple-output (MIMO) LTE–Advanced (LTE–A) optical fronthaul systems including carrier aggregation. The experimental test-bed comprises a 150-m optical fronthaul of four-core homogeneous MCF, which can be configured with different bending radius for analysis. Performance of linear crosstalk in MCF media is evaluated by simulation and validated by the experimental work in the laboratory with the radio-over-fiber transmission of full-standard LTE–A wireless signals considering both SISO and MIMO configurations. The optical fronthaul analysis evaluates both same-propagation and counter-propagation spatial multiplexing, i.e., all cores propagating in the same direction and two versus two cores propagating in opposite directions, corresponding to a dual LTE–A roof-mounted system. The performance of the MCF optical fronthaul system is evaluated using a single 20–MHz LTE–A carrier and also carrier-aggregated signals with 16QAM and 64QAM subcarrier modulation. The experimental results indicate that intercore crosstalk (IC–XT) increases up to 9.6 dB when raising the bending radius from 35 to 67 cm, being the bending radius a key parameter in MCF fronthaul systems. The demonstration performed at 1550.12 nm reported an increase of IC–XT of 4.6 dB with the bending radius, which deteriorates the quality of the SISO LTE–A signal up to 8.1 dB error vector magnitude (EVM). This IC–XT impairment can be mitigated using 2×2 MIMO processing, which reduces the EVM impact to less than 1 dB when increasing the MCF bending radius from 35 to 67 cm. The experimental results indicate that 3GPP MIMO LTE–A algorithms already in-place in the wireless LTE–A standard can be used to compensate the IC–XT in the 150-m multicore optical fronthaul system with spatial multiplexing.

Time and Modulation Frequency Dependence of Crosstalk in Homogeneous Multi-Core Fibers

This study presents an evaluation of the time and modulation frequency-varying power transfer of crosstalk in a homogeneous multi-core fiber (MCF). Experimental observations using a seven-core MCF over a period of 10 h show that, unlike localized crosstalk, distributed crosstalk power transfer has a pronounced modulation frequency response which changes substantially over time. This response is described using an adaptation of previous crosstalk models for MCFs showing that it results from the random time-varying interference between crosstalk contributions generated at a discrete number of phase matching points along the fiber. The model is used to produce simplified empirical models to characterize its average and variance. It is shown that both these quantities are nonzero and nearly constant for modulation frequencies above the inverse of the time skew between cores. These observations are validated by experimental data.

Compact three-core fibers with ultra-low differential group delays for broadband mode-division multiplexing.

An approximate explicit condition for the achievement of zero differential group delay (DGD) in a homogeneous multicore fiber (MCF) is presented and verified numerically for a step-index three-core fiber. This condition is explored for the study of compact three-core fibers with low DGDs. To achieve an ultra-low DGD in the C-band, a three-core fiber with a central refractive-index dip in each core is proposed and analyzed in detail. A specific design with three touching cores and a core-cladding index difference of 0.3% yields a maximum DGD smaller than 3.2 ps/km in the C-band. The fiber is suitable for broadband mode-division multiplexing applications and the design approach could be applied to MCFs with more cores.

Crosstalk analysis in homogeneous multi-core two-mode fiber under bent condition.

We analyze the inter-core crosstalk in homogeneous multi-core two-mode fibers (MC-TMFs) under bent condition by using the coupled-mode equations. In particular, we investigate the effects of the intra-core mode coupling on the inter-core crosstalk for two different types of MC-TMFs at various bending radii. The results show that the inter-core homo-mode crosstalk of LP(11) mode is dominant under the gentle fiber bending condition due to its large effective area. However, as the fiber bending becomes tight, the intra-core mode coupling is significantly enhanced and consequently makes all the inter-core crosstalk levels comparable to each other regardless of the mode. A similar tendency is observed at a reduced bending radius when the difference in the propagation constants between modes is large and core pitch is small.