Toshiki Taru

Design and fabrication of ultra-low crosstalk and low-loss multi-core fiber

We designed and fabricated a multi-core fiber (MCF) in which seven identical trench-assisted pure-silica cores were arranged hexagonally. To design MCF, the relation among the crosstalk, fiber parameters, and fiber bend was derived using a new approximation model based on the coupled-mode theory with the equivalent index model. The mean values of the statistical distributions of the crosstalk were observed to be extremely low and estimated to be less than -30 dB even after 10,000-km propagation because of the trench-assisted cores and utilization of the fiber bend. The attenuation of each core was very low for MCFs (0.175-0.181 dB/km at 1550 nm) because of the pure-silica cores. Both the crosstalk and attenuation values are the lowest achieved in MCFs.

Space Division Multiplexed Transmission of 109-Tb/s Data Signals Using Homogeneous Seven-Core Fiber

We achieved record 109-Tb/s transmission over 16.8 km, using space division multiplexing (SDM) together with conventional multiplexing technology. 7-core SDM, 97 WDM (100-GHz spacing), 2 × 86 Gb/s PDM-QPSK signals were used. The spectral efficiency was 11.2 b/s/Hz. SDM transmission was realized using a multi-core fiber with ultra-low-crosstalk (less than -90.0 dB/km at 1550 nm) and high performance SDM MUX/DEMUX. The overall SDM crosstalk of -53 dB caused almost no penalty for the PDM-QPSK transmission.

Coupling in dual-core photonic bandgap fibers: theory and experiment

We have theoretically and experimentally investigated dual-core photonic bandgap fibers (PBGFs), which consist of a cladding with an array of high-index rods and two cores formed by omitting two nearby rods. We find novel features in their coupling characteristics such as maxima and minima in coupling length, complete decoupling of the cores, and an inversion of the usual ordering of supermodes so that the odd supermode has the higher propagation constant. This behavior is understood by considering the field distribution in the rods between the cores.

Characterization of Crosstalk in Ultra-Low-Crosstalk Multi-Core Fiber

We describe a newly-developed method to measure inter-core crosstalk of single-mode multi-core fiber (MCF) statistically, and characteristics of the crosstalk of a previously fabricated MCF. To measure the crosstalk lower than -60 dB, we suppressed power floor of the measurement system by employing trench-assisted fiber as launching and receiving fibers. We measured statistical crosstalk distributions by utilizing wavelength dependence of the crosstalk, and the mean crosstalk between the neighboring cores was observed to be about -70 dB after 17.4-km propagation at 1625 nm. The measurement results were in good agreement with the previously proposed model. Based on the measurement results and the model, the crosstalk after 10000-km propagation was estimated to be less than -30 dB. We also confirmed that the crosstalk was varied stochastically depending on polarization state, and on mechanical shock and stress on the MCF.

Ultra-low-crosstalk multi-core fiber feasible to ultra-long-haul transmission

Inter-core crosstalk of fabricated multi-core fiber was statistically evaluated to be less than −77.6 dB after 17.4-km propagation for λ=1550 nm using newly-developed method. The crosstalk was estimated to be less than −30 dB after 10,000 km.

MIMO-Based Crosstalk Suppression in Spatially Multiplexed 3

We experimentally demonstrate simultaneous transmission of three spatially multiplexed 56-Gb/s polarization-division-multiplex quadrature phase-shift keying (PDM-QPSK) channels over 24 km of a strongly coupled three-core fiber. The resulting crosstalk of about -4 dB is almost completely removed by coherent multiple-input multiple-output (MIMO) processing.

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We designed and fabricated a multi-core fiber whose attenuation is less than 0.18 dB/km and effective area is about 80 µm². Crosstalk was observed be less than −55.5 dB after 17.6 km propagation.

56-Gb/s PDM-QPSK Signals for Strongly Coupled Three-Core Fiber

Low-loss optical fibers are now indispensable transmission media for transmission systems. Recently, the ultralow-loss performance for long transmission systems, the water-loss-free performance for wide-band wavelength-division-multiplexing (WDM) systems, the hydrogen-loss-insensitive performance for system reliability, and the bending-loss-insensitive performance for access or indoor applications have attracted much interest. In this regard, pure-silica-core fibers (PSCFs) are suitable, and unprecedented low-loss PSCFs have been successfully fabricated. This paper introduces the recent progress of low-loss PSCFs and their possible impact on transmission systems.

Low-crosstalk and low-loss multi-core fiber utilizing fiber bend

We designed and fabricated a low-crosstalk seven-core fiber with transmission losses of 0.17 dB/km or lower, effective areas larger than 120 μm(2), and a total mean crosstalk to the center core of -53 dB after 6.99-km propagation (equivalent to -42.5 dB after 80 km), at 1550 nm. We also investigated the signal-to-noise ratio (SNR) achievable in uncoupled multi-core transmission systems by regarding the crosstalk as a virtual additive white Gaussian noise. The SNR under existence of crosstalk in the fabricated multi-core fiber (MCF) was estimated to be 2.4 dB higher than that in a standard single-mode fiber (SSMF) in the case of 80-km span, and 2.9 dB higher in the case of 100-km span; which are the best values among MCFs ever reported, to the best of our knowledge. The SNR penalties from crosstalk in this MCF were calculated to be 0.4 dB for 80-km span and 0.2 dB for 100-km span. We also investigated SNR penalty from crosstalk in the more ordinary case of an MCF with SSMF cores, and found that the total mean crosstalk to the worst core after one 80-km span should be less than about -47 dB for 0.1-dB penalty, about -40 dB for 0.5-dB penalty, and about -36 dB for 1-dB penalty.

Low-loss pure-silica-core fibers and their possible impact on transmission systems

Loss and crosstalk variations of an indoor multi-core fiber (MCF) cable were evaluated during mechanical and thermal tests. Furthermore, for the first time, the crosstalk of the MCF in the straightened cable was observed. The results demonstrate optical-characteristics robustness of the MCF cable.