Eisuke Sasaoka

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.

Measurement of the nonlinear refractive index in optical fiber by the cross-phase-modulation method with depolarized pump light

Depolarized pump light is used in measurements of the nonlinear refractive index, n2, in optical fiber by the cross-phase-modulation method. High measurement repeatability within ±1% is obtained by this method. The nonlinear refractive index is determined for dispersion-shifted fiber, standard single-mode fiber, pure silica-core single-mode fiber, and dispersion-compensating fiber as 3.35 × 10−20, 2.96 × 10−20, 2.79 × 10−20, and 4.44 × 10−20 m2/W, respectively, at 1.55 μm. This shows that the nonlinear refractive indices of the optical fibers differ greatly according to glass composition.

Hole-assisted lightguide fiber for large anomalous dispersion and low optical loss

Hole-assisted lightguide fiber (HALF) is a microstructured fiber comprising a material index profile for waveguiding and air holes for modifying optical properties. Anomalous dispersion larger than those of the conventional fibers can be realized without severe degradation in optical loss, because of low power fraction in the holes and structural simplicity. We investigate into the causes of the loss of the fabricated HALFs, and show that a GeO2-doped core, in addition to the low power fraction, is desirable for low loss. The fabricated HALF exhibits a loss as low as 0.41 dB/km and a large anomalous dispersion of +35 ps/nm/km at 1550 nm.

Crosstalk variation of multi-core fibre due to fibre bend

It was experimentally and theoretically confirmed for the first time that inter-core crosstalk can be varied more than 20dB depending on bending radius. Simulation results based on coupled-mode equation with equivalent index model are in good agreement with measurement results.

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.

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

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.

Physical interpretation of intercore crosstalk in multicore fiber: effects of macrobend, structure fluctuation, and microbend

We have derived an intuitively interpretable expression of the average power-coupling coefficient for estimating the inter-core crosstalk of the multicore fiber. Based on the derived expression, we discuss how the structure fluctuation and macrobend can affect the crosstalk, and organize previously reported methods for crosstalk suppression. We also discuss how the microbending can affect the crosstalk in homogeneous and heterogeneous MCFs, based on the derived expression and previously reported measurement results.

Novel hole-assisted lightguide fiber exhibiting large anomalous dispersion and low loss below 1 dB/km

We propose a novel holed fiber based on lightguiding by glass index difference and dispersion enhancement by holes. Due to the reduced power fraction in the holes, a record-breaking low loss of 0.82 dB/km at 1550 nm is achieved with extraordinarily large anomalous dispersion of +34 ps/nm/km.

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

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.