Broadband design of silica-PLC mode-dependent-loss equalizer for 2LP-mode transmission systems.

A new configuration of mode-dependent-loss (MDL) equalizer for two linearly-polarized mode transmission systems using the silica planar lightwave circuit platform is proposed. This device acts as an LP01-mode attenuator (precisely, LP01/LP21 mode converter) to adjust the MDL keeping a high transmission of the LP11 modes. Almost all components constructing the device are based on the adiabatic mode conversion, which brings broadband operation. Especially, a newly proposed E12/E22 mode converter plays a key role in broadband MDL equalization. It is numerically revealed that the flattened spectra with designated transmission can be obtained for the wavelength from 1200 nm to 1650 nm.

Design of silica-PLC LP11 mode rotator based on adiabatic mode conversion.

A silica-based LP11 mode rotator, which is one of the basic and indispensable optical components for space division multiplexing, with multiple tapered trenches is proposed. Compared with the conventional interference-based LP11 mode rotator with a simple L-shape waveguide, the proposed LP11 mode rotator has many advantages in a mode conversion efficiency, an insertion loss, and a fabrication tolerance because the operation principle is based on the adiabatic mode conversion. By using an approach of the shortcut to adiabaticity, the proposed device is effectively miniaturized rather than the standard tapered structures. Among the LP11 mode rotators in the silica-based mode multi/demultiplexers, the proposed type will be a considerably promising candidate.

Reduction of differential modal gain in a two-mode amplifier using a void-inscribed EDF.

We propose a technique for reducing the differential modal gain (DMG) that occurs in a two-mode erbium-doped fiber (2M-EDF) by inscribing voids in the core center of a 2M-EDF with a femtosecond laser. We show that an empty void inscribed at the core center can attenuate the linearly polarized (LP01) mode selectively while suppressing excess loss for the LP11 mode. We also reveal that DMG can be controlled by means of void diameter. The longitudinal position dependence of the void in a 2M-EDF was also investigated considering its influence on gain and noise figure (NF) characteristics. Finally, we realize a sufficiently low DMG of less than 0.5 dB in the full C-band as well as a sufficient gain and NF by using the proposed technique.

Design of small mode-dependent-loss scrambling-type mode (de)multiplexer based on PLC.

A general design rule of scrambling-type mode (de)multiplexers (mode scramblers) based on silica planar lightwave circuit (PLC) with small mode-dependent-loss (MDL) is presented for a mode-division multiplexing (MDM) system. First, we consider four- and eight-mode scramblers and demonstrate that if the number of modes is 2N, it is possible to construct small-MDL mode scramblers by using Y-branch waveguides and mode rotators. Next, a 6-mode scrambler, which can be used for four linearly polarized (LP) mode transmission in MDM system, is considered, and the MDL is large if Y-branch waveguides are cascaded simply, originating from the radiation loss of unwanted modes at the Y-branch. We propose a 2 + 4-type mode scrambler by combining 2- and 4-mode scramblers and demonstrate that it is possible to design a small MDL 6-mode scrambler.

A novel Si four-wavelength multiplexer for 100/400GbE using higher-order mode composed of (a)symmetric directional couplers and TE1-TM0 mode converter.

A novel Si four-wavelength multiplexer (MUX) for 100/400GbE composed of (a)symmetric directional couplers and a rib-waveguide TE1-TM0 mode converter is proposed and experimentally demonstrated. Two-lane signals are multiplexed as TE0 modes via symmetric directional coupler (DC) and other two-lane signals are multiplexed as TE1 modes via asymmetric directional couplers (ADCs). Finally, TE1 modes are converted to TM0 modes and the device acts as a 4-lane MUX. The proposed device is fabrication-tolerant due to the removal of the 1600-GHz filter used in conventional two-stage Mach-Zehnder MUXs, and the insertion loss is smaller than those of previously reported Si-based MUXs.

Effective mode-field diameter for few-mode fibers for considering splice loss characteristics.

We experimentally and numerically investigate the applicability of the conventional near-field pattern, far-field pattern (FFP), and variable aperture (VA) methods for measuring the effective mode-field diameter (MFD) of the higher-order modes, which can be used for accurately evaluating splice loss. We confirmed that the variation in the MFD values obtained with the FFP and VA methods can be converged by considering the minimum scanning angle and the minimum aperture angle, respectively, for both fundamental and higher-order modes. We reveal that the FFP and VA methods provide adequate effective MFD values for few-mode fiber (FMF), which can be used for evaluating the splice loss based on the conventional Gaussian fitting model. We also show the applicability of the effective MFD to four linearly polarized mode FMFs for considering the splice loss characteristics of each mode.

Optimal design of 4LP-mode multicore fibers for high spatial multiplicity.

High spatial multiplicity fiber designs are presented for homogeneous and heterogeneous 4LP-mode multicore fibers (MCFs) that support six spatial modes per core. The high-spatial-density 4LP-mode MCF design methodology is explained in detail. The influence of the number of cores on the cladding diameter (Dcl) and relative core multiplicity factor (RCMF) is investigated. The optimal core designs and MCF layouts with square and triangular lattices maintain glass fiber reliability (maximum Dcl = 250 µm). For homogeneous 4LP-mode MCFs, a 19-core triangular-lattice fiber gives the highest RCMF of 61.7. For heterogeneous 4LP-mode MCFs, an RCMF of 65.4 is obtained for a 21-core square-lattice fiber.

Guided acoustic-wave Brillouin scattering characteristics of few-mode fiber.

We experimentally investigate the modal dependence of the guided-acoustic wave Brillouin scattering (GAWBS) characteristics in a few-mode fiber (FMF). We clarify that launching higher-order modes into FMF induces different peak frequencies from those of single-mode fiber (SMF) in the GAWBS spectrum and that the GAWBS spectrum strongly depends on the mode excitation power ratio between the LP(01) and LP(11) modes. We evaluate the mode excitation power ratio at a splice between an FMF and an SMF by utilizing the GAWBS spectrum, which we realize by detecting only the fundamental mode at the output of the FMF and without the use of a mode demultiplexer.

Experimental and numerical evaluation of inter-core differential mode delay characteristic of weakly-coupled multi-core fiber.

We investigated the inter-core differential mode delay (DMD) characteristic of a weakly-coupled homogeneous multi-core fiber with a view to utilizing inter-core crosstalk compensation with MIMO processing. We confirmed experimentally that the bend induced inter-core DMD is lower than the simulated results, which we expected owing to the twist of the fiber. We also revealed numerically that the refractive index profile variation of each core greatly increases inter-core DMD. Finally, we conducted a 4 × 4 MIMO transmission experiment using a weakly-coupled 4-core fiber and successfully compensated for the inter-core crosstalk.

Technique for characterizing single-mode operability in optical fibers by utilizing variable-aperture technique.

We propose a technique, new to our knowledge, for characterizing single-mode operability by monitoring far-field patterns via a variable aperture. We clarify the applicability of our proposed technique to cutoff wavelength measurement, which agrees well with cutoff wavelength obtained by a conventional multimode reference technique. We also show the applicability of our proposed technique to holey fibers including photonic crystal fibers (PCFs) with uniform air-hole structure and effectively W-shaped air-hole structure. We can clearly characterize the single-mode operability of the PCF. The bend dependence of the single-mode bandwidth is also revealed by utilizing our proposed technique.

Structural dependence of guided acoustic-wave Brillouin scattering spectra in hole-assisted fiber and its temperature dependence.

We describe the guided acoustic-wave Brillouin scattering (GAWBS) characteristics of hole-assisted fiber (HAF). We clarify numerically and experimentally that the GAWBS spectrum corresponding to a particular acoustic mode is observed for HAF and that the efficiency for that mode can be controlled simply by designing the air-hole size and position. We also reveal the temperature dependence of the GAWBS characteristics in HAF.

Effective Raman gain characteristics in germanium- and fluorine-doped optical fibers.

We describe the dopant dependence of Raman gain in germanium- and fluorine-doped optical fibers. We clarify, both theoretically and experimentally, the effective Raman gain characteristic in an optical fiber, which is closely related to the fiber's refractive-index profile and electromagnetic field profile. We also show that this experimentally determined relationship can be used to evaluate the effective Raman gain characteristic in a germaninum- or a fluorine-doped optical fiber with an arbitrary index profile.