Kunimasa Saitoh

We design and demonstrate a dual-mode power splitter that equally splits both the fundamental (TE0) and higher-order (TE1) modes, based on an ultra-compact multimode interference (MMI) coupler utilizing slotted subwavelength grating structures. The fabricated MMI coupler occupies a footprint of only 33 × 5 µm ² , which, to the best of our knowledge, is the smallest footprint reported for a 2 × 2 dual-mode power splitter. Furthermore, experimental results demonstrate favorable power-splitting characteristics over a 60-nm bandwidth, with worst-case excess losses of 0.98 dB for TE0 input and 1.38 dB for TE1 input, and modal crosstalk below −16.4 dB.

We demonstrate a broadband and compact silicon polarization splitter-rotator (PSR) designed to mitigate fabrication-induced performance variations by combining an all-rib configuration and an optimized taper structure. By employing the all-rib configuration, we inherently reduce performance sensitivity to double-step etching variations. Furthermore, we introduce the shortcuts-to-adiabaticity (STA) approach and multi-parameter adiabaticity engineering. Consequently, experimental results demonstrate that the proposed PSR enables broadband operation over the S, C, and L bands while substantially reducing the device length.

We investigate weakly-coupled 3-mode 4-core fibers for the C + L band, comparing graded-index (GI) and dual-step-index (DSI) core profiles with a focus on differential mode delay (DMD) reduction. While both profiles are effective for minimizing DMD in 3-mode transmission, we found that it is difficult for GI profiles with a profile exponent of around 2 to satisfy both the cutoff and low-loss conditions for 3-mode transmission across the entire C + L band. In contrast, the DSI profile offers a higher degree of design freedom and superior controllability over the wavelength dependence of DMD. Furthermore, we demonstrate that a common-depressed cladding layer effectively relaxes cutoff conditions for both GI and DSI core profiles. By replacing a previously reported weakly-coupled 3-mode 4-core step-index fiber with the DSI profile, we achieved inter-core crosstalk (XT) < −50 dB/km while maintaining DMD < 60 ps/km across the entire C + L band.

We propose a cascaded long-period fiber grating (LPFG)-based gain and mode equalizer for erbium-doped fiber amplifiers (EDFAs) supporting two linearly polarized (LP) modes (LP ₀₁ and LP ₁₁ ) in step-index few-mode fibers. By exploiting distinct phase-matching conditions for LP ₀₁ and LP ₁₁ modes, we design a three-stage architecture comprising independent LP ₀₁ and LP ₁₁ mode equalizers followed by a fine-tuning stage. Numerical results demonstrate that this configuration, with a total device length of less than 232 mm, suppresses the gain ripple to ≤1.06dB and the differential mode gain to ≤0.36dB across the C-band. The proposed scheme exhibits robustness against initial gain variations, offering a practical all-fiber solution for mode-division multiplexing transmission systems.

We propose and numerically demonstrate a method to achieve ultra-low birefringence in trench-assisted (TA) multi-core fibers (MCFs) by engineering the thermally induced stress field through the incorporation of doped silica stress rods. The approach is evaluated in two structures: a 4-core TA-MCF employing a single boron-doped rod at the cladding center, and an 8-core TA-MCF using one central and four peripheral fluorine-doped rods. By optimizing the rod positions, radii, and dopant concentrations, the method identifies parameter sets that reduce birefringence into the ultra-low regime by counteracting the stress asymmetries induced by neighboring cores. The optimized designs exhibit birefringence beat lengths exceeding 10 m and are expected to yield polarization-mode dispersion (PMD) on the order of 0.01 ps/√km with strong resilience to external perturbations. Moreover, the same stress balancing mechanism is expected to mitigate inter-core crosstalk (IC-XT) in certain MCF designs, thereby addressing the two critical impairments of PMD and IC-XT with a single approach.

We propose a nonvolatile and broadband 2 × 2 silicon photonic tunable coupler with phase change material (PCM). By arranging broadband 3-dB couplers point-symmetrically, the tunable coupler achieves broadband characteristics at any splitting ratio. We consider two types of broadband 3-dB couplers for the point-symmetric tunable coupler: one based on asymmetric waveguide widths and the other based on asymmetric PCM states. Both have high fabrication tolerance, but the latter additionally allows for fabrication error compensation. The proposed tunable coupler with a device length of 109.8 µm provides less than 1.6% deviation from the ideal power-splitting ratio and less than 0.032 dB excess loss within the wavelength range of 1.53–1.57 µm. This device contributes to the realization of low-power programmable photonic circuits.

We propose a group delay spread (GDS) calculation for randomly coupled multi-core fiber (RC-MCF) based on simple core-mode analysis for a single-core structure, which avoids the time-consuming two-dimensional supermode calculation. The differential mode delay (DMD) and coupling efficiency between supermodes required for GDS calculation are derived from the individual core-mode properties through the mode basis transformation from the core-modes to the supermodes, which is applicable for an arbitrary number of cores and layout. After calculating the GDS properties of 4-, 7-, and 12-core RC-MCFs, we found that the proposed method well describes the GDS dependence on fiber parameters such as core pitch, bending radius, and core structure, which are essential for optimizing RC-MCF design.

The so-called effective bending radius (EBR) for a stress-effect correction is required in order to fit calculated bend losses in single-core fibers with measured results; however, the EBR has been hardly used for evaluating the stress effects on the inter-core crosstalk (XT) and inter-core skew (ICS) in bent multicore fibers (MCFs). In this paper, first, revisiting and comprehensively reviewing the EBR derivation method, we confirm that there exist two different EBR values, ${{R}_{\mathrm{eff}}} \approx $1.28$R$ and ${{R}_{\mathrm{eff}}} \approx $1.4$R$ with $R$ being the bending radius, depending on the underlying assumptions. Next, incorporating the EBR into the XT and ICS formulae, we evaluate the stress effects on the XT and ICS in bent MCFs, and reveal that the EBR ${{R}_{\mathrm{eff}}}$= 1.4$R$ leads to more consistent results between calculations and measurements.