Kunimasa Saitoh

Kunimasa Saitoh
Hokkaido University | Hokudai · Graduate School of Information Science and Technology

PhD

About

505
Publications
21,558
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
11,617
Citations

Publications

Publications (505)
Article
A novel direct-binary-search algorithm for the design of fabrication-tolerant photonic-crystal-like subwavelength structures is proposed. In the proposed method, structures based on holes with different diameters are considered to enhance the device performance under diameter variation. Usefulness of the proposed method is demonstrated through the...
Article
A simple and efficient method for estimating modal amplitude and phase of multimode near field patterns (NFPs) based on artificial-neural-network (ANN) with the help of the optimization method is proposed. The inferred amplitude and phase of measured NFPs based on ANN are refined by using a grey-wolf optimizer (GWO). By using the proposed method, t...
Article
Analytical expression is derived for the mode-coupling coefficient between non-identical step-index (SI) cores, which enables a quick estimate of the crosstalk in the heterogeneous SI multi-core fibers (Hetero-SI-MCFs). The analytical results are in good agreement with the results obtained by numerical simulations by finite-element method (FEM). Us...
Article
We numerically reveal the feasibility of designing multi-core fibers (MCFs) with the simple step-index (SI) profile within the standard 125-m cladding diameter. We show that the standard cladding diameter enables us to incorporate five identical cores for C-band operation, and the number of cores can be further increased with the heterogeneous core...
Article
Using orbital angular momentum (OAM) as a spatial information channel attracts a lot of attention due to its infinite multiplexing capability. The research on OAM carrying fibers is intensively studied and ring-core fibers are promising candidates for them. At the same time, generating OAM modes in those fibers are also important topic. Here, the e...
Article
We propose a broadband silicon four-mode multi/demultiplexer which consists of three asymmetric directional couplers, and the asymmetric directional couplers were designed by a wavefront matching method to operate as mode multi/demultiplexers for TE1, TE2, and TE3 modes, respectively. Simulated results show that the -0.5-dB bandwidths of normalized...
Article
A novel Si four-wavelength multiplexer (MUX) for CWDM system composed of two types of Mach-Zehnder (MZ) MUX, and a polarization splitter rotator (PSR) based on a tapered asymmetric directional coupler (ADC) and TE1-TM0 mode converter (MC) is proposed and experimentally demonstrated. To make the ADC fabrication-tolerant and broadband, a new design m...
Article
An efficient design method of mosaic-based power splitters based on Bayesian optimization technique is proposed. First, learning characteristics of Gaussian process (GP), which is one of the Bayesian inference techniques, is investigated to show its high regression performance. The transmission characteristics of 1×2 mosaic-based power splitters ca...
Article
We demonstrate tunable broadband two-mode power dividers based on a wavelength-insensitive coupler for a mode-division multiplexing system. Ultra-small TE0−TE1 and TE0−TE2 mode dividers based on Si-wire waveguides are designed based on the coupled-mode theory. Tunability is achieved by using the thermo-optic effect. In both dividers, arbitrary bran...
Article
A broadband and compact TE0-TE1 mode converter for a mode division multiplexing system designed using a wavefront matching method is realized. We present the first experimental demonstration of a silicon waveguide device designed by a wavefront matching method. In order to achieve broadband operation of the silicon mode converter, seven wavelengths...
Article
Two silicon, 4λ O-band multiplexers (MUXs) for 400GbE using two Mach–Zehnder interferometers (MZIs) having 90-degree phase shift in their output with the help of on-chip polarization multiplexing are demonstrated. By using 2 × 2 and 2 × 1 MZIs for first 3200-GHz filters, the relative position of their spectra has 90-degree phase difference, leading...
Article
The evolutions of polarization and orbital angular momentum (OAM) states of light in helically twisted birefringent photonic crystal fibers (TB-PCFs) are analyzed. It is shown that a circular polarization (CP) component (S3 of a Stokes parameter) is periodically excited when usual linearly polarized (LP) modes of PCF are launched. The excitation or...
Article
A Few-mode fiber design suitable for long-haul dense space division multiplexing (SDM) transmission is proposed, where the spatial density and the splicing characteristics of the fiber is optimized. We numerically investigate the impact of the number of modes per core in a multi-core structure on the total number of spatial channels possible and th...
Article
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 const...
Article
We present a high spatial density space division multiplexed fiber amplifier (i.e. 6-mode 7-core multicore fiber amplifier) supporting 42 spatial channels. A high average modal gain of 23.1 dB is obtained in the L-band with an average differential modal gain of 3.2 dB and a minimum output power of 17.5 dBm per core per mode. The total power consump...
Conference Paper
2×4λ Si-photonics multiplexers for 400GbE composed of Mach-Zehnder filters and a polarization-splitter-rotator are proposed and experimentally demonstrated for the first time. Relative spectral position of two filters is locked by using 2×2 and 2×1 configurations.
Conference Paper
A tunable TE0-TE1 mode divider based on wavelength-insensitive-coupler is experimentally demonstrated for the first time. Arbitrary branching ratios can be realized by using thermo-optic heaters. The proposed device is useful for gain-equalization in MDM networks.
Conference Paper
A first six-mode exchanger based on one sidewall grating-like waveguide is successfully designed with the help of strong optimization algorithm. Fabricated device compensates for mode-dependent-loss caused by fiber-waveguide junctions, showing the proof-of-concept operation.
Article
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...
Conference Paper
We present an L-band 6-mode 7-core amplifier supporting 42-spatial channels with a new record low power consumption factor of 0.36 and demonstrate simultaneous amplified SDM transmission with the highest reported spatial density and energy efficiency.
Article
The effect of core offset on the mode converting characteristics in twisted single mode fiber (SMF) is investigated. If the core position of SMF is slightly shifted from its rotation center, multiple spectral dips appear in the transmission spectra due to the coupling between core and cladding modes. Guided mode solver and beam propagation method (...
Article
We numerically investigate the group delay spread (GDS) characteristics in few-mode coupled multicore fibers (FM-CMCFs) in weak random-bending conditions by using a coupled-wave theory. We qualitatively show the modal coupling between modes in the intra- and the inter-mode group generated by bending. The results indicate that the modes of FM-CMCFs...
Article
This paper extends a recent demonstration of 1.2 Pb/s transmission using a few-mode multi-core fiber with a cladding diameter of 160 $\mu$ m. The fiber was a 3.37 km, 4-core, 3-mode few-mode multi-core fiber with spatial multiplexing and demultiplexing performed with especially designed mode-selective couplers. We transmitted 368 wavelength divis...
Article
The papers included in this special section were presented at the 44th European Conference on Optical Communication (ECOC 2018) that was held from September 23–27, 2018, in Rome, Italy.
Article
An ultrabroadband silicon polarization beam splitter (PBS) based on a wavelength-insensitive coupler is fabricated by a standard complementary metal oxide semiconductor technology. The PBS has three identical directional couplers and two identical delay lines with a point symmetric layout. We experimentally demonstrate the broadband operation of th...
Article
Threshold carrier densities of GeSn quantum well (QW) lasers and the physical reason of low-temperature lasing of current GeSn laser are investigated through the comparison of threshold carrier densities of conventional III-V QW lasers. Electrons distributed over L-band is the main cause of decreased gain for GeSn QWs. To increase the gain (and imp...
Article
We propose a novel multiple-input multiple-output (MIMO) detection scheme based on iterative unreplicated parallel interference cancelling (UPIC) technique for mode dependent loss (MDL)-impaired few-mode fibre (FMF) transmission. An optical MIMO channel over a FMF exhibits a mode-dependent property where each mode division multiplexed signal is pas...
Conference Paper
A Si four-wavelength multiplexer for 100/400GbE composed of (a)symmetric directional couplers and a rib-waveguide mode-converter is experimentally demonstrated. Proposed device is fabrication-tolerant due to the removal of 1600-GHz filter used in conventional two-stage Mach-Zehnder multiplexers.
Conference Paper
We investigate crosstalk fluctuations between cores and modes in a few-mode-MCF, observing almost 20dB power fluctuations for CW signals. We then measure increased crosstalk penalty for LP11 modes compared to the more confined LP01 modes.
Conference Paper
A broadband TE0-TE1 mode divider with arbitrary-branching-ratio based on multimode wavelength-insensitive coupler is proposed for flexible mode manipulation in modedivision-multiplexing system. Measured spectra of fabricated devices in Si-photonics platform are in excellent agreement with theory.
Conference Paper
Ultrasmall TE0-TE1 and TE1-TE2 mode converters based on Si-wire waveguides designed by wavefront matching (WFM) method are demonstrated both theoretically and experimentally. This is the first demonstration of WFM-designed Si waveguide devices.
Article
We present an all-optical diode suppressing broadband backward transmission using single-port and four-port photonic crystal (PC) cavities and its operation principle. We find that, by combining a four-port PC cavity and two power-dependent phase shifters that are composed by a 3-dB power divider and two single-port PC cavities, the backward transm...
Article
We demonstrate differential mode delay (DMD)- unmanaged 2500-km 12-core × 3-mode multicore fewmode fiber (MC-FMF) and 6300-km 3-mode FMF transmission. Mode division multiplexed signals exhibit different transmission behavior depending on excited spatial modes in the presence of moderelevant physical phenomena including DMD and mode dependent loss (...
Article
The transmission characteristics of helically twisted photonic crystal fibers (PCFs) having off-axis cores are theoretically investigated for the first time, to the best of our knowledge. Due to the modified equivalent permittivity and permeability tensors induced by helical twisting, the transmission characteristics of helically twisted PCFs depen...
Article
Combined use of wavelength division multiplexing (WDM) and mode division multiplexing (MDM) attracts a lot of attention for expanding transmission capacity. For WDM/MDM transmission, a broadband mode multi/demultiplexer (MUX/DEMUX) is necessary. In this work, we propose a silica-PLC-based Mach–Zehnder (MZ) interferometer mode MUX. The MZ filter for...
Article
Non-circular multi-core fibers (NC-MCFs) to accommodate more than 100 cores per fiber are proposed for super-dense space-division multiplexing. The proposed MCFs have the potential for easier splicing/connection and their applicability to highly-dense ribbon MCFs for ultra-high capacity optical transport systems.
Article
Presents corrections to the paper, “Full-vector finite-element beam propagation method for helicoidal waveguides and its application to twisted photonic crystal fibers,” (Fujisawa, T., et al), J. Lightw. Technol., vol. 35, no. 14, pp. 2894–2901
Article
Transmission characteristics of a helically twisted photonic crystal fiber (PCF) are theoretically investigated by a recently developed beam propagation method for a helicoidal system. Experimentally reported features of the twisted PCF, such as periodical transmission dips, optical activity discontinuity, and circular dichroism, are successfully r...
Article
A PLC based mode multi/demultiplexer based on asymmetric directional coupler has advantages in terms of compactness, mass productivity, low insertion loss, and matured reliability. However, it has relatively large wavelength dependence due to the difference of coupling length. To expand the bandwidth, we have designed two-mode (LP01/LP11a) multi/de...
Article
We propose and experimentally demonstrate a low-loss and low-crosstalk Mach-Zehnder mode/wavelength multi/demultiplexer for WDM/MDM transmission based on a Si-photonics platform. A broadband 3-dB mode divider, which is also newly devised here, makes it possible to compose a Mach-Zehnder filter for “mode” and “wavelength” simultaneously. Transmissio...
Article
Fully three-dimensional (3D) wavefront matching method (WFM) based on finite-element scheme for optical waveguide devices including waveguide discontinuities is newly developed. In contrast to the conventional WFM method based on the beam propagation method, the developed method, in principle, can be applied to optimize any geometries including ref...
Article
We propose taper coupler electroabsorption modulator (EAM) composed of GeSn/SiGeSn multiple-quantum-well (MQW) on Ge-on-Si platform for mid-infrared (2 μm) integrated optical active devices. The epitaxial design is performed by calculating the absorption spectra of GeSn/SiGeSn MQW using many-body theory to investigate the extinction characteristics...
Article
A novel scrambling-type mode multiplexer (MUX) is proposed for future large-mode-number mode-division-multiplexing. The proposed MUX does not pursue high mode extinction ratio as in the conventional mode MUX, rather, all the modes are moderately mixed with the assumption of using MIMO receiver. The proposed configuration is superior to conventional...
Article
We propose the optimum design of high spatial density few-mode multi-core fiber (MCF) for repeated space division multiplexed (SDM) transmission. Numerical analyses reveal that a hexagonally arranged 6-mode 7-core structure is potentially better than the other possibility, the 6-mode multi-core structure, in terms of realizing high spatial density...
Conference Paper
We demonstrate DMD-unmanaged 2500-km 12-core × 3-mode MC-FMF and 6300-km 3-mode FMF transmission. DMD-induced pulse broadening and MDL accumulation were significantly suppressed by transmission with cyclic mode permutation. Interference cancellation mitigated MDL-impact with > 2-dB Q-factor improvement.
Article
A high-forward-transmission all-optical diode based on cascaded photonic crystal cavities is proposed. To obtain a high forward transmission and a large nonreciprocity, we generate a box-like spectrum by cascading two side-coupled photonic crystal cavities. We find that by appropriately adjusting the distance between two cavities, a high contrast o...