Hokkaido University · Engineering
Professor Takeshi Fujisawa's research lab specializes in the design and optimization of advanced photonic integrated circuits and optical components for high-speed, low-loss, and energy-efficient optical communication systems. The lab focuses on novel waveguide structures—such as slot waveguides, photonic crystal fibers, and compact bend configurations—enabling polarization-insensitive and broadband functionalities essential for next-generation optical interconnects. Key research directions include nonlinear optical devices, dispersion compensation in fiber systems, and ultra-compact components for dense integration. The lab combines rigorous full-vector finite-element simulations with innovative structural designs to achieve breakthrough performance in optical signal processing and transmission.
Figures are computed from collected data and may differ slightly.
A novel design of all-optical logic gates based on nonlinear slot-waveguide couplers is proposed. NOT, OR, and AND logic gates can be realized by a simple single optical-directional coupler configuration. Strong polarization dependencies of slot waveguides are effectively utilized for realizing polarization-independent optical-directional couplers in the linear regime and polarization-dependent all-optical switches in the nonlinear regime. All the simulations performed in this paper were perform
A polarization-independent optical directional coupler based on slot waveguides is proposed and analyzed by using rigorous full-vectorial analysis methods based on a finite-element scheme. Properly choosing materials and structural parameters makes the coupling length for quasi-TE modes become equal to that for quasi-TM modes. Tolerances to operating wavelength and structural parameters are also discussed. The proposed coupler can be used for highly integrated optical circuits without polarizati
Chromatic dispersion profile of dual-concentric-core photonic crystal fibers is optimized for broadband dispersion compensation of single mode fibers (SMFs) by using genetic algorithm incorporated with full-vector finite-element method. From the numerical results presented here, it is found that by increasing the distance between central core and outer ring core, larger negative dispersion coefficient and better dispersion slope compensation are possible. There is a tradeoff between the magnitud
Ultimately low-loss 90° waveguide bend composed of clothoid and normal curves is proposed for dense optical interconnect photonic integrated circuits. By using clothoid curves at the input and output of 90° waveguide bend, straight and bent waveguides are smoothly connected without increasing the footprint. We found that there is an optimum ratio of clothoid curves in the bend and the bending loss can be significantly reduced compared with normal bend. 90% reduction of the bending loss for the b
A 1.3-μm, 4 × 25-Gbit/s, EADFB laser array module with large output power and low driving voltage is developed for 100GbE. A novel rear grating DFB laser is introduced to increase the output power of the laser while keeping the single mode lasing, which is desirable for a monolithic integration. Also, InGaAlAs-based electroabsorption modulators make very-low-driving-voltage operation possible due to their steep extinction curves. With the module, very clear 25-Gbit/s eye openings are obtained fo
A polarization-insensitive 1times2 multimode interference beam splitter based on sandwiched structures is proposed and investigated theoretically for the components of ultrasmall optical integrated circuits. Strong polarization dependencies arising in Si-wire waveguide systems are compensated by introducing sandwiched structures
A time-domain beam propagation method (BPM) based on a finite-element scheme is newly formulated for nonlinear optical propagation analysis. In order to obtain steady-state solutions, a way of continuous-wave (CW) excitation is also described. The validity of this method is verified by numerical examples: self-focusing guiding phenomena and nonlinear gratings. Furthermore, this approach is also applied to characterizing nonlinear photonic crystal circuits. Specifically, a grating structure desig
A 1.3- monolithically integrated light source for metro area 100-Gb/s Ethernet is developed. Four 25-Gb/s electroabsorption modulators integrated with distributed-feedback lasers and their multiplexer are monolithically integrated on one chip. A shallow ridge waveguide is used for lasers and modulators for large modulation bandwidth, and a deep ridge waveguide is used for the multiplexer region due to its low bending and radiation losses. The integration of hetero-waveguide devices enables the v
A high-performance 1.3 µm InGaAlAs electroabsorption modulator integrated with a DFB laser for the metro-area 100 Gbit/s Ethernet system (100GBASE-LR4 and -ER4) has been developed. 40 km transmission under 25 Gbit/s operation on singlemode fibre that is required for 100GBASE-ER4 application at 40°C is demonstrated for the first time.
A 1.3 µm, 50 Gbit/s electroabsorption modulator integrated with a DFB laser for future parallel LAN, the total data rate of which is over 100 Gbit/s, is developed. Very clear eye openings after 40 km transmission under 50 Gbit/s operation on singlemode fibre are demonstrated for eight wavelengths with discrete packages for the first time.
Guided modes of nonlinear slot waveguides are investigated for the first time and their unusual modal properties are demonstrated. Nonlinear phase shifts of slot waveguides induced by optical Kerr effect exhibit saturable behavior for strong optical intensity. This is because a self-localization effect of electromagnetic fields induced by optical Kerr effect is compensated with a field broadening originating from a unique confinement mechanism of slot waveguides. Nonlinear phase shifts on variou
A full-vector finite-element beam propagation method (VFE-BPM) in terms of all the components of slowly varying electric fields is described for the analysis of three-dimensional (3-D) nonlinear optical waveguides. Electric fields obtained with this approach can be directly utilized for evaluating nonlinear refractive index distributions. To eliminate nonphysical, spurious solutions, hybrid edge/nodal elements are introduced. Furthermore, to avoid spurious reflections from the computational wind
This paper treats the synthesis of nondissipative low-pass ladders without mutual induction. Necessary and sufficient conditions for physical realizability of mid-series or mid-shunt nondissipative low-pass ladders without mutual induction are presented here and the complete proof is given. The essence of the conditions imposed upon the input impedance of the network terminated in a pure resistance is a relation between the finite frequencies at which the loss is infinite and the roots of a poly
Chromatic dispersion characteristics of nonlinear photonic crystal fibers are, for the first time to our knowledge, theoretically investigated. A self-consistent numerical approach based on the full-vector finite-element method in terms of all the components of electric fields is described for the steady-state analysis of axially-nonsymmetrical nonlinear optical fibers. Electric fields obtained with this approach can be directly utilized for evaluating nonlinear refractive index distributions. T
A nonlinear Mach-Zehnder interferometer based on photonic-crystal waveguides is proposed and modeled by using rigorous finite-element-based numerical scheme. Guided modes of nonlinear photonic-crystal waveguides are investigated to determine a length of nonlinear arm and a switching power, and further, intensity-dependent switching characteristics of the nonlinear Mach-Zehnder interferometer, for the first time, as far as the authors know, are demonstrated. Effects of saturable nonlinearity and
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