Takeshi Fujisawa

Bio: Takeshi Fujisawa is an academic researcher from Hokkaido University. The author has contributed to research in topics: Distributed feedback laser & Photonic crystal. The author has an hindex of 27, co-authored 234 publications receiving 2469 citations. Previous affiliations of Takeshi Fujisawa include Nippon Telegraph and Telephone.

50-Gb/s Direct Modulation of a 1.3-μm InGaAlAs-Based DFB Laser With a Ridge Waveguide Structure

Abstract: We demonstrate 50-Gb/s direct modulation by using 1.3-μm distributed-feedback lasers with a ridge waveguide structure. We employed InGaAlAs material for a multiple-quantum well to obtain a low damping factor K, and fabricated a ridge waveguide structure buried in benzocyclobutene to realize a structure with a low parasitic capacitance. In addition, to obtain high maximum frequency relaxation oscillations fr, we designed the cavity length L), and achieved a 3-dB-down frequency bandwidth of 34 GHz. We realized 50-Gb/s clear eye openings with a back-to-back configuration, and achieved a mean output power of over 5.0 dBm, and a dynamic extinction ratio of 4.5 dB. We measured the 50-Gb/s transmission characteristics, and obtained clear eye openings for transmissions over 20-, 40-, and 60-km single-mode fibers (SMF). We also measured the bit-error-rate performance, and obtained an error-free operation and a power penalty of less than 0.5 dB after a 10-km SMF transmission.

All-optical logic gates based on nonlinear slot-waveguide couplers

Abstract: 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 performed for three-dimensional nonlinear channel waveguide structures by using rigorous numerical schemes based on the full-vector finite-element method specially formulated for nonlinear optical waveguides.

Chromatic dispersion profile optimization of dual-concentric-core photonic crystal fibers for broadband dispersion compensation

Abstract: 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 magnitude of negative dispersion coefficient and dispersion slope compensation due to the concave dispersion profile of dual-concentric-core photonic crystal fibers. In spite of the tradeoff, dual-concentric-core photonic crystal fibers having larger negative dispersion coefficient as well as compensating for dispersion slope of SMFs in the entire C band with large effective area can be designed.

Polarization-independent optical directional coupler based on slot waveguides

Abstract: 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 polarization diversity schemes.

Design and Fabrication of 10-/40-Gb/s, Uncooled Electroabsorption Modulator Integrated DFB Laser With Butt-Joint Structure

Abstract: This paper describes the wide temperature range operation of an electroabsorption modulator (EAM) integrated with DFB laser diodes (LDs) (EML) designed to reduce the power consumption and size of optical transmitters. We optimized the multiquantum wells (MQWs) for LD and EAM separately to realize uncooled operation. We employed a conduction band offset (?Ec) of around 250 meV for the LD and 150 meV for the EAM. The number of well layers was set at 6 for the LD and 12 for the EAM, respectively. We fabricated the EML using a butt-joint (BJ) process to allow us to design the LD and the EAM independently. We introduced a ridge waveguide structure for the LD and EAM waveguides, and designed the width of the LD and EAM mesa to achieve a high optical coupling efficiency between the LD and the EAM. We then used the 200-? m-long EAM for 10-Gb/s operation and the 150- ?m-long EAM for 40-Gb/s operation, and thus obtained a dynamic extinction ratio of over 9 dB at 10 Gb/s from -25 °C to 100 °C and of 8.2 dB at 40 Gb/s from -15 to 80°C. We achieved a power penalty of less than 2 dB after an 80-km single-mode fiber (SMF) transmission at 10 Gb/s and a 2-km SMF transmission at 40 Gb/s over a wide temperature range. These results confirm the suitability of this EML with a BJ structure for use as a 10-Gb/s or 40-Gb/s uncooled light source.

Supercontinuum generation in photonic crystal fiber

Abstract: A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Nonlinear silicon-on-insulator waveguides for all-optical signal processing

Abstract: Values up to gamma=7 x 10(6)/(W km) for the nonlinear parameter are feasible if silicon-on-insulator based strip and slot waveguides are properly designed This is more than three orders of magnitude larger than for state-of-the-art highly nonlinear fibers, and it enables ultrafast all-optical signal processing with nonresonant compact devices At lambda=155 microm we provide universal design curves for strip and slot waveguides which are covered with different linear and nonlinear materials, and we calculate the resulting maximum gamma

Fundamentals of optical waveguides

Abstract: Preface 1. Wave Theory of Optical Waveguides 2. Planar Optical Waveguides 3. Optical Fibers 4. Couple Mode Theory 5. Nonlinear Optical Effects in Optical Fibers 6. Finite Element Method 7. Beam Propagation Method 8. Staircase Concatention Method 9. Planar Lightwave Circuits 10. Theorems and Formulas Appendix

Optical sensing by optimized silicon slot waveguides

Abstract: A theoretical investigation of silicon-on-insulator nanometer slot waveguides for highly sensitive and compact chemical and biochemical integrated optical sensing is proposed. Slot guiding structures enabling high optical confinement in a low-index very small region are demonstrated to be very sensitive to either cover medium refractive index change or deposited receptor layer thickness increase. Modal and confinement properties of slot waveguides have been investigated, considering also the influence of fabrication tolerances. Waveguide sensitivity has been calculated and compared with that exhibited by other silicon nanometer guiding structures, such as rib or wire waveguides, or with experimental values in literature.