Masanori Koshiba

Hole-assisted lightguide fiber for large anomalous dispersion and low optical loss

Hole-assisted lightguide fiber (HALF) is a microstructured fiber comprising a material index profile for waveguiding and air holes for modifying optical properties. Anomalous dispersion larger than those of the conventional fibers can be realized without severe degradation in optical loss, because of low power fraction in the holes and structural simplicity. We investigate into the causes of the loss of the fabricated HALFs, and show that a GeO2-doped core, in addition to the low power fraction, is desirable for low loss. The fabricated HALF exhibits a loss as low as 0.41 dB/km and a large anomalous dispersion of +35 ps/nm/km at 1550 nm.

Hole-Assisted Lightguide Fiber - A Practical Derivative of Photonic Crystal Fiber

Usage of air holes in optical fibers has become a hot subject in fiber optics because of the possibilities for novel transmission properties. Although photonic crystal fibers based on photonic bandgap guidance are the most drastic innovation in this subject, optical fibers containing air holes but not having photonic crystal structures are also being intensively studied. Such air-silica microstructured fibers are more practical than the photonic bandgap fibers because the lack of photonic crystal structure makes the fabrication far easier. Even without the photonic bandgap, the microstructured fibers can exhibit valuable properties in terms of group velocity dispersion and nonlinearity, because the index contrast between air and silica is 10 or more times as large as that of the conventional optical fibers based on doped silica glasses. However, one of the major challenges for practical applications of the air-silica microstructured fibers has been their high transmission losses, which have been several tens to hundreds times higher than those of the conventional fibers. As a solution to this problem, we have proposed a more practical structure called hole-assisted lightguide fiber (HALF). In addition to the air holes for realizing novel optical properties, this structure has a material index profile for waveguiding, and hence is closer to the conventional fibers than the other microstructured fibers are. As a result, novel optical properties can be realized without severe degradation in transmission loss. In experiments, an anomalous group velocity dispersion as large as +35 ps/nm/km at 1550 nm wavelength, which would be unattainable in the conventional fibers, has been realized with a loss of 0.41 dB/km, which is comparable to those of the conventional fibers. Analyses of the losses of the fabricated HALFs suggest that the loss should be lowered by mitigating the effect of the drawing tension and minimizing the power fraction in the holes. It is also shown that the full-vector finite element method realizes accurate modeling of the properties such as dispersion and macrobend loss.

Design of Ultra-Small Polarization Splitter Based on Silicon Wire Waveguides

We propose a novel design of ultra-small polarization splitter based on silicon wire waveguides. Numerical simulations show that a 12-μm-long polarization splitter with the extinction ratio better than -20 dB in entire C-band is achievable.

Highly-Nonlinear Horizontal Slot Waveguideswith Low and Flat Dispersion

We present an optimum design of highly-nonlinear horizontal slot waveguides with flat dispersion characteristics. Numerical simulations show that 6000 W-1m-1nonlinear coefficient and flat dispersion on a 260-nm bandwidth can be achieved.

Three-Dimensional Vector Finite Element Analysis of Leakage Losses in One-Dimensional Photonic Crystal Coupled Resonator Optical Waveguides

We evaluated leakage losses of one-dimensional photonic-crystal coupled-resonator-optical waveguides (1-D PC-CROW). We show design methods of low-loss 1-D PC-CROW and leakage losses of proposed structure are one order of magnitude lower than normal 1-D PC-CROW.

Design of Ultra-Small Polarization Splitter Based on a Resonant Coupling between Silicon Wire and Slot Waveguides

We propose an ultra-small polarization splitter based on a resonant tunneling phenomenon. Numerical simulations show that a 16-?m-long polarization splitter with extinction ratio better than -20 dB on the entire C-band is achievable.

Optical Characterization of Planar Photonic Crystal Structures with Elliptically-Elongated Veins: A Generalized Fourier-Mathieu Multipole Expansion Technique

An efficient analytical approach based on an extension of the scattering-matrix technique is presented for characterizing planar photonic crystal resonant-cavities composed of elliptical veins. Extensive numerical results for various lattices are presented.

Numerical Investigation of Raman Amplification Properties in Photonic Crystal Fibers

A full-vectorial finite-element method is used to investigate Raman amplification properties of photonic crystal fibers. Raman gain of 9 dB is obtained in 4-km length of PCF with a high optical signal-to-noise ratio.

Finite element design of single-polarization single-mode photonic crystal fibers

A new structure of single-polarization single-mode photonic crystal fiber (PCF) is proposed and designed through a full-vector finite element method. The PCF propagates only the slow-axis mode within the wavelengths ranging from 1500nm to 1600nm.