Kazuo Shiraishi

A Two-Port Single-Mode Fiber–Silicon Wire Waveguide Coupler Module Using Spot-Size Converters

In this paper, the performance of a two-port single-mode fiber-silicon wire waveguide coupler module which utilizes an identical spot-size converter (SSC) at the input and output ports is reported. Each of the silicon (Si)-based SSCs comprised cascaded horizontal linear and vertical nonlinear up-tapers measured 300 and 200 mum in length, respectively, in a common silicon-on-insulator (SOI) substrate. The structural parameters of the tapers were designed for compactness and relaxed tolerance to fabrication errors. The total length of the two-port coupler module was 1000 mum plus the variable length of the wire waveguide connecting the two SSCs. The mode-field diameter (MFD) of the Si-wire waveguide, 0.32 times 0.46 mum2, was transformed to the diameter of 2.8 times 8.0 mum2 at the wavelength of 1.55 mum (corresponding to an area expansion of about 150 times) and vice versa by the SSCs with a net transmission loss of 4.1 dB/port. The field-mismatch loss between the SSC and the single-mode fiber with the MFD of 5.2 mum was 2.1 dB/port.

A high-performance micro-GRIN-chip spot-size converter formed with focused ion beam

A high-performance spot-size converter composed of a micrograded-index slab chip with a hemi-cylindrical endface for conversion between single-mode fibers (SMFs) and fine-core waveguides has been fabricated using the focused ion beam etching technique. The microchip has the index difference as much as 1.8 in the index-gradient direction and the input hemi-cylindrical endface of curvature radius as small as 6.3 /spl mu/m. The measured full width at the 1/e/sup 2/ maximum of the resulting focused spot intensity distribution is 1.8/spl times/2.5 /spl mu/m/sup 2/ when the input end is excited by an SMF of 10.8-/spl mu/m-spot diameter at the wavelength of 1.55 /spl mu/m.

A two-port polarization-insensitive coupler module between single-mode fiber and silicon-wire waveguide.

A two-port polarization-insensitive single-mode fiber-silicon wire-waveguide coupler module, 5.3 × 3.4 × 0.7 mm(3) in size, is realized. The spot-size converter (SSC) involved utilizes a concatenated horizontal up-taper and vertical down-taper. Measured coupling losses between the fiber and the silicon-wire waveguide of the E(11)(y) and E(11)(x) modes of the SSC are 2.8 and 2.7 dB/port, respectively. The device platform is planar, robust, and easy to fabricate with conventional lithography.

High-Index-Layer Coating on a Lensed Fiber Endface for Enhanced Focusing Power

A new scheme for lensed fibers with a high focusing power is proposed. The new lensed fiber has a high-index layer coated (HILC) on a convex-shaped endface of a single-mode fiber (SMF). Numerical investigation shows that, by coating a 10-mum-thick film with refractive index of 3.7, the lensed fiber can contract the beam diameter of the propagating mode from 10.4 mum at the wavelength of 1.55 mum, to 1.2 mum. The enhanced focusing effect of the lensed fiber has been confirmed experimentally by coating an a-Si:H film on the lensed endface. The measured focused spot diameter was as small as 1.6 mum, much the same as the wavelength.

Reduction of light reflection at silicon-plate surfaces by means of subwavelength gratings in terahertz region

We investigate triangular surface-relief gratings for reducing reflection at a planar silicon surface for light in the terahertz frequency region of 0.3-3.0THz. Structural parameters of the one- and two-dimensional (1D 2D) subwavelength gratings required for the antireflection (AR) layer were obtained numerically. Experimental results for the AR effects agreed well with those obtained numerically, except for gratings fabricated with laser ablation, which causes structural fluctuations of the grating. In the 1D grating, a high transmittance exceeding 90% for the TM wave was obtained. 2D gratings comprised of arrayed micropyramids were experimentally confirmed to be polarization insensitive.

Infrared polarizer employing multiple metal-film subwavelength gratings

A multiple thin metal-film subwavelength grating is proposed for polarizers in the infrared wavelength region of 10-20 μm. The dependence of the transmission characteristics of the polarizers on structural parameters was obtained numerically, and the potential for high performance was confirmed experimentally. The measured TE-wave losses in a polarizer comprising a triangular triple Al-film grating are more than 45 and 35 dB for the wavelength ranges of 10-16 and 16-20 μm, respectively, while the net TM-wave losses are lower than 1.5 dB in the wavelength rage of 15-20 μm.

Triangular surface-relief grating for reduction of reflection from silicon surface in the 0.1–3 terahertz region

A triangular surface-relief grating is proposed to reduce reflection from a silicon surface for the light in the terahertz frequency region. Optimum structural parameters of the grating have been numerically determined and confirmed experimentally.

Vertical-tunneling spot-size converters for silicon-wire waveguides

A new spot-size converter utilizing tunneling phenomenon in the vertical direction for coupling between a silicon-wire waveguide and a single-mode fiber is proposed. Numerical calculations predict a miniaturized spot-size converter shorter than 50μm.

High Focusing Power Lensed Fibers Employing Graded-Index Fiber With Eigen-Beam Diameter

Two advanced structures of lensed fiber each employing a graded-index fiber with eigen-beam diameter (EB-GIF) to provide superior focusing power are reported. The high-index-layer (a-Si:H)-coated lensed fiber and the plano-convex lensed fiber focus the 10.4-μm mode-field diameter of a standard single-mode fiber to 2.7 and 1.1 μm , respectively, even in oil with refractive index of 1.501 at the wavelength of 1.55 μm with the corresponding transmission losses as low as 0.78 and 0.08 dB, respectively. Thus, the new lensed-fiber structures would facilitate efficient coupling between the standard single-mode fibers and the high-index-contrast silicon-oxynitride or the super high-index-contrast silicon-rib waveguides. Employment of the EB-GIF not only significantly simplifies fabrication of the lensed fibers, but also greatly improves their reproducibility.

A Lensed Fiber for Butt Coupling Between High-Index Contrast Waveguides and Single-Mode Fibers

A lensed fiber with a convex-piano silicon microlens is proposed for butt-coupling between standard single-mode fibers and high-index contrast waveguides with fine mode-field diameters. The theoretical focused spot diameter is 0.56 μm at the wavelength 1.55 μm.