Hidehiko Yoda

Beam quality factor of higher order modes in a step-index fiber

The beam quality factor (or M2-parameter) for linearly polarized (LP)-modes of a step-index fiber is calculated in a closed form, as a function of the fiber V-number. It is shown that M2 sharply peaks for all fiber modes when they are close to cutoff. Particularly simple expressions are derived in the limit Vrarrinfin. Two practically important coherent superpositions of modes are considered for which the degree of degradation of the beam quality due to the higher order mode content is calculated. The reported results can be useful for designing large-core high-power fiber lasers, amplifiers, and fiber-based beam delivery systems, when preservation of the spatial beam quality is important

A silicon-based spot-size converter between single-mode fibers and Si-wire waveguides using cascaded tapers

Theoretical and experimental performances of a robust Si-based spot-size converter (SSC) for efficient coupling between a single-mode fiber and a photonic Si-wire waveguide are reported. The SSC is comprised of cascaded horizontal linear and vertical nonlinear up tapers measured 2.0mm and 86μm in length, respectively, in a silicon-on-insulator substrate. The fine modal field of a Si-wire waveguide, 0.54×0.38μm2 in diameters, was expanded to diameters of 5.1×9.2μm2 with measured net transmission loss of 0.5dB at the wavelength of 1.55μm by the miniature SSC.Theoretical and experimental performances of a robust Si-based spot-size converter (SSC) for efficient coupling between a single-mode fiber and a photonic Si-wire waveguide are reported. The SSC is comprised of cascaded horizontal linear and vertical nonlinear up tapers measured 2.0mm and 86μm in length, respectively, in a silicon-on-insulator substrate. The fine modal field of a Si-wire waveguide, 0.54×0.38μm2 in diameters, was expanded to diameters of 5.1×9.2μm2 with measured net transmission loss of 0.5dB at the wavelength of 1.55μm by the miniature SSC.

A new scheme of a lensed fiber employing a wedge-shaped graded-index fiber tip for the coupling between high-power laser diodes and single-mode fibers

A novel lensed fiber employing a wedge-shaped graded-index fiber (GIF) is proposed. The characteristic of coupling loss as a function of the aspect ratio is numerically investigated. When the aspect ratio is seven, the coupling loss of a wedge-shaped GIF and a conventional wedge-shaped SMF are 0.22 dB and 0.60 dB, respectively, and when the aspect ratio is 13, they are 0.5 dB and 2.4 dB. The tolerance to laser diode (LD)-fiber displacement is also numerically investigated, and it is confirmed that the proposed fiber has larger tolerance than a conventional wedge-shaped single-mode fiber (SMF). The performance of the new lensed fiber is also experimentally verified.

Cascaded GI-fiber chips with a wedge-shaped end for the coupling between an SMF and a high-power LD with large astigmatism

A novel lensed fiber for high-efficiency coupling between a single-mode fiber (SMF) and a high-power laser diode (LD) with large astigmatism of 176 /spl mu/m is proposed and demonstrated. We experimentally confirmed that the proposed lensed fiber had a minimum coupling loss of 2.5 dB including a reflection loss of 0.3 dB at both ends of the fiber. In addition the new configuration had a long working distance of 67 /spl mu/m. We also investigated the tolerance characteristics for various displacements.

Blue-light emission from sputtered Ti:SiO2 films without annealing

Blue-light emission from Ti:SiO2 sputtered films was observed at room temperature without annealing and it could be seen by the naked eye. The peaks of photoluminescence spectra were located at 3.03–3.05 eV and full width at half maximum ranged from 0.38–0.40 eV, which were almost the same in samples having different energies of absorption edge. SiOx layers may contribute to emission, which are interfacial regions between Ti particles and surrounding SiO2 medium, and the size of Ti particles may affect the efficiency of emission.

A lensed GIO fiber with a long working distance for the coupling between laser diodes with elliptical fields and single-mode fibers

The experimental performance of a new lensed fiber employing a graded-index oval-core fiber is presented for the first time. The new scheme has a long working distance and a high coupling efficiency compared with conventional wedge-shaped lensed fibers. Experiments show that the coupling loss between a laser diode, operating at a wavelength of 980 nm with a field profile aspect ratio of 5.9, and single-mode fiber is as low as 1.3 dB with the long working distance of 30 /spl mu/m. Measured tolerance characteristics against lateral displacement and tilt are acceptable for practical active alignment.

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.

Single-mode fiber with a plano-convex silicon microlens for an integrated butt-coupling scheme

A single-mode fiber with plano-convex silicon microlens is proposed for butt-coupling between single-mode fibers and high-index-contrast waveguides with fine mode-field diameter. The lensed fiber has two specific features, high focusing power, and null working distance. The theoretical focus-spot diameter at the endface of the lensed fiber is calculated to be as small as 0.56 microm at a wavelength 1.55 microm. A simple fabrication method for the lensed fiber employing chemical etching and rf-sputtering is presented. Experiments showed that the mode-field-diameter of a single-mode fiber was successfully contracted from 10.5 to 1.3 microm with the lensed fiber.

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.