Hiroshi Ito

Light-induced self-written three-dimensional optical waveguide

Three-dimensional (3D) optical waveguides were fabricated in a photopolymerizing resin mixture solution by using a multimode optical fiber, without any moving parts. The core portion has formed by the selective photopolymerization of a higher refractive index monomer by Ar+ laser irradiation through the optical fiber. A continuous, straight waveguide was grown by the self-trapping of a guided laser beam. We demonstrated automatic 3D optical circuit formation that enables regrowth after passing through thick transparent glass plates. This growth mechanism also enables automation of the optical interconnection and packaging process, and could potentially contribute to future expansion of optical fiber communications networks.

Waveguide shape control and loss properties of light-induced self-written (LISW) optical waveguides

We show the feasibility of automatic waveguide formation by means of the self-trapping effect of multimode optical fiber irradiation into a photopolymerizing resin. By using a graded-index multimode optical fiber, we experimentally obtained a straight waveguide of over 20 mm in length. It is shown that its growth properties, such as waveguide shape and diameter, depend on the propagation modal distribution along the optical fiber used for the waveguide formation. Moreover, an all-solid polymer optical waveguide that relies on the selective photopolymerization proceeding into the photopolymerizing resin mixture is also demonstrated. We call this type of waveguide a light-induced self-written optical waveguide. The measured propagation loss in the waveguide is 1.0 dB/cm or less for the wavelength range 0.8/spl sim/1.0 /spl mu/m. The proposed technology is capable of eliminating both costly lenses and the need for an alignment system from optical waveguide devices.

Mode selective polymer channel waveguide defined by the photoinduced change in birefringence

A transverse electric (TE) mode selective polymer waveguide was fabricated by a photobleaching process using a urethane–urea copolymer. The output power of the TE modes through the waveguide was higher than that of the transverse magnetic (TM) modes, when the TE and TM modes were excited equally, the TE mode to TM mode extinction ratio being 27 dB. Mode selectivity was explained by the following relation of the refractive index: the refractive index for the TE modes in the core region was higher than that in the cladding region, while the refractive index for the TM modes in the core was lower than that in the cladding. This relation was attained by the photoinduced change in birefringence of the polymer layer.

Laser emission from a solar-pumped fiber

We report the realization of a solar-pumped fiber laser (SPFL) using a double-clad (a center core/ an inner clad working also as optical waveguide/ an outer clad) Nd-doped fluoride optical fiber as a laser medium. With a compact off-axis parabolic mirror of 5 cm in aperture diameter, the natural sunlight is concentrated by a factor 10⁴, and introduced partly into the core of the fiber and partly into the inner clad in which the light is guided in some distance and transferred to the core after all. We have obtained clear laser spectrum characteristics with approximately 0.01 nm full-width-half-maximum of the laser line at the peak wavelength of 1053.7 nm, a low-lasing threshold of 49.1 mW, a slope efficiency of 6.6%, and a total efficiency of 1.76%. Further optimization of the medium properties, optical cavity, and concentration technique will yield higher efficiency and lower threshold.

Fabrication of large-core, high-Δ optical waveguides in polymers

The realization of polymer optical waveguides that have a large core size and high refractive-index difference (LCHD) Δ transmission characteristics is presented. A fabrication procedure for the waveguide based on vertical dip coating and reactive ion etching has been studied. To achieve the lower propagation loss, this procedure includes two original techniques, i.e., the lamination of thick polymer films and sidewall flattening. With these techniques, Δ of 5.4% and a 80 µm × 83 µm core polymer waveguide with 1.4-dB/cm propagation loss were achieved at 680 nm. The LCHD polymer waveguides are useful for practical power-transmission devices.

Thermal conversion and hydrogenation effects in AlGaAs

Thermal conversion of molecular‐beam epitaxially (MBE) grown AlGaAs from semi‐insulating to p‐type caused by annealing has been studied using Hall‐effect and photoluminescence measurements. The presence of C impurities causes thermal conversion of the AlGaAs layers. A key observation is that residual C impurities can be greatly reduced by increasing growth temperature. Thermodynamic evaluation of adsorption kinetics suggests that C incorporation during AlGaAs MBE is chemisorptive. The effects of hydrogenation in MBE AlGaAs have also been studied and indicate that thermally activated C acceptors and nonradiative recombination centers can be passivated with this treatment.

Simultaneous fabrication of optical channel waveguides and out-of-plane branching mirrors from a polymeric slab structure

Recently, polymeric multimode optical waveguides have been widely studied for low-cost interconnection use. We describe fabrication processes for a low-loss poly(methyl methacrylate) (less than 0.1 dB/cm at 675 nm) waveguide based on the reactive-ion-etching technique and for a slope (45 degrees mirror face) formed on the waveguide. To obtain a ridge core with a crack-free and extremely smooth surface, we applied a heating process at a temperature greater than the glass transition point and a smoothening process by solvent. Furthermore, to fabricate simultaneously both vertical and sloped sidewalls, we applied a unique phenomenon in the process that decreases the etching rate directly under a narrow opening of etching mask. By using the above fabrication techniques, we demonstrated an out-of-plane branching mirror.

Silicon photovoltaic cells coupled with solar-pumped fiber lasers emitting at 1064 nm

We have designed silicon (Si) photovoltaic (PV) cells coupled with solar-pumped fiber lasers (SPFLs), by considering the unique illumination conditions for PV cells: (1) monochromatic (1064 nm) and normal incidence, (2) large intensity, and (3) small area. We have revealed that a multilayered bandpass filter formed on the front surface of a PV cell equipped with a diffuse reflector on the back surface sufficiently traps the illumination. This, in turn, allows us to use a thin Si wafer to eliminate the detrimental effect of the series resistance in the cell under intense illumination, along with a small lateral size that is suitable for direct connection with an SPFL. Simulated conversion efficiency of the newly designed 20 μm-thick Si PV cell with the 14-layerd bandpass filter and Lambertian back reflector is 61% under normally incident 1064 nm illumination at 1 kW/cm2, which is twice as high as that of a conventional surface-textured Si solar cell of 75 μm in thickness.

Spectroscopic investigation of Nd 3+ -doped ZBLAN glass for solar-pumped lasers

We evaluated the optical properties required for the design of a solar-pumped laser for Nd3+-doped ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass. The quantum efficiency (QE) of near-infrared emission from the 4 F3/2 state of Nd3+ using sunlight as an excitation source was 70%. The product of the stimulated emission cross section and the radiative lifetime (σseτr) of the 4F3/2→4I11/2 was 1.5×10−23 cm2·s. The integrated absorption strength in the 450–900 nm region was the largest among Nd3+-doped fluoride glasses. The high QE, large σseτr product, and large integrated absorption strength indicate that Nd3+-doped ZBLAN is one of the most promising materials for solar-pumped lasers.

Polymer optical waveguide devices for low-cost WDM module

In order to simultaneously realize high-speed data transmission, low-cost and lightweight network in automotive applications, a 3(lambda) WDM has been considered to be a suitable solution. We have developed two types of low-cost WDM modules using a polymer optical circuit. As the first method, we developed a planar optical circuit for interconnection between an optical fiber and an LD/PD chip using a PMMA waveguide having out-of-plane branching mirrors, fabricated simultaneously with a waveguide from a polymer film, using a newly-developed temperature-controlled RIE (Reactive Ion Etching) technique. The waveguide has a large diameter (100 x 100micrometers ) and a high-(Delta) (NA=0.5) structure. Transmission losses were 0.1 dB/cm (660nm), and 0.3 dB/cm (1320nm). As the second method, we employed a light-induced waveguide technology for the WDM module fabrication. When a GI optical fiber is soaked in two kinds of light-hardening resin blends in which the hardening wavelength differs ((lambda) 1>(lambda) 2) , the resin is irradiated with (lambda) 1 light through the fiber, a hardened polymer waveguide having uniform diameter geometry would grow from the fiber tip. This method does not need any fiber alignment and packaging process; therefore, cost-effective module fabrication can be expected. This technique utilizes the self-focusing phenomenon of the optical fibers outgoing beam by a refractive index increase in the material hardening process. We could also confirm two key component formations necessary for the WDM module: regrowth of the waveguide form the backside of a WDM filter after passing through it, and a 90 degree(s) reflected waveguide using a 45 degree(s) mirror.