Hisaharu Yanagawa

Index-and-dimensional taper and its application to photonic devices

A mode-field-converting optical channel waveguide with tapered refractive index and cross-sectional dimensions is proposed. In this waveguide, both the refractive index and cross-sectional dimensions of a core are adiabatically varied along a propagation direction in such a way that the normalized frequency V of the waveguide may be kept constant. Since radiation loss caused by waveguide imperfections is strongly dependent on the V value, the waveguide has good mode-field-converting capability without causing high radiation loss. Layer thickness and index of a core of a silica waveguide were tapered by modulating a raw material gas flow in a flame hydrolysis deposition method, and a channel width taper was formed by a photolithographical technique. In order to confirm the usefulness, taper waveguides have been used (1) to achieve good field matching between a fiber and a guided-wave optical switch, and (2) to fabricate a fiber-compatible monolithic 32*32 star coupler in a small chip. >

Polarization- and wavelength-insensitive guided-wave optical switch with semiconductor Y junction

A polarization- and wavelength-insensitive semiconductor guided-wave optical switch with a Y junction is proposed. The switch exhibits a digital response with respect to current, allowing its use as a wavelength-insensitive 1*2 optical switch. The switching characteristics are analyzed by using the beam propagation method, and a design example is given. The polarization- and wavelength-insensitive switching operation has been confirmed with a fabricated GaAs/GaAlAs switch at wavelengths of 1.3 and 1.55 mu m, and the on/off ratio exceeded 20 dB at an injection current of 250 mA at these wavelengths. >

Filter-embedded design and its applications to passive components

A design in which a filter is embedded in a fiber fixed on a substrate is proposed. It was successfully applied to the fabrication of low loss fiber optic components for a single-mode fiber transmission system. These components have a wide operational temperature range. The design is easily adapted to a multifilter or multifiber component. Filter embedding is achieved in two ways. One is to insert a thin filter chip in a slit which is machined into a fiber fixed on a substrate by a dicing saw (filter inserted type). The other is to evaporate a filter film directly on a fiber end face and butt this fiber to another fiber on a guide groove on a substrate (filter evaporated type). >

Broad-band high-silica optical waveguide star coupler with asymmetric directional couplers

A broadband optical waveguide star coupler with asymmetric directional couplers is proposed. A coupling or splitting ratio of the coupler is flattened with respect to wavelength by choosing appropriate values for Delta beta and complete power transfer length. The wavelength characteristics of uniform and alternating Delta theta b directional couplers are investigated, and their broadband operation is shown theoretically. To demonstrate the feasibility of the new proposal, 8*8 star couplers composed of such asymmetric directional couplers are fabricated by using high-silica waveguides on a silicon chip. A star coupler composed of asymmetric directional couplers exhibited 2-dB loss variation in the wavelength range of 1.3-1.55 mu m, and this value was almost one fourth of that of a star coupler composed of symmetric directional couplers. >

High-extinction directional coupler switches by compensation and elimination methods

A conventional guided-wave optical directional coupler switch always presents a low-extinction switching property less than 30 dB. One main reason is that coupling occurs between input and output lead regions of the coupler, causing low extinction in the through switching state despite the coupling strength being very weak. Another reason may be due to the asymmetric structural designs of couplers in optical integrating circuits, making low extinction result in the cross switching state. In this paper, we propose two methods which can solve this crosstalk problem easily. One is by the compensation technique in which we design an optimum structural construction for the reversal /spl Delta//spl beta/-directional coupler switch, and for which theoretical prediction shows that infinity extinction can be achieved where the fabricated switch element gave above 36 dB extinction ratio for both the through and cross switching states. In another method, coupling effect between lead waveguides are eliminated by means of refractive index control of the waveguides. Theoretical prediction shows that if the change of refractive index /spl delta/n is greater than 3/spl times/10/sup -3/, over 60 dB extinction can result, where the fabricated switch element also gave more than 38 dB extinction in both the through and cross switching states. Fabrication error, existence of propagation loss, and stray lights due to scattering may be the causes of difference between predicted and experimental values. However, both proposed methods show evidence that it is possible to obtain an extinction ratio as high as nearly 40 dB. >

Fused taper/filter hybrid three-channel single-mode wavelength division multi/demultiplexer

The dominant choice of wavelengths in wavelength division multiplexing has been so far 1.3 and 1.55 μm, because these two bands correspond to the low-loss and wideband (electrical) regions of a single-mode fiber in the optical spectrum. Consequently, such interband wavelength division multi/demultiplexers (WDMs) have been developed by using a fused taper coupler technique or an interference filter technique,1-3 and they are widely available now. To increase the number of channels further, however, these two techniques encounter very difficult problems, if not insoluble, in terms of channel spacing in the case of a fused taper WDM or insertion loss in the case of a filter WDM.

Polarization and Wavelength Independent Four-Port Optical Routing Switch with Semiconductor Y-Junction Optical Cross-Points

We propose a polarization and wavelength independent four-port routing switch with semiconductor Y-junction optical switching cross-points, and report the performance of the fabricated routing switch.