Coupling characteristics between two curved waveguides are studied using a two-core fiber. A tunable monochromatic light source is used to measure the wavelength dependence of the coupling efficiency. Measured values of the coupling efficiency agree well with theory. It is found that (1) the coupling efficiency can be changed from 0 to 1 by changing the radius of curvature, for example, from 100 mm to infinity in the measured coupler; and (2) when phase shift due to curvature sbeta/2R exceeds the coupling coefficient K, where s is the center-to-center core separation, beta the propagation constant, and R the radius of curvature, coupling between the curved cores does not occur, and there is only the phase shift between the cores.
We developed a new optical add/drop multiplexer composed only of a grating-folded directional coupler. We found that in this multiplexer, the power drop becomes maximal when the grating-coupling strength is equal to that of directional coupling. At this maximum power, both power transfer between the waveguides and power reflection take place simultaneously at the grating. The power drop approaches 1 when the grating length increases.
Embedded single and multimode waveguides on fused quartz substrates have been fabricated by high-speed silica deposition and accurate reactive sputter etching by a C2F6 gas plasma. These waveguides have rectangular cross sections with an aspect ratio close to unity. Total waveguide loss was approximately 5 dB/cm, and absorption loss measured by laser calorimetry at 637-nm wavelength was less than 0.1 dB/cm.
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