[OPTICAL REVIEW Vol. 6, No. 3 (1999) 173-179]
Characteristics of Fused Optical Fiber Grating Couplers with Tapered Shapes
Hirohisa YOKOTA, Takahiro HASEGAWA, Yoshitaka SATODA, Eiichi SUGAI and Yutaka SASAKI
Department of Systems Engineering, Faculty of Engineering, Ibaraki University, 4-12-1, Nakanarusawa-cho, Hitachi, Ibaraki, 316-8511 Japan
(Received November 19, 1998; Accepted January 22, 1999)
Fused optical fiber grating couplers (FGCs), in which Bragg gratings are written in the coupling regions of fused optical fiber couplers with tapered shapes along them are theoretically and experimentally investigated. By coupled mode analysis considering these tapered shapes, the drop efficiencies of the FGCs, which are inherently zero for the normal fiber couplers without grating, are calculated. The grating offset, which is the difference between the centers of grating and the coupler taper, is found to be effective to increase the drop efficiency of the FGC. It is determined that the tapered shapes influence the wavelength dependence of the drop efficiency and tolerate the grating offset precision. Apodizations for the grating of FGC are found to be effective in suppressing side lobes of the drop efficiency. Measurement of the wavelength characteristics of drop efficiency and the temperature characteristics of drop wavelength of the fabricated fused FGC confirm that the lightwave with specific wavelength can be dropped by the FGC and that the temperature dependence of the drop wavelength is similar to that of Bragg wavelength changes of usual fiber Bragg gratings.
Key words : optical fiber coupler, fiber Bragg grating, add-drop multiplexer, WDM transmission system, coupled mode analysis