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Optical Signal Is Coupled Out Using Fiber Grating

The development of high speed optical telecommunication systems has been driven by a need for bandwidth. Currently, the main focus is on getting fiber into metro and access networks and to individual residences (fiber-to-the-home, FTTH). In particular, for FTTH to succeed, the cost of any optical communication systems to be installed must be reasonable. Small and cheap optical devices, such as receivers and transmitters, are therefore required. Fiber gratings are well-suited to this purpose: they have many attractive features that make them viable alternatives to conventional optical devices. In particular, their small size makes them ideal for integrated optics, and their wavelength-selective nature makes them a good match for wavelength-division-multiplexed communications systems.

Our goal was to design a fiber grating that could be use to replace optics for signal out-coupling. We noted that tilted fiber Bragg gratings (TFBGs) are good for extracting light from the tightly-confined core mode into the cladding (less-confined) and/or radiation (free-space) modes. This property has been used to sense the core-mode power: by directly coupling the core mode to the radiation mode, in-line fiber spectrometers1¨C3 and monitors4 have been implemented. However, these have mainly had coupling efficiencies in the 0.1??3% range: unsuitable for high sensitivity optical receivers.

To overcome this problem, we etch the cladding. As a result, we take advantage of two kinds of coupling processes. First, the strong core-to-cladding mode coupling is forced by the TFBGs. Then, a highly efficient out-coupling of the cladding mode through the v-groove structure takes place. Unfortunately, the core-to-cladding mode coupling creates an undesirable crosstalk effect that results in some degradation of to the optical communication,5 but so far this cannot be avoided.6
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