TUNABLE, MECHANICALLY INDUCED LONG-PERIOD FIBER GRATING WITH ENHANCED POLARIZING CHARACTERISTICS

摘要

A new type of fiber filter is useable in optical communication systems. In particular, the fiber filter may be used to flatten the gain of erbium-doped fiber amplifiers (EDFAs). Such gain flattening is important for long-haul, dense (wavelength dependent multiplexed) WDM communication systems. The filter includes a periodic mechanical structure pressed against the side of a singlemode fiber to induce a wavelength-dependent loss in a signal propagating in the fiber core by coupling the signal to fiber cladding modes. The mechanical structure is a periodic comb of small ridges. Each ridge induces a local index change in the fiber via the photoelastic effect. For coupling to the right cladding modes, the period of the grating (and the comb) is in the range of few hundreds of microns. Thus, the grating is easy to fabricate with standard machining equipment. Such filters show low loss (e.g., <0.5 dB) and exhibit the type of filter functions required to flatten the gain of an EDFA. Compared to known photosensitive long-period fiber gratings, which are currently in favor for this application, the filter is simpler and faster to fabricate. The filter does not require capital intensive UV lasers. The filter is adjustable and can be removed easily. The filter is potentially easy to mass-produce. In certain embodiments, the polarization effects caused by asymmetric application of force to polarized light are reduced or eliminated by utilizing a groove to receive the fiber. In alternative embodiments, the polarization effects are enhanced to provide an all-fiber polarizer.