Optical filters have numerous applications in various optical systems due to their wavelength based discriminative transmission losses. These applications include detection of an emitted weak signal while avoiding strong excitation signal in any spectroscopy system, laser beam diagnostics, and tailoring the spectrum of a broadband light source etc in different sectors such as astrophysics, chemical analysis, imaging systems, communication, aerospace, and optics labs etc. A fiber based filter is highly desirable as it can be easily spliced to other fibers leading to an integrated optical systemthanks to the low splice losses. Several fiber based filters have been demonstrated based on birefringence , tapering , dispersion , and absorption [4-5] etc. The most important characteristic of a filter is a steep absorption edge with a low insertion loss, therefore it must offer a very high absorption in attenuated band compared to transmission band. The rare-earth absorption based fiber filters can offer very high absorptions with steep edge. Moreover, they are superior in quality than others as they can afford to handle much higher power thanks to the long length of the fiber. A first demonstration of fiber filter based on holmium doped silica fiber was demonstrated by Farries et al. .
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