a broadband tunable in - fiber filter includes a grating with divergent ridges which can be translated transversely of a side - polished optical fiber to vary the periodicity at an exposed evanescent field . the divergence is gradual so that at any given transverse position of the grating , the ridge interacting with the evanescent field are effectively parallel . the divergence is great enough so that a tuning - to - reflected bandwidth ratio of about 33 : 1 is demonstrated . the grating is fabricated in an amorphous silicon film on a fused quartz substrate . the film is coated with photoresist which is exposed to a holographic interference front . the substrate is tilted with respect to an interference front created by two spherically diverging beams to achieve the desired divergence . subsequent processing , including etching are standard .