A method and a device in optical fiber based spectral filtering. A length of an optical fiber including at least a core region surrounded by a cladding region is coiled over its length in whole or in part to subject the fiber to longitudinal curvature in order to affect the optical transmission properties of the fiber. The fiber is arranged to have radially asymmetric refractive index distribution and in addition to coiling the fiber lengthwise, the fiber is over its length in whole or in part also twisted around its longitudinal axis. The method and device can be used to significantly improve the performance of fiber based filtering devices.