A tunable optical filter is formed by the longitudinal alignment of two opposing end sections of single-mode optical fibers. On at least one of the end sections is a collimator fiber section which is formed from a section of a graded-index, multimode optical fiber which is an odd number of quarter pitches long. The collimator fiber section has an angled end surface which joined to the reciprocally angled end surface of the at least one single-mode optical fiber end section. Piezoelectric material controls the separation between the first and second single-mode optical fiber end sections and sets the wavelengths of optical signals carried through the first and second single-mode optical fiber end sections.