Birefringent optical filters in single-mode fiber

In-line fiber-optic spectral-bandpass filters with one, two, three, and four elements are designed in analogy to the Sole filters of bulk optics by bending the fiber into the form of small coils, which are rotated to suitable angles and arranged between polarizers. One four-element filter has a bandwidth of 3.5% at a center wavelength of 704 nm and a free spectral range of 17%.