Passive components in fiber-compatible form for single-mode systems are important in both long-haul and local distribution communications applications. Among these components are isolators, attenuators, filters, splitters, and various forms of couplers. A common feature in the construction of many of these components is the insertion of an optical element or elements into the single-mode free-space beam. This rules out the direct use of fibers except for the insertion of the thinnest elements due to the prohibitive losses incurred when two fibers are separated axially. One solution to this problem, and the one generally adopted, is to utilize beam expansion. By expanding the size of the single-mode beam its divergence due to diffraction is reduced, and hence greater axial separation between the expanding elements is afforded. All current beam expansion techniques, however, require the attachment of an additional element to the fiber with the concomitant problems of critical alignment, joint stability, and aberration losses due to the expanding element itself.
© 1989 Optical Society of AmericaPDF Article