UC San Diego

Narrow channel fused fiber wavelength division multiplexers, produced by laying two optical fibers side by side, heating, and fusing them together, are discussed. Methods and equipment for repeatable production of devices with channel spacings from approximately 30 nm to 5 nm have been developed. The polarization asymmetry in the coupling of such devices is explained, and is shown to be the limiting factor in achieving high isolation in highly overcoupled fused fiber WDMs, and the primary parameter limiting the ultimate scalability of the technology. A method for reducing the polarization dependent coupling in narrow channel fused fiber WDMs by halting fiber tapering at the peak of the observed polarization envelope is demonstrated. Techniques allowing the manipulation of this envelope based on altering fusion conditions to change the fused region cross-sectional geometry are described, allowing fabrication of polarization independent WDMs over a continuous range of channel spacings. The performance of higher order (greater then 2 wavelengths) WDMs produced by cascading multiple narrow channel fused fiber devices is discussed, and low loss, high isolation devices capable of mux/demux operation with up to 8 wavelengths have been demonstrated. A simple theoretical model is shown which has been used to successfully describe the behavior of narrow channel fused fiber WDMs, and this model is extended to allow prediction of optical behavior based on a few parameters determined after coupler fabrication. A method is demonstrated for fine-tuning the optical characteristics of the devices by altering the index of refraction of the glass within the fused region by applying high intensity UV light. The use of both single couplers and multiple, cascaded devices has been demonstrated in several selected applications which highlight the importance of polarization independent coupling for such devices