Mode-field expansion and reduction in dispersive fibers by local heat treatments

A novel method is proposed to make the mode-field expander and reducer using dispersive fibers which are fabricated from optical glasses with different transformation temperatures and index reductions caused by rapid cooling. The dispersive fibers are heated locally at low or high temperatures, and the index difference is decreased or increased at the heated part. The mode-field is confined weakly or strongly in the core owing to the reduced or raised index difference, and is expanded or reduced partially.