Hollow-core anti-resonant fiber (HAF) shows promising applications. Nevertheless, there has been a persistent problem when it comes to all-fiber integration due to lack of HAF based fiber components. Interconnecting a solid core based fiber component with HAFs remains limited solutions. As a result, most of the HAF based optical systems rely on free space optical components that make the system cumbersome and increases the complexity of the system.
In response to this remained challenge, we investigate a reliable, versatile, and efficient method to convert a HAF into a fiber filter. By locally heating a HAF with a CO2 laser, the fiber structure gets deformed and cladding capillaries shrink to produce a thicker wall. This process is analogous to "writing" a new fiber with a thicker wall on the original fiber, resulting in creating new high loss regions (resonant wavelengths) in the original transmission bands. Thus, construction of a fiber filter is realized by “writing” a new fiber on the original fiber. Feasibility of this method is confirmed through experiments, adopting both one and two-layer HAF. The HAF based fiber filters are found to have transmission spectra consistent with simulation prediction. Both band pass and band reject fiber filters with more than 20 dB extinction ratio are obtainable without extra loss. Thus, an in-fiber HAF filter is demonstrated by CO2 writing process. Its versatile approach promises controlled band selection, and would find interesting applications to be discussed.
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