Systems and methods for a hollow core optical fiber resonant filter

Systems and methods for a hollow core resonant filter are provided. In one embodiment, a hollow-core fiber resonant cavity (100) comprises: a hollow-core fiber (110) having a first and second ends (120,122); a first and a second pigtail fiber (130,132) each of solid core fiber material. A tip (131) of the first pigtail is optically aligned with the first-end to couple light from the first pigtail to the hollow core fiber across a first free-space gap (125). A tip (133) of the second pigtail is optically aligned with the second-end to couple light from the second pigtail to the hollow-core fiber across a second free-space gap (125). The tip (133) of the second pigtail is coated (142) to reflect light received from the second-end back across the second free-space gap into the second-end. The tip of the first pigtail is coated (140) to reflect light received from the first-end back across the first free-space gap into the first-end.