Anti-resonant hollow core optical assembly having diagnostic ring structure and method

A method of diagnosing a defect in a hollow core fiber assembly comprising a central longitudinal axis extending from a first end to a second end, a diagnostic ring structure through which the central longitudinal axis extends, the diagnostic ring structure extending longitudinally from the first end to the second end, disposed azimuthally around the central longitudinal axis, comprising an outer surface at an outer radius from the central longitudinal axis and an inner surface at an inner radius from the central longitudinal axis, and having a first refractive index, a plurality of anti-resonant elements in contact with the inner surface, the anti-resonant elements extending longitudinally from the first end to the second end and surrounding the central longitudinal axis to define an effective core region, and an outer cladding surrounding the diagnostic ring structure, the outer cladding having a second refractive index less than the first refractive index, the method comprising the steps of:

• launching a light signal into the diagnostic ring structure of the hollow core fiber assembly;
• and
• monitoring the light signal passing through the diagnostic ring structure.