Optical signal in a hollow core photonic bandgap fibre is guided in an air core surrounded by a high air filling factor PBG microstructured region (>90%). Added to the low bend sensitivity due to the core high NA, this confers to this fibre design significantly reduced material nonlinearities since more than 95% of optical power is propagating in air. In addition air/undoped silica provides excellent temperature immunity critical for high performance fibre sensing and metrology applications.
