How to make the propagation time through an optical fiber fully insensitive to temperature variations

Propagation time through standard (solid core) optical fibers changes with the temperature at a rate of 40 ps/km/K. The thermo-optic effect in silica glass accounts for about 95% of this change, and thus, hollow-core fibers, in which the majority of optical power propagates through an air rather than a glass core, can have this sensitivity greatly reduced. To date, we have demonstrated a sensitivity as low as 2 ps/km/K, this value being limited by thermally induced fiber elongation. In this paper, we predict and experimentally demonstrate that the thermal sensitivity of the propagation time can be reduced to zero (or even made negative) in hollow-core photonic bandgap fibers by compensating the thermally induced fiber elongation with an equal and opposite thermally induced group velocity change (i.e., by making the light travel faster through the elongated fiber). This represents the ultimate fiber solution for many propagation time-sensitive applications.