The thermal phase sensitivity of both coated and uncoated standard and hollow core fibers down to cryogenic temperatures

The thermal phase sensitivity of an optical fiber quantifies the degree to which a change in ambient temperature modifies the accumulated phase of light propagating through it. This sensitivity is often the limiting factor to the performance of fiber-based interferometers. Here we compare the thermal phase sensitivity of a hollow core fiber (HCF) and a standard single mode fiber (SMF-28) from -180 °C up to the room temperature. We report measurements on fibers both with and without acrylate coating that enables an accurate estimation of the coating contribution. The thermal phase sensitivity of fibers without any coating decreases at low temperatures. For SMF-28 it is reduced by a factor of four at -180 °C as compared to the room temperature. For HCF, the thermal phase sensitivity becomes negative at low temperatures, crossing zero around -70 °C, making the HCF operated at that temperature fully insensitive to small temperature fluctuations. The coating significantly influences a fibers overall thermal phase sensitivity, especially at low temperatures, since it goes through a phase transition from a rubbery state at room temperature to stiff glassy state at low temperatures. We quantify the coating contribution and suggest fiber coating design rules to obtain fibers with reduced or even zero thermal phase sensitivity.