Study of Brillouin gain spectrum in standard single-mode optical fiber at low temperatures (1.4–370 K) and high hydrostatic pressures (1–250 bars)

A single laser pump and probe experiment demonstrating the first very low temperature Brillouin gain spectrum measurement in standard telecommunication single-mode optical fibers at 1.55 μm is presented. A novel theoretical expression of the Brillouin gain spectrum depending on the laser sources linewidth is detailed. This makes possible measurements to extend down to a temperature of a few Kelvin where the Brillouin gain spectrum linewidth becomes equivalent to the laser source. Brillouin frequency and linewidth measurements are presented at atmospheric pressure from 1.4 to 370 K. These measurements open the way for optical fiber temperature sensors for cryogenic applications. An analytical simple modelling of non-linear Brillouin gain linewidth based on the two asymetric quantum-wells model fitting the measurements from 370 down to 20 K is presented. Measurements at ambient temperature under hydrostatic pressure equivalent to 0–2500 m water depth are presented as well.