Impact of fundamental thermodynamic fluctuations on light propagating in photonic waveguides made of amorphous materials

To improve the detection limit of optical sensors, it is of paramount importance to understand light–matter interaction processes at a fundamental level. At room temperature, the ultimate detection limit is governed amongst others by fundamental thermodynamic fluctuations. Their effect on the properties of light that propagates in amorphous materials is not well understood. Here, we unveil and model for the first time a dominating high-frequency (terahertz-range) noise contribution in the phase of laser light having propagated in optical waveguides, leading to Raman-like sidebands in the optical output spectrum. A salient feature of our approach is to consider a mean relaxation time of the spontaneous random heat flux in the medium, which leads to a spatial correlation of the thermo-refractive noise. The resulting phase noise can be several orders of magnitude larger than what was predicted by earlier models. Our model allows us to explain the origin and specificities of the background that is observed in the Raman optical spectra of silicon nitride waveguides and silica optical fibers. Not only do these findings add a previously unknown dimension to the fundamental knowledge about noise in light–matter interaction, they also need to be taken into account in any optical system in which dynamic fluctuations at the picosecond (ps) or sub-ps level play a role.