Photonics sensing at the thermodynamic limit

We report a slow-light fiber Bragg grating strain sensor with a resolution limited by the extremely low thermodynamic phase fluctuations of the fiber. This was accomplished by using a short grating (4.5 mm) to enhance the thermal phase noise, an ultra-stable interrogation laser to lower the laser frequency noise, and a slow-light mode with a high group index (similar to 533) to suppress all other noise sources. We demonstrate that in a similar but longer grating (21 mm), the phase noise is suppressed in inverse proportion to the square root of the length, in accordance with theory, leading to a strain resolution as low as 130 f epsilon/root Hz and a minimum detectable length of similar to 3 x 10(-15) m at 1.5 kHz. (C) 2017 Optical Society of America