Measuring attostrains in a slow-light fiber Bragg grating

We report a new generation of slow-light FBG strain sensor with a strain resolution (or minimum detectable strain) as low as 30 fepsilon/√Hz at 30 kHz, which is one order of magnitude lower than the record held by the previous generation. This sensor has an ultra-stable output (no drift in 4 days) and is capable of resolving an absolute strain of ~250 attostrains by integrating its output for ~8 hours, which is also a new record for an FBG fiber sensor. These improvements were accomplished by first maximizing the slope of the slow-light resonances, and hence the strain sensitivity. To this end the apodized FBG was written in a deuterium-loaded fiber with a femtosecond infrared laser, then thermally annealed. The three main sources of noise in the sensor system were also carefully reduced. The dominant source of noise, laser frequency noise, was reduced by interrogating the FBG with an ultra-stable laser (linewidth under 200 Hz) with a low intensity noise. The phase noise was minimized by selecting the proper FBG length (~25 mm). When used as an acoustic sensor, the same grating had a measured average pressure resolution of 50 μPa/√Hz between 3 kHz and 6 kHz, one order of magnitude lower than the previous lowest reported value for an FBG sensor.