Slow light in fiber Bragg gratings for sensing thermal phase noise, attostrains and other applications [electronic resource]
Keywords: Electrical Engineering.

Strain sensors have many applications in structural health monitoring, civil engineering, geoscience, and gravitational wave detection. To address the needs of high-end applications, and to push the resolution of fiber strain sensors well beyond their current picostrain/√Hz level, we have explored the potential of utilizing the narrow slow-light resonances that exist in strong fiber Bragg gratings (FBGs) and the noise limitations of this kind of sensor. Prior to this work the strain resolution of slow-light FBG strain sensors were limited by the frequency noise of the laser used to interrogate them. The main goals of this thesis were first to reduce this source of noise by utilizing a probe laser with a much narrower linewidth, which necessitated the design of new gratings matched to the new laser; and second, to study theoretically and experimentally the next noise source below the laser frequency noise, which is thermodynamic phase noise. This work was broadly divided into four major tasks. First, we studied theoretically, from basic thermodynamic principles, the magnitude and frequency dependence of the thermal phase-noise in Fabry-Perot-like resonances similar to the slow-light resonances available in strong FBGs. This study showed that the thermal phase noise is proportional to the group index of the resonance, and that when expressed in units of strain it is proportional to 1/√L, where L is the length of the sensor. Thus the shorter the FBG the higher the thermal phase noise in units of strain. Second, we improved the design and the fabrication of our FBGs in order to achieve very high group delays, and hence very high sensitivities in short fibers. This step was crucial because to measure thermal phase noise in units of strain (normalized output power noise to input power times the sensitivity), a short FBG with high sensitivity is needed for the sensor to be limited by thermal phase noise, otherwise it would be limited from other noise source, i.e. laser intensity noise. We achieved this result by using strongly apodized FBGs written with a femtosecond laser in deuterium-loaded fiber, and thermally annealing the FBG optimum sensitivity was achieved. Using this technique, we were able to achieve a 42-ns group delay, an eight-fold improvement compared to what was reported previously in similar FBGs. Third, we had to modify our experimental setup to improve its stability and reduce the dominant laser frequency noise of the sensor. To this end we probed our sensors with a new low-noise laser from OrbitsLightwave with a 200-Hz linewidth, we placed our sensor in an anechoic enclosure, and we used a low-noise photo-detector. The last effort was to use these various developments and improvements to design, fabricate, and test two FBGs, one to measure the thermal phase noise in an FBG for first time, and the other to observe the smallest strain ever measured in an FBG-based sensor (an minimum detectable strain (MDS) of 110 fε/√Hz at 2 kHz and 30 fε/√Hz at 30 kHz). This sensor was so stable that it exhibited no drift in its Allan variance after a four-day measurement. By integrating a 4-day output trace with an 8-hour integration time, we were able to measure an absolute MDS of 250 attostrains, the lowest value ever measured in an FBG. While we were aiming to measure thermal phase noise and reduce the MDS, some other applications presented themselves. Because these devices confine light not only temporally but also spatially, they can be used in applications that benefit from extremely high intensities and confinement, in particular in quantum electrodynamic experiments and nonlinear optic applications such as optical signal processing. As a proof of concept, I report in the end of this thesis the performance of two FBGs optimized for maximum field enhancement, maximum Purcell factor, maximum group delay, and minimum group velocity (a record of 300 km/s). The measured values for these parameters are the highest reported in an all-fiber device. These properties enable robust novel devices that are simple to fabricate and in which light can be coupled easily and efficiently.