Study of Optical Fiber Sensors for Cryogenic Temperature Measurements.

In this work, the performance of five different fiber optic sensors at cryogenic temperatures has been analyzed. A photonic crystal fiber Fabry-Pérot interferometer, two Sagnac interferometers, a commercial fiber Bragg grating (FBG), and a π-phase shifted fiber Bragg grating interrogated in a random distributed feedback fiber laser have been studied. Their sensitivities and resolutions as sensors for cryogenic temperatures have been compared regarding their advantages and disadvantages.

Thermal wavelength stabilization of Bragg gratings photowritten in hole-filled microstructured optical fibers.

We demonstrate that the resonance wavelength of fiber Bragg gratings photowritten in the core of microstructured optical fibers can be efficiently stabilized versus temperature by inserting suitable refractive index materials with a negative thermal sensitivity into the holes. By these means, the effective index of the guided mode undergoes thermal variations which counterbalance the effect of the grating period thermal drift. The residual excursion of the resonance wavelength can be limited to less than +/- 10 pm over a 70 degrees C range of temperature into Microstructured Optical Fibers (MOFs) having realistic geometrical parameters, and using existing refractive index materials.