Last year a group from Los Alamos National Lab published two papers that detail experimental studies involving the use of a Micron Optics si155 optical sensing interrogator and Fiber Bragg Gratings (FBGs) in extreme sample environments. Some of these environments included cryogenic temperatures, large magnetic fields and applied hydrostatic pressures.
The first paper titled, “An FBG Optical Approach to Thermal Expansion Measurements under Hydrostatic Pressure”, discusses an optical technique for measuring thermal expansion and magnetostriction at cryogenic temperatures and under applied hydrostatic pressures. Because different materials exhibit different changes in length in response to variations in temperature, the study of thermal expansion can provide important information to a wide range of fields and can be crucial to the design of structures including bridges, engines and space shuttles. However, precise thermal expansion measurements under applied hydrostatic pressure can be impossible to obtain with only a standard capacitance dilatometer technique. Thus, an alternative technique was tested, one which used FBGs placed in a clamp-type pressure chamber to measure the strain in a millimeter-sized sample of CeRhIn5, a highly tunable and exceptionally impurity-free antiferromagnetic metal. To interrogate the FBGs, an si155 HYPERION optical sensing instrument was used. The study revealed that FBGs can be used as pressure sensors in applications where hydrostatic pressure is examined. These results show the possibility of performing high-resolution thermal expansion measurements under pressure and open new possibilities for a wide range of materials.
(a) (Left) Cross-section of experimental setup. (Right) Schematics of the interrogation system. (b) Picture of the experimental setup for pressure-dependent measurements.
The second paper, “Fiber Bragg Grating Dilatometry in Extreme Magnetic Field and Cryogenic Conditions” reviews three different experimental approaches to single mode SiO2 fiber Bragg techniques for dilatometry studies of small millimeter-long single-crystalline samples in extreme conditions of high magnetic fields and low temperatures. These experimental approaches discussed in this paper are examined for their technique applicability, sensitivity and advantages over traditional methods, such as capacitance dilatometers and resistive foil strain gauges. For sensing under extreme conditions, FBGs are an ideal choice due to their high accuracy and immunity to electromagnetic interference. Furthermore, their sensitivity to mechanical vibration can be reduced to much less than what can be seen in capacitance dilatometers. To interrogate the small FBGs used, an si155 Hyperion optical sensing unit was used once again. The study concluded that small samples in the mm range perform better than predicted by the available models that were discussed in the literature. Additional studies performed with a broader selection of adhesives and sample lengths will be conducted in the future.
Experimental setup for FBG interrogation with a swept laser source, such as the Micron Optics HYPERION instrument. λB is the Bragg wavelength, n is the fiber’s index of diffraction, and d is the grating spacing. fiber’s index of diffraction, and d is the grating spacing.