High-speed fiber Bragg grating measurement of extreme material dynamics

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An example plot of processed coherent time domain FBG waveform data from a pulsed magnetic field (150 T) driven magnetostriction strain experiment on the 2-­‐mm long magnetic perovskite LaCoO3 system followed by launch of an acoustic wave after elongation of the sample. The graphic shows a time-­‐time plot of the 1555 nm 1-­‐mm FBG sensor versus for 50.036 km of dispersion. The left ordinate axis is a window of 6.5 ns within one laser clock period time slice (10 ns). The abscissa axis is the event time beginning from the trigger of the NHMFL single-­‐turn capacitor bank and has not been time corrected for the various delays in the signal and trigger paths. Conversion of the left ordinate time axis into FBG wavelength shift yields a wavelength-­‐shift time plot as shown in the right ordinate axis labels. A positive shift to longer wavelength indicates that the strain resulted in elongation of the sensor/sample combination. A mean FBG wavelength shift of over 4 nm is observed.

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Lab researchers developed and demonstrated a novel fiber Bragg grating (FBG) interrogation systemfor dynamical extreme conditions. The system can be used ions where traditional fiber Bragg grating sensing approaches are too slow to capture the events. An Optical Society of America “Spotlight on Optics” article in Optics Express called this “an important step towards real-time measurement and analysis of process dynamics” for understanding material behavior in extremes.

 

Fiber Bragg grating based sensors are used for research and commercial measurements of strain, temperature, and pressures. The sensors provide a good basis for distributed sensing because their operational modality is rooted in multiplexing approaches from optical telecommunications technology. However, much less is known about their performance in high-speed dynamic events that push the limits of FBG sensing to extreme environments under sub-microsecond transient loading conditions where both sensor and readout rates demand performance currently unavailable commercially.

 

In the Optics Express article, George Rodriguez (Center for Integrated Nanotechnologies, MPACINT) and co-workers describe very fast sensors for difficult environments, such as time-resolved ultrahigh magnetic field measurements and pressure dynamics analysis in thermal ignition of explosives. The general response of an FBG to temperature, strain and pressure is well known. The team’s key advance was the development of high speed (100 MHz) interrogation rate methodssurpassing previously existing interrogation speed techniques by several orders of magnitude. This development enables real-time diagnosis of dynamical events. The investigators use a broadband pulse from an ultrafast mode locked fiber laser to probe the FBG. The team mapped the spectralresponse of the FBG into time using a long dispersive fiber on a single element photodetector coupled to a transient digitizer. This method allows the researchers to capture the optical spectrum of the FBG sensor at the repetition rate of the 100 MHz laser source approach. The approach enables FBG strain and pressure sensing on experiments with 10 ns resolution. Moreover, the FBG sensor has an unobtrusive small diameter and noise-free isolation from other electrical based diagnostics.

 

Two examples demonstrate this interrogation technology in dynamical extreme conditions. 1) The team applied this technique to measure the dynamics of the strain in a magnetostrictive material at extremely high magnetic fields (approximately 150 T) produced at the National High Magnetic Field Laboratory (NHMFL). The system’s high time domain allows the observation of acoustic effects and phase transitions in the material. 2) The researchers conducted pressure measurements during the thermal ignition of the high explosive PBX9501. The high temperature and material flow that occurs during the sub-sonic blast have previously made such measurements challenging. The new technique enables observation of material phase changes. Different process phases such as heating and pressurization can be observed and analyzed in detail. The example measurements would not have been possible with traditional FBG interrogation approaches that are too slow to resolve such events.

 

Reference: “Coherent Pulse Interrogation System for Fiber Bragg Grating Sensing of Strain and Pressure in Dynamic Extremes of Materials,” Optics Express 18 (11), 14219 (2015); doi: 10.1364/OE.23.014219, Optical Society of America “Spotlight on Optics”www.osapublishing.org/spotlight/summary.cfm?id=318940. Authors: George Rodriguez and Abul Azad (MPA-CINT), Laura Smilowitz and Bryan Henson (Physical Chemistry and Applied Spectroscopy, C-PCS) for the thermally driven explosives work, and Marcelo Jaime, Fedor Balakirev, and Chuck H. Mielke (Condensed Matter and Magnet Science, MPA-CMMS) for the strain measurement in an ultrahigh magnetic field. Early concept development for the approach was in collaboration with Brandon La Lone and Bruce Marshall from the Special Technologies Laboratory (National Security Technologies, LLC) in Santa Barbara, CA.

 

Laboratory Directed Research and Development (LDRD) and National Science Foundation NHMFL programs funded different aspects of the work. The National Science Foundation and the DOE Office of Science enabled the 100 T magnet capability at the NHMFL in Los Alamos. The research supports the Lab’s Nuclear Security mission area and Science of Signatures and Materials for the Future Science pillars by enhancing materials dynamics diagnostic capability for complex materials and energetic materials under conditions of extremes. It also enhances the Laboratory’s weapons physics experimental diagnostic capability. Technical contact: George Rodriguez