Real-time distributed monitoring of pressure and shock velocity by ultrafast spectrometry with Chirped Fiber Bragg Gratings: Experimental vs calculated wavelength-to-pressure sensitivities in the range [0–4 GPa]

Abstract

Fiber Bragg Gratings (FBGs) are gaining acceptance as velocity/pressure gauges in the fields of detonation and shock physics on account of their sensitivity, small size, flexibility, electromagnetic immunity, and wavelength-encoded feature. Chirped FBGs (CFBGs) are investigated as wavelength-to-position discriminators with the purpose of monitoring pressure/velocity profiles over a distance range of typically 100 mm. The use of CFBGs simplifies both sensor deployment and data retrieval and finally improves the accuracy due to the increased number of measurement data. In this paper, the metrological performance of CFBGs used as in situ distributed shock pressure/velocity gauges is investigated both theoretically and experimentally in a planar shock loading configuration with an aluminum-based flyer and target. In the intermediate range for shock stress, i.e., less than the Hugoniot Elastic Limit (HEL) of silica, CFBGs provide simultaneous measurements of both shockwave velocity and stress within the target material. A Bragg wavelength-to-stress model is proposed that takes into account (i) the state-of-stress within the target material, (ii) the stress coupling coefficient due to imperfect impedance matching between the target material and the silica fiber, (iii) the conversion of the state-of-stress into a state-of-strain within the silica fiber, and (iv) the conversion of strain data into observable Bragg wavelength shifts. Finally, the model also takes into account the pressure dependence of constitutive parameters for silica and aluminum. Experiments were performed in planar shock loading using CFBGs as stress gauges, bonded along the target axis with Araldite glue. 6061-T6 aluminum flyers were launched at several velocities by a gas gun onto targets of the same material. A free-space Czerny-Turner (CT) spectrometer and an integrated-optics Arrayed-Waveguide Grating (AWG) were both used as dynamic spectrum analyzers. Experimental Bragg wavelength shifts agree well with theoretical predictions for both elastic and hydrodynamic planar shock loading of 6061-T6 aluminum, opening up large perspectives for shock physics experiments.

Development of a Shock and Detonation Velocity Measurement System Using Chirped Fiber Bragg Gratings

“…Such a system was developed to measure pressure levels in inert and HE materials using monochromatic FBGs, but it can also detect any defect or delay in the shortening process of a CFBG during an SDT, for instance. Hydrodynamic simulations were also conducted to better understand the coupling of the shock between an HE material and the glass fiber [29].…”

“…We inverted this orientation for the shorter CFBG (20 mm long) so that it became more sensitive at lower shock pressure levels. Indeed, if the CFBG experiences a shock pressure which is lower than a detonation pressure, part of its spectrum is shifted toward the lower wavelengths [29]. Then, to avoid any overlap of wavelengths between the CFBG's pristine section and the shocked section, the shorter wavelength side should be exposed to the upcoming shock front.…”

Shocks Sensing by Fiber Bragg Gratings and a 100 MHz Dynamic Dispersive Interrogator

“…The measured wavelength shift, +4.7 nm/GPa, is a bit lower than expected; nevertheless this would facilitate measurements of higher pressure levels in the C+L band. A theoretical study on the shock coupling like in [14] for Aluminum should be performed to fully discuss the wavelength shifts function in both configurations in the case of PMMA.…”

“…The wavelength-shift to pressure dependency is not linear and depends on the target material. In Aluminum [14], the shock coupling to Silica is near 75% and wavelength-pressure slope starts roughly at -10 nm/GPa and it reduces as function of pressure.…”

“…The two phenomena are of opposite sign but the reduction of the grating period is dominant and at the end the FBG peak wavelength shifts towards the lower wavelengths as function of pressure [4]. We deeply studied this configuration theoretically for FBGs integrated in Aluminum targets [14] in the range of [0 -4] GPa. The wavelength-shift to pressure dependency is not linear and depends on the target material.…”