Dynamic measurements of detonation velocity profiles are performed using long Chirped Fiber Bragg Gratings (CFBGs). Such thin probes, with a diameter of typically 150 µm, are inserted directly into a high explosive sample or simply positioned laterally. During the detonation, the width of the reflected optical spectrum is continuously reduced by the propagation of the wave-front, which physically shortens the CFBG. The reflected optical intensity delivers a ramp down signal type, which is directly related to the detonation velocity profile. Experimental detonation velocity measurements were performed on the side of three different high explosives (TNT, B2238 and V401) in a bare cylindrical stick configuration (diameter: 2 inches, height: 10 inches). The detonation velocity range covered was 6800 to 9000 m/s. The extraction of the detonation velocity profiles requires a careful calibration of the system and of the CFBG used. A calibration procedure was developed, with the support of optical simulations, to cancel out the optical spectrum distortions from the different optical components and to determine the wavelength-position transfer function of the CFBG in a reproducible way. The 40-mm long CFBGs were positioned within the second half of the three high explosive cylinders. The excellent linearity of the computed position-time diagram confirms that the detonation was established for the three high explosives. The fitted slopes of the position-time diagram give detonation velocity values which are in very good agreement with the classical measurements obtained from discrete electrical shorting pins.

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A new closed-form expression is derived in this paper for computing the electromagnetic field reflected by two moving parallel interfaces. Next, the interfaces velocities and refractive index between interfaces are derived from measurable frequencies and amplitudes embedded in the reflected electromagnetic field. As an application, the remote analysis of the shock wave phenomenon in solids is reported and shock wave descriptors are estimated. Based on the proposed approach, physical insights on measurement published results are reported. Moreover, new measurement data on shocked polymethyl methacrylate material are presented and discussed.

High pressure shock profiles are monitored using a long Fiber Bragg Grating (FBG). Such thin probe, with a diameter of typically 150 µm, can be inserted directly into targets for shock plate experiments. The shocked FBG's portion is stressed under compression, which increases its optical group index and shortens its grating period. Placed along the 2D symmetrical axis of the cylindrical target, the second effect is stronger and the reflected spectrum shifts towards the shorter wavelengths. The dynamic evolution of FBG spectra is recorded with a customized Arrayed Waveguide Grating (AWG) spectrometer covering the C+L band. The AWG provides 40 channels of 200-GHz spacing with a special flattop design. The output channels are fiber-connected to photoreceivers (bandwidth: DC-400 MHz or 10 kHz-2 GHz). The experimental setup was a symmetric impact, completed in a 110-mm diameter single-stage gas gun with Aluminum (6061T6) impactors and targets. The FBG's central wavelength was 1605 nm to cover the pressure range of 0-8 GPa. The FBG was 50-mm long as well as the target's thickness. The 20-mm thick impactor maintains a shock within the target over a distance of 30 mm. For the impact at 522 m/s, the sustained pressure of 3.6 GPa, which resulted in a Bragg shift of (26.2 ± 1.5) nm, is measured and retrieved with respectively thin-film gauges and the hydrodynamic code Ouranos. The shock sensitivity of the FBG is about 7 nm/GPa, but it decreases with the pressure level. The overall spectra evolution is in good agreement with the numerical simulations.

Fiber Bragg Gratings (FBGs) are used to measure shock velocity, detonation velocity, shock wave profile or pressure profile in inert and energetic materials. Such thin probe, with a diameter below 150 µm, can be inserted directly into materials without disturbing the physical phenomena. Chirped FBGs are used to track the shock wave in the grating using wavelengths. The velocity (few km/s) and shock wave profile measurements are realized by recording the CFBG's reflected spectral width. Pressure measurements at few GPa levels use dynamic spectrometers, two approaches are compared: parallel acquisition using an Arrayed-Waveguide-Grating and time-multiplexing by wavelength-to-time conversion using dispersion.