Polymer Chirped Fiber Bragg Grating for more Sensitive Shock Velocity Measurements

The performances of a single-mode polymer chirped fiber Bragg grating (CFBG) made in a microstructured PMMA fiber is investigated to measure shock and detonation velocities. The polymer material makes the fiber sensor more sensitive at lower pressure values than with silica. The optical characterization of the microstructured polymer CFBG is discussed. The single-mode fiber offers a stable reflected optical spectrum over time and the manufacturing process used provides a constant chirp rate along the grating. The maximum reflectivity of the microstructured polymer CFBG provides enough signal for the photoreceiver without significantly increasing the optical source power. The actual limitations include the global shape of the reflected spectrum, which comprises many dips, and the optical losses that reduce sensor sensitivity towards the end of the grating. Anyhow, a first detonation experiment intended to measure shock and detonation velocities in a wedge test was completed. The X-T diagram showing the shock and detonation wave positions as a function of time presents two slopes corresponding to a shock velocity of 4695 m/s and a steady-state detonation velocity of 8392 m/s. These values are very similar to the ones obtained with 48 piezoelectric pins, but the uncertainties remain high (>2%) for this first experiment. Nevertheless, the experience proves that the PMMA material is suitable for detonation physics experiments. Technical solutions were identified to improve sensor performance. First, optical losses could be reduced within the grating and a more constant reflectivity level could be obtained. Sensitivity would be similar along the full length of the grating. The second point of focus will be to prevent any dips in the reflected optical spectrum. With these improvements, we should achieve uncertainties of less than +/- 1% (at k=2) for shock and detonation velocity measurements.