Characterization of 100 Micron Long Fiber Bragg Gratings and Applications in Ultrafast Shock Wave Experiments

Ultrashort Fiber Bragg Gratings (FBGs) may be very useful whenever highly localized pressure measurements are desired. Obviously, very short FBGs will exhibit lower, as well as spectrally broader, reflections; however, these deficiencies may be outweighed by the advantage of the potential for highly localized measurement. In addition, with such short gratings, their accurate localization along the fiber is of great importance. In this paper, commercial 100 μm FBGs are characterized, and their location along the fiber is accurately determined by two methods: Spectral and white-light interferometry. We also demonstrate the advantage of ultrashort gratings in a shock-wave experiment; when a solid target is hit by a projectile, the resultant pressure waves may change rapidly in both location and time. In this paper, the suitability of a 100 μm grating to this scenario is demonstrated, revealing features in good agreement with theory-in particular, an approximately 0.7 mm long constant pressure region. Although these pressure waves may be monitored by ordinary (~6-8 mm long) FBGs, due to the high-pressure gradients involved, the FBG response will be very complicated and data retrieval may be ambiguous. Thus the advantage of ultrashort FBGs is demonstrated.