Optical fiber sensors for measurements in difficult environments have been intensively studied for many years. Some of their most important advantages are high sensitivity, immunity to electromagnetic interference and high speed. Among others, sensor setups to measure temperature, strain and pressure have gained significant importance, and fiber Bragg grating (FBG) sensors in particular have been widely investigated and used for these applications.

In this Optics Express article, Rodriguez et al. concentrate on very fast sensors for very difficult environments, namely time-resolved ultra-high magnetic field measurements and pressure dynamics analysis in thermal ignition of explosives. The general response of a FBG to temperature, strain and pressure has been well known for years. A fiber with a sensing FBG placed inside it enables fast measurements, since the thermal and mechanical masses are very small.

The first problem to solve is therefore the realization of a high-speed FBG interrogation system. This system should provide the Bragg wavelength shift with a high repetition rate. Using the group velocity dispersion of a fiber is one of the key techniques to be used in such a case. The interrogation system, consisting of a fs-laser providing a broadband signal at high repetition rate (100 MHz), a stretching fiber performing a spectral-domain-to-time-domain conversion of the sensing signal, a detector, and a high speed sampling oscilloscope (50 GS/s), opens the door for studying dynamic effects in FBG sensor applications.

Two impressive examples are given to demonstrate this interrogation technology. The first deals with the measurement of magnetic-field-driven magnetostrictive effects. Here we see the dynamics of the strain in a magnetostrictive material at extremely high magnetic fields in the order of 150 T. The system’s high resolution in the time domain allows the observation of, e.g., acoustic effects and phase transitions in the material. In addition to these results, some new questions come up that will stimulate future research. The second example is in the field of pressure measurement in thermal ignition of high explosives. In the presented measurements, effects such as material phase changes become visible. Different process phases like heating and pressurization can be observed and analyzed in detail.

This article discusses new applications for a sophisticated interrogation system for FBG sensors in extreme environments, but it also addresses the very important question of probe preparation and intelligent probe placement, as well as the challenge of dealing with the cross sensitivity of such fiber sensors to different measures like temperature, pressure or strain. Despite these remaining challenges, the demonstration of the high-speed interrogation system presented here is an important step towards real-time measurement and analysis of process dynamics.

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