Fiber-optic gyroscope for rotational seismic ground motion monitoring of the Campi Flegrei volcanic area.

The real-time monitoring of densely populated areas with high seismic and volcanic risk is of crucial importance for the safety of people and infrastructures. When an earthquake occurs, the Earth surface experiences both translational and rotational motions. The latter are usually not monitored, but their measurement and characterization are essential for a full description of the ground motion. Here we present preliminary observational data of a high-sensitivity rotational sensor based on a 2-km-long fiber-optic Sagnac gyroscope, presently under construction in the middle of the Campi Flegrei Volcanic Area (Pozzuoli, Italy). We have evaluated its performance by analyzing data continuously recorded during an acquisition campaign of five months. The experimental setup was composed of a digital nine-component seismic station equipped with both a rotational sensor and conventional seismic sensors (seismometers, accelerometers, and tiltmeters). During this experiment we detected seismic noise and ground rotations wavefield induced by small to medium local earthquakes (M D<3). The prototype gyroscope shows a very promising sensitivity in the range of 5×10-7-8×10-9 r a d/s/H z over the frequency bandwidth 5 mHz-50 Hz. Future upgrades and perspectives are discussed.

All-fiber high-resolution incoherent broadband spectrometer.

Portable optical spectrometers are crucial devices for bio-chemical sensing and spectroscopic applications whereby robust, compact and cost-effective set-ups are desirable. However, existing miniaturized instruments typically struggle to achieve broad wavelength operation and high spectral resolution at the same time. Here, an all-fiber optical spectrometer based on two cascaded Bragg gratings is devised and demonstrated, showing a record resolution and a wavelength span-to-resolution ratio larger than that of most miniature broadband spectrometers reported to date. Thanks to a synchronous control of the grating lengths and to a unique combination of their reflection features, spectral analysis of incoherent light within 1 pm is achieved. On the other hand, fast and reproducible wavelength tuning over several nanometers on a millisecond-timescale is ensured by mechanical stretching of the internal fiber, limited only by the actuator's dynamic range. A striking evidence of the spectrometer capabilities is provided with Doppler-limited spectroscopy of gas absorption bands performed with a near-infrared LED source. The observed spectra exhibit lineshapes comparable with those obtained by laser-based set-ups and the retrieved gas-line parameters are in agreement with existing spectroscopic databases. The spectrometer lends itself to applications in high-resolution interrogation of multiple fiber-optic sensors as well as broadband imaging with supercontinuum light.

Automatic Alignment Method for Controlled Free-Space Excitation of Whispering-Gallery Resonances.

Whispering-gallery mode microresonators have gained wide popularity as experimental platforms for different applications, ranging from biosensing to nonlinear optics. Typically, the resonant modes of dielectric microresonators are stimulated via evanescent wave coupling, facilitated using tapered optical fibers or coupling prisms. However, this method poses serious shortcomings due to fabrication and access-related limitations, which could be elegantly overcome by implementing a free-space coupling approach; although additional alignment procedures are needed in this case. To address this issue, we have developed a new algorithm to excite the microresonator automatically. Here, we show the working mechanism and the preliminary results of our experimental method applied to a home-made silica microsphere, using a visible laser beam with a spatial light modulator and a software control.

Difference frequency generation in the mid-infrared with orientation-patterned gallium phosphide crystals.

We report on the generation of coherent mid-infrared radiation around 5.85 µm by difference frequency generation (DFG) of a continuous-wave Nd:YAG laser at 1064 nm and a diode laser at 1301 nm in an orientation-patterned gallium phosphide (OP-GaP) crystal. We provide the first characterization of the linear, thermo-optic, and nonlinear properties of OP-GaP in a DFG configuration. Moreover, by comparing the experimental efficiency to Gaussian beam DFG theory, we derive an effective nonlinear coefficient d=17(3) pm/V for first-order quasi-phase-matched OP-GaP. The temperature and signal wavelength tuning curves are in qualitative agreement with theoretical modeling.