Ultrashort fiber Bragg gratings (FBGs) possess significant potential as weak sensing units for distributed measurements, the exploitation of which, however, have been usually limited by their inherent wide spectra which is undesirable for most traditional wavelength measurements. In this letter, we propose to use shifted optical Gaussian filters to interrogate such wide spectrum gratings. However, instead of acting like Gaussian edge filters as previously done, to take advantage of spectral feature of ultrashort gratings, here they have a role which is more like shifted matched filters. The measurement inherits the important features of a common shifted Gaussian filter interrogation technique, namely its high flexibility, natural insensitivity to intensity variations, promising future for multichannel interrogation, and a potentially wider measuring range relative to common intensity-based approaches. We believe that this simple concept for ultrashort Bragg gratings should offer the strong foundation for their future applications in distributed measurements. Interrogation of a strain-tuned ultrashort grating was accomplished, with a wide sensitivity tuning range from 2.8 to 10.4 dB/nm achieved.
A novel vibration sensor based on the single mode (SM)-no core (NC)-SM fiber structure is proposed and experimentally sensing results demonstrated. By numerical simulation, we use an NC fiber (NCF) as the multimode waveguide structure for the multimode interference (MMI) sensing. Through intensity demodulation, the vibration sensing structure can detect continuous vibration disturbances with the frequencies of ranging from 100 Hz to 29 kHz and the inherent frequency of cantilever slab of 700 Hz. The frequency sensing resolution is 1 Hz in real-time monitoring. The proposed compact structure is easy to fabricate and has low cost.
We report a large-scale multi-channel fiber sensing network, where ultra-short FBGs (USFBGs) instead of conventional narrow-band ultra-weak FBGs are used as the sensors. In the time division multiplexing scheme of the network, each grating response is resolved as three adjacent discrete peaks. The central wavelengths of USFBGs are tracked with the differential detection, which is achieved by calculating the peak-to-peak ratio of two maximum peaks. Compared with previous large-scale hybrid multiplexing sensing networks (e.g., WDM/TDM) which typically have relatively low interrogation speed and very high complexity, the proposed system can achieve interrogation of all channel sensors through very fast and simple intensity measurements with a broad dynamic range. A proof-of-concept experiment with twenty USFBGs, at two wavelength channels, was performed and a fast static strain measurements were demonstrated, with a high average sensitivity of ~0.54dB/µƐ and wide dynamic range of over ~3000µƐ. The channel to channel switching time was 10ms and total network interrogation time was 50ms.
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