Design of low frequency fiber optic Fabry–Perot seismometer based on transfer function analysis

We develop a low frequency fiber Fabry–Perot(F-P)seismometer based on transfer function analysis.The seismometer structure and demodulation system accuracy are limitations of low frequency seismic monitoring.The transfer function of the F-P seismometer is analyzed,and the mass displacement spectrum(MDS)is introduced.MDS provides guidance for mechanical structure design and optical interferometer analysis to achieve low noise.The F-P seismometer prototype is built.The experiment shows that the prototype has an average noise of 6.74 ng=p Hz below 50 Hz,and its noise is less than that of the global new high noise model within 0.16–50 Hz,whose potential is considerable.

Random Bragg-grating-based wavelength-tunable random fiber laser with a full-open cavity

A full-open-cavity wavelength-tunable random fiber laser(WT-RFL) with compact structure and hundreds of picometers tuning range is proposed and demonstrated. A π fiber Bragg grating(FBG) is used in the WT-RFL as a filter to select lasing wavelengths. The two random Bragg grating arrays(RBGAs) and a section of high gain erbium-doped fiber result in a low lasing threshold and high stability. A numerical model to analyze the tunable characteristics is developed. The results show that the laser threshold is 22 m W, and the maximum peak-power fluctuation is 0.55 d B. To the best of our knowledge, it is the first time that a compact and full-open-cavity WT-RFL with two RBGAs and a π-FBG is proposed.

Fiber optic strain rate sensor based on a differentiating interferometer

Strain rate is an important basic physical parameter in the fields of deformation observation, geodetic measurement,and geophysical monitoring. This paper proposes a novel fiber optic strain rate sensor(FOSRS) that can directly measure the strain rate through a differentiating interferometer that converts the strain rate to the optical phase.The sensing principle, sensitivity, resolution, and dynamic range of the proposed FOSRS are theoretically analyzed and verified by experiment. The experimental results show that the developed FOSRS with a 12.1 m sensing fiber has a flat sensitivity of 69.50 d B, a nanostrain rate(nε∕s) resolution, and a dynamic range of better than 95 d B.An ultrahigh static resolution of 17.07 pε∕s can be achieved by using a 25.277 km sensing fiber for long baseline measurements. The proposed method significantly outperforms existing indirect measurement methods and has potential applications in geophysical monitoring and crustal deformation observation.

DFB fiber laser sensor for simultaneous measurement of acoustic and magnetic fields

A novel distributed feedback（DFB） fiber laser sensor, which can measure acoustic and magnetic fields simultaneously, is proposed. The magnetic field can be measured by detecting the change of resonant frequency of the fiber laser, and the acoustic pressure can be measured by detecting the phase shift of the fiber laser. Both of the signals can be simultaneously demodulated in the frequency domain without affecting each other. Experimental studies show that the acoustic pressure sensitivity of this sensor is about-130 d B（0 dB re 1 pm∕μPa） and the sensor has a good linearity with a magnetic field sensitivity of 0.57 Hz∕mT.