The temperature and salinity parameters of seawater are important observation elements in the marine environment. In this paper, the feasibility of applying the whispering gallery modes of a single mode fiber (SMF) ring with small radius to sensing is proposed. The temperature and salinity are mainly perceived by physical method. The variation of the bending loss with temperature and salinity of the SMF under small radius is studied. In order to solve the problem of cross sensitivity of temperature and salinity, the double-loop intercalibration method is proposed to achieve high-sensitivity salinity sensing under a specific bending radius. The experimental results show that the bending loss of the SMF ring with the circumference of 28.5 mm and 31 mm has not changed obviously in the temperature range of 10 ~ 40°C, but the salinity sensitivity is better. At room temperature, when the monocyclic circumference is 31 mm, the bending loss with the salinity change rate is -5.773 ×10-2 dB/‰, the residual R2 < 0.01. When the ring circumference is 28.5 mm, the bending loss with the salinity change rate is 5.256 ×10-2 dB /‰, and the residual R2 < 0.01. The high-sensitivity sensing of salinity can be realized by the double-ring SMF through intercalibration.
In this paper, nonlinear noises that characterize the performance of a long-haul optical fiber sensing system were investigated. In a 50 km transmission system, when stimulated Brillouin scattering (SBS) occurs seriously, the phase noise of the interferometer increases from -102dB (0dB=1rad/sqrt(Hz)) to -84dB due to the enlargement of the laser linewidth and the deterioration of the signal-to-noise ratio (SNR). While the phase modulation (PM) and the Phase-generated carrier (PGC) modulation to the laser frequency are applied simultaneously, the suppression of SBS is 35dB and 10dB respectively in the backscattering spectra and the interferometric phase noise caused by SBS is completely eliminated. When the input power continues to increase and exceeds the modulation instability (MI) threshold, the system performance also deteriorates significantly. The forward output spectra of the 50 km optical fiber and phase noise of the interferometer are measured. The results show that with the increase of the injection power, the increase trend of the MI component in the total power of the spectrum is approximately consistent with that of the phase noise. It can be concluded that the phase noise introduced by MI is mainly caused by the increase of light intensity noise and the deterioration of optical SNR. Therefore, in order to reduce the impact of MI in the sensor system, it is needed to avoid the generation of serious MI as far as possible, and then the ultra-narrow band filter should be used to filter the MI sideband for the improvement of the system SNR.
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