Distributed multi-parameter sensing utilizing Brillouin frequency shifts contributed by multiple acoustic modes in SSMF.

A multi-parameter optical-fiber sensor, which is based on multiple acoustic modes in stimulated Brillouin scattering (SBS) effect, for distributed measurement of temperature and strain utilizing standard single-mode fiber (SSMF) is proposed and experimentally demonstrated. By manipulating doping level and refractive index profile of the fiber, the properties of Brillouin gain spectrum (BGS) related to the guided optical and acoustic modes are analyzed. The simulated results indicate that multiple acoustic modes can be excited in single-mode fiber (SMF) and the BGS is composed of multiple peaks corresponding to multiple acoustic modes. Moreover, the temperature and strain sensitivities of different acoustic modes are unequal and the capability of discriminative measurement between temperature and strain can be proved. Simultaneously, the mode field diameter, the dispersion parameter, and the cutoff wavelength are calculated and the results show that parts of SSMF can be used for multi-parameter measurement. However, the accuracy of measurement is varied with the fiber structure parameters. Consequently, in experimental section, two different SSMFs are put into test and both have multiple-peak BGSs although the BGSs show a great difference to each other. The discrimination of temperature and strain is successfully demonstrated by analyzing the coefficients of the Brillouin frequency shifts introduced by different acoustic modes. In the fiber which has a better measurement result, the sensitivities of the fundamental acoustic mode are 1.19 MHz/°C and 62.28 kHz/με with an accuracy of 0.98 °C and 19.6 με in 20 km sensing range.