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Brillouin fiber optic sensing has been attracting much attention as one of the best ways of monitoring the temperature and/or strain distribution of large structures, such as bridges or pipelines. To detect abnormal sections of such structures at an earlier stage, improvement of measurement sensitivity is required. In the standard Brillouin fiber optic sensing, sensitivity given by the temperature or strain coefficient of Brillouin frequency shift (BFS) is about 1 MHz/℃ or 0.05 MHz/με, respectively. In this study, we introduce a new method utilizing two-frequency pump and probe light to enlarge these coefficients without using special fibers. In this method, Brillouin gain spectrum is measured by sweeping the two frequencies of probe light in the opposite directions, where the measured spectrum has two peaks. The separation between the two peaks linearly changes with BFS, and so it has a linear relation to temperature and strain of an optical fiber. Since the changing rate of the separation between the two peaks is twice as large as that of BFS, the temperature or strain sensitivity is doubled to 2 MHz/℃ or 0.1 MHz/με. The enhanced sensitivity was experimentally confirmed in the proof-of-concept experiment.
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