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A magnetic field sensor based on multilongitudinal mode fiber laser (MMFL) is proposed and demonstrated. The MMFL contains two fiber Bragg gratings (FBGs), one of which is fixed on a magnetostrictive alloy (MA) and works as the sensing FBG. With a magnetic field applied, the MA stretches and transforms the magnetic field into strain due to the magnetostrictive effect of the MA. In this case, the wavelength of sensing FBG and the length of the MMFL cavity both shift with the magnetic field, ultimately resulting in the frequency shift of the longitudinal modes of the MMFL. By sending these longitudinal modes of the MMFL to a photodetector, the longitudinal mode beat signals (LMBSs) are generated, whose frequency would shift with the magnetic field. We experimentally verify that the magnetic field can be demodulated via the LMBS and demonstrate a sensitivity of -47 kHz/mT when selecting an LMBS at 1.608 GHz for demodulation. We also demodulate in optical domain by means of tracking the wavelength of the sensing FBG, a sensitivity of 1.5 pm/mT is achieved. Compared with the conventional fiber optic magnetic field sensors demodulated in the optical domain, radio-frequency demodulation is used in our work, which enhances the sensitivity and resolution. It also provides a potential way for high-speed demodulation. Moreover, the sensing head is a conventional FBG without any elaborate transducer, which enables the features of simple structure, easy fabrication, and compact size.
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