A compact optical magnetic field sensor based on a fiber laser oscillator circuit merged with a Sagnac loop that contains a polarization-maintaining fiber (PMF) and a magnetostrictive rod is proposed. The combination of the PMF-Sagnac loop and fiber laser oscillation circuit significantly increases the signal-to-noise ratio of the reflection spectrum in the system, which is a mixed spectrum that includes interference fringes and lasers. For the proposed system, an erbium-doped fiber amplifier is used to provide gain, a dispersion compensating fiber is used to compensate for dispersion in the system, a PMF is inserted in the Sagnac loop as a microwave filter, while a section of the PMF is bonded to a magnetostrictive rod to achieve magnetic field sensing. The proposed system is shown experimentally to produce a fine reflection interference spectrum. The highest intensity reflection interference peak (from the laser) is over 40 dB with a –3 dB line-width of about 0.03 nm. The system can provide magnetic field sensitivities of 0.07 nm/mT and 0.076 nm/mT for sensor head lengths of 1 m and 2 m, respectively, and exhibits a stability of ±0.029 nm over 1 h. The proposed sensing system has advantages of low cost, high sensitivity, compact structure, and can produce a fine and stable reflection spectrum.

We propose, analyze, and experimentally demonstrate a highly sensitive and stable fiber-laser pressure-sensing system based on an unequal-arm Mach-Zehnder interferometer (MZI) merged with a Sagnac interferometer (SI). The unequal-arm MZI structure consists essentially of two single-mode fibers of different lengths, and the SI structure uses a section of polarization-maintaining fiber, which acts as a sensing head. Moreover, in the proposed sensing system, the MZI and SI structures both act as a cascade filter. Given the asymmetric MZI and cascade filter structure laser mode hopping is effectively suppressed, so minor variations in environmental parameters cause a stable shift of the interference spectrum. The experimental results show that the proposed system can be used to make accurate pressure measurements. The −3 dB linewidth of the reflection interference peak produced by the proposed system is less than 0.02 nm, and the signal-to-noise ratio (SNR) can exceed 45 dB. With a 1-m-long sensor head, the proposed system provides a pressure sensitivity of 29.275 nm/MPa, and the interference spectrum of the sensing system fluctuates less than ±0.02 nm over 1 h. The proposed sensing system thus offers the attractive characteristics of good sensing linearity and stability, high SNR, and high sensitivity.

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