1) The differences in the system PM noise when the compensation interferometer and the sensor are adjacent and separated are analyzed theoretically. The results reveal that the amplitude difference in the system PM noise under the two structures is 20lg(τ2/τ).2) When the compensation interferometer is adjacent to the sensor, the PM noise and random noise on the transmission fiber can be greatly reduced by the optical differential effect. A smaller path-matched difference indicates a more significant optical differential effect, and the returned PM noise on the transmission fiber can be effectively suppressed.3) When the compensation interferometer is separated from the sensor, the PM noise is only related to the arm difference of the sensor, which is independent of that of the compensation interferometer or their optical path-matched difference. As the arm difference of the sensor grows, the PM noise also increases.4) The optimal PMDI structure is obtained. In the design of a remotely interrogated OFS system with internal modulation of PGC, the compensation interferometer should be well shielded from sound and vibration. When the requirements of the internal modulation of PGC and PM for arm differences are satisfied, the optical path-matched difference should be as small as possible. Meanwhile, the compensation interferometer should be adjacent to the sensor. The optical differential effect of this structure can be used to suppress the PM noise and low-frequency random noise on the transmission fiber.

The interferometric optical fiber sensing (OFS) system based on the beam interference principle has the advantages of a wide working frequency band, small volume, and invulnerability to electromagnetic interference. It is of great significance for national security, oil and natural gas exploration, seismic detection and warning, etc. The Mach-Zehnder and Michelson interferometer structures are commonly used in practical OFS systems. Among them, the OFS system with path-matched differential interference (PMDI) is one of the research hotspots for its simple structure, high optical energy utilization rate, and flexible demodulation schemes. Up to now, the interference of the conventional PMDI structure is usually perfectly matched, and the compensation interferometer is located at the emitter or receiver end. This structure has the following problems when it is applied to the remotely interrogated OFS system: 1) the pickup noise accumulates along the remote transmission fiber and will be finally converted into great system noise by the optical differential effect; 2) in the remotely interrogated OFS system, optical frequency modulation or phase modulation (PM) is required to suppress the phase noise induced by stimulated Brillouin scattering (SBS). As the interference of the conventional PMDI structure is usually perfectly matched, external carrier signals cannot be directly loaded into the interferometer, which means the conventional PMDI structure is no longer applicable. Although the carrier signals could be loaded on one arm of the interferometer by a piezoelectric transducer (PZT) or an electro-optic modulator (EOM), this will destroy the passive property of the system and also introduce some electrical noise. In view of the above-mentioned defects, the paper proposes an improved PMDI structure for remotely interrogated OFS applications. Through numerical simulation and experimental verification, the optimal PMDI structure for remotely interrogated OFS systems is obtained. We hope that the conclusions drawn in this paper can provide a theoretical and experimental reference for the comprehensive design and noise suppression of remotely interrogated OFS systems.

This paper studies the influence of the PMDI structure on the noise of a remotely interrogated OFS system with phase generated carrier (PGC) techniques. Firstly, the influence of different PMDI structures on PM noise is analyzed. Through numerical simulation and experimental verification, the optimal PMDI structure for the remotely interrogated OFS system is obtained. Then, the influence of different path-matched differences on the PM noise of the system is studied. Finally, the phase noise background with different optical path-matched differences is measured experimentally.

The differences in the system PM noise when the compensation interferometer and the sensor are adjacent and separated are analyzed theoretically. The results show that the amplitude difference in the system PM noise under the two structures is . When the compensation interferometer is adjacent to the sensor, noise caused by environmental disturbance to the lead fiber can be greatly reduced by the optical differential effect (Fig. 2). When the compensation interferometer is separated from the sensor, the PM noise is only related to the OFS arm difference, which is independent of the arm difference of the compensation interferometer or their optical path-matched difference. When the OFS arm difference increases, the PM noise also rises (Fig. 3).

This paper first theoretically analyzes the influence of different structures and arm differences of the compensation interferometer and the sensor on the noise in the remotely OFS system of PMDI. Then, it designs an experiment for verification. The results show that the theory is in good agreement with the experiment, and the following conclusions are drawn.