Objective  Although conventional electronic sensing technologies are mature, current marine environmental monitoring that primarily relies on them still faces limitations, such as poor interference immunity and high system complexity. While fiber optic sensing technology offers advantages like immunity to electromagnetic interference and structural flexibility, existing research has largely focused on single-parameter detection or wavelength-interrogation methods, leaving challenges such as cross-sensitivity in the simultaneous decoupling measurement of multiple parameters unresolved. This study aims to construct an all-fiber multi-parameter sensing probe that achieves high-sensitivity synchronous detection and effective decoupling of temperature, salinity, and pressure, thereby providing a reliable and easily integrable fiber optic sensing solution for in-situ marine environmental monitoring.

Methods This study adopts a systematic experimental approach, which primarily consists of two parts: sensor fabrication and performance testing. The sensor fabrication procedure is outlined as follows. First, a photonic crystal fiber is spliced with a single-mode fiber using a fusion splicer, and a tapered region is formed through a tapering process to construct a Mach–Zehnder interferometer. Subsequently, a hollow-core fiber is spliced to an SMF, cleaved approximately 80 μm from the splice point, and a polymer film is formed using UV-curable adhesive to fabricate a Fabry–Pérot interferometric cavity. Finally, a fiber Bragg grating is connected in series with the above structures via single-mode fiber to form the complete composite sensor. Performance testing was conducted on a dedicated optical platform. A broadband light source with a wavelength range of 1250–1650 nm and an optical spectrum analyzer with a resolution of 0.04 nm were employed. For salinity testing, a series of NaCl solutions with concentrations ranging from 0.5 to 3.0 mol/L were prepared, and the solutions were injected via a syringe into a glass capillary tube containing the packaged sensor to complete the measurements. Pressure testing was performed by adjusting the immersion depth of the sensor in a 1.6 m water column to achieve continuous pressure variation within the range of 2.94–8.82 kPa. Temperature testing involved placing the sensor in a water environment on a temperature-controlled stage, with heating and cooling tests carried out in the range of 29–39 ℃ with a step size of 1 ℃. Based on the test results, a sensitivity matrix was constructed to enable synchronous decoupling measurement of temperature, salinity, and pressure.

Results and Discussions The influence of salinity, pressure, and temperature on the spectral characteristics of the composite fiber sensor is summarized as follows. For salinity sensing, the intensities of the interference dips Dip1 and Dip2 in the transmission spectrum increase linearly with rising salinity, with Dip1 exhibiting a sensitivity of −25.1557 dB/(mol/L), while the FBG reflection peak remains stable, confirming its temperature-sensing independence. Under pressure variation, the depths of the interference dips decrease regularly with increasing pressure. The pressure sensitivities of Dip1 and Dip2 are 2.2878 dB/kPa and 1.456 dB/KPa, respectively, and the FBG reflection peak shows negligible response to pressure change, indicating good pressure-temperature decoupling capability. Temperature experiments reveal a stable response of the FBG reflection peak with a sensitivity of −0.0465 dB/℃, whereas the interference dips exhibit piecewise-linear temperature dependence. Based on the above experimental results, a sensitivity matrix was established, which successfully enables simultaneous decoupling measurement of salinity and pressure. The average measurement errors for salinity and pressure are no more than 0.029 mol/L and 0.28 kPa, respectively, demonstrating reliable multi-parameter sensing performance of the sensor for marine environmental monitoring.

Conclusions This paper proposed and experimentally validated an MZI sensor for simultaneous measurement of temperature, liquid salinity, and pressure. By tapering a PCF used for salinity sensing and splicing it with an FBG for temperature compensation on one side and a polymer-film FPI for pressure sensing on the other, a complete MZI sensor capable of multi-parameter measurement was constructed. The transmission of input light through the fiber sensor was analyzed theoretically in detail, and its performance was demonstrated through comparative experimental validations. The results show that the fabricated sensor exhibits linear spectral responses to variations in liquid salinity and pressure, with salinity and pressure sensitivities reaching −25.1557 dB/(mol/L) and 2.2878 dB/kPa, respectively, while temperature compensation via the FBG achieves a temperature sensitivity of −0.0465 dB/℃. By constructing a sensing matrix based on the salinity–pressure sensitivities of two selected interference dips and the FBG reflection peak, simultaneous measurement of the three parameters is enabled.

Although this sensor demonstrates excellent sensing performance in laboratory conditions, systematic research on environmental stability and long-term reliability is still required for practical marine applications. Subsequent work will be conducted at three levels: 1) material level: conduct long-term seawater immersion and accelerated aging tests on polymer films to evaluate their service life; simultaneously explore inorganic film alternatives (e.g., Al₂O₃, SiO₂) to fundamentally enhance the chemical stability of the sensing unit. 2) Encapsulation level: design a multi-layer encapsulation structure comprising “flexible buffer-rigid protection-anti-fouling coating” with a focus on micro-area coating protection for vulnerable areas like tapered PCFs. Conduct high hydrostatic pressure cycling tests and seal integrity verification. 3) System validation layer: construct a multi-parameter coupled environmental simulation chamber to conduct long-term drift and cross-sensitivity re-evaluation tests. Plan real-sea deployment trials to validate the sensor's long-term reliability under typical oceanic conditions. In summary, this sensor demonstrates strong potential in structural design, sensing performance, and decoupling capability. Future work will follow a three-pronged research approach integrating materials, packaging, and systems to progressively advance it from laboratory prototyping to practical in-situ ocean monitoring applications. This will provide a novel all-fiber optic sensing solution for high-precision, interference-resistant, long-term continuous observation of ocean temperature, salinity, and depth.