基于光纤激光的海洋传感技术研究

  光纤传感器凭借体积小、灵敏度高、抗电磁干扰及在恶劣环境下的稳 定性而备受研究关注。然而，传统的宽带光源调制的干涉仪存在解调精度 低、成本高等方面的不足，难以满足特定应用的需求。本文聚焦于海洋传 感技术中对温度和盐度测量的核心需求，通过融合光纤激光传感原理及多 样化的光纤类型，设计出多种传感结构，并结合波长解调和散斑解调技术 进行了实验研究。具体研究内容包括：

  在材料增敏方面，利用掺铒光纤（Er-Doped Fiber, EDF）的特性提升 灵敏度，并通过低成本熔接技术制备了基于双花生内嵌拉锥的马赫曾德尔 干涉结构传感器。该结构利用纤芯失配技术增强了系统的灵敏性，同时拉 锥结构进一步强化了光与物质的相互作用，实验结果显示其折射率灵敏度 高达 441.56 nm/RIU，相较于普通单模光纤结构提升了 115.22 nm/RIU。 更进一步实验证明该结构在海洋传感领域应用的潜力，测得温度和盐度灵 敏度分别为－0.046 nm/℃和 0.2368 nm/‰。

  在结构增敏方面，利用游标效应有效提高灵敏度的原理，提出并验证 了 Sagnac 内嵌拉锥保偏光纤结构，通过游标效应显著提升温度和盐度测量 的灵敏度。实验结果表明，该传感器的温度和盐度灵敏度分别达到－3.992 nm/℃和 0.2687 nm/‰。此外，利用单模光纤-空芯光纤-多模光纤级联结构 制备了高稳定性的光纤温度传感器，该传感器温度灵敏度为 69.2 pm/℃。 进一步实验中，设计了一种基于散斑图像识别的单模光纤-空芯光纤-EDF的游标效应传感器，通过零均值归一化互相关算法处理散斑场变化以实现 测量。其温度灵敏度为－0.0224 /℃，盐度灵敏度为－0.0439 /%。这种散 斑解调方式数据处理简单，且结果具有唯一性。

  在系统优化方面，利用光纤激光的高 Q 值、高分辨力特点，设计了两 款光纤环形激光传感器。其中，基于拉锥边孔光纤的马赫曾德尔干涉结构 在激光器中作为滤波器与传感器使用，实现了－0.4270 nm/℃的温度灵敏 度和 0.3347 nm/‰的盐度灵敏度。另一种基于 Sagnac 干涉仪内嵌拉锥边 孔光纤的光纤环形激光传感器，实现了－0.3041 nm/℃和 0.2867 nm/‰的 温度和盐度灵敏度。

  在降本增效方面，利用散斑解调的高分辨力特性，提出了一种多模光纤散斑解调 DBR 光纤激光器的解调方案。利用倾斜光纤光栅的温度盐度传 感特性以及散斑解调的高分辨率优势进行测量。实验测得，该传感器温度 灵敏度为－0.0150 /℃，盐度灵敏度为－0.0158 /‰。

  综上，这四个维度在提升光纤传感器性能上相互关联。材料增敏通过 选用 EDF 材料提高灵敏度，结构增敏则通过设计游标效应结构进一步放大 响应信号，系统优化确保这些增强的信号能够被精确捕捉与处理，而降本 增效则在保持高性能的同时，实现了技术的经济性和广泛应用的可能性。

Fiber optic sensors have gained significant attention due to their small size, high sensitivity, anti-electromagnetic interference, and stability in harsh environments. However, the traditional interferometer modulated by a wideband light source has some drawbacks, such as low demodulation precision, high cost, and difficulty in meeting specific application needs. This thesis focuses on the essential requirements of ocean sensing technology, namely, temperature and salinity measurement. The study integrates the principle of fiber laser sensing and various fiber types to design several sensing structures. The research also carries out experimental investigations by combining wavelength demodulation and speckle demodulation techniques. The specific research topics include:

The utilization of detection sensitivity for material sensitization can be achieved through the use of Er-doped fiber (EDF) properties. A Mach-Zehnder interferometer (MZI) sensor based on a peanut embedded taper structure is developed using low-cost fusion technology. The taper structure enhances the interaction between light and matter, resulting in a refractive index sensitivity of 441.56 nm/RIU, which is higher than that of the ordinary single-mode fiber structure by 115.22 nm/RIU. Further experiments demonstrate the potential of the structure in the field of ocean sensing, the temperature and salinity sensitivity reach 0.046 nm/℃ and 0.2368 nm/‰, respectively.

In terms of structural sensitization, the principle of using Vernier effect can effectively improve the sensitivity. A Sagnac embedded taper polarization-maintaining fiber structure has been proposed and verified. The structure significantly improves the sensitivity of temperature and salinity measurements through the vernier effect. The experimental results show that the temperature and salinity sensitivity of the sensor reach －3.992 nm/℃ and 0.2687 nm/‰, respectively. Additionally, a high-stability optical fiber temperature sensor has been developed, which employs a cascade structure of single-mode fiber, hollow core fiber, and multi-mode fiber. This sensor has a temperature sensitivity of 69.2 pm/℃. Furthermore, a single-mode fiber-hollow fiber-EDF Vernier effect sensor based on speckle image recognition has been designed. The zero-mean normalized cross-correlation algorithm is utilized to process speckle field changes for measurement. The experimental results show that the temperature and salinity sensitivity of the sensor reach －0.0224 /℃ and －0.0439 /%, respectively. The data processing of this speckle demodulation method is simple and the results are unique.

Two fiber ring laser sensors have been designed for system optimization, utilizing the high Q-value and high resolution of fiber laser. One of them uses the MZI structure based on side-hole fiber as a filter and sensor in the laser, achieving a temperature sensitivity of －0.4270 nm/℃ and a salinity sensitivity of 0.3347 nm/‰. The other fiber ring laser sensor employs a Sagnac interferometer embedded with a taper side-hole fiber, resulting in temperature and salinity sensitivities of －0.3041 nm/ °C and 0.2867 nm/‰.

A new method for demodulating multimode fiber speckle demodulation DBR fiber lasers has been proposed to increase efficiency while reducing cost. The proposal utilizes the temperature and salinity sensing characteristics of inclined fiber grating, along with the high-resolution advantage of speckle demodulation for measurement. The sensor has a temperature sensitivity of －0.0150 /℃ and a salinity sensitivity of －0.0158 /‰.

Overall, these four dimensions are interrelated in improving the performance of optical fiber sensors. Material sensitization improves the sensitivity by selecting EDF materials, and structural sensitization further amplifies the response signal by designing a cursors effect structure. System optimization ensures that these enhanced signals can be accurately captured and processed, and cost reduction and efficiency increase while maintaining high performance. The economy of the technology and the possibility of wide application are realized.

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