Structure Design of Disposable Ocean Temperature Depth Sensing Probe Based on F-P Cavity and FBG

The basic principle of sensing is the combination of F-P cavity interference and fiber grating reflection. A hybrid structure sensor probe has been designed based on the combination of an F-P cavity of liquid-filled thermometer structure, and a fiber grating with an elastic diaphragm, herein F-P cavity is used for temperature sensing, and the fiber grating is used for pressure sensing. By adopting the dual optical path structure, the dual-parameter detection of temperature and depth is realized, which solves the problem of low accuracy caused by the cross response of temperature and pressure of a single sensor device and the calculation of the depth information of the ocean with empirical formulas. Compared with traditional sensors, the sensitivity is effectively improved. Theoretical analysis shows that the sensitivity of the F-P cavity with a thermometer structure filled with kerosene reaches 1.334 nm/ ̊C, and the depth sensitivity of the fiber grating is 284.6 pm/Mpa within the ocean depth range of 0 400 m.


Introduction
Temperature and depth are the most basic and important parameters in marine physics. With the continuous development of optical technology, optical sensors have been used in marine resource development, climate monitoring, military industry and other fields [1] [2] [3] [4]. Among them, the fiber grating and F-P cavity structure are the most widely studied objects in optical sensing due to their strong anti-interference ability, high sensitivity, small size, intrinsic insulation, etc. [5]. Expendable bathythermograph (XBT) is a commonly used structure in ocean temperature depth sensing, its sensitivity and response time are the most important technical parameters. For traditional sensing probes, which generally use electrical sensing methods, and its response time is on the order of 100ms, there is a risk of large errors and short circuits because the depth detection is estimated by the falling time of the probe [6] [7].
In 2018, Yang Yu [8] and others prepared an ultra-fine fiber coupler (OMC) using two twisted traditional communication fibers, providing a promising method for improving the temperature sensitivity. Experiments show that the sensor can measure salinity, temperature and depth in seawater under a pressure of 0 -25 Mpa, with temperature sensitivity is 2.326 nm/˚C, and pressure sensitivity is 169 pm/Mpa.
In 2019, Yong Zhao et al. [9] used a gold film coated on the surface of an optical fiber to excite the plasmon resonance effect, and two sensitive films (PDMS, SU-8) were plated on the outside of the Au film at different positions. The temperature sensitivity measured by the experiment is −1.82 nm/˚C, and the depth sensitivity is 2.838 nm/MPa. This structure has great practical significance.
In 2020, Liang Cao [10] and others used ultra-fine fiber couplers combined with fiber optic loop (MCFL) reflective photonic devices for seawater temperature and depth sensing experiments. MCFL's reflective composite sensing optical path design eliminates the effect of MC free port pair on the sensor, and the temperature sensitivity of 248.2 pm/˚C and the depth sensitivity of 122.6 pm/Mpa is obtained. This structure provides a new design for the marine environmental parameter sensor based on the micro-fiber coupler.
In 2021, Hongchang Deng [11] and others proposed a high-sensitivity fiber grating strain and temperature sensor composed of input SMF, HCOF and output SMF cascade structure. Experiments show that the strain sensitivity can reach −9.29 pm/μ and then the temperature sensitivity can reach 17.14 pm/˚C. If this structure is further improved, it will have the potential for high-sensitivity detection of ocean depths.
The above research shows that the use of optical fibers with different structures and special processes for coupling and coating will greatly improve the sensitivity of the sensor device, but the response time of sensing is seldom addressed, and the complex process will reduce the stability and repetition of the sensor device. The temperature and pressure in the same device will be sensitive to cross transmission, which will affect the overall sensing accuracy.
This paper proposes an ocean temperature depth sensing structure for XBT application. The structure is based on the parallel connection of FP cavity and Fiber-Brag-Grating (FBG). The temperature and depth are measured by the FP cavity part of the thermometer structure and the FBG incorporated with the elastic diaphragm. Theoretical analysis and calculation, a sensor structure with good temperature sensitivity and depth sensitivity have been designed, which provides a feasible solution for further application on XBT. Journal of Applied Mathematics and Physics

Sensor Probe Design
The F-P cavity and the FBG are connected in parallel, and the specific optical path structure of the sensing system is shown in Figure 1.
The part of the FP cavity, used to measure temperature, is first filled with liquid into the microcapillary. One end of the thin tube is inserted with SMF and the port is welded, and the other end is welded with a cylindrical quartz tube filled with water to form a seal. The liquid surface and the end face of the SMF constitute the FP cavity structure. In the FBG part used to measure pressure, the FBG with one end connected to the spring is connected to the elastic diaphragm, as indicated. Since the outside will be encapsulated by a steel cylinder, the lateral stress is largely overcome, so this article ignores the influence of the lateral stress, the probe structure diagram as shown in Figure 2.
In Figure 2

Theoretical Analysis of Temperature Sensing Structure
In the fiber optic FP temperature sensing structure, two reflective surfaces (the reflectivity of which are 1 R and 2 R respectively) form an F-P cavity with a length of L. If 1 2 R R R = = and the refractive index of the F-P cavity is n, when the intensity and When a light beam with wavelengths λ , intensity 0 I , enters the F-P cavity, the reflected light intensity R I is  According to the sensing range, the overall wavelength drift is deduced, then according to the above formula, the temperature-wavelength relationship for the three different sensitizing liquids can be obtained as shown in Figure 3.

Theoretical Analysis of Depth Sensing Structure
In the FBG part used for pressure sensing, the central reflection wavelength of the fiber Bragg grating depends on the period of the fiber grating and the effective refractive index of the fiber core. The equation can be expressed as The change of external stress will cause the center wavelength of the fiber Bragg grating to shift, and its expression is In the formula, eff n represents the effective refractive index. When the external uniform axial stress acts on the fiber grating, it will cause the axial strain to change the effective refractive index of the fiber core. At the same time, the axial strain will also change the grating period Λ, the expression is Then the wavelength change caused by the axial strain can be expressed as P ε in the equation is the effective elasticity coefficient. The parameters of the fiber grating to be used are: the central reflection wavelength is 1550 nm, the core diameter is 8.2 μm, the cladding diameter is 125 μm, the grating length is 10 mm, and the grating tensile limit is 3000 με. 0.22 P ε = . So its strain sensitivity is Journal of Applied Mathematics and Physics

pm/με.
We choose to use a stainless steel elastic diaphragm with a thickness of h = 0.45 mm and a radius of R = 5 mm. The effective length of the fiber grating is 3 mm. For the ocean pressure range of 0.1 -5 MPa, the pressure sensitivity of the FBG to be designed is calculated as 284.6 pm/Mpa.

Conclusion
In this paper, an expendable ocean temperature probe structure based on F-P cavity and FBG is designed. According to the application requirements of XBT fast sensing, the theoretical analysis and simulation of temperature sensitivity, depth sensitivity and response time are carried out. The sensitivity effects of several different sensitizing liquids and the addition of elastic diaphragms on the sensing structure were compared, and the temperature sensitivity of 277.4 pm/˚C, 666.8 pm/˚C, 1.334 nm/˚C, and the depth sensitivity of 284.6 pm/Mpa were obtained for pure water, glycerol and kerosene as the sensitizing liquids, respectively. It testified the theoretical feasibility of using this structure to obtain good sensitivity and response time. Under the condition of linear expansion of the liquid in the cavity, the use of a liquid with a high expansion coefficient can enhance the sensitivity of the sensing structure; therefore it is suitable for marine expendable depth and temperature sensor.