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A seawater temperature sensing method based on polydimethylsiloxane-coated (PDMS-coated) microfiber knot resonator (MKR) is proposed, which has the advantages of high sensitivity and weak salinity dependence. The dependences of the temperature sensitivity on fiber diameter, coating thickness and probing wavelength are theoretically investigated and the range of coating thickness for weak salinity dependence is obtained. By optimizing the parameters of the seawater temperature sensing system, when the probing wavelength is 1550 nm, the fiber diameter is 1 μm, and the coating thickness is 5 μm, the sensitivity can reach to 0.197 nm/°C. Results shown here are beneficial to find the optimal parameters for the temperature sensors with high sensitivity and weak salinity dependence.

Seawater temperature is one of the most important parameters of the ocean detection, which is important in oceanography, meteorology, biology, and marine military. Several seawater temperature detection methods have been reported, including conductivity-temperature-depth (CTD) system, microwave remote sensing and sensors based on fiber bragg grating [

In recent year, microfiber resonator (MR) has attracted more and more attention because of fast response, compact size, low loss and high sensitivity. The MRs can be used as refractive index, salinity, and temperature sensors, which have been demonstrated successfully and show high sensitivity [

In this paper, we propose a temperature sensor based on polydimethylsiloxane-coated microfiber knot resonator with high sensitivity and weak salinity dependence. The dependences of the sensitivity on fiber diameter, coating thickness and probing wavelength are theoretically investigated and the range of coating thickness for weak salinity dependence is obtained. Results shown here may offer helpful references for assembling micro-photon-ics device used in seawater temperature sensing and detection.

The schematic diagram of the seawater temperature sensor based on PDMS-coated microfiber knot resonator is shown in

The polymer PDMS is chosen because the thermo-optical coefficients of the PDMS is negative and_{p} = −1.8 × 10^{−4}/˚C [_{s} _{=} −1.0 × 10^{−4}/˚C [_{f} = 1.14 × 10^{−}^{5}/˚C [

The temperature can influence the refractive indices of the seawater and the material, then the effective refractive index (n_{eff}). The n_{eff} can be written as n_{eff} ≈ n_{1}η_{1}+ n_{2}η_{3}+ n_{3}η_{3} [_{1}, n_{2}, n_{3} are the refractive indices of the silica, PDMS and seawater, respectively. The η_{1}, η_{2}, η_{3} are the fractional power transmitted in the silica, PDMS, and seawater, respectively. Since the refractive index of the seawater is the function of temperature, salinity and probing wavelength [_{eff}. It is necessary to remove the influence of the salinity to detect the temperature accurately. In this paper, the coating method is proposed.

The sensitivity of the sensor is defined as the shift of resonance wavelength to the change of the temperature, which can be defined as

where T is the seawater temperature, _{eff} first, then the shift of the resonance wavelength. Thus, Equation (1) can be written as

It can be seen that the n_{eff} should be calculated first to obtain the sensitivity.

Using the finite element method, the n_{eff} is calculated by COMSOL, and the sensitivity is calculated by MATLAB. Since the n_{eff} is related to the probing wavelength, fiber diameter and coating thickness. We study the dependences of the sensitivity on these three parameters in next section.

Similar to the temperature sensitivity, the salinity sensitivity can be described as [

The dependence of salinity sensitivity on coating thickness is shown in

After coating, the most power transmits in the polymer, and little in the seawater. With the increasing coating thickness, the power in seawater decreases gradually, and the contribution of seawater refractive index to the n_{eff} decreases gradually. And the salinity only influences the refractive index of seawater, not the PDMS’s. Thus, with the increasing coating thickness, the power in seawater decreases. Since salinity changes do little to n_{eff}, the resonance wavelength can be regarded as no change. So, the coated MKR can detect the seawater temperature without the influence of salinity.

First, we calculate the temperature sensitivity before and after coating.

Because of the thermo-optical coefficient of the silica and the seawater, the bared microfiber can be used in seawater temperature sensing. When the fiber diameter is small, the power mostly transmits in the seawater due to the evanescent field. So the refractive index of seawater has a large influence on the n_{eff}. The refractive

index of seawater decreases with the increasing temperature. For this reason, to increase the sensitivity, the influence of the negative thermo-optical coefficient should be increased. So, we choose the polymer (PDMS) with a negative thermo-optical coefficient and its absolute value is larger than that of seawater.

As mentioned above, when the coating thickness ranges from 3 to 5 μm, the influence of salinity to temperature detection can be ignored. So we investigate the temperature sensitivity when coating thicknesses range from 3 to 5 μm.

Above simulations indicate that coating can increase the temperature sensitivity. To find the optimal coating thickness is the next problem. Since the temperature sensitivity increases with the increasing probing wavelength, the probing wavelength is chosen to be 1550 nm.

Besides the fiber diameter and coating thickness, the probing wavelength also influences the temperature sensitivity. As shown in Figures 3(b) and Figures 4(b), the temperature sensitivity increases with the increasing probing wavelength.

Combine the above results, the parameters for seawater temperature sensor can be chosen. Considering the loss and high sensitivity, the probing wavelength is assumed to be 1550 nm. When coating thickness ranges from 3 to 5 μm, the influence of the salinity on temperature detection can be ignored. The 5 μm coating thickness can meet the demand. Since the temperature sensitivity increases with the decreasing fiber diameter, the fiber di-

ameter should be small. Considering the problem of drawing microfiber, the fiber diameter is chosen to be 1 μm. Thus, the parameters of the sensor are wavelength 1550 nm, fiber diameter 1 μm, the coating thickness 5 μm, and the calculated sensitivity is 0.197 nm/˚C.

In conclusion, we propose a temperature sensor based on polydimethylsiloxane-coated microfiber knot resonator with high sensitivity and weak salinity dependence. The dependences of the sensitivity on fiber diameter, coating thickness and probing wavelength are theoretically investigated and the range of coating thickness for weak salinity dependence is obtained. When coating thickness ranges from 3 to 5 μm, the influence of the salinity on temperature detection can be ignored. So the range of coating thickness for weak salinity dependence is 3 - 5 μm. The temperature sensitivity increases with the decreasing fiber diameter, the increasing coating thickness and the increasing probing wavelength. The most important thing is that the polymer should have a negative thermooptical coefficient and its absolute value is larger than that of seawater. To obtain a high sensitivity, large probing wavelength, small fiber diameter and large coating thickness should be chosen. By optimizing the parameters of the seawater temperature sensing system, when the probing wavelength is 1550 nm, the fiber diameter is 1 μm, and the coating thickness is 5μm, the sensitivity can reach to 0.197 nm/˚C. Results shown here are beneficial to find the optimal parameters for the temperature sensors with high sensitivity and weak salinity dependence, and offer helpful references for assembling micro-photonics device used in seawater temperature sensing and detection.

We would like to thank financial supports from the Natural Science Foundation of China (No. 61171161) and the Fundamental Research Project of Qingdao (No. 12-1-4-1-(26)-jch).