Whispering gallery mode (WGM) optical microcavity is recognized for its high sensitivity and quality factor, making it invaluable in fields such as fundamental physics and biochemistry. However, traditional fabrication methods pose several challenges, including issues with chemical etching processes on silicon substrates that can adversely affect other optical devices and present health hazards. We address these challenges by employing a three-dimensional (3D) printing technique using two-photon polymerization to fabricate the sensor. This method offers high processing resolution, low fabrication costs, excellent repeatability, and sensitive sensing capabilities, presenting a viable solution for designing and manufacturing high-quality WGM microcavities. In addition, traditional preparation materials such as glass and crystal are limited by glass’s restricted tunability and crystal’s high processing costs. By utilizing polymer materials, our study overcomes these limitations and enhances the sensitivity of the sensor.

We use Lumerical Mode Solution software to simulate the optical field in the coupling region. The results show that the coupling gap of 100 nm could achieve the ideal resonance effect under the selected waveguide size. The proposed micro-ring resonator is prepared on silicon substrate by two-photon polymerization 3D printing technology with suitable laser intensity and scanning speed, and developed by propylene glycol methyl ether acetate solution and isopropyl alcohol solution. After cleaning the input and output optical fibers with a cutting knife and an optical fiber welding machine, the conical waveguides are precisely aligned using a six-dimensional displacement platform. The structure is then solidified with ultraviolet (UV) glue under ultraviolet light.

The supercontinuum light source and spectrograph are connected with the cured package structure, and the transmittance spectra in the air are 8.96 nm and 14.0 dB respectively. The structure is placed in a temperature and humidity-controlled chamber and tested across a temperature range of 5?35 ℃. The linear fitting results of the transmission spectrum show that the structure has a good linear temperature sensitivity of 243 pm/℃ in the measured temperature range (Fig. 6), which is 2?3 times higher than that of the same type of structure and close to the temperature sensitivity of the cascaded micro-ring resonator. To further explore the salinity sensitivity of the proposed structure, it is placed in a beaker of standard seawater sample solution, and the temperature and humidity are kept constant. When the concentration of standard seawater changes from 20‰ to 230‰, the transmission spectrum of the structure is recorded. The linear fitting results demonstrate good linearity over this range, with a salinity sensitivity of 28.2 pm/‰. These findings highlight the proposed micro-ring resonator's excellent temperature and salinity sensitivity, alongside its compact structure and reproducibility.

Our study demonstrates the successful fabrication of a micro-ring resonator sensor on a silicon substrate using polymer materials and two-photon polymerization 3D printing technology. The sensor leverages the strong evanescent field effect of the WGM microcavity and precise wave coupling enabled by the 3D printing process, achieving highly sensitive temperature and salinity measurements. Within a temperature range of 5?35 ℃, the sensor exhibits a temperature sensitivity of 243 pm/℃, significantly outperforming similar structures and approaching the sensitivity of cascaded micro-ring resonators. In standard seawater from 20‰ to 230‰, the salinity sensitivity is 28.2 pm/‰. This kind of optical sensor, characterized by its compact design, high sensitivity, and ease of fabrication, shows considerable potential for applications in fundamental physics, biochemistry, and related fields.