Ocean internal-wave phenomena occur with the variation in seawater vertical temperature, and most internal-wave detections are dependent on the measurement of seawater vertical temperature. A seawater temperature sensor based on a microfiber knot resonator (MKR) is designed theoretically and demonstrated experimentally in this paper. Especially, the dependences of sensing sensitivity on fiber diameter and probing wavelength are studied. Calculated results show that sensing sensitivity increases with the increasing microfiber diameter with the range of 2.30–3.91 μm and increases with the increasing probing wavelength, which reach good agreement with results obtained by experiments. By choosing the appropriate parameters, the maximum sensitivity measured can reach to be 22.81 pm/°C. The seawater temperature sensor demonstrated here shows advantages of small size, high sensitivity, easy fabrication, and easy integration with fiber systems, which may offer a new optical method to detect temperature of seawater or ocean internal-wave phenomenon and offer valuable reference for assembling micro sensors used for other parameters related to seawater, such as salinity, refractive index, concentration of NO3− and so on.

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The salinity and temperature of seawater are important parameters in oceanography. Based on the directional coupler assembled by polyimide-coated micro/nanofibers, optical sensors with high sensitivity for simultaneous salinity and temperature sensing in seawater are proposed. Dependences of sensitivities on wavelength, salinity, and temperature are investigated theoretically, with which performances of such sensor under general sea conditions can be evaluated. Results show that salinity and temperature sensitivities can reach levels of nm/‰ and nm/°C, which are much higher than those of fiber Bragg gratings, knot resonators, and photonic crystal fibers. Other considerations for system design such as the length of the coupling area, the diameter difference between two fibers, and the thickness of polyimide coatings are also discussed. Sensors proposed here suggest a simple approach to realize high-sensitivity micro/nanofiber optical sensing of salinity and temperature in seawater simultaneously and may find applications in developing miniature sensors used in seawater.