Self-heated fiber thermal lever sensor based on temperature gradient inside a hermetic chamber

Fiber-optic sensors are increasingly being used in several applications owing to their remarkable high sensitivity and large dynamic range properties, low cost, and ability to transmit signals from remote sensors to the signal processors. In particular, fiber-optic lever sensors offer many advantages over conventional lever sensors and have attracted significant attention in various sensing applications. For instance, they are highly preferred for liquid lever measurements due to their non-electrical sensing nature. A typical fiber-optic lever sensor requires a proof mass to provide the necessary inertia force. Based on the type of proof mass used, fiber-optic lever sensors can be classified into two, namely, solid mass and liquid mass.

Unlike fiber-optic lever sensors based on a solid mass, those based on liquid masses are compact and exhibit high directional sensitivity and good linearity. Nonetheless, they both suffer from low shock survival rating when exposed to strong shock and vibration conditions, attributed to large inertia induced when the proof masses are acted upon by gravitational forces. This has necessitated the need for alternative and more efficient fiber optical lever sensors. Recently, proof-mass-free thermal convection-based lever sensors exhibiting high shock reliability have been identified as a promising solution. Unfortunately, these sensors are based on electronic heaters and detectors, which results in more challenges such as electromagnetic interference leading to inaccurate results.

To this account, Professor Ran Gao from Beijing Institute of Technology, Dr. Jiansen Ye from Zhengzhou Institute of Technology, and Dr. Danfeng Lu from Xi’an University of Technology designed a new thermal convection-based fiber lever sensor. The authors also looked at the operating principle as well as the properties of the proposed lever sensor. Their work is currently published in the research journal, Fiber Optics.

In their approach, a self-heated Co²⁺-doped microfiber heater replaced the solid and liquid masses used in conventional lever sensors as the source of heat. The doped-microfiber was heated via pump laser to generate symmetrical temperature profiles inside the hermetic chamber through thermal convection. Additionally, a Michelson interferometer was constructed by placing two micro-single mode fibers orthogonal to the Co²⁺-doped microfiber. The feasibility of the Michelson interferometer was validated by detecting the tilt angles and temperature distribution at different tilts angles.

The authors found the proposed fiber lever sensor worked based on the thermal convection principle. Due to natural convection effects, the convection currents remained in the same state at different tilt angles while the two micro-single mode fibers were not located on the same isothermal line. Therefore, it was possible to detect the tilt angles by interrogating the Michelson interferometer. For instance, a tilt angle sensitivity of up to 95 pm/deg could be achieved. Furthermore, the proposed lever sensors exhibited a compact size, large dynamic range, high sensitivity properties desired for various applications.

In summary, the study proposed a fiber lever sensor based on the thermal convection principle. The proposed design utilized a Co²⁺– doped microfiber as a heat source instead of proof masses used in conventional lever sensors. Results showed that the Michelson interferometer could detect various tilt angles in different directions. Moreover, the proposed sensor exhibited remarkable properties, including a large dynamic range, high sensitivity, and low-cost. In a statement to Advances in Engineering, Professor Ran Gao said that the fiber thermal lever sensor is a promising candidate for numerous applications such as civil engineering and aviation.