Ultra-Sensitive Strain Sensor Based on Femtosecond Laser Inscribed In-Fiber Reflection Mirrors and Vernier Effect

One of the efficient techniques to enhance the sensitivity of optical fiber sensor is to utilize Vernier effect. However, the complex system structure, precisely controlled device fabrication, or expensive materials required for implementing the technique creates the difficulties for practical applications. Here, we propose a highly sensitive optical fiber strain sensor based on two cascaded Fabry–Perot interferometers and Vernier effect. Of the two interferometers, one is for sensing and the other for referencing, and they are formed by two pairs of in-fiber reflection mirrors fabricated by femtosecond laser pulse illumination to induce refractive-index-modified area in the fiber core. A relatively large distance between the two Fabry–Perot interferometers needs to be used to ensure the independent operation of the two interferometers. The fabrication of the device is simple, and the cavity's length can be precisely controlled by a computer-controlled three-dimensional micromachining platform. Moreover, as the device is based on the inner structure inside the optical fiber, good robustness of the device can be guaranteed. The experimental results obtained show that the strain sensitivity of the device is ∼28.11 pm/μϵ, while the temperature sensitivity achieved is ∼278.48 pm/°C.