High-sensitivity strain sensors play a crucial role in various fields such as aerospace engineering, structural health monitoring, human healthcare, and motion tracking. Over the past few decades, a wide range of optical strain sensors have been developed using different methods and structures. However, many of these sensors require costly equipment or specialized optical fibers, increasing both expense and fabrication complexity. In recent years, the Vernier effect has attracted significant attention due to its ability to substantially enhance the sensitivity of various optical fiber sensors. A typical Vernier-effect-based fiber sensor consists of two interferometers with similar free spectral ranges (FSRs). A minor spectral shift in one interferometer leads to a pronounced shift in the superimposed spectrum, which significantly improves the sensitivity by several orders of magnitude. Various Vernier-effect-based sensor structures have been proposed for measuring physical parameters such as temperature, refractive index, displacement, and strain. However, traditional fiber-optic sensors often rely on broadband light sources, resulting in low optical transmission power and wide 3 dB bandwidths, which compromise sensing accuracy. If a light source with higher power and a narrower 3 dB bandwidth is used, sensing accuracy can be significantly improved. Fiber laser sensors offer a promising solution to these limitations. In such systems, the sensing element acts as a filter, and its central wavelength determines the operating wavelength of the laser. Changes in external conditions lead to a shift in the central wavelength of the filter, which in turn alters the operating wavelength of the laser. While the integration of the Vernier effect with fiber laser sensing has enhanced the performance of strain sensors, challenges such as limited stability and high temperature cross-sensitivity remain. To address these limitations, we propose a high-sensitivity fiber laser strain sensor based on a Vernier effect filter.

The core of the proposed sensor is a Vernier-effect-based filter composed of two cascaded Mach-Zehnder interferometers (MZIs), where one functions as the reference arm and the other as the sensing arm. The FSRs of the two MZIs are matched by carefully adjusting the length difference between the arms, ensuring the generation of a Vernier effect. When external stress causes a change in the optical path of the sensing MZI, the laser’s central wavelength experiences a significant shift, enabling high-sensitivity strain detection.

The sensing MZI is stretched in increments of 72 nm, corresponding to a strain change of 0.72 με. As strain increases, the peak wavelength of the laser shifts accordingly. Within the strain range of 0?3.6 με, the sensor exhibits a sensitivity of 4.15 nm/με (Fig. 6). In repeatability tests, the maximum deviation in strain sensitivity across four experiments is 0.023 nm/με (Fig. 6). During a 1-h stability test, the wavelength drift remains between 0.04 nm and 0.06 nm (Fig. 7). After temperature compensation, the temperature sensitivity is 0.097 nm/℃, and the temperature cross-sensitivity is 0.023 με/℃(Fig. 9).

We present a high-sensitivity strain fiber laser sensor based on a Vernier effect filter, which comprises two cascaded MZIs serving as reference and sensing arms. The FSR of the two MZIs is closely matched to generate a Vernier effect. By integrating the Vernier effect filter into a ring fiber laser, the sensor achieves a high strain sensitivity of 4.15 nm/με over a range of 0?3.6 με. The system also shows excellent repeatability, with a maximum strain deviation of 0.023 nm/με across four trials, and good stability, with a wavelength drift of only 0.04?0.06 nm over 1 h. After temperature compensation, the temperature sensitivity reaches 0.097 nm/℃, and the temperature cross-sensitivity is 0.023 με/℃. To the best of our knowledge, this sensor demonstrates the highest strain sensitivity among reported MZI-based fiber sensors. By leveraging the inherent advantages of fiber lasers, the proposed sensor exhibits excellent repeatability and stability, making it highly suitable for applications across a wide range of fields.