Optical Fiber Sensors Based on Advanced Vernier Effect: A Review

The optical Vernier effect has emerged as a powerful tool for enhancing the sensitivity of optical fiber interferometer-based sensors, ushering in a new era of highly sensitive fiber sensing systems. While previous research has primarily focused on the physical implementation of Vernier effect-based sensors using different combinations of interferometers, conventional Vernier sensors face several challenges. These include the stringent requirements on the sensor fabrication accuracy to achieve a large amplification factor, the necessity of using a source with a very large bandwidth and a bulky optical spectrum analyzer, and the associated complex signal demodulation processes. This article delves into recent advances and developments in the advanced Vernier effect aimed at addressing these challenges. It begins by introducing the basic concept of the optical Vernier effect and outlining the challenges faced by conventional Vernier sensors. Subsequently, it reviews the concept of the virtual Vernier effect. Then, the use of machine learning as a tool for demodulating Vernier sensors is discussed, presenting new avenues for developing compact and simplified Vernier sensor interrogators. Furthermore, the article explores an extension of the optical Vernier effect known as the microwave-photonic Vernier effect, which offers unique features such as distributed sensing capability and ultra-high-speed update rates. Finally, the article provides perspectives on future research directions in this area, highlighting the continued potential for innovation and advancement in Vernier effect-based sensing technologies.