In recent years, optical fiber sensors based on multimode interference (MMI) have attracted increasing interest and developed into various sensors used in many practical applications. This review presents MMI-based fiber sensors with a specific focus on the probe structures, measurement methods, and sensing properties of different structures. The fundamentals of MMI-based fiber sensors are briefly described. Further, five main categories of SMS structure-based fiber sensors are reviewed, including conventional SMS fiber sensors, MMI-based sensors with no-core fiber (NCF), MMI-based sensors with etched/polished fiber, MMI-based sensors with tapered fiber, and cascades of MMI-based fiber sensors. Besides, their probe structures, sensing properties, practical fields, and measurement sensitivities are summarized. Finally, the current challenges and technique outlook of the MMI-based fiber sensors are pointed out, which gives the potential solutions for the existing problems and future development. This work indicates that the MMI-based fiber sensors bring the possibilities of applying multi-mode fiber (MMF) in different measurement fields with high sensitivities, easy fabrication, and low cost. With the appearance and development of new SMS fiber sensors, it will contribute significant value to scientific research and industrial applications.

Absolute distance measurement is a field of research with a large variety of applications. Laser triangulation is a well-tested and developed technique using geometric relations to calculate the absolute distance to an object. The advantages of laser triangulation include its simple and cost-effective setup with yet a high achievable accuracy and resolution in short distances. A main problem of the technology is that even small changes of the optomechanical setup, e.g., due to thermal expansion, lead to significant measurement errors. Therefore, in this work, we introduce an optical setup containing only a beam splitter and a mirror, which splits the laser into a measurement beam and a reference beam. The reference beam can then be used to compensate for different error sources, such as laser beam dithering or shifts of the measurement setup due to the thermal expansion of the components. The effectiveness of this setup is proven by extensive simulations and measurements. The compensation setup improves the deviation in static measurements by up to 75%, whereas the measurement uncertainty at a distance of 1 m can be reduced to 85 μm. Consequently, this compensation setup can improve the accuracy of classical laser triangulation devices and make them more robust against changes in environmental conditions.

Multimode fiber (MMF) sensors have been extensively developed and utilized in various sensing applications for decades. Traditionally, the performance of MMF sensors was improved by conventional methods that focused on structural design and specialty fibers. However, in recent years, the blossom of machine learning techniques has opened up new avenues for enhancing the performance of MMF sensors. Unlike conventional methods, machine learning techniques do not require complex structures or rare specialty fibers, which reduces fabrication difficulties and lowers costs. In this review, we provide an overview of the latest developments in MMF sensors, ranging from conventional methods to those assisted by machine learning. This article begins by categorizing MMF sensors based on their sensing applications, including temperature and strain sensors, displacement sensors, refractive index sensors, curvature sensors, bio/chemical sensors, and other sensors. Their distinct sensor structures and sensing properties are thoroughly reviewed. Subsequently, the machine learning-assisted MMF sensors that have been recently reported are analyzed and categorized into two groups: learning the specklegrams and learning the spectra. The review provides a comprehensive discussion and outlook on MMF sensors, concluding that they are expected to be utilized in a wide range of applications.

A variety of specialty fibers such as no-core fiber (NCF) have already been studied to reveal their sensing abilities. In this work, we investigate a specialty fiber, square-core fiber, for temperature and strain sensing. A simple single-mode–multimode–single-mode (SMS) fiber sensor was fabricated, consisting of a 30-cm-long square-core fiber. The experimental results indicate that the maximal wavelength-temperature and wavelength-strain sensitivities are −15.3 pm/${}^{\circ }$C and −1.5 pm/$\mathsf{\mu }\epsilon$, respectively, while the maximal power-temperature and power-strain sensitivities are 0.0896 dBm/${}^{\circ }$C and 0.0756 dBm/$\mathsf{\mu }\epsilon$. Analysis of the results suggests that the fiber sensor has the potential to be used as a high-sensitivity temperature sensor with a low strain sensitivity.

A simple, inexpensive, and high-sensitivity temperature and strain sensor based on a single-mode–multimode–single-mode (SMS) structure with core offset is developed and experimentally characterized. This sensor does not require specialty fibers and can be fabricated using a standard fiber fusion splicer. The dependencies of the temperature and strain sensitivities on the core-offset amplitudes at the input and output single-mode/multimode fiber boundaries are investigated. The results indicate that the maximum temperature and strain sensitivities are two times and eight times higher than those of the standard SMS structure, respectively. The limit of the sensitivity enhancement by core offset is also revealed.

A simple, stable, and high-sensitivity temperature sensor based on multimode interference in a polymer optical fiber (POF) with higher-order mode excitation has been developed. In a single-mode–multimode–single-mode (SMS) structure, one end of the multimode POF with physical-contact (PC) connectors is connected to a silica single-mode fiber with an angled-PC (APC) connector. We compare the temperature sensing characteristics of the three configurations (no PC-APC, PC-APC at input, and PC-APC at output) and obtain the highest temperature sensitivity of 219.2 pm °C⁻¹, which is more than double the value of the standard (no PC-APC) SMS structure.