Optical fiber sensors based on multimode interference using square-core fiber for temperature measurement

Optical fiber sensors based on multimode interference (MMI) have been widely used and developed into various applications. The sensing principle is mostly based on the induced wavelength shift of selected dips (or peaks) in the transmission spectrum. A simple structure to obtain MMI devices is a so-called single-mode-multimode-single-mode (SMS) fiber structure, which is composed of a short section of multimode fiber (MMF) fusion spliced between two single-mode fibers (SMFs). However, most of these MMI-based fiber sensors are related to circular core MMFs. Although the sensitivity is enhanced and the fabrication process is improved, some common problems still exist and need to be solved. To solve current challenges, such as linearity, crosstalk, and compactness, it is essential to find solutions like using a new structure or a new type of fiber. Recently, different new fibers, like hollow annular core fiber (HACF) and square no-core fiber, have been demonstrated to have advantages to overcome some of these limits. The applications of these fibers provide possibilities to study specifically shaped core multimode fibers. In this contribution, we propose a compact MMI-based fiber sensor for temperature measurement. Instead of the standard MMF used in the SMS structure, a square-core fiber (SCF) with a circular cladding is implemented as the sensing element. To the best knowledge of the authors, the SCF has not been investigated yet for sensing. Therefore, the sensing characteristics are studied experimentally. The proposed fiber sensor reaches a sensitivity of 45 pm/°C. It proves the sensing capability of SCF is promising, provides great potential for further works.