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This paper presents a systematic review concentrating on the design and extensive applications of optical fiber sensors based on embedded long-period fiber gratings (LPFGs). First of all, the preparation procedures and the working principle is introduced in detail. Then, the applications of different types of sensors based on the embedded LPFG are investigated, which is explored to detect physical parameters such as temperature, strain, refractive index (RI), bending, torsion, liquid level. Also, the method for enhancing sensing performance is presented. Compared with LPFGs proposed in these years, the embedded LPFGs sensors with high selectivity and flexibility have proved to behave excellent sensing characteristic, especially in the recognition in bending and RI measurement. Furthermore, the sensors are briefly described according to the design scheme and sensing performance. Finally, the drawbacks of the sensors are analyzed and the future prospect is discussed. [ABSTRACT FROM AUTHOR]

• A highly sensitive strain optical fiber sensor is achieved by means of side-polishing and high-temperature oxyhydrogen flame heating. • The highest strain sensitivity of the M-LPFG reaches to −19.05 pm/ με. • A compact length is obtained, which is 5 mm. • Low cost, simple structure and easy fabrication. Optical sensors for strain measurement have been significantly concerned due to their unique characteristics of quick response, anti-electromagnetic interference and compact size. In this article, a brand-new type of "M"-shaped core long-period fiber grating (M-LPFG) based on core-modulated method is proposed and experimentally testified. By means of side-polishing and high-temperature oxyhydrogen flame heating, the cladding of fiber tends to be circular and the core will bend slightly towards the polished area. The micro-bending core is suffered strong refractive index modulation and this phenomenon also excites the evanescent field adequately. By analyzing the simulated light transmission and the transmission spectrum, considerable degree of micro-bending core is achieved. In the experiment, the proposed sensor with 10 periods (5 mm) displays a high strain sensitivity of −19.05 pm/ με with range of 0–500 με, and the temperature sensitivity of which is obtained as 51.16 pm/°C. Due to relatively small influence on cladding and self-circularity of structure, the M-LPFG possesses a high mechanical strength. With the above advantages of high sensitivity, compact size, low-cost and strong mechanical strength, the method of fabricating M-LPFG has great potential in strain measurement. [ABSTRACT FROM AUTHOR]

• A high sensitive curvature optical fiber sensor is achieved by using CO 2 laser polished method. • The highest bending sensitivity of the AP-LPFG reaches to 30.85 nm/m−1. • A compact length is obtained, which is 7.39 mm. • Low cost, simple structure and easy fabrication. In this paper, we experimentally propose and theoretically investigate a two-dimensional vector curvature sensor based on asymmetrically polished long period fiber grating (AP-LPFG). This sensor is fabricated by a high-frequency CO 2 laser in order to polish periodic asymmetrical V-shaped grooves on a single mode fiber (SMF). Such grooves efficiently enhances the bending sensitivity of sensor and shorten the length of the structure to 7.39 mm. Experimental results show that the sensitivity of AP-LPFG reaches 30.85 nm/m−1 in the direction of the X-axis and −0.97 nm/m−1 in the direction of the Y-axis. Meanwhile, the temperature response of AP-LPFG is carried out, which is 76 pm/℃ in range of 30-160℃. Moreover, the AP-LPFG has the features of compact length and simple structure, which shows the potential in curvature sensing. [ABSTRACT FROM AUTHOR]