Contents I. Introduction
Electric field (-field) sensors play an important role in various areas, such as the fault diagnosis of electric power equipment [1], electromagnetic pollution control [2], and -field monitoring in medical apparatus [3]. Compared with the traditional metal structure -field sensor, the photonic -field sensor has a small size, compact structure, high sensitivity, and immunity to the electromagnetic coupled interference [4], [5]. By far, numerous fiber-optic -field sensors have been proposed. Usually, optical fibers do not exhibit second-order nonlinear refractive effects due to the parity symmetry of silica. Hence, fiber-optic -field sensor sensing methods are usually based on electro-optic (EO) crystals, liquid crystal (LC) materials, and organic EO polymer materials. The most typical EO crystal is lithium niobate (LN), whose refractive index (RI) varies with an applied -field [6]. For example, Qiu et al. [7] designed photonic crystal Bragg grating on a thin film LN slot waveguide for small -field detection. Calero et al. [8] achieved an all-dielectric -field sensor by the combination of LN, laboratory-on-fiber technologies, and photonic crystals. However, the LN-based -field sensors involve the fabrication and integration of fiber optics with LN crystal and exist disadvantages of high coupling loss, complex structure, and high cost due to the hard and unbendable physical properties of LN crystal [9], [10]. For this reason, all-fiber -field sensors based on LC materials and organic EO polymer materials are reported [11], [12], [13]. Thanking their advantages of high sensitivity, stability, and ease of fabrication, all-fiber -field sensor attracts great interest.
