Abstract
We propose and experimentally demonstrate a hybrid grating, in which an excessively tilted fiber grating (Ex-TFG) and a fiber Bragg grating (FBG) were co-inscribed in a reduced-diameter fiber (RDF). The hybrid grating showed strong resonances due to coupling among core mode and a set of polarization-dependent cladding modes. This coupling showed enhanced evanescent fields by the reduced cladding size, thus allowing stronger interaction with the surrounding medium. Moreover, the FBG’s Bragg resonance confined by the thick cladding was exempt from the change of the surrounding medium’s refractive index (RI), and then the FBG can work as a temperature compensator. As a result, the Ex-TFG in RDF promised a highly sensitive RI measurement, with a sensitivity up to ~1224 nm/RIU near the RI of 1.38. Through simultaneous measurement of temperature and RI, the temperature dependence of water’s RI is then determined. Therefore, the proposed hybrid grating with a spectrum of multi-peaks embedded with a sharp Bragg resonance is a promising alternative for the simultaneous measurement of multi-parameters for many RI-based sensing applications.
Highlights
Refractive index (RI) measurement has an enormously broad range of applications, from biochemical industry to environmental monitoring and life science, where the sensing of many parameters can be converted to the RI measurement [1]
We present a hybrid grating to achieve the high-sensitivity RI sensing with temperature calibration
Where, i represents TE or TM modes, neff,core and nm are the effective indices of the core mode i,eff,clad and the mth cladding mode, respectively, ΛB and ΛG are the grating period of the fiber Bragg grating (FBG) and nominal period of the excessively tilted fiber grating (Ex-TFG), respectively, and θ is the tilt angle
Summary
Refractive index (RI) measurement has an enormously broad range of applications, from biochemical industry to environmental monitoring and life science, where the sensing of many parameters can be converted to the RI measurement [1]. Coating nanomaterials with high-complex RI can extend the measurement range, especially in the low-RI region [25,26,27,28] Another special kind of so-called excessively tilted fiber grating (Ex-TFG) is characterized by a large tilted angle usually >65◦ and a set of polarization-dependent cladding mode resonances. Ex-TFG is recognized for the highly sensitive nature of the light that interacts with the surrounding medium [29,30] They have been developed and applied in the sensing of twist [31], refractive index [32], liquid-level [33], and biochemical parameters [34,35]. The proposed hybrid grating promises applications in simultaneous measurement of multi-parameters
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