## 1 Introduction  
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]. Compared with Abbe refractometers based on Snell's law and measurement of the critical angle of total reflection, optical fiber-based RI sensors have many unique advantages, for example their compact size, remote and distributed interrogation, and high sensitivity. A number of attempts employing fiber gratings and interferometers have been made to improve the RI sensitivity for many RI-based sensing applications [2; 3; 4; 5; 6; 7; 8; 9]. Among these, long-period gratings (LPGs) and tilted fiber Bragg gratings (TFBGs) with abundant cladding mode resonances have demonstrated excellent performance in highly sensitive RI measurement by using several distinct demodulation techniques [10; 11; 12; 13; 14]. In order to further enhance the sensitivity and enlarge the sensing range, a few important approaches have been employed, i.e., increasing the tilted angle, reducing core or cladding's size, and coating functional materials onto the TFBGs. For instance, increasing the tilted angle and reducing the fiber diameter can shift the TFBG's cladding mode towards a shorter wavelength and enhance the evanescent field for a larger RI measurement range and higher sensitivity [15; 16; 17; 18]. Similarly, coating TFBGs with noble metals to excite surface plasmon resonance (SPR) can effectively improve the RI sensitivity for biochemical and gas sensing [2; 19; 20; 21; 22; 23; 24]. 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\({}^{\circ}\) and a set of polarization-dependent cladding mode resonances. This 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]. However, in these sensing applications using an Ex-TFG, the temperature cross-talk is a critical issue because there is no Bragg resonance to serve as a temperature reference like that of a TFG with small tilted angles.  
In this work, we present a hybrid grating to achieve the high-sensitivity RI sensing with temperature calibration. An Ex-TFG and an FBG were UV-inscribed in the same segment of reduced-diameter fiber (RDF), showing a wide and multi-notch spectrum together with a sharp Bragg resonance. The use of an RDF for the Ex-TFG enhanced the accessible evanescent field for interacting with an external medium. The mode coupling of the FBG only occurred in the fiber core, with the light field was isolated from the surrounding by the cladding, and was thus exempt from disturbance by the change of surrounding medium's RI. As a result, the Ex-TFG in RDF provided a highly sensitive RI measurement, and the FBG was used as a temperature compensator. In the sensing experiments of temperature and RI, the hybrid grating demonstrated enhanced sensing ability and was able to be employed to examine the temperature dependence of water's RI. Therefore, the proposed hybrid grating promises applications in simultaneous measurement of multi-parameters.