## 1 Introduction  
Fiber sensors are characterized by their compact size, remote sensing ability and immunity to electromagnetic interference [1]. Currently, most fiber refracometers operate based on evanescent field interaction with a medium surrounding the fiber [2]. Usually, for such sensors, the dynamic range for refractive index detection is naturally limited by the refractive index of silica fiber to satisfy the total internal reflection condition. This gives rise to another issue of exciting and collecting evanescent light, or cladding modes, when higher index fluid samples are used. Moreover, it is also difficult to achieve multi-channel detection in practical applications with composite fluids like blood. In contrast, fiber-integrated sensors can be good candidates to realize both a wide dynamic index sensing range and multi-channel detection ability. Tiny open Fabry-Perot (FP) cavities, fabricated by, e.g., the splicing of fiber segments or precision dicing close to or directly at the fiber tip, have attracted a great deal of attention due to their very high sensitivities [3, 4]. However, their multiplexing abilities are typically limited to a small number of sensing elements located at different spatial positions along the fiber [5]. To date, this has hindered their use in biomedical applications where small amounts of liquid samples need to be tested. The proposed RWG sensor on a fiber tip represents an interesting alternative to other types of fiber sensors. Although typical sensitivities are lower than that of, e.g., the FP cavities' sensor, this concept provides the principal advantage of multiplexing many sensors at a single fiber tip for testing the same small volume of a sample liquid. RWGs, sometimes named guided mode resonant gratings (GMRG) or leaky mode resonant gratings (LMRG), are structures composed of higher refractive index waveguiding layers with a grating on the surface of lower index substrates. The study of this resonant grating phenomenon can be traced back to 1941, when [PERSON] suggested that some of [PERSON]'s anomaly may be attributed to the excitation of surface waves [6]. This was further theoretically demonstrated by [PERSON] and [PERSON], in 1965, by applying a metallic grating model [7]. Later in 1973, [PERSON] et al. studied these anomalies in dielectric gratings and related them to the excitation of leaky waveguide modes [8]. When a wave is incident on an RWG, the first-order diffracted light can be coupled into a leaky waveguide mode for a certain incident angle and wavelength. Consequently, leaky waves interfere destructively with the 0 th-order transmitted wave to result in sharp resonant peaks in the reflection spectrum, which can reach up to 100% modulation [9]. By selecting proper waveguide and grating parameters, both the peak position and its width can be adjusted. Benefiting from its unique dispersion characteristics and resonant properties, various RWG applications have been developed, including narrowband and broadband wavelength filters, polarizers, color mixers, and devices for the enhancement of nonlinear processes [10; 11; 12; 13; 14]. RWGs can also be applied as refractive index sensors because the resonant wavelength is sensitive to index changes in an ambient environment [15; 16; 17; 18]. The sensitivity of RWG sensors is usually in the range from several tens to several hundreds of nanometers per refractive index unit (nm/RIU), which is sufficient for most bio-sensing applications [19]. In addition, RWG sensors offer a nearly linear response for relatively large refractive index ranges (depending on waveguide and grating materials) and suffer less from cross-sensitivity due to temperature fluctuations [20]. Consequently, RWG sensors are widely studied for on-chip devices and high-throughput detection systems that are available for living cell sensing [21]. The proposed fiber refracometers can be realized by depositing a RWG directly on the fiber facet, offering a much larger index detection range compared to lateral detection fiber sensors.