Surface plasmon resonance (SPR) has been widely used for the high accuracy and fast detection of multiple physical, biomedical, and biochemical reactions or parameters^[1,2]. So far, several different structures have been utilized to achieve the SPR, such as a prism type coupling based on the Kretschmann configuration^[3] and Otto configuration^[4], grating couplers^[5,6], and optical waveguides^[7,8]. Compared with the traditional prism configuration, optical fiber-based SPR sensors are more flexible and compact, and could even be inserted into human tissue or blood vessels for real-time analysis or monitoring, which could be a convenient way to achieve remote sensing and become fundamental characteristics in current applications. Therefore, reducing the size while increasing the sensitivity of the sensors is a constant challenge. There are several optical fiber architectures-based SPR sensors, which include etched multimode optical fibers^[9], side-polished optical fibers^[10], D-shaped optical fibers^[11,12], tapered optical fibers^[13], U-shaped optical fibers^[14,15], photonic crystal fibers^[16], and tilted fiber Bragg gratings (TFBGs)^[17,18]. Among them, the TFBGs have a comb-like cladding mode, which is an ideal in-fiber prism to excite the SPR at the interface of metal and fiber cladding. Meanwhile, the asymmetrical structure of TFBGs could also induce the polarization-dependent cladding mode coupling, which includes P-polarization and S-polarization cladding modes. The TFBG could directly detect the surrounded refractive index (SRI) by monitoring the variation of the transmission spectrum of the TFBG. However, the sensitivity of bare TFBGs is around 25 nm/RIU^[19,20], due to the backward cladding mode coupling. In 2010, Albert and Shevchenko proposed a TFBG-based SPR sensor, which dramatically improves the sensitivity of the TFBG^[21]. Since then, massive works based on such technology have been reported, which included ultra-sensitive gas sensing^[22–24], biochemical and electrochemical sensing^[25,26], heavy metal ion detection^[27,28], humidity and twist sensing^[29,30], and so on. TFBG-based SPR sensors have indicated broad development potential. However, one inconvenience of using this technique is that only the evanescent field of P-polarized light can excite the SPR. In the traditional TFBG-based SPR sensing system, a polarizer and a polarization controller (PC) need to be employed to excite the P-polarization cladding mode of the TFBG. During the measuring process, to ensure the measuring accuracy, we must adjust the PC at all time to make sure that SPR is excited maximally, which is not convenient for the applications, since any alteration of the environment could compromise the measurement. The development of fiber sensors is toward low cost, miniaturization, and simplification. In our previous works, we have proposed a 45° tilted fiber grating (45° TFG)-based in-fiber polarizing grating device^[30]. In this work, we propose a hybrid tilted fiber grating-based SPR sensor, which is composed of a 45° TFG and a silver (Ag)-coated 10° TFBG that are concatenated together by using UV-inscription technology. Such a sensor has taken advantages of the TFBG cladding modes resonance and conjointly had the property of the 45° TFG to act as a polarizer to filter the S-polarization cladding mode^[31], exciting the SPR, which does not need the PC to control the polarization any more. The hybrid tilted fiber gratings greatly simplify the interrogation of the SPR sensor, due to both the 45° TFG and 10° TFBG being concatenated without any gap by UV-inscription technology. Finally, we have demonstrated the high sensitivity hemoglobin (Hb) concentration detection by using the hybrid tilted fiber gratings-based SPR sensor. Such a Hb sensor can achieve a sensitivity of 8.144 nm per mg/mL within the sensing range from 0.1 mg/mL to 1.0 mg/mL.

The configuration of the proposed hybrid tilted fiber gratings-based SPR sensor is illustrated in Fig. 1, which consists of a 45° TFG and a 10° TFBG coated with a 50 nm Ag layer in the TFBG area to excite the SPR.

The proposed hybrid tilted fiber gratings are fabricated by applying the phase mask method. During the fabrication process, a 10° TFBG (L≈10mm) is firstly UV inscribed by using a 248 nm excimer laser through a 1068 nm period phase mask with a 7.5° tilt angle to achieve a 10° TFBG into a H₂ loaded SM-28 fiber; the optical fiber can be treated as a cylindrical lens that only compresses the light in the direction perpendicular to its central axis. For the inscription of the fiber Bragg grating (FBG), the interference fringe of the UV beam is perpendicular to the fiber axis. So, the fringe inside the fiber is still perpendicular to the fiber axis. When the interference fringe is tilted at an angle with respect to the fiber axis, the interference fringe in the core will be distorted. Thus, the tilt angle of the grating is not the same as that outside the fiber. The relationship between these two angles is described in Ref. [32]. After the fabrication process, the transmission spectrum in the air of the TFBG is tested by using the traditional method [see the inset of Fig. 2(a)], and the two separately polarized cladding mode resonances (S-polarization and P-polarization) are obtained by controlling the polarizer controller (PC), as shown in Fig. 2(a); the transmission spectrum could be improved by applying a better light source as well as avoiding as much as possible the losses of the connectors. Afterward, a 45° TFG, with a length of L≈30mm, is inscribed in the front of the 10° TFBG to filter the S-polarization cladding mode without any gap between them, while allowing the P-polarization cladding mode to pass through the 45° TFG since the TFBG-SPR could only be obtained by the P-polarized light. Comparing the conventional interrogation system, the utilization of the 45° TFG simplifies the interrogation system of TFBG by reducing a PC and an optical polarizer, which significantly improves the mobility of the sensor. Once the inscription of the 45° TFG is completed, the transmission spectrum of the new hybrid tilted fiber gratings-based sensor is also tested in air, obtaining only the P-polarization mode, as presented in Fig. 2(b). The hybrid tilted fiber gratings are also tested by using the traditional method shown in inset of Fig. 2(a). By adjusting the PC, we do not see the power exchange of two polarization cladding modes, which do not have polarization-dependent transmission peak alternation, and only the intensity level of the whole spectrum is changing, as shown in Fig. 3. It is because the PC is working as a hand-adjustable attenuator. In the experiment, the broadband light source we used is a super continuum source (SC-5, purchased from YSLP), which has a broadband output with around a −45 dBm intensity level in the wavelength range from 1400 nm to 1600 nm.

Finally, the optical fiber is coated with an Ag layer (approximately 50 nm of thickness) by using the physical vapor deposition method. During the coating process, the optical fiber is in constant rotation to get a homogeneous layer surrounding the fiber cladding, and the area of the 45° TFG is blocked to avoid being coated with the Ag film (if the 45° TFGs are coated with a reflective film, there would be an interference between the radiated light and the guided light). During the coating process, the film thickness is monitored continuously by a quartz crystal.

To evaluate the refractive index (RI) sensitivity of the proposed sensor, the hybrid tilted fiber gratings-based SPR sensor is subjected into RI oil with different RI values that are massed by mixing glycerine and water in different concentrations within a range between 1.33 RIU and 1.37 RIU, and calibrated by a 2AW-J Abbe refractometer (Shanghai Tianxing Instrument Co., Ltd.). The experiment setup is shown in Fig. 4. Compared with the traditional interrogation system^[18], the proposed sensor is connected directly to a broadband light, without the usage of both the PC and optical polarizer.

The sensor, fixed to a platform, is subjected to the different solutions, and such a transmission spectrum is monitored by an optical spectrum analyzer. Figure 5(a) shows the transmission spectrum of the hybrid tilted fiber gratings-based SPR sensor subjected to media with different RI values. As shown in Fig. 5(a), the cladding mode of the 10° TFBG is not able to excite the SPR in the air because the surface plasmon (SP) has its higher moment at the air–Ag interface. Furthermore, once the sensor is subjected to water, the SPR could be excited at the wavelength of 1493.7 nm. As the RI increases, the resonance moves to a longer wavelength; see Fig. 5(a). Finally, we could plot and calculate the RI sensitivity of the proposed sensor, which is around 522.8 nm/RIU in a range between 1.33 and 1.37 RIU [Fig. 5(b)], where the SPR wavelength shows a linear behavior, according to the variation of the surrounding RI. In this work, we apply the hybrid tilted fiber gratings-based SPR sensor for the detection of Hb. The sensing setup is similar to the configuration used for the RI sensing characterization, replacing only the type of the solutions to be measured, of which different concentrations of Hb and deionized water massed in the range of 0.1 mg/mL and 1.0 mg/mL are detected. The transmission spectrum of the hybrid gratings-based SPR sensor with different Hb concentrations is shown in Fig. 6(a). Compared with the spectrum in Fig. 5(a), the SPR absorption dip is becoming weaker and broader, which is because the Hb solution is not an isotropic solution, and, microcosmically, the Hb molecule is heterogeneous. From the results in Fig. 5, the sensing solution is a water +glycerin solution, which is a pure solution.

We also compare the sensing results by using a bare TFBG at similar conditions. Figure 6(b) shows that the resonance wavelength of the hybrid gratings-based SPR sensor is clearly moving to the longer wavelength as the Hb concentration increases; and the transmission spectrum of the bare TFBG almost stays the same at the wavelength and intensity of the cladding mode [see Fig. 6(b)] due to a very weak index change in the range from 0.1 mg/mL to 1.0 mg/mL of Hb concentration. According to previous works^[27], bare TFBG is almost insensitive to the surrounding RI changing with only 1.3 nm/RIU sensitivity, due to its forward cladding mode coupling behavior, which is not suitable for highly sensitive biosensing. Additionally, the large RI range from 1.30 to 1.44, can only affect the attenuation of the cladding mode of TFBG. However, once the TFBG is combined with the SPR, the RI sensitivity of sensor could be greatly improved.

The wavelength shift of the proposed sensor caused by Hb solutions with different concentrations is plotted in Fig. 7, which has a sensitivity of 8.144 nm per mg/mL with Hb solutions between 0.1 mg/mL and 1.0 mg/mL. As shown in Fig. 7, the wavelength shift is linearly proportional to the variation of the Hb concentrations; on the contrary, the bare TFBG is insensitive to the variation of the Hb concentrations.

In this work, we have proposed a hybrid 45° TFG and 10° TFBG-based SPR sensor and demonstrated its application for Hb concentration detection. Due to the compact grating structure and S-polarization filtering function of the 45° TFG, the proposed hybrid tilted fiber gratings-based SPR sensor has a simple interrogation system. In the experiment, the sensor is tested by sensing SRI with an index value from 1.33 RIU to 1.37 RIU, which has a sensitivity of 522.8 nm/RIU. Finally, we have applied the proposed hybrid TFGs-based SPR sensor for Hb concentration detection. The sensor is subjected to several solutions composed of water and Hb with different concentrations within a range from 0.1 mg/mL to 1.0 mg/mL. The experimental results reveal that the proposed sensor responds with a linear behavior with a sensitivity of 8.144 nm per mg/mL to Hb concentrations between 0.1 mg/mL and 1.0 mg/mL. Such a system could be potentially applied in the medical field.