Hybrid tilted fiber gratings-based surface plasmon resonance sensor and its application for hemoglobin detection Download: 815次
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[2224" target="_self" style="display: inline;">–
The configuration of the proposed hybrid tilted fiber gratings-based SPR sensor is illustrated in Fig.
The proposed hybrid tilted fiber gratings are fabricated by applying the phase mask method. During the fabrication process, a 10° TFBG () 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.
Fig. 2. (a) Transmission spectrum of the TFBG with two orthogonal polarization cladding modes; (b) transmission spectrum of hybrid tilted fiber gratings with only the P-polarization cladding mode.
Fig. 3. Transmission spectrum of hybrid grating with different incident polarization states.
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.
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
Fig. 5. (a) SRI sensing test of the hybrid tilted fiber gratings-based SPR sensor in different RI; (b) sensitivity of the hybrid tilted fiber gratings-based SPR sensor.
Fig. 6. SRI sensing comparison between (a) the hybrid tilted fiber gratings-based SPR RI sensor and (b) bare TFBG for different Hb concentrations.
We also compare the sensing results by using a bare TFBG at similar conditions. Figure
The wavelength shift of the proposed sensor caused by Hb solutions with different concentrations is plotted in Fig.
Fig. 7. Hb concentration sensitivity of the proposed hybrid tilted fiber gratings-based SPR sensor and bare TFBG-based sensor.
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.
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Yarien Moreno, Qingguo Song, Zhikun Xing, Yuezhen Sun, Zhijun Yan. Hybrid tilted fiber gratings-based surface plasmon resonance sensor and its application for hemoglobin detection[J]. Chinese Optics Letters, 2020, 18(10): 100601.