The dual-parameter detection of temperature and refractive index (RI) plays a crucial role in various fields, which provides comprehensive information for facilitating real-time monitoring and control of diverse processes, thereby enhancing efficiency, quality, and reliability, particularly in such areas as medical diagnosis, industrial manufacturing, and food safety. Researchers have explored the implementation of dual-parameter sensing for temperature and RI based on the Mach-Zehnder interferometer (MZI) principle, where multiple optical fibers are fused. Some have proposed the integration of MZI with tilted fiber Bragg gratings (TFBGs) for temperature and RI dual-parameter sensing. However, the relatively complex manufacturing and detection processes involved in these sensors pose challenges to practical production applications. Photonic crystal fiber (PCF) sensors feature real-time detection and strong interference resistance. By adjusting the periodic arrangement of air holes in the PCF, the RI distribution across the fiber’s cross-section can be modified to influence the fiber’s transmission characteristics. Different sensing functionalities can be achieved by filling the PCF with various advanced liquid materials. The surface plasmon resonance (SPR) phenomenon significantly enhances the sensitivity and detection range of PCF, among other sensing performances. We design a double-D-type SPR-PCF filled with liquid crystal E7 to enable dual-parameter sensing for temperature and RI. The double-D-type structure is designed to extend and further enhance the characteristics of the D-type structure, with a larger proportion of the fiber core close to the external environment for improving sensing performance.
The internal air holes of the PCF are arranged in a hexagonal pattern. The central large hole is filled with the temperature sensitive material liquid crystal E7 to form the fiber core. The upper and lower sides of the PCF are polished, and gold films deposited on the large open loops on both sides enable SPR external sensing. Selective deposition of gold films on the second layer of air holes achieves SPR internal sensing, further enhancing the SPR effect and sensing performance. In the PCF, when the optical wave enters the metal film surface and forms the cladding material, total internal reflection occurs if the incident angle is greater than the critical angle. Along the direction parallel to the critical interface, evanescent waves are generated. When the real part of the surface plasmon polariton (SPP) wave propagating along the interface of the metal and analyte matches the phase-matching condition, the SPR phenomenon occurs. The energy of the incident light is absorbed by the free electrons on the metal surface, leading to a sharp intensity decrease in the reflected light, and a pronounced loss peak appears at the resonance wavelength. The loss peak generated by SPR is highly sensitive to changes in the external environment. Variations in the fiber core and cladding materials, metal film, and analyte can all cause the loss peak to shift. The shift detection in the loss peak allows for the RI and temperature measurement.
When the RI of the analyte is set to 1.5, and the temperature increases from 15 ℃ to 50 ℃, the relationship between the core mode loss and operating wavelength is calculated (Fig. 4). With the temperature elevation, the loss peak significantly increases, corresponding to a red shift in the resonance wavelength of the loss peak. The ordinary and extraordinary refractive indexes of the liquid crystal are determined by the temperature coefficients, making the crystal a nonlinear thermosensitive material. At temperatures of 45-50 ℃, the refractive index undergoes substantial changes, leading to significant movement of the resonance wavelength. This overall increase in temperature detection sensitivity comes at the cost of reduced linearity. Sensitivity segmentation facilitates fitting sensitivity curves to the actual detection range, allowing for a more accurate reflection of the sensor’s performance at specific temperatures. Second-order polynomial fitting from 15-50 ℃ and linear fitting from 25-45 ℃ are separately performed to obtain corresponding wavelength sensitivity and amplitude sensitivity (Fig. 5). The sensor exhibits higher temperature sensitivity within the detection range and exceptionally high sensitivity between 45 ℃ and 50 ℃, thus providing practical significance by enabling the fitting of required curves within the actual detection range. By setting the temperature to T=25 ℃, the relationship between core mode loss and operating wavelength is calculated when the RI of the analyte varies from 1.48 to 1.55 (Fig. 6) to obtain corresponding wavelength sensitivity and amplitude sensitivity (Fig. 7). By adjusting the periodic arrangement of air holes in the photonic crystal fiber (PCF), the RI distribution of the fiber cross-section can be altered, thereby affecting the fiber’s transmission performance. The effects of varying the central liquid crystal hole diameter d0, small air hole diameter d1, and regular air hole diameter d2 on the fiber’s loss spectrum and resonance wavelength are studied separately (Figs. 8-10) to determine the optimal parameters. The sensor’s sensitivity is further enhanced after parameter adjustment, with good linearity (Fig. 11). Following parameter adjustments, the sensor demonstrates a maximum temperature sensitivity of 13.79 nm/℃ within the temperature range of 15-50 ℃, with a corresponding linear fitting constant of 0.99066. In the temperature range of 25-45 ℃, the sensor exhibits good linearity, with an average temperature sensitivity of 7.6 nm/℃ and a corresponding linear fitting constant of 0.98539. For analyte RI in the range of 1.48-1.55, the average RI sensitivity is 2904.76 nm/RIU, with a corresponding linear fitting constant of 0.98179.
We present a novel double-D-type liquid crystal filled SPR-PCF sensor enabling simultaneous detection of temperature and RI. The temperature and RI are measured by filling the temperature sensitive material liquid crystal E7 in the central large air hole and the RI analysis solution with large open loops contact on both sides. By varying the environmental temperature and analyte RI, the transmission characteristics of the optical fiber are systematically investigated by adopting the finite element method. The effects of the central liquid crystal hole diameter d0, small air hole diameter d1, and regular air hole diameter d2 on sensor performance are studied. Following parameter adjustments, the sensor demonstrates a maximum temperature sensitivity of 13.79 nm/℃ within the temperature range of 15-50 ℃, with a corresponding linear fitting constant of 0.99066. In the temperature range of 25-45 ℃, the sensor exhibits good linearity, with an average temperature sensitivity of 7.6 nm/℃ and a corresponding linear fitting constant of 0.98539. For analyte RI in the range of 1.48-1.55, the average RI sensitivity is 2904.7 nm/RIU, with a corresponding linear fitting constant of 0.98179. The designed double-D-type SPR-PCF sensor optimizes beyond the traditional D-type PCF and possesses enhanced sensitivity. Meanwhile, its capability for dual-parameter detection of environmental temperature and RI analyte brings the applicability to various fields such as environmental monitoring, healthcare, and biochemistry.
