Differential Refractive Index Sensor Based on Coupled Plasmon Waveguide Resonance in the C-Band

Abstract

We proposed a differential fiber-optic refractive index sensor based on coupled plasmon waveguide resonance (CPWR) in the C-band. The sensor head is a BK7 prism coated with ITO/Au/ITO/TiO2 film. CPWR is excited on the film by the S-polarized components of an incident light. The narrow absorption peak of CPWR makes it possible to realize dual-wavelength differential intensity (DI) interrogation by using only one incident point. To implement DI interrogation, we used a DWDM component to sample the lights with central wavelengths of 1529.55 and 1561.42 nm from the lights reflected back by the sensor head. The intensities of the dual-wavelength lights varied oppositely within the measurement range of refractive index, thus, a steep slope was produced as the refractive index of the sample increased. The experimental results show that the sensitivity is 32.15/RIUs within the measurement range from 1.3584 to 1.3689 and the resolution reaches 9.3 × 10−6 RIUs. Benefiting from the single incident point scheme, the proposed sensor would be easier to calibrate in bio-chemical sensing applications. Moreover, this sensing method is expected to be applied to retro-reflecting SPR sensors with tapered fiber tip to achieve better resolution than wavelength interrogation.

Active Differential Fiber Coupled Plasmon Waveguide Resonance Sensor Based on the Mode Competition Effect

“…An ITO/Au/ITO film was prepared by sputtering pure gold target and ITO target (In 2 O 3 :SnO 2 = 9:1) using two ion sputtering equipment (SBC-12, KYKY Technology Co., Ltd., Beijing, China). The coating process is similar to what we previously reported [17]. For each layer, the background pressure is 4 Pa, the sputtering pressure is 5.5 Pa, and the target-substrate distance of each ion sputtering equipment is set to 30 mm.…”

“…Figure 2 shows the fiber probe with ITO/Au/ITO/TiO 2 film. The ITO layers serve as adhesion layers to improve the stability of the sensing film on the fiber probe [17].…”

“…Using the measurement function of the microscope, the angle between the axial direction of the fiber and the polished plane was measured to be approximately 15°. To achieve dual-wavelength DI interrogation using CPWR in the C-band and considering the double excitation of CPWR on the fiber probe, we used an ITO/Au/ITO/TiO2 film to excite P-CPWR due to its spectral width being narrower than that of S-CPWR [17], which is more advantageous for achieving high DI sensitivity. An ITO/Au/ITO film was prepared by sputtering pure gold target and ITO target (In2O3:SnO2 = 9:1) using two ion sputtering equipment (SBC-12, KYKY Technology Co., Ltd., Beijing, China).…”

“…Figure 2 shows the fiber probe with ITO/Au/ITO/TiO2 film. The ITO layers serve as adhesion layers to improve the stability of the sensing film on the fiber probe [17]. To display the structure of the probe more clearly, we drew a sketch diagram of the probe.…”

“…Similar to conventional SPR, the resonant wavelength redshifts with the increase in n s , which is the principle of the CPWR sensor with wavelength interrogation. Compared with other modes of SPR, CPWR provides a narrower resonant spectrum; thus, dual-wavelength DI interrogation in the C-band can be achieved by using a CPWR structure with a single incident point [17]. The reflectivity of two wavelengths located on both sides of the resonant wavelength shows opposite changes when n s varies within the measurement range.…”