## 1 Introduction  
Sensors based on the surface-plasmon-resonance (SPR) effect can be applied in many detection fields because they possess high sensitivity, simple operation, and online detection [1; 2; 3; 4; 5]. Different from traditional Kretschmann prism-type SPR sensors, fiber-based sensors are widely acclaimed due to the excellent virtues of micro-nano dimensions, low cost, anti-interference, and ease of remote real-time monitoring [6; 7; 8]. Generally, there are two main methods to implement fiber SPR sensors. First, in order to produce a sensor with specific performance, we need to process the carrier fiber, and common processing techniques such as bending, taper, engraved grating, and side polishing. Second, in order to stimulate surface plasmons (SPs) for sensors, excitation-metal wire filling, inner and outer coating, and fiber end coating can be selected. Unfortunately, conventional glass-based fibers are not only less flexible in design, which results in limited sensing performance, but also very fragile, which makes conventional fibers unable to be excessively processed [9]. Therefore, it is necessary to find a flexible and easily fabricated scheme. In view of the lack of design flexibility in conventional fibers, photonic crystal fiber [10; 11; 12] (PCF), which is known as the catalyst for the fiber industry, is a promising candidate and may become a satisfactory sensing platform. For a PCF-based sensor, more superior sensing characteristics are produced by combining the SPR phenomenon because of its small dimension and flexible design advantages. Depending on the PCF structural parameters used to control light transmission, many optical properties in PCFs, such as nonlinearity, dispersion, effective mode field area, and mode birefringence, can easily be tailored. Accordingly, sensing characteristics can also become rich. In our scheme, side polishing was selected for fiber processing because it greatly simplified the fabrication of the sensor. In recent years, an increasing number of D-shaped PCF sensors were designed and investigated thanks to the SPR effect. For instance, in 2012, [PERSON] et al. reported a D-shaped SPR sensor based on an all-solid PCF, showing at least 7300 nm/refractive index unit (RIU) for spectral sensitivity, and 216 for figure of merit (FOM) in the visible band [13]. In 2014, [PERSON] et al. designed a side-polished fiber sensor with a gold layer and liquid filling operating from 500 to 750 nm, and sensitivity over 6430 nm/RIU [14]. In 2015, [PERSON] et al. proposed a D-shaped fiber sensor with a coated gold layer operating from 500 to 900 nm that obtained sensitivity of \(10\times 10^{3}\) nm/RIU with a resolution of \(9.8\times 10^{-6}\) RIU [15]. In 2016, [PERSON] et al. analyzed a highly sensitive D-shaped sensor. The sensor showed wavelength sensitivity of 5200 nm/RIU and resolution of \(1.92\times 10^{-5}\) RIU in the wavelength range of 1600-2100 nm [16]. In 2017, a novel D-shaped sensor based on endless single-mode PCFs and SPR was experimentally demonstrated by [PERSON] et al. that obtained sensitivity of \(6.53\times 10^{-5}\) RIU in the visible band [17]. [PERSON] et al. proposed a D-shaped PCF-SPR sensor with average sensitivity of 7700 nm/RIU and resolution of 1.30\(\times 10^{-5}\) RIU for analyte refractive index (RI) varying between 1.43 and 1.46 [18]. [PERSON] et al. theoretically and experimentally analyzed a D-shaped sensor with maximal sensitivity of 21,700 nm/RIU in the visible band [19]. In 2018, [PERSON] et al. numerically investigated a novel D-shaped sensor. Sensitivity of 31,000 nm/RIU was obtained in the near-infrared band [20]. [PERSON] et al. demonstrated a novel D-shaped PCF-SPR sensor for low RI detection; maximal spectral sensitivity of 11,055 nm/RIU and RI resolution of \(9.05\times 10^{-6}\) RIU were obtained in the range from 2.35 to 2.65 \(\upmu\)m [21]. Two common characteristics can be found from the designs described above. For sensing performance, modern sensors are moving toward high sensitivity and high resolution.