Objective Photonic crystal fibers (PCFs) have wide applications in gas sensing, all-optical switching, four-wave mixing, and other fields due to its unique light control characteristics. In recent years, with the reduction of loss and cost, the sensors based on PCF have the characteristics of short absorption path and sensitive detection, which have become the focus of attention of researchers. However, many scholars have only optimized optical parameters such as sensitivity and limiting loss, ignoring the influence of the dispersion effect on the optical signal. When the dispersion is too large, it is very likely that the optical pulse will be broadened, causing the overlap of adjacent pulses. Therefore, the increase of the bit error rate will not be conducive to the propagation of light, so it is a challenge to reduce the dispersion and improve the detection sensitivity. Therefore, we have studied the octagonal PCF structure with multi-slot waveguides, which provides the possibility to achieve accurate detection of harmful gases, and at the same time achieves low-dispersion and flat-bandwidth optical signal transmission in the near-infrared wavelength range.

Methods In the infrared spectrum of various gases, the uniqueness of the absorption band is of great significance for the trace amount of the gas. When a beam of light with an intensity of I₀ passes through the photonic crystal fiber, the intensity of light passing through the gas to be measured changes due to the absorption of the light field by the gas to be measured, and the output light intensity I satisfies the Lambert-Beer law. The optimal structure is obtained through parameter optimization, and two grooves close to the core are filled with optical fluid to study the influence of optical fluid technology on optical properties. In order to ensure that the optical fluid only enters the core and not the cladding pores, the optical fluid can be slowly injected into the fiber through a small tip device such as a hypodermic needle or a cladding layer can be deposited on one end of the fiber to block the entry of the optical fluid. With the advent of optical fluids, manipulating light and fluids on a microscopic scale to change the optical capabilities of the medium has become an important means of manufacturing highly sensitive sensors. We adjust the optical characteristics of the photonic circuit through changing the refractive index n, so that the photonic device has tunability and reconfigurability, which can be used for gas sensing detection. In addition, different fluid materials are used to achieve the control of light on substances, and optical fluid technology is introduced into the pores of the optical fiber to dynamically adjust the mode, which reduces the cost caused by replacing the optical fiber and facilitates the construction of a highly sensitive integrated sensor.

Results and Discussions The larger the numerical aperture (NA), the more advantageous it is for sensing applications. When the refractive index difference between the core and the cladding of the PCF is larger, the NA is closer to 1. In this paper, under the best design parameters, we get the NA of 0.11 (Fig. 5), and most of the previously proposed sensor structures ignore this characteristic of PCF. The incident light in the Y polarization direction has higher sensitivity (Fig. 8), so a polarization controller is added between the light source and the sensor to control the light entering the sensor to be linearly polarized light along the Y direction. Since the optical fluid is filled to increase the effective refractive index (A_eff), the A_effis significantly moved up compared to the unfilled structure, the NA is decreased, but the change of NA is small compared to the unfilled structure (Fig. 10). In practical applications, it is necessary to consider the dispersion. The dispersion range obtained in the range of 1.55--1.64 μm is (0.041±0.023) ps·THz ^-1·cm ^-2 (Fig. 11). Compared to the previously proposed structure (Table 1), the dispersion of the proposed method is very low and the flatness is significantly improved.

Conclusions This paper designs a new type of PCF, uses finite element method to study the optical characteristics in the near infrared range, and analyzes the influence of filling on relative sensitivity, limiting loss, and dispersion parameters of the optimized structure combined with optical fluid technology. The results show that in the wavelength range of 1.55--1.64 μm , it has obvious flat dispersion characteristics close to zero, and the relative sensitivity is above 65%. By filling the optical fluid in a wide near-infrared wavelength range, the optical signal propagation with a low loss of 1.52×10 ^-2-2.8×10 ^-2 dB·m ^-1 and a ultra-low dispersion of 0.018 ps·THz ^-1·cm ^-2 is realized. In addition, due to the flexibility of the structure, it is expected to be applied to gas sensing detection in the THz range by adjusting the structural parameters.