Photonic crystal fiber (PCF) has a variety of unique optical properties, such as single-mode transmission in all wavelength ranges, large effective mode area, high nonlinearity, and dispersion control, which make it have a wide range of applications. The transmission ability of a traditional PCF is restricted by air core collapse and structured cladding deformation during optical fiber fabrication. The all-solid-state structure can solve the problems of air hole collapse and cladding deformation of traditional PCF. The large mode area (LMA) PCF can solve the nonlinear effects such as four-wave mixing and Brillouin scattering caused by the small mode field area of the traditional PCF. Therefore, large mode area single-mode all-solid-state PCF is a good carrier for high-power fiber lasers and amplifiers and the only effective way to improve the power processing capability of chalcogenide fibers while maintaining beam quality. In this study, a large mode area single-mode all-solid-state chalcogenide PCF is prepared by extrusion-stacking method, which solves the problem that traditional fibers could not achieve high power transmission and high beam quality at the same time.

In this study, we first establish a theoretical model for large mode area single-mode all-solid-state chalcogenide PCF. Mid-infrared chalcogenide glass Ge₁₀As₂₂Se₆₈ and As₂S₃ are chosen as high and low refractive index materials. The fiber is prepared by extrusion-stacking method. Firstly, the two materials are extruded into rods, and then the rods are stacked according to the theoretical model to obtain preforms. Finally, the preform is drawn into fiber. The cross section, mode area, near-field energy distribution, fiber loss and bending loss of the fiber are calculated and analyzed.

According to the simulation results, the optimal structural parameters of large mode area single-mode all-solid-state chalcogenide PCFs are obtained. The experimental results show that large mode area single-mode chalcogenide PCF can simultaneously achieve high power transmission and high beam quality. Based on the cross section of the large mode field single-mode all-solid-state chalcogenide PCF [Fig. 8(a)], the mode field area of the fiber is about 5400 μm². From the spot diagram [Fig. 8(b)] and energy distribution diagram [Fig. 8(c)] measured by the near-infrared fiber field analyzer at 1.55 μm, it can be seen that the optical fiber transmission intensity distribution is Gaussian distribution, which proves that the large mode area all-solid-state chalcogenide PCF realizes single-mode transmission. The fiber losses of the prepared large mode area single-mode all-solid-state chalcogenide PCF and the cladding-free Ge₁₀As₂₂Se₆₈ fiber are tested. The truncation method is used for multiple measurements and the average value is calculated. The fiber end face is kept intact with a precision fiber cutter to obtain the fiber loss diagram (Fig. 9). It can be seen that the PCF loss is much larger than the loss of the cladding-free fiber, that is, the loss of the matrix glass. The reason for this additional loss may be the leakage of light energy caused by the structural defects of the fiber and the defects formed by the stacking interface. Through the bending loss performance test of the large mode area single-mode all-solid-state chalcogenide PCF (Fig. 10), it is found that the loss in the fiber core increases with the decrease of the bending radius, and the change is obvious at 12 cm bending radius. The low bending loss of 1 dB is observed in the fiber with a bending radius of 14 cm, which is consistent with the simulation results of COMSOL. It can be seen that the prepared fiber has good bending resistance.

Aiming at the current problem that the high-power transmission and high beam quality are difficult to achieve at the same time in chalcogenide fiber, we simulate the properties of the fiber by COMSOL software, and design the structural parameters of PCF that satisfies single-mode transmission and has a large mode field area. The designed fiber satisfies the single-mode condition that the confinement loss ratio of the high-order mode (LP₁₁) to the fundamental mode (LP₀₁) is greater than 10² in the range of 1.5?10 μm, and the effective mode area of the fundamental mode is about 5362 μm² at 2 μm. According to the structural parameters of the simulation, high-purity chalcogenide glass is prepared by a new combination of dynamic and static distillation. Combined with a novel extrusion-stacking method, an all-solid-state single-mode large-mode-area PCF is prepared based on chalcogenide glass. The optical energy distribution and intensity test results of the fiber at a wavelength of 1.55 μm confirm the single-mode transmission characteristics. At the same time, a low bending loss of 1 dB is observed in a 1.5 m long fiber with a bending radius of 14 cm, which is basically consistent with the simulation results. It is shown that this all-solid-state PCF has excellent single-mode transmission and bending resistance, and has the potential for mid-infrared high-power laser applications.