Abstract:A polarization-maintaining hollow-core negative curvature fiber is proposed， which can be used to transmit mid-infrared laser with an operating wavelength of 2.94 μm. By adjusting the thickness of the cladding tube in the y-direction and the maximum distance from the nested tube to the inner diameter of the cladding tube， the polarization mode in the y-direction is weakly coupled with the surface mode of the cladding tube. By optimizing the core diameter and the outer diameter of the cladding tube， the y-direction polarization mode is strongly coupled with the cladding tube surface mode， causing the y-direction polarization mode to leak out of the core. The gap between the cladding tubes guides the high-order mode to leak out of the core， while the x-direction polarization mode keeps the low confinement loss due to the coupling suppression effect of the thickness of the cladding tube and the nested tube. Finally， a polarization-maintaining hollow core negative curvature fiber with an operating wavelength of 2.94 μm is obtained. The confinement loss of the x-direction polarization mode is 2.8×10-2 dB/m， the polarization extinction ratio is greater than 2×103， the high-order mode rejection ratio is greater than 100， and the birefringence is1.4×10-5. When bending in the y direction with a radius of 25 cm， the bend loss is 0.62 dB/m.