High-performance interferometers, gyroscopes, frequency combs, quantum information experiments and optical clocks rely on the transmission of light beams with the highest possible spatial and polarization purity. Free-space propagation in vacuum unlocks the ultimate performance but becomes impractical at even modest length scales. Glass optical fibres offer a more pragmatic alternative, but degrade polarization purity and suffer from detrimental nonlinear effects. Hollow-core fibres have been heralded for years as the ideal compromise between the two, but achieving high modal purity in both spatial and polarization domains has proved elusive thus far. Here, we show that carefully designed, low-nonlinearity hollow-core antiresonant fibres can transmit a single pair of orthogonal polarization modes with cross-coupling on the scale of 10^–10 m^–1; that is, orders of magnitude lower than any other solution. This free-space-like optical guidance can immediately provide a leap in performance for photonics-enabled sensors and instruments.

高性能干涉仪，陀螺仪，频率梳，量子信息实验和光钟依赖于具有最高空间和偏振纯度的光束传输。真空中的自由空间传播会释放出最终的性能，但即使是适度的长度尺度也变得不切实际。玻璃光纤提供了更为实用的替代方案，但会降低偏振纯度并遭受有害的非线性影响。多年来，中空纤维一直被认为是两者之间的理想折衷方案，但是迄今为止，在空间和偏振域中实现高模态纯度已被证明是遥不可及的。在这里，我们表明经过精心设计的低非线性中空反谐振光纤可以传输一对正交极化模，并且交叉耦合的比例为10^{– 10}  m ^{– 1} ; 也就是说，比其他任何解决方案都低几个数量级。这种类似自由空间的光学导引可立即为启用光子学的传感器和仪器提供性能上的飞跃。