We measured the transmission of tapered and untapered optical fibres as a function of input beam numerical aperture at 635 nm. The tapered fibres were fabricated with an adiabatic tapering process from graded and step-index fibres with 50 $\mathrm{\mu }$m core diameters to form a 100 mm long taper with 5:1 taper ratio. We tested tapered graded-index and step-index fibres fabricated from commercial Thorlabs products and a custom graded-index taper. The 5:1 tapered graded-index fibre can give a transmission greater than 0.4 for Thorlabs and 0.6 for the custom taper. We simulated the transmission of the tapered fibres and found reasonable agreement with the measured graded-index tapered fibre results across the numerical aperture range of interest. Experimentally, step-index tapered fibres performed relative poorly and considerably below modelling expectations. Based on our examinations this arises because the properties of step-index fibre were not robust to the tapering process. Suitably tapered graded-index fibres may offer a new route for efficient focal ratio reduction of fibre optic signals, e.g., in fibre-fed spectrographs, though we stress that our measurements have been limited to monochromatic light in this work.

我们测量了锥形和非锥形光纤的传输作为 635  nm输入光束数值孔径的函数。锥形纤维是由梯度和阶跃折射率的纤维采用绝热锥形工艺制成的，其折射率为 50 $\mathrm{\mu }$m 芯直径形成 100 mm 长锥度，锥度比为 5:1。我们测试了由商业 Thorlabs 产品和定制的渐变折射率锥度制成的锥形渐变折射率和阶跃折射率光纤。5:1 锥形渐变折射率光纤可为 Thorlabs 提供大于 0.4 的透射率，为定制锥形提供大于 0.6 的透射率。我们模拟了锥形光纤的传输，并发现在感兴趣的数值孔径范围内与测量的渐变折射率锥形光纤结果具有合理的一致性。实验上，阶跃折射率锥形光纤的性能相对较差，远低于建模预期。根据我们的研究，出现这种情况是因为阶跃折射率纤维的特性对锥形过程不稳健。合适的锥形渐变折射率光纤可以为有效降低光纤信号的焦比提供一条新途径，例如在光纤馈送光谱仪中，