Cruz J L, Barmenkov Y O, Díez A, Andres M V
Appl Opt. 2021 Apr 1;60(10):2824-2832. doi: 10.1364/AO.418049.
In this work we report on the measurement, with record accuracy, of the absolute modal effective refraction index (phase index) of single-mode optical fibers by using Bragg gratings. We also demonstrate a new method to measure the group index of the fibers from the grating's Bragg wavelength. We present as well the characterization of the thermo-optic and strain-optic coefficients as a function of the wavelength; the values we have obtained are the closest to those of pristine fiber measured with gratings technology so far. The phase index is measured with a set of gratings in the wavelength ranges from 1509 to 1563 nm, and the group index is obtained from the wavelength dependence of the phase index. Very weak gratings with reflectivity down to 10 have been used in order to minimize the perturbation of the pristine fiber. Results are presented at a temperature of 22°C and zero strain after preliminary calibration of the thermo-optic and strain-optic coefficients as a function of the wavelength. Accuracies better than 3×10 and 7×10 have been achieved for the phase and group indices, respectively. It is also shown that the main source of error relates to the uncertainty in pitch of the phase masks used for grating inscription. The technique is useful for testing different kinds of fibers including telecommunication, amplifying, and polarization maintaining.
在这项工作中，我们报告了利用布拉格光栅以创纪录的精度测量单模光纤的绝对模式有效折射率（相位折射率）。我们还展示了一种从光栅的布拉格波长测量光纤群折射率的新方法。我们还给出了热光系数和应变光系数随波长变化的特性；我们获得的值是迄今为止用光栅技术测量的最接近原始光纤的值。在1509至1563纳米的波长范围内用一组光栅测量相位折射率，并从相位折射率的波长依赖性获得群折射率。为了使原始光纤的扰动最小化，使用了反射率低至10的非常弱的光栅。在将热光系数和应变光系数作为波长的函数进行初步校准后，在22°C的温度和零应变下给出结果。相位折射率和群折射率的精度分别达到了优于3×10和7×10。还表明，主要误差源与用于光栅写入的相位掩膜间距的不确定性有关。该技术可用于测试包括电信、放大和保偏在内的不同类型的光纤。
