A graded-index (GI)-type photonic crystal fiber (PCF) that has an α-power effective refractive index profile operates as a multimode optical fiber and can be applied to a collimator for single-mode PCFs. The α-power effective index distribution is realized by an α-power air hole diameter distribution with uniform air hole pitch. However, a pressure controlling in fiber drawing is difficult due to the difference of air hole diameters in fiber cross section. In this paper, we propose a GI-PCF with uniform air hole diameter while the air hole pitch varies in radial direction. Light propagation characteristics in the Gaussian beam excited GI-PCF with uniform air hole diameter were theoretically studied. The period and amplitude of mode field diameter variation along the propagation direction for the innermost air hole pitch, air hole diameter, and radial distribution profile of the pitch were clarified. The collimator application of GI-PCF with uniform air hole diameter for a conventional single-mode PCF was investigated. The suitable GI-PCF structure to obtain high optical coupling efficiency and good tolerance for GI-PCF length error was clarified. Wavelength characteristics of the GI-PCF collimator were also studied. It was clarified that the appropriate GI-PCF length was longer for shorter operating wavelength in collimator application.

具有α次有效折射率分布的渐变折射率(GI)型光子晶体光纤(PCF)作为多模光纤工作，并且可以应用于单模PCF的准直器。α次有效指数分布是通过具有均匀气孔间距的α次气孔直径分布来实现的。然而，由于纤维横截面的气孔直径不同，纤维拉伸中的压力控制很困难。在本文中，我们提出了一种具有均匀气孔直径而气孔间距沿径向变化的GI-PCF。对气孔直径均匀的高斯光束激发GI-PCF中的光传播特性进行了理论研究。阐明了最内气孔节距、气孔直径和节距径向分布轮廓的模场直径沿传播方向变化的周期和幅度。研究了具有均匀气孔直径的 GI-PCF 准直器在传统单模 PCF 中的应用。阐明了获得高光耦合效率和良好的 GI-PCF 长度误差容限的合适 GI-PCF 结构。还研究了GI-PCF准直器的波长特性。已澄清，在准直器应用中，对于较短的工作波长，适当的 GI-PCF 长度较长。