通过有效折射率模型分析了光子晶体光纤的无限截止单模特性, 数值模拟表明只要光纤微结构包层中空气孔直径与孔间距之比足够小, 光子晶体光纤就可以在任意波长支持单模传输。具体计算了光子晶体光纤在大于200 nm的波长范围内支持单模传输的结构参数, 为实现光子晶体光纤单模传输紫外激光提供了理论依据。实验采用光纤激光器四次谐波260 nm飞秒激光和一段小空气孔直径的光子晶体光纤, 研究了紫外激光在光子晶体光纤中的传输特性。其中光子晶体光纤纤芯直径为4.54 μm, 耦合效率大于31%。

Properties of the endlessly single mode of photonic crystal fiber(PCF) are analyzed theoretically with the effective-index model. Numerical calculation shows that PCF can be single mode for any wavelength if the proportion of the diameter of the air holes to the pitch in the cladding is small enough. Structure parameters of a single-mode PCF for wavelength above 200 nm are calculated exactly, which provide us with theoretical instruction for the ultroviolet (UV) light single-mode delivery of PCF. Based on 260 nm femtosecond pulses output from a frequency-quadrupled fiber laser, transmission of the UV laser is experimentally demonstrated using a piece of PCF with small air hole. With a core diameter of 4.54 μm, the transmission loss and the coupling efficiency of the PCF are measured to be 0.043 dB/cm and above 31%, respectively.