空间激光与单模光纤和光子晶体光纤的耦合效率

为了设计最优光纤耦合系统,利用高斯模场近似单模阶跃光纤的模场和大模面积光子晶体光纤的模场,推导出了理想情况下空间激光与这两种光纤的耦合效率解析表达式以及光纤端面相对于耦合系统存在横向偏移和端面倾斜时的耦合效率解析表达式。基于上述理论表达式计算了空间激光与光纤的耦合效率,并通过实验验证了此理论表达式的有效性。理论计算和实验均证实了单模阶跃光纤对于横向偏移更敏感,当横向偏移量等于单模光纤的纤芯半径时所对应的耦合效率只有20.25%,为理论最大值的1/4;而大模面积光子晶体光纤对于端面倾斜更加敏感,当端面倾斜2°时对应的耦合效率只有40.5%,为理论最大值的1/2。所提出理论表达式和实验方法完全可以为设计光纤耦合系统提供准确的参数。

Coupling efficiency of free-space laser coupling into single mode fiber and photonic crystal fiber

It is necessary to confirm the system parameters by theoretical analysis and experiments for desig- ning a optimum fiber-coupling system. By approximating the mode fields of a Single Mode Fiber（SMF） with a step index and a Photonic Crystal Fiber（PCF） with a Large Mode Area（LMA） using the Gaussian mode field, two analytical expressions for fiber-coupling efficiency in an ideal case and in the presences of lateral offset and fiber-end tilt are derived. Based on the two theoretical expressions, the efficiency for free-space laser coupling into the fibers is computed. Moreover, the validity of the two expressions are verified by experiments. As a re- sult, the theoretical computation and experiments demonstrate that the SMF with the step index is more sensirive to the lateral offset compared to the PCF. When the offset is equal to the radius of the fiber core, the effi- ciency is only 20. 25%, which is a quarter of the theoretical maximum efficiency. Besides, the PCF with the LMA is more sensitive to the fiber-end tilt. When the tilt angle is 2°, the efficiency is only 40.5%, which is a half of the theoretical maximum efficiency. In conclusion, the proposed theoretical expressions and experi- ments can provide accurate parameters for designing fiber-coupling systems.