基于模式耦合的低弯曲损耗大模场带隙光纤研究

Study on low bending loss and large mode area bandgap fiber based on mode coupling

提出一种大模场带隙光纤,由排布在正方结构网格中的高折射率介质柱形成导光机制。采用有限元法分析了直光纤与弯曲光纤下的模式损耗与模场面积等特性。研究结果表明:这种光纤具有较宽的带隙,可同时支持基模和高阶模的传输,两种模式的泄漏损耗均低于1×10-3 dB/m。当光纤弯曲时,其包层会产生具有强泄露损耗的包层模,并在一定的弯曲半径下与纤芯的高阶模发生强耦合。当弯曲半径在15~20cm之间时,基模弯曲损耗小于0.01dB/m,而高阶模损耗大于1dB/m,因而光纤可以经弯曲实现大模场单模传输。在1064nm波长处,其直光纤的基模模场面积为1 319.62μm2,而在弯曲状态下的模场仍可达到975.00μm2以上,因而可实现大模场的低弯曲损耗传输。

A large mode area bandgap fiber is fully investigated, in which the guiding system is formed by high-refractive index rods. These rods are arranged in a mesh grid of the square structure. Losses and mode field area of the straight fiber and bending fiber are calculated by applying the finite-element meth- od. The investigation results show that the transmission of higher-order modes and fundamental modes can be supported by this fiber simultaneously, owing to a wide bandgap. In addition, leakage losses of these modes are less than 10-3 dB/rn. The calculation results indicate that cladding modes with strong leakage losses are generated from cladding, when bending the fiber. Under the condition of a certain ben- cling radius, cladding modes are coupled with higher-order modes in the core. In particular, when the ben- ding radius is in the range of 15 cm to 20 cm , the bending losses of fundamental modes are less than 0. 01 dB/m,whereas losses of higher-order modes are more than 1 dB/rn. As a result,the large mode ar- ea single-mode transmission can be realized by bending the fiber. At 1064 nm,the mode field area of the straight fiber can reach 1 319.62 μm2, but the mode field area of bending fiber can be more than 975 μm^2 ,which confirms that the large mode area and low bending loss can be achieved.