Zhao Xiaofan, Li Hongye, Rao Binyu, Wang Meng, Wu Baiyi, Wang Zefeng
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China.
Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China.
Sensors (Basel). 2021 May 10;21(9):3278. doi: 10.3390/s21093278.
We study here the spectral characteristics of square-wave-modulated type II long-period fiber gratings (LPFGs) inscribed by a femtosecond laser. Both theoretical and experimental results indicate that higher-order harmonics refractive index (RI) modulation commonly exists together with the fundamental harmonic RI modulation in such LPFGs, and the duty cycle of a square wave has a great influence on the number and amplitudes of higher-order harmonics. A linear increase in the duty cycle in a series of square wave pulses will induce another LPFG with a minor difference in periods, which is useful for expanding the bandwidth of LPFGs. We also propose a method to reduce insertion loss by fabricating type II LPFGs without higher-order harmonic resonances. This work intensifies our comprehension of type II fiber gratings with which novel optical fiber sensors can be fabricated.
我们在此研究由飞秒激光写入的方波调制II型长周期光纤光栅（LPFG）的光谱特性。理论和实验结果均表明，在此类LPFG中，高阶谐波折射率（RI）调制通常与基波谐波RI调制同时存在，且方波的占空比对高阶谐波的数量和幅度有很大影响。一系列方波脉冲中占空比的线性增加会诱导出周期略有差异的另一个LPFG，这对于扩展LPFG的带宽很有用。我们还提出了一种通过制造无高阶谐波共振的II型LPFG来降低插入损耗的方法。这项工作加深了我们对II型光纤光栅的理解，利用其可制造新型光纤传感器。
