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Communication

Spectral Characteristics of Square-Wave-Modulated Type II Long-Period Fiber Gratings Inscribed by a Femtosecond Laser

by 1,2,†, 1,3,†, 1,2, 1,2,3, 1,2,3 and 1,2,3,*
1
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
2
Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China
3
State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Academic Editor: Kyriacos Kalli
Sensors 2021, 21(9), 3278; https://doi.org/10.3390/s21093278
Received: 24 March 2021 / Revised: 30 April 2021 / Accepted: 7 May 2021 / Published: 10 May 2021
(This article belongs to the Special Issue Advanced Fiber Photonic Devices and Sensors)
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. View Full-Text
Keywords: long-period fiber gratings; higher-order harmonics; femtosecond laser long-period fiber gratings; higher-order harmonics; femtosecond laser
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MDPI and ACS Style

Zhao, X.; Li, H.; Rao, B.; Wang, M.; Wu, B.; Wang, Z. Spectral Characteristics of Square-Wave-Modulated Type II Long-Period Fiber Gratings Inscribed by a Femtosecond Laser. Sensors 2021, 21, 3278. https://doi.org/10.3390/s21093278

AMA Style

Zhao X, Li H, Rao B, Wang M, Wu B, Wang Z. Spectral Characteristics of Square-Wave-Modulated Type II Long-Period Fiber Gratings Inscribed by a Femtosecond Laser. Sensors. 2021; 21(9):3278. https://doi.org/10.3390/s21093278

Chicago/Turabian Style

Zhao, Xiaofan, Hongye Li, Binyu Rao, Meng Wang, Baiyi Wu, and Zefeng Wang. 2021. "Spectral Characteristics of Square-Wave-Modulated Type II Long-Period Fiber Gratings Inscribed by a Femtosecond Laser" Sensors 21, no. 9: 3278. https://doi.org/10.3390/s21093278

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