{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,22]],"date-time":"2026-07-22T18:34:22Z","timestamp":1784745262641,"version":"3.55.0"},"reference-count":34,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2021,5,10]],"date-time":"2021-05-10T00:00:00Z","timestamp":1620604800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Outstanding Youth Science Fund Project of Hunan Province Natural Science Foundation","award":["2019JJ20023"],"award-info":[{"award-number":["2019JJ20023"]}]},{"name":"National Natural Science Foundation of China (NSFC)","award":["11974427, 12004431"],"award-info":[{"award-number":["11974427, 12004431"]}]},{"name":"State Key Laboratory of Pulsed Power Laser","award":["SKL-2020-ZR05"],"award-info":[{"award-number":["SKL-2020-ZR05"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>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.<\/jats:p>","DOI":"10.3390\/s21093278","type":"journal-article","created":{"date-parts":[[2021,5,10]],"date-time":"2021-05-10T10:49:51Z","timestamp":1620643791000},"page":"3278","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["Spectral Characteristics of Square-Wave-Modulated Type II Long-Period Fiber Gratings Inscribed by a Femtosecond Laser"],"prefix":"10.3390","volume":"21","author":[{"given":"Xiaofan","family":"Zhao","sequence":"first","affiliation":[{"name":"College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"},{"name":"Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hongye","family":"Li","sequence":"additional","affiliation":[{"name":"College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"},{"name":"State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Binyu","family":"Rao","sequence":"additional","affiliation":[{"name":"College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"},{"name":"Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Meng","family":"Wang","sequence":"additional","affiliation":[{"name":"College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"},{"name":"Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China"},{"name":"State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Baiyi","family":"Wu","sequence":"additional","affiliation":[{"name":"College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"},{"name":"Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China"},{"name":"State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Zefeng","family":"Wang","sequence":"additional","affiliation":[{"name":"College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"},{"name":"Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China"},{"name":"State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2021,5,10]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"58","DOI":"10.1109\/50.476137","article-title":"Long-period fiber gratings as band-rejection filters","volume":"14","author":"Vengsarkar","year":"1996","journal-title":"J. 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