DOI: 10.1002/ecj.12275 TRANSLATED PAPER Cladding mode loss effect on transmission characteristics of mechanically induced long-period fiber gratings Yasuhiro Tsutsumi 1, 2 Kenta Kitahara 2 Masaharu Ohashi 2 Hirokazu Kubota 2 Yuji Miyoshi 2 Ikuo Yamashita 3 1 Ritsumeikan University, 1-1-1, Nojihigashi, Kusatsu, Shiga 525–8577, Japan 2 Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan 3 Kansai Electric Power Co., Inc., 3-11-20, Nakouji, Hyougo, Amagasaki 661–0974, Japan Correspondence Yasuhiro Tsutsumi, Ritsumeikan Uni- versity, 1-1-1, Nojihigashi, Kusatsu, Shiga 525–8577, Japan. Email: tutumi@fc.ritsumei.ac.jp Translated from Volume 140, Number 7, pages 682–688, DOI: 10.1541/ieejeiss.140. 682 of IEEJ Transactions on Electronics, Information and Systems (Denki Gakkai Ronbunshi C) Abstract In the mechanically induced long-period fiber grating (MLPFG) fabricated by applying the periodical pressure on an optical fiber, the small bending occurs due to the periodical pressure, which influences the losses of the core and cladding modes. We propose the theoretical model of the MLPFG for estimating its trans- mittance based on the transfer matrix method and investigate the effect of the losses of the core and cladding modes in the MLPFG on the transmittance, the- oretically and experimentally. We numerically clarify that the transmitted light spectrum of MLPFG shows only the main attenuation lobe, no side lobe, and the attenuation bandwidth is broadened as the cladding mode loss increases. We also clarify that the coupling coefficient, and the losses of the core and cladding modes in the fabricated MLPFG can be estimated from the measured transmitted spectrum based on our model. Moreover, we measure the transmittance of two types of MLPFGs that fabricated with a screw, weights, and metallic plates, and that fabricated with a screw and a heat-shrinkable tube. The measured spectra show good agreements with the calculated ones using our model. Our model will be useful for designing MLPFGs applied to the sensors. KEYWORDS cladding mode loss, mechanically induced long-period fiber gratings, optical fiber sensor, trans- fer matrix model 1 INTRODUCTION Optical fiber sensors are immune to electromagnetic noise, and sensor heads do not conduct electricity; hence, excellent explosion proofness and other merits. Partic- ularly, mechanically induced long-period fiber gratings (MLPFGs) feature a simple structure and relatively inex- pensive manufacture, while their light transmission spec- tra vary with MLPFG temperature, strain, external refrac- tive index, and other parameters; a number of studies were conducted on sensor applications. 1–5 MLPFG is an optical device provided with diffraction grating formed through periodical compression so that refractive index of optical fiber periodically changes. 6,7 In MLPFG, cou- pling occurs between core and cladding modes at spe- cific wavelengths that meet phase matching conditions. 8 In a cladding mode, attenuation readily occurs due to tiny bends and distortions of optical fiber or external refractive index, so that signals disappear during propagation from MLPFG to an optical receiver. 9 Therefore, light transmis- sion spectra of MLPFG show strong attenuation at specific wavelength. MLPFG is fabricated by applying periodical pressure to optical fiber; as a result, tiny bends occurring in MLPFG 46 © 2020 Wiley Periodicals LLC Electron Comm Jpn. 2020;103:46–53. wileyonlinelibrary.com/journal/ecj