In 1880, American inventor Alexander Graham Bell invented the "optical telephone," which loaded sound signals onto a vibrating mirror for transmission, and was hailed as the beginning of contemporary optical wireless communication technology (Chen in Opt Commun Technol 17 (5):288–289 (1993)). In 1960, Theodore Maiman, an American physicist, invented the world's first ruby laser, marking the entry of optical wireless communication into experimental exploration and research (Wang and Fang in Progress in Laser and Optoelectron 54(9):090,005 (2017)). Professor Kao Kun proposed using optical fibers as communication media in 1966, after which, coherent optical communication developed more significantly in the field of optical fiber, compared to the field of free space.