講演情報
[23a-P05-14]Mechanically Induced Long-Period Fiber Gratings for Higher-Order Mode Generation
〇Wen-Hsuan Kuan1, Kuei-Huei Lin1 (1.Univ. of Taipei)
キーワード：
Long-Period Fiber Gratings、Doughnut Beam、Higher-Order Mode
By using a constant-period V-grooved plate and a flat plate, we have made ultrabroadband long-period fiber gratings (LPGs) in a two-mode fiber upon which the mechanical stress gradient is employed. In this work, we show that in addition to the doughnut beam, the Mexican-hat beam and the crater-lake beam can also be generated in a mechanically induced few-mode LPG via the coupling of the fundamental core mode to a series of co-propagating higher-order modes. A schematic diagram of mode tuning based on the mechanically induced LPG is shown in Fig. 1, in which a Corning HI1060 few-mode fiber (FMF) is sandwiched between a V-grooved plate having a period of 400 mm and a flat plate, where the force gradient is applied for producing quasi-periodic stress on the FMF. The 532-nm green light from a frequency-doubled Nd:YVO4 laser is launched into the fiber via an objective lens, and the output intensity distribution from the FMF is recorded by an optical beam profiler. The cutoff wavelength of HI1060 is about 920 nm, therefore, while operating at the wavelength 532 nm, it supports LP01, LP11, LP02, and LP21 modes. By tuning the angle of the grooved plate against the axis of straight optical fiber, the effective period of the gratings and the corresponding resonance wavelength can be adjusted. The polarization controllers are used to adjust the coupling fraction among four eigenmodes and their relative phases. Figures 2(a)-(d) demonstrate the experimentally generated fundamental beam, the doughnut beam, the Mexican-hat beam, and the crater-lake beam, and the simulation results of these beams by superposition of the four eigenmodes are shown in Fig. 2(e)-(h).