Optimal design and fabrication of multichannel helical long-period fiber gratings based on phase-only sampling method

In this study, a novel and efficient method enabling fabrication of a multichannel helical long-period fiber grating (HLPG) with almost the same channel spacing and the same rejection depth has been proposed and demonstrated for the first time both theoretically and experimentally, which is realized based on utilization of the so-called phase-only sampling technique. Unlike the previous amplitude-type sampling method where either a strong rectangular-type or a Sinc-like index-modulation distribution is generally demanded, the proposed method requires the minimum as well as a uniform index-modulation to the designed multichannel gratings, which considerably facilitates the fabrication process and makes the multichannel HLPGs to be fabricated in even a conventional single-mode fiber (SMF) by using the CO₂ laser writing technique. As examples, a 3-channel and a 9-channel HLPGs have been successfully demonstrated numerically and experimentally. The experimental results agree with the designed ones, which represents the first realization of a multichannel HLPG, to the best of our knowledge. It is believed that the proposed multichannel HLPG could find potential applications to multi-wavelength orbit-angular-momentum (OAM) mode converter as well as the wavelength division multiplexing (WDM) sensors.