Ultra-broadband multi-channel orbital angular momentum mode generator using a helical long-period grating pair

We demonstrate an ultra-broadband multi-channel orbital angular momentum (OAM) mode generator using a helical long-period grating (HLPG) pair in a few-mode fiber. The numbers of the channel of the OAM mode generator can be flexibly adjusted by designing the cascaded length of the HLPG pair. When the cascaded length is 50 cm, the maximum channel number of OAM mode generator reaches 46, and the mode convert efficiencies of the channels are more than 90 %, which is, to the best of our knowledge, the highest number of channels among OAM mode converters. By reducing grating length and enhancing the refractive index modulation depth, the 3 dB bandwidth of the HLPG is significantly increased, and thus the wavelength range of the HLPG pair can be expanded. The maximum wavelength ranges of the channels with coupling efficiency greater than 90 % are measured to be 379 nm experimentally. By adjusting the spiral direction of the helical refractive index modulation, we can achieve multi-channel +/- 1 order OAM mode conversion for each channel. The multi-channel OAM mode generator holds substantial potential for applications in mode division multiplexing systems, wavelength filters, and fiber sensing.