Demonstration of the mode-selection rules obeyed in a single-helix helical long-period fiber grating.

ABSTRACT

A simple method enabling the validation of the mode-selection rules obeyed in a single-helix helical long-period fiber grating (SHLPG) has been demonstrated both theoretically and experimentally, which is realized by investigating and analyzing the circular dichroism (CD) and the polarization dependence loss (PDL) spectra of the utilized SHLPG. It is confirmed for the first time, to the best of our knowledge, that in terms of the SHLPG's helicity, the core mode ${{\rm HE}_{\textbf{11}}}$HE11 with either the left circular polarization or the right circular polarization will be selectively coupled into the higher HE or the TE/TM mode, respectively. As a result, the SHLPG would exhibit an enhanced CD, as well as an enhanced PDL near the resonant wavelengths of the HE and TE/TM modes, respectively. The results presented in this Letter will be helpful for efficiently designing and producing SHLPG-based circular polarizers and orbital-angular-momentum generators.