Efficient Mutual Conversion of High-Order Core Mode in Few-Mode Fiber Employing Long Period Fiber Gratings

Mode division multiplexing technology (MDM) shows a bright prospect in the process of solving the capacity crisis, and mode converters play an important role in it. In recent years, how to excite higher-order modes with high efficiency, low insertion loss and low complexity has attracted much attention, but the problem of conversion between two higher-order modes to achieve higher-order mode excitation has been rarely discussed. In this paper, an efficient mutual conversion method between high-order angular core modes in few-mode fiber based on long period fiber gratings (LPFGs) is proposed and demonstrated. By presenting an optimizing refractive index modulation model, the coupling coefficients between fundamental mode and high-order core mode as well as that between different higher-order core modes are calculated in detail. Experimentally, LPFGs converting from LP
₁₁
to LP
₂₁
, from LP
₂₁
to LP
₃₁
, from LP
₃₁
to LP
₄₁
mode with conversion efficiencies of 95.6%, 95.2% and 94.7% and insertion losses of less than 0.15 dB are fabricated by using conventional CO
₂
laser side exposure methods, respectively. By adjusting polarization controllers, the selective excitation of
$\mathbf{LP}_{{\bm{l1}}}^{{\bm{a/b}}}{\bf (}{\bm{l}}{\bf = 2,3,4)}$
modes and orbital angular momentum (OAM) modes of different topological charges (±2, ±3, ±4) is realized, respectively. To the best of our knowledge, this is the first systematic investigated on mutual conversion between adjacent high-order core modes, and simultaneously achieve higher-order mode excitation up to
l
= 4 with high efficiency, high purity and low insertion loss. This work provides a possibility for data exchange in optical communication networks based on any high-order mode transformation in the future.