Dual-Pitch staggered Long-Period fiber grating for mode conversion and permutation

Mode division multiplexing (MDM) technology enhances transmission capacity by multiplexing modes that carry distinct information. However, as transmission distance increases, the differential mode group delay (DMGD) between modes becomes more pronounced, posing a critical challenge to long-haul MDM transmission. Introducing strong mode coupling through long-period fiber gratings has proven to be an effective approach to mitigating DMGD and enabling extended transmission. In this work, we propose and demonstrate a dual-pitch staggered long-period fiber grating (DSP-LPFG) capable of facilitating mode conversion between two distinct mode pairs. By strategically distributing three sets of DSP-LPFGs along the fiber, this method achieves periodic mode permutation of four mode groups along the entire length of fiber. The combined grating generates four target modes with a purity exceeding 90 %, while the device’s insertion loss remains below 0.7 dB. Theoretically, this approach balances energy ratio variations among modes within the transmission link and mitigates associated processing capability fluctuations. Consequently, it reduces the risk of overload in the MIMO equalizer at the receiving end, effectively addressing a key limitation of long-haul MDM transmission.