Impact of capillary material intrusion on the photonic lantern performance

Photonic lanterns (PL) fabricated by the fiber bundle fusion tapering technique offer a low-loss and highly efficient interconnection module between single-mode fiber (SMF) and few-mode fiber (FMF). However, due to the lower fusion temperature, the fluorine-doped silica at the PL cladding is prone to intruding into the core region, resulting in the core deformation of the PL. In the current submission, we present a comprehensive study of the impact of capillary material intrusion (CMI) on the PL performance. Theoretical analysis reveals that CMI leads to a petal-shaped core at the few-mode end of the PL. As for the 3-mode PL, the asymmetry induced by the petal-shaped core breaks the degeneracy between mode groups, leading to the enhancement of the PL mode selectivity (MS) by 2∼3 dB. In addition, CMI brings a distortion of the mode field, leading to a performance penalty when the PL is coupled with the traditional FMF, particularly for higher-order modes. As the number of mode groups increases, the reduction of asymmetry induced by the mode field mismatch effect becomes more dominant. As for the 6-mode PL, CMI causes an MS reduction of 3∼5 dB. Higher-order modes of both the 3-mode and 6-mode PLs experience an insertion loss (IL) deterioration of 0.45 dB and 0.64 dB, respectively. Meanwhile, the mode conversion efficiency (MCE) decreases by 10% and 14%, in comparison with the scenario without the CMI. Furthermore, CMI strengthens the wavelength dependence of PL, leading to the compression of the operation wavelength range. As the number of modes increases, the CMI effect gradually diminishes as the petal-shaped core becomes more circular. Those findings provide valuable insights for the design and fabrication of PL, with the potential to enhance the PL performance for ultra-wideband mode division multiplexing transmission.