Superior ultrafast laser-inscribed photonic-lantern mode (de)multiplexers using trajectory-asymmetry with uniform waveguides

Femtosecond laser fabrication technology has been applied to photonic-lantern mode (de)multiplexers owing to its 3D fabrication capability. Current photonic-lantern mode (de)multiplexer designs based on femtosecond laser fabrication technology mostly follow a fibre-type photonic lantern design, which uses trajectory-symmetry structures with non-uniform waveguides for selective mode excitation. However, non-uniform waveguides can lead to inconsistent waveguide transmission and coupling losses. Trajectory-symmetry designs are inefficient for selective-mode excitation. Therefore, we optimised the design using trajectory asymmetry with uniform waveguides and fabricated superior ultrafast laser-inscribed photonic-lantern mode (de)multiplexers. Consistent waveguide transmission and coupling losses (0.1 dB/cm and 0.2 dB/facet, respectively) at 1550 nm were obtained on uniform single-mode waveguides. Based on the trajectory-asymmetry design for photonic-lantern mode (de)multiplexers, efficient mode excitation (\begin{document}$ {LP}_{11}^{a} $\end{document},\begin{document}$ {LP}_{11}^{b} $\end{document}, and\begin{document}$ {LP}_{01} $\end{document}) with average insertion losses as low as 1 dB at 1550 nm was achieved, with mode-dependent losses of less than 0.3 dB. The photonic-lantern design was polarisation-insensitive, and the polarisation-determined losses were less than 0.2 dB. Along with polarisation multiplexing realised by fibre-type polarisation beam splitters, six signal channels (\begin{document}$ {LP}_{11x}^{a} $\end{document},\begin{document}$ {LP}_{11y}^{a} $\end{document},\begin{document}$ {LP}_{11x}^{b} $\end{document},\begin{document}$ {LP}_{11y}^{b} $\end{document},\begin{document}$ {LP}_{01x} $\end{document}, and\begin{document}$ {LP}_{01y} $\end{document}), each carrying 42 Gaud/s quadrature phase-shift keying signals, were transmitted through a few-mode fibre for optical transmission. The average insertion loss of the system is less than 5 dB, while its maximum crosstalk with the few-mode fibre is less than −12 dB, leading to a 4-dB power penalty. The findings of this study pave the way for the practical application of 3D integrated photonic chips in high-capacity optical transmission systems.