Spatial multiplexers (SMUXs) for mode division multiplexing often involve multiple strategies for mode-selective excitation and the minimization of insertion and other losses. Haoshuo Chen, Roy van Uden, Chigo Okonkwo and Ton Koonen, working at the COBRA Institute at the Eindhoven University of Technology in The Netherlands, have reported a SMUX for mode division multiplexing that packages multiple strategies into compact components with a small footprint (Opt. Express 22, 31582–31594; 2014). They demonstrated photonic integrated SMUXs on both silicon-on-insulator (SOI) and indium phosphide (InP) platforms for selectively exciting modes.

Credit: HAOSHUO CHEN

The use of two-dimensional top-coupling provided the coupling interface between a few-mode fibre and the photonic integrated SMUX. A 3-spot SMUX realized using the InP platform with 45° vertical mirrors and laser-inscribed waveguide technology was demonstrated as an option for a fully-packaged dual-channel 6-mode SMUX including two 6-core photonic lantern structures serving as a mode multiplexer and a demultiplexer, respectively. The scanning electron microscope image (pictured) shows the coupling section of an InP SMUX with six vertical mirrors that support operation with six spatial modes.

Haoshuo Chen, now at Bell Labs in the USA, told Nature Photonics that the aim was to use vertical 'emitters', such as vertical grating couplers on SOI and 45° vertical mirrors on InP devices, to realize two-dimensional top-coupling between few-mode spatial division multiplexing fibres and photonic integrated components.

“Previous spatial multiplexers were mostly based on bulky free-space optics with large footprints, which can be challenging to scale up to support more spatial-mode channels,” Chen explained. “Integrated photonic SMUXs can be incorporated with other functionality on the same chip, which is cost- and energy-efficient.”

As always, there were some hurdles for the authors. Chen noted that the insertion losses of the experimentally fabricated vertical emitters can be greater than those predicted by the design. However, Chen said that future work will focus on optimizing designs and reducing insertion losses. The team would also like to integrate other functionality with the SMUX such as lasers, optical modulators for spatial division multiplexing transmitters, and create hybrids — photodetectors for spatial division multiplexing coherent receivers. The photonic SMUXs can be mass-manufactured and packaged for high reliability.