Spatial Mode Mixing Device Printed on Fiber Facet

Experiments have lately demonstrated the feasibility of mode division multiplexing using spatial modes as independent data channels over few mode fiber (FMF). FMF is an attractive method of increasing capacity over that afforded by SMF. FMF transmission requires signal processing at the receiver in order to unravel the mixed channels. Under strong mode coupling, there is a frequent energy exchange between the modes. Statistically, this fact reduces the required DSP complexity.

In order to obtain mode-mixing over all modes, together with low loss, we designed a custom 3D spatial phase mask. The non-uniform phase acquired by the mask induces mode mixing between all mode groups, while maintaining low and equal loss over the mode groups. The mode scrambler design reached as low IL as -1 dB and average XT values > -5 dB for all mode groups. For similar mode mixing by conventional methods, the insertion loss increases significantly, makes it inefficient and non-scalable.

The complex phase pattern was printed on fiber tip (fig. 1c), using a 3D laser lithography system of “Nanoscribe GmbH” employing immersion technology. By varying the height of polymer, we achieved the required spatial phase delay. Since the device is located on the fiber`s facet, we eliminate the loss of free space optics and obtain small footprint device which can be directly coupled to another fiber (Fig. 1b).

In order to characterize the mode mixing device we measured the mode mixing of two back-to-back selective photonic lanterns with and without the device (see fig. 1a) to evaluate the degree of mode mixing obtained as well as the excess loss. We built the transfer matrix of the photonic lanterns and the mode mixing device, fig. 1-d,e, anf found the initial selectivity disapeared in the presence of our device. Hence, we succeeded in mixing all mode-groups.