Detailed study of modal gain in a multimode EDFA supporting LP01 and LP11 mode group amplification

There are concerns that radically new fiber designs and network architectures will soon be required in order to keep up with exponential growth of internet traffic in the backbone network in a cost effective manner [1]. Spatial division multiplexing (SDM) schemes employing either multi core fibers (MCFs) or multi-mode fibers (MMFs) have been recently proposed and successfully demonstrated [2,3]. SDM in principle allows upscaling of the capacity per fiber by a factor proportional to the number of cores or modes. However suitable in-line optical amplifiers will clearly need to be developed if SDM is ever to be used in long haul networks. We very recently reported the demonstration of a multimode fibre amplifier for use in SDM systems. In these first experiments we demonstrated the simultaneous amplification of two signals propagating on two separate modes of different order and with orthogonal polarization (namely the LP01 and LP11a modes). Gains of > 22dB were obtained for both modes using a fibre with an optimized refractive index/erbium ion distribution profile and suitable pump profile [4]. Clearly, in order to exploit the full potential of such an amplifier it is essential to be able to simultaneously amplify signals in all spatial and polarization modes supported by the amplifier i.e. 6 modes altogether in our particular fiber (which supported two transverse mode groups). Here we extend our previous work to demonstrate the simultaneous amplification of LP11a and LP11b (different polarization, different high-order transverse mode) as well as LP01 and LP11 (same polarization, different transverse mode group), achieving similar gains and differential modal gains to those achieved for the mode combinations investigated previously. Our results highlight the potential for simultaneously achieving well matched gains for all 6 guided modes through fiber design and suitable pump excitation.