Tunable Orbital Angular Momentum Converter Based on Integrated Multiplexers

A four-orbital angular momentum (OAM) modes converter consisting of the cascade of an OAM demultiplexer and an OAM multiplexer is implemented and characterized. The OAM multiplexer is realized with an integrated photonic circuit based on four tunable OAM emitters, i.e. the order of the emitted OAM mode can be selected by thermal tuning. It works on linearly polarized signals. The OAM demultiplexer, completely passive, consists of the cascade of two patterned refractive elements followed by a lens and can work independently of the polarization of the input signals. The proposed OAM converter can receive at its input a bundle of spatially multiplexed beams mapped on different OAM modes, each one carrying a set of wavelength division multiplexed (WDM) channels. Each WDM channel can be independently converted to a different OAM mode and all the OAM beams are multiplexed at the converter output. The performance of the proposed scheme is assessed as a function of the order of the OAM beam before and after the conversion, as well as of the wavelength of the input signal. The measurements show a power penalty on the bit error-rate (BER) curves of less than 1 dB in all the considered cases. The scheme is tested also with 100 Gb/s real data-traffic generated with commercial network cards, showing full operability. The cascading ability of the proposed scheme is also investigated in an experiment where the signal to be converted is generated by an OAM multiplexer, i.e. the output stage of the OAM converter, showing a penalty of less than 2 dB. Moreover, considerations on scalability, bandwidth and power consumption are also included along with a possible application of OAM conversion within add/drop nodes in an OAM-wavelength multiplexed transmission scenario.