Data exchange between different mode channels is essential in the optical communication network with mode-division multiplexing (MDM). However, there are challenges in realizing mode exchange with low insert loss, low mode crosstalk, and high integration. Here, we designed and fabricated a mode exchange device based on multiplane light conversion (MPLC), which supports the transmission of LP01, LP11a, LP11b, and LP21 modes in the C-band and L-band. The simulated exchanged mode purities are greater than 85%. The phase masks were fabricated on a silicon substrate to facilitate the integration with optical systems, with an insert loss of less than 2.2 dB and mode crosstalk below -21 dB due primarily to machining inaccuracies and alignment errors. We carried out an optical communication experiment with 10 Gbit/s OOK and QPSK data transmission at the wavelength of 1550 nm and obtained excellent performance with the device. It paves the way for flexible data exchange as a building block in MDM optical communication networks.

Space-division multiplexing based on few-mode multi-core fiber (FM-MCF) technology is expected to break the Shannon limit of a single-mode fiber. However, an FM-MCF is compact, and it is difficult to couple the beam to each fiber core. 3D waveguide devices have the advantages of low insertion loss and low cross talk in separating various spatial paths of multi-core fibers. Designing a 3D waveguide device for an FM-MCF requires considering not only higher-order modes transmission, but also waveguide bending. We propose and demonstrate a 3D waveguide device fabricated by femtosecond laser direct writing for various spatial path separations in an FM-MCF. The 3D waveguide device couples the LP01 and LP11a modes to the FM-MCF with an insertion loss below 3 dB and cross talk between waveguides below -36dB. To test the performance of the 3D waveguide device, we demonstrate four-channel multiplexing communication with two LP modes and two cores in a 1-km few-mode seven-core fiber. The bit error rate curves show that the different degrees of bending of the waveguides result in a difference of approximately 1 dB in the power penalty. Femtosecond laser direct writing fabrication enables 3D waveguide devices to support high-order LP modes transmission and further improves FM-MCF communication.

Multiplexing and demultiplexing of optical orbital angular momentum (OAM) are critical operations in mode-division multiplexing communications. Traditional Dammann gratings, spiral phase planes, and optical geometric transformations are regarded as convenient methods for OAM mode (de)multiplexing. However, crosstalk between the different modes and the difficulty of mode multiplexing greatly limit their application to mode-division multiplexing communications. Here, using a set of inversely-designed phase planes, we demonstrate an OAM (de)multiplexer based on multiphase plane light conversion that can enable perfect OAM multiplexing communication. The sorted patterns are Gaussian-like and can be coupled easily into single-mode fiber arrays. Inputs from the fiber array are turned into coaxial OAM modes after the phase planes. OAM mode crosstalk generated by the multiplexer is less than -20dB, with insertion loss of less than 2.6 dB. OAM modes are sorted by the demultiplexer with mode crosstalk below -10dB, and the sorting results are coupled to the fiber array. OAM modes carrying 10 Gbit/s on–off keying signals were transmitted in a 5 km few-mode fiber. The measured bit-error-rate curves have power penalties of less than 10 dB. The proposed configuration is highly efficient and convenient and will be beneficial for potential applications in quantum information, information processing, and optical communications.

Stokes vector direct detection is a promising, cost-effective technology for short-distance communication applications. Here, we design and fabricate a spin-dependent liquid crystal grating to detect light polarization states. By separating the circular and linear components of incident light, the polarization states can be resolved with accuracy of up to 0.25°. We achieved Stokes vector direct detection of quadrature phase-shift keying (QPSK), 8PSK, and 16-ary quadrature amplitude modulation signals with 32, 16, and 16 GBd rates, respectively. We integrated the system, including the grating, photodetectors, and optical elements, on a miniaturized printed circuit board and demonstrated high-speed optical communications with 16 GBd rate QPSK signals.