Abstract
With the amplitude, time, wavelength/frequency, phase, and polarization/spin parameter dimensions of the light wave/photon almost fully utilized in both classical and quantum photonic information systems, orbital angular momentum (OAM) carried by optical vortex modes is regarded as a new modal parameter dimension for further boosting the capacity and performance of the systems. To exploit the OAM mode space for such systems, stringent performance requirements on a pair of OAM mode multiplexer and demultiplexer (also known as mode sorters) must be met. In this work, we implement a newly discovered optical spiral transformation to achieve a low-cross-talk, wide-optical-bandwidth, polarization-insensitive, compact, and robust OAM mode sorter that realizes the desired bidirectional conversion between seven co-axial OAM modes carried by a ring-core fiber and seven linearly displaced Gaussian-like modes in parallel single-mode fiber channels. We further apply the device to successfully demonstrate high-spectral-efficiency and high-capacity data transmission in a 50-km OAM fiber communication link for the first time, in which a multi-dimensional multiplexing scheme multiplexes eight orbital-spin vortex mode channels with each mode channel simultaneously carrying 10 wavelength-division multiplexing channels, demonstrating the promising potential of both the OAM mode sorter and the multi-dimensional multiplexed OAM fiber systems enabled by the device. Our results pave the way for future OAM-based multi-dimensional communication systems.
Highlights
Spatial division multiplexing exploiting the spatial degree of freedom plays a significant role in current efforts toward higher information capacity in optical communication systems [1,2]
To further prove the device performance and demonstrate its potential applications in fiber-based multi-dimensional Orbital angular momentum (OAM) information systems, the compact mode sorter is applied in a 50-km ring-core fiber (RCF)-based data transmission system with simultaneous OAM mode-division multiplexing (MDM) and wavelength-division multiplexing (WDM) [46]
A compact and robust OAM mode sorter has been demonstrated based on a novel nonparaxial spiral coordinate transformation
Summary
Spatial (mode) division multiplexing exploiting the spatial degree of freedom plays a significant role in current efforts toward higher information capacity in optical communication systems [1,2]. If low cross talk between OAM modes are to be suppressed to levels compatible with the stringent OAM information system requirements, the nonparaxiality must be corrected We solve this problem by analytically deriving and experimentally implementing a new phase distribution for the second mask to correct the phase-front distortions caused by the nonparaxiality, thereby achieving the high resolution between adjacent modes in a compact and robust fashion as promised by the spiral transformation. As the azimuthal phase excursion of the OAM mode is intercepted multiple times by an endless spiral, the available effective phase excursion along it is multiple times of 2π (only limited by the width of the input OAM beam) and high-resolution mode sorting can be achieved with low cross talk, which is the major advantage of the spiral transformation compared to the log-polar one—the latter only achieves phase excursion of 2π along one single circle [35]. It should be noted that the phase profile of the unwrapper in Eq (2) is still derived in the paraxial regime in order to obtain an analytical solution, which could be further extended to the nonparaxial regime by numerical optimization in future work
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