Abstract
Multi-channel structured light with orbital angular momentum (OAM) can be applied in different applications. For example, OAM modulation and OAM multiplexing in fiber optics communications, high-dimensional quantum cryptography-based OAM states for transmitting secure information across free-space, and independent data streams through OAM beams multiplexed free-space optical links. Using a simple and efficient system consisting of a spiral phase element (SPE) and a multi-channel vortex filter (MVF), we have converted input Gaussian beams into multi-channel OAM-based vortex beams for infrared wavelengths. An SPE has been designed, which generates optical vortices with wavelength-dependent topological charge (including fractional values). The resulting complex fields are optically relayed on a binary MVF designed by modulo- $2\pi $ phase addition of multiple fork gratings with topological charges 1, 2, and 3 and azimuthal orientations. In this way, the MVF generates beams with different OAM states for different carrier waves with different angles and maps them at desired locations in the detector plane. In this study, both $3\times 3$ , as well as a hexagonal configuration, were used. Furthermore, the experimentally obtained OAM spectrum qualitatively agrees with the results of numerical simulations, thus verifying our approach. The presented approach opens a new pathway for developing an efficient multi-channel OAM beams generator designed for a specific wavelength and illuminated by an input beam of different wavelengths over a broad spectral range.
Highlights
Vortex beams have numerous applications and have been widely reviewed [1]
We presented a robust and flexible approach for orbital angular momentum (OAM) mode generation, detection, and demultiplexing with the combination of optical elements spiral phase element (SPE) and multi-channel vortex filter (MVF) that are fabricated with well-established manufacturing technologies
We have experimentally established this approach that offers cost-effective OAM modes generation and mapping at desired locations and with desired geometry is achieved with the combination of simple and effective optical elements (SPE, MVFs) that can generate efficient outputs for input laser beams over a broad wavelength range
Summary
Vortex beams have numerous applications and have been widely reviewed [1]. Such beams with orbital angular momentum (OAM) have potential in increasing communications capacity [2]–[5] and revolutionize the field of freespace optical communication. The commercial and dynamic tools were discussed for OAM mode formation and detection All those studies were based on active devices such as spatial light modulators, liquid crystals, and digital mirror devices sensitive to laser threshold damage and not suitable for high power applications. These devices are relatively expensive and responding to specific polarization states because their dimensions and combinations can control the OAM beams into different configurations and dimensions, making the approach more expensive [18]–[20]. We presented a robust and flexible approach for OAM mode generation, detection, and demultiplexing with the combination of optical elements SPE and MVF that are fabricated with well-established manufacturing technologies. The proposed MVFs are fabricated using a well-controlled and straightforward laser writing technique
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