Photonic-lantern-based coherent LIDAR system

Revolutionizing Free-Space Optics: A Survey of Enabling Technologies, Challenges, Trends, and Prospects of Beyond 5G Free-Space Optical (FSO) Communication Systems

As the demand for high-speed, low-latency communication continues to grow, free-space optical (FSO) communication has gained prominence as a promising solution for supporting the next generation of wireless networks, especially in the context of the 5G and beyond era. It offers high-speed, low-latency data transmission over long distances without the need for a physical infrastructure. However, the deployment of FSO systems faces significant challenges, such as atmospheric turbulence, weather-induced signal degradation, and alignment issues, all of which can impair performance. This paper offers a comprehensive survey of the enabling technologies, challenges, trends, and future prospects for FSO communication in next-generation networks, while also providing insights into the current mitigation strategies. The survey explores the critical enabling technologies such as adaptive optics, modulation schemes, and error correction codes that are revolutionizing FSO communication and addressing the unique challenges of FSO links. Also, the integration of FSO with radio frequency, millimeter-wave, and Terahertz technologies is explored, emphasizing hybrid solutions that enhance reliability and coverage. Additionally, the paper highlights emerging trends, such as the integration of FSO with artificial intelligence-driven optimization techniques and the growing role of machine learning in enhancing FSO system performance for dynamic environments. By analyzing the current trends and identifying key challenges, this paper emphasizes the prospects of FSO communication in the evolving landscape of 5G and future networks. In this regard, it assesses the potential of FSO to meet the demands for high-speed, low-latency communication and offers insights into its scalability, reliability, and deployment strategies for 5G and beyond. The paper concludes by identifying the open challenges and future research directions critical to realizing the full potential of FSO in next-generation communication systems.

Compact and efficient photonic lanterns through multi-stage tapering

Photonic lanterns (PLs) have been recently used in mode-division multiplexed systems with a low insertion loss, a low mode-dependent loss (MDL), and a wide bandwidth. However, the cross talk (XT) performance of the PLs requires further enhancement within a short taper length. In this Letter, a multi-stage cascaded scheme for short PLs is proposed to further improve the performance on losses and XT. The XT of the optimized 6-mode 3-stage PL is below -23.4 dB, while the total length is only 6 cm. To the best of our knowledge, this is the first quantitative optimization of a 3-stage tapered PL, resulting in a compact structure and excellent performance. Furthermore, we experimentally validate the feasibility of the 3-stage tapering process.

Optimal broadband starlight injection into a single-mode fibre with integrated photonic wavefront sensing

In astronomy and related fields there is a pressing need to efficiently inject light, transmitted through the atmosphere, into a single-mode fibre. However this is extremely difficult due to the large, rapidly changing aberrations imprinted on the light by the turbulent atmosphere. An adaptive optics system must be used, but its effectiveness is limited by non-common-path aberrations and insensitivity to certain crucial modes. Here we introduce a new concept device - the hybrid mode-selective photonic lantern - which incorporates both focal plane wavefront sensing and broadband single-mode fibre injection into a single photonic package. The fundamental mode of an input multimode fibre is directly mapped over a broad (1.5 to 1.8μm) bandwidth to a single-mode output fibre with minimal (<0.1%) crosstalk, while all higher order modes are sent to a fast detector or spectrograph for wavefront sensing. This will enable an AO system optimised for maximum single-mode injection, sensitive to otherwise 'blind' modes and avoiding non-common-path wavefront-sensor aberrations.

The Effect of Spatial Mode Distribution on Coupling Efficiency of Single-Mode Fiber: Theoretical Analysis and Experimental Verification

In this study, Zernike polynomials and optical fiber field theory are applied to build a mathematical model of coupling efficiency (CE) and spatial mode of aberrations. The theory built in this paper can be used to quickly calculate the CE affected by a single aberration as well as the aberrations caused by atmospheric turbulence. The aberrations are classified based on Zernike polynomials and the effects of aberrations on CE of different types and different spatial frequencies are analyzed. The influence of the effects of AO system residual errors is also analyzed. Adaptive optics (AO) equipment is applied to build a system on which the proposed theory was tested; the experimental results validate the theoretical analysis.

Multiplexed single-mode wavelength-to-time mapping of multimode light

When an optical pulse propagates along an optical fibre, different wavelengths travel at different group velocities. As a result, wavelength information is converted into arrival-time information, a process known as wavelength-to-time mapping. This phenomenon is most cleanly observed using a single-mode fibre transmission line, where spatial mode dispersion is not present, but the use of such fibres restricts possible applications. Here we demonstrate that photonic lanterns based on tapered single-mode multicore fibres provide an efficient way to couple multimode light to an array of single-photon avalanche detectors, each of which has its own time-to-digital converter for time-correlated single-photon counting. Exploiting this capability, we demonstrate the multiplexed single-mode wavelength-to-time mapping of multimode light using a multicore fibre photonic lantern with 121 single-mode cores, coupled to 121 detectors on a 32 × 32 detector array. This work paves the way to efficient multimode wavelength-to-time mapping systems with the spectral performance of single-mode systems.

Photonic lanterns are made by merging several single-mode cores into one multimode core. Here, the authors show this type of structure can both perform wavelength-to-time mapping of multimode states of light and couple such light to an array of single-photon avalanche detectors.