In a pre-proof article posted in Optics Communications, researchers proposed a novel angle-of-arrival (AoA) tracking device that eliminates the need for beacon lights and beam splitters by employing an integrated beam alignment positioning sensor with a data photo-detector (PD).
Mitigating angle-of-arrival variations, which mainly result from receiving end vibration and atmospheric turbulence, is one of the leading technical problems with free-space optical communication (FSOC) systems. Using an angle-of-arrival tracking device at the receiver is a typical strategy. However, since beacon-based angle-of-arrival tracking needs the beacon light when deployed, the system becomes complex and extensive.
The number of necessary optical parts can be significantly reduced using the novel approach of beaconless angle-of-arrival tracking. Typically, an optical beam splitter is needed for this tracking to direct the optical signal received to both the beam alignment position sensor and data PD. However, the beam splitter causes optical power loss and may result in inaccurate tracking.
This paper discussed an experimental demonstration of angle-of-arrival tracking systems over 104 m for the 10-Gb/s free-space optical communication link, contrasting its performance with the traditional approach. The demonstration showed that the suggested design could reduce loss by more than 2.9 dB.
It demonstrated that for a target bit-error ratio (BER) of 10-3, the proposed model increased the tolerance to angle-of-arrival fluctuation by a factor of at least two. Angle-of-arrival fluctuation of 250 mrad corresponded to a tracking time of 171 ms. Finally, the study also demonstrated that the tracking time's key factors were the fast-steering mirror (FSM) bandwidths and the loop controller.
Beaconless AoA Tracking and the Future of Free-Space Optical Communication
The capacity of free-space optical communication to wirelessly transmit high-speed data over a lengthy free-space optical communication channel without dissipating the limited radio-frequency resources has recently attracted much attention.
Numerous applications, including space, inter-satellite, terrestrial, and airborne links, have demonstrated the benefits of free-space optical communication. Free-space optical communication may be a potential option for enhancing the flexibility and scope of fronthaul/backhaul networks for emerging mobile communication systems that go beyond 5G or 6G.
The main technological issue of free-space optical communication systems is the exact beam alignment of the transmitter and receiver since even a little beam alignment error could result in a significant signal power loss at the reception. Angle-of-arrival variation is one of the common beam alignment defects. The main contributors to angle-of-arrival fluctuations are the receiver's vibration and the optical signal's wavefront tilt caused by atmospheric turbulence. AoA variations lead to randomly altering attenuations of the detected signal over time, which significantly impairs the performance of free-space optical communication systems.
This work focussed on a different group of approaches that employed the real-time tracking device at the receiver to simplify implementation and promote quick beam alignment. AoA tracking using beacons was one such design. The beacon-based angle-of-arrival tracking system used two optical links, one for beam alignment and the other for data transmission, to achieve beam alignment using a beacon light. However, since this design involved added optical components, the resulting AoA tracking system was complex. Hence, the beacon-based design was undesirable, particularly for aerial free-space optical communication devices with strict size, weight, and power (SWaP) restrictions.
AoA tracking without beacons was another design. Here, beam alignment and data transmission were accomplished over a single optical link, which reduced the number of optical components needed for implementation. The integrated beam alignment positioning sensor and data PD were expanded upon in this paper, and a new AoA tracking system design was proposed. While the center PD, which had a comparatively small size relative to the four peripheral PDs, was employed for high-speed data detection. Four peripheral PDs with larger sizes were used for better tracking range and sensing performance.
Experiments were conducted to show how the traditional and proposed AoA tracking systems worked for a 10-Gb/s transmission of data over a 104-m indoor free-space optical communication link. It also investigated the elements influencing the AoA tracking speed, including the loop controller's bandwidth and the FSM's steering speed.
Proof-of-Concept Investigations on Beaconless AoA
This work used a single laser beam for beam alignment and data transmission. The receiver utilized a lens to concentrate the optical beam on the focal plane. AoA fluctuation adjustment was achieved by employing a beam steering device such as an FSM to route the received signal to the detector. On the focus, a plane was where the integrated beam alignment positioning sensor and data PD were positioned. The (i) sensor gap (g), (ii) sensor diameter (ds), (iii) data PD diameter (dpd), and (iv) beam diameter (db) on the device were used to describe the design of an integrated sensor-and-detector device.
The electro-absorption modulated laser (EML), which operated at 1.53 mm, received the electrical binary signal. An optical signal was transmitted to the free-space optical communication channel through a standard single-mode fiber (SSMF) one meter long and followed by a fiber collimator. This collimator offered a 7 mm waist diameter and a 0.35 mrad beam divergence angle.
The system's performance was assessed in static AoA circumstances. The tracking effectiveness was evaluated in the presence of time-varying AoA fluctuations. Fluctuations of the received power were measured to be ~5 dB when the AoA fluctuation frequency increased to 2.1 Hz, which was only ~1.4 dB lower than when AoA tracking was OFF. Such an observation was a result of the tracking system's bandwidth restriction. By increasing the system's bandwidth, the problem could be resolved.
Beaconless AoA Tracking System Proves Beneficial Over Conventional Techniques
This paper proposed a novel design of an AoA tracking system where the beam positioning sensor and photodetector were integrated. Therefore, it eliminated the requirement for a beam splitter and beacon light and made the AoA tracking system's deployment quick and easy. The suggested system eliminated the technical issues related to the photodetector and boosted the received optical power at the detector.
Compared to the conventional design, the proposed method boosted the received optical power by 2.9 dB, according to the experimental demonstrations. Additionally, it demonstrated that in a 104 m free-space optical communication system of 10-Gb/s, the suggested tracking system increased the tolerance to AoA variation by >2 times at the desired BER of 10-3.
The tracking time was measured to be 171 ms at an AoA of 250 mrad, constrained by the fast-steering mirror and the loop controller's bandwidth. The researchers believe that the suggested architecture could be employed to reduce the adverse effects of AoA variations in FSOC systems needing reliable, accurate, and compact tracking.
Mai, V., Kim, H. (2022). Beaconless angle-of-arrival tracking with improved receiver sensitivity and tracking precision for free-space optical communications. Optics Communications. https://www.sciencedirect.com/science/article/pii/S0030401822006101
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