The coupling efficiency between free-space optics and fiber optics is highly susceptible to environmental disturbances (e.g., beam displacement, angular jitter, and wavefront aberrations), which has become a critical bottleneck in high-precision optical transmission systems. To address this issue, a cascaded adaptive feedback control system integrated into the optical transmission path is proposed. Targeting the dual requirements of "rapid response" and "long-term stability" in practical optical transmission, a power-feedback-based ramp-up algorithm is developed to simultaneously meet these needs. The algorithm dynamically adjusts piezoelectric-driven reflective mirrors to achieve multi-dimensional autonomous beam alignment and real-time optimization of coupling states. The system is validated on a rubidium Raman photon source experimental platform, demonstrating its applicability to demanding quantum and sensing platforms. Experimental results show that the system increases multimode (single-mode) optical coupling efficiency from below 1% to over 80% (70%) within 10 s (20 s) and enters a long-term stable transmission state after 65 s (70 s). Additionally, real-time link monitoring effectively suppresses coupling attenuation caused by beam jitter, achieving a balance between response speed and transmission stability without the need for independent mode optimization. This scheme provides practical technical support for efficient coupling in high-precision optical transmission systems such as laser communication and optical sensing.
The coupling efficiency between free-space optics and fiber optics is highly susceptible to environmental disturbances (e.g., beam displacement, angular jitter, and wavefront aberrations), which has become a critical bottleneck in high-precision optical transmission systems. To address this issue, a cascaded adaptive feedback control system integrated into the optical transmission path is proposed. Targeting the dual requirements of "rapid response" and "long-term stability" in practical optical transmission, a power-feedback-based ramp-up algorithm is developed to simultaneously meet these needs. The algorithm dynamically adjusts piezoelectric-driven reflective mirrors to achieve multi-dimensional autonomous beam alignment and real-time optimization of coupling states. The system is validated on a rubidium Raman photon source experimental platform, demonstrating its applicability to demanding quantum and sensing platforms. Experimental results show that the system increases multimode (single-mode) optical coupling efficiency from below 1% to over 80% (70%) within 10 s (20 s) and enters a long-term stable transmission state after 65 s (70 s). Additionally, real-time link monitoring effectively suppresses coupling attenuation caused by beam jitter, achieving a balance between response speed and transmission stability without the need for independent mode optimization. This scheme provides practical technical support for efficient coupling in high-precision optical transmission systems such as laser communication and optical sensing.
Intermodal quantum key distribution at telecom wavelengths provides a hybrid interface between fiber connections and free-space links, both essential for the realization of scalable and interoperable quantum networks. Although demonstrated over short-range free-space links, long-distance implementations of intermodal quantum key distribution remain challenging, due to turbulence-induced wavefront aberrations which limit efficient single-mode fiber coupling at the optical receiver. Here, we demonstrate a real-time intermodal quantum key distribution field trial over an 18 km free-space link, connecting a remote terminal to an urban optical ground station equipped with a 40 cm-class telescope. An adaptive optics system, implementing direct wavefront sensing and high-order aberration correction, enables efficient single-mode fiber coupling and allows secure key generation of 200 bit/s using a compact state analyzer equipped with room-temperature detectors. We further validate through experimental data a turbulence-based model for predicting fiber coupling efficiency, providing practical design guidelines for future intermodal quantum networks.
Time-bin encoded photon pairs enable robust, decoherence-resistant transmission through optical fibers for long-distance quantum communication, where phase noise poses a critical limitation to stable operation. Here, we implement an adaptive perturbation-and-observe algorithm on a fully digital FPGA platform operating with real-time feedback at 1 Hz. The control signal is derived from the coincidence counts of correlated photon pairs. This adaptive approach reduces the rise time by 70 % and the coincidence noise by 30 %, resulting in visibility improvements sustained for more than 600 s. These results provide an efficient solution for long-term phase stabilization in quantum and photonic systems.
An underground fiber-based quantum network currently connects Brookhaven National Laboratory and Stony Brook University. Brookhaven's goal is to enhance the network by developing a system to detect or distribute single entangled photons across a 21-km free space optical link. Since the new link is through free space, it uses telescopes instead of fibers. The light sent through the system must couple to a single mode fiber whose opening is only 5 microns in diameter. The beam, having been corrected for atmospheric effects by an adaptive optics system, will have residual drift over time that must be corrected for continuously, and thus a precise and automatic fiber alignment system is needed as part of the active optics system. An alignment algorithm was developed, which will adjust the optical fiber using a three-axis translation stage, controlled by a piezo controller attached to a Raspberry Pi. Testing was conducted in the Instrumentation Division at Brookhaven. This automatic alignment system was also used for a quantum astrometry experiment at the Center for High Angular Resolution Astronomy Array at Mount Wilson, California. The final system consisted of a coarse alignment algorithm that attempted to find the focal point of a fiber coupling lens mathematically by scanning cross sections of the beam's intensity, and a fine alignment algorithm that searched a small volume of points around the previously calculated point. The automated system was as good at finding best focus as a human, and achieved a coupling efficiency greater than 50%, which was the desired outcome.
In practical applications to free-space quantum communications, the utilization of active beam coupling and stabilization techniques offers notable advantages, particularly when dealing with limited detecting areas or coupling into single-mode fibers(SMFs) to mitigate background noise. In this work, we introduce highly-enhanced active beam-wander-correction technique, specifically tailored to efficiently couple and stabilize beams into SMFs, particularly in scenarios where initial optical alignment with the SMF is misaligned. To achieve this objective, we implement a SMF auto-coupling algorithm and a decoupled stabilization method, effectively and reliably correcting beam wander caused by atmospheric turbulence effects. The performance of the proposed technique is thoroughly validated through quantitative measurements of the temporal variation in coupling efficiency(coincidence counts) of a laser beam(entangled photons). The results show significant improvements in both mean values and standard deviations of the coupling efficiency, even in the presence of 2.6 km atmospheric turbulence effects. When utilizing a laser source, the coupling efficiency demonstrates a remarkable mean value increase of over 50 %, accompanied by a substantial 4.4-fold improvement in the standard deviation. For the entangled photon source, a fine mean value increase of 14 % and an approximate 2-fold improvement in the standard deviation are observed. Furthermore,the proposed technique successfully restores the fidelity of the polarization-entangled state, which has been compromised by atmospheric effects in the free-space channel, to a level close to the fidelity measured directly from the source. Our work will be helpful in designing spatial light-fiber coupling system not only for free-space quantum communications but also for high-speed laser communications.
Orbital angular momentum (OAM) of photons provides a promising approach to enhance the capacity, robustness, and functionality of quantum communication systems in optical fibers. With many twist states available, OAM enables high-dimensional encoding, significantly increasing data rates and noise tolerance. This review explores sources, channels, and system architectures for OAM-based quantum communication in fibers. It compares free-space and integrated emitters, discussing in-fiber pair generation and mode conversion pathways that produce fiber-ready single photons and entangled states. For the channels, it details design principles and trade-offs for step-index and ring-core fibers that minimize intermodal coupling and reviews milestones in kilometer-scale single-photon transmission and high-dimensional entanglement transport. On the system side, the review examines coupling, multiplexing, and detection techniques—including mode (de)multiplexers, lanterns, and programmable sorters—and discusses transmission-matrix-based control methods to mitigate dispersion and bending effects. The review concludes with practical guidance and open challenges, such as improving mode selectivity and stability under real-world conditions; developing compact, efficient detectors for many modes; and integrating co-designed error correction schemes. These directions aim to bridge laboratory demonstrations with the deployment of scalable, high-dimensional fiber networks that harness OAM to significantly boost capacity, resilience, and functionality.
Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest are ever increasing. Same is true for satellite free-space optical communications (FSOC). In order to enable a robust channel for transmission it is indispensable to account for static and dynamically changing misalignments between the transmitter and receiver pair. This work will focus on the transmitter terminal (Alice) and the design and verification process of the active beam steering system. The novelty is a recently developed variable reluctance fine steering mirror (FSM) including eddy current sensors (ECS) to measure its tip and tilt. A cascaded architecture was chosen in order to combine the optical stabilization objective with the dynamics of the mirror platform. The inner control loop makes use of an observer model whose estimated output is fed into a state controller allowing for an increased responsiveness. While high gains increase the closed loop bandwidth the eigenfrequency of the system introduces a pole to the plant which has to be avoided by the controller output. A digital notch filter was introduced to reject the excitation of the critical frequency band which gets obsolete in a system with high frequency sampling capabilities. The outer loop is engaged when a valid optical signal is received and a transition from a closed loop pointing to a closed loop tracking mode is performed. A proportional-integral (PI) controller keeps the received beam at the 4-quadrant-diode (4QD) whose center is used as the main reference through prior calibration with the transmit beam launching on the same path. The presented cascaded control scheme allows improvements in system performance and reliability.