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IEEE MIT Undergraduate Research Technology Conference (URTC)

Automatic Optical Fiber Alignment for a Free Space Optical Link

Aimee Linebarger, Justine Haupt

Published Oct 10, 2025• 0 citations

Abstract

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.

DOI: 10.1109/urtc68753.2025.11533107

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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.

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