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Towards Drift-Free Frequency Transfer Over Fiber Baseline for Optical Very-Long-Baseline-Interferometry

Tess R. Ekblad

Published Mar 11, 2026• 0 citations

Abstract

Optical very-long-baseline-interferometry (VLBI) aims to increase angular resolution capabilities beyond those of the radio regime and enable advanced spectroscopic performance. VLBI requires a distributed network of coherence to properly synthesize the apertures together. Fiber optics have revolutionized precision frequency-and-time transfer for applications such as communication, ranging, and timing and offer a platform for distributing coherent optical references for VLBI. However, signals transmitted through optical fibers are subject to the noise environment in which the fiber is laid. This is particularly problematic for precision optical frequency transfer over long distances and for precision timing across large baselines for applications such as VLBI. To combat the added phase noise due to fiber-piston action and refractive index changes in optical fibers, a phase cancellation method is required. In this thesis, we detail the evolution of a one-way optical frequency stabilization technique to increase the stability by limiting the out-of-loop path in a traditional phase-tracking heterodyne interferometer. Each apparatus’s stability was first tested over a 4.09 km SMF-28 fiber spool. The second and third apparatus iterations were individually deployed over a telecom SMF-28 fiber link of 2923 m in round-trip length. The final evolution involved a custom micro-optic component to reduce the out-of-loop to 1.4 cm and achieved in-loop instability of 4x10-19 at 1 ks and a worst-case out-of-loop timing-jitter below 200 attoseconds for up to 1 ks over the telecom link. The performance of the final apparatus is more than sufficient to distribute a coherent optical reference to perform distributed thermal heterodyne measurements.

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SummaryPrecise and stable coherent optical frequency transfer over long-haul fiber link is required by diverse scientific applications, such as large-scale optical clock networks, global positioning and timing, very-long baseline interferometry, and high-sensitivity tests of fundamental physics. To meet the needs of the above applications, active compensation schemes for cancelling fiber-induced phase noise has developed considerably over the past ten years. However, optical signal severely reduces across distant fiber-optic networks, which degrades optical beat signal and feedback control.Here, we adopt active phase compensation and bidirectional optical frequency locking to achieve ultra-stable optical carrier transfer. The phase noise of the fiber link is effectively suppressed, meanwhile, the ultra-stable optical frequency signal is regenerated accurately at the remote site.Preliminary experiment over 100 km fiber link indicates that the additional frequency stability exhibits 1.9×10-16and 2.5×10-17at an averaging time of 1 s and 100 s, respectively. This approach has a capacity of extending to the parallel transfer of microwave and optical frequency, and free-space optical frequency transfer.

The maximum baseline, and therefore resolution, of optical astronomical interferometers is limited by attenuation and phase noise within the optical path between the apertures and beam combiner, as well as the practical challenges of constructing optical delay lines more than a few hundred meters in length. We implement off-band phase stabilization on two fiber optic links of 85 km, creating a total baseline of 170 km. We show that the system is able to effectively phase stabilize signals from an incoherent pseudo-thermal source with a bandwidth of 11.2 nm. We are able to reduce the phase noise by 4-5 orders of magnitude between 1 and 100 Hz, such that we could resolve an applied phase difference of 0.16 cycles per second with continuous measurement. We show that, with phase stabilization active, the interferometer is able to recover both first-order and second-order photon correlations. These results demonstrate the feasibility of this technique for long-baseline optical and quantum astronomical interferometers. The present results are limited by chromatic dispersion within the fiber, which can be mitigated using dispersion-compensating modules.

We report an optical link of 540 km for ultrastable frequency distribution over the Internet fiber network. The phase-noise compensated link shows a fractional frequency instability in full bandwidth of 3 × 10 −14 at one second measurement time and 2 × 10 −18 at 30 000 s. This work is a significant step towards a sustainable wide area ultrastable optical frequency distribution and comparison network. Time transfer was demonstrated simultaneously on the same link and led to an absolute time accuracy (250 ps) and long-term timing stability (20 ps) which outperform the conventional satellite transfer methods by one order of magnitude. Current development addresses the question of multiple users distribution in the same metropolitan area. We demonstrate on-line extraction and first results show frequency stability at the same level as with conventional link. We also report an application to coherent frequency transfer to the mid-infrared. We demonstrate the frequency stabilisation of a mid-infrared laser to the near-infrared frequency reference transferred through the optical link. Fractional stability better than 4 × 10 −14 at 1 s averaging time was obtained, opening the way to ultrahigh resolution spectroscopy of molecular rovibrational transitions.

We have investigated the performance of an optical carrier transfer system for a remote comparison of optical clocks. In the frequency transfer system, technical problems and fundamental principles limit the coherent transfer on a long-haul optical fiber. They contribute to noise processes that come from the propagation delay in the fiber, the design of an interferometer, burst length fluctuations of the fiber, asymmetrical propagation characteristics, and so on. The feasibility of a full optical link with high stability is discussed. The phase detection with a high degree of precision and cancellation of the fiber-induced noise on the round-trip light are achieved with a well-known standard technique using an all-fiber-based interferometer. We report the characteristics of these noise processes with results on a spooled fiber. An Allan deviation of the compensated link which is extended to 90 km reaches 5 × 10-18 close to the theoretical limit imposed by the delay-unsuppressed fiber noise at averaging time of 103s.

Coherent Regeneration of Optical Frequency with 1-s Instability at

10^{-17}

The remote regeneration of coherent optical frequency technique finds increasing applications in the fields of large-scale optical clock networks, precision spectroscopy, very-long baseline interferometry, and high-sensitivity tests of fundamental physics. Meanwhile, with the improvements of coherent optical frequency generation, the requirement of stable transfer is more and more demanding. Here, we demonstrate a regeneration system of coherent optical frequency based on bidirectional phase locking, in which the locking loop at the local site is for eliminating optical phase noise over the link, and the locking loop at the remote site is for locking to the incoming coherent optical frequency. Preliminary experiment over a 1.5 km fiber link indicates that the additional frequency stability can reach

\mathbf{5.9}\times \mathbf{10}^{-\mathbf{17}}

and

\mathbf{7.0} \times \mathbf{10}^{-\mathbf{19}}

at an averaging time of 1 s and 103s, respectively. This method can be further extended to remote optical frequency comparisons for multiple users.