Weilin Xie

Frequency noise of frequency-swept laser fundamentally governs the performance of optical frequency-modulated continuous-wave (OFMCW) interferometry based technologies. Here, we proposed general and versatile analytic model capable of throughout analyzing and accurately predicting the performance of OFMCW system for lasers without assuming specific analytical forms of frequency noise spectrum (FNS). Compared to conventional noise models that rely on specific analytical expression or empirical parameter fitting, this approach establishes a direct link between the laser FNS and the performance of OFMCW system. It allows for highly accurate prediction of the beat note spectrum regardless of laser FNS characteristics with a computational complexity of O(NlogN), and thus enables assessing the impact of laser FNS on key performance metrics such as carrier-to-noise ratio, effective distance, and spatial resolution. Both theoretical and experimental demonstrations for laser with irregular FNS by incorporating optical phase-locked loop (OPLL) have validated an excellent agreement between the predictions and actual measurements with an average R-square of 0.9910. For a practical usage, the analysis permitted by this model reveals the dominance of low-frequency noise, confirming the important role of OPLL in improving the performance. With the verified accuracy, versatility, efficiency, and reliability, the proposed model provides a comprehensive and practical tool for design, characterization, and optimization of OFMCW systems.

Propriétés non-linéaires d'amplificateurs paramétriques à fibre optique sensibles à la phase pour le traitement du signal: La capacité et les performances des systèmes à fibres optiques et photoniques dépendent fortement du bruit et des non-linéarités des amplificateurs optiques. Dans ce contexte, les amplificateurs paramétriques à fibre optique (PS-FOPA), reposant sur le mélange à quatre ondes dans les fibres optiques, surpassent les amplificateurs conventionnels insensibles à la phase. En effet, leur sensibilité à la phase peut être exploitée pour l'amplification sans bruit et al compensation de la distortion non-linéaire. En conjonction avec leur large spectre de gain et d'autres fonctionnalités telles que la conversion de longueur d'onde, ils sont considérés comme des candidats prometteurs pour la prochaine génération d'amplificateurs optiques pour les communications et le traitement de signal tout optiques.Le PS-FOPA est classiquement décrit par les équations d'ondes couplées, dérivées de l'équation de Schrödinger non-linéaire qui ne contiennent que trois ou quatre ondes en interaction. Cependant, dans un cas plus général, l'apparition de fréquences supplementaires d’ordre plus élevé affectera inévitablement la sensibilité en phase.L’objectif de cette thèse est d'étudier de manière approfondie les propriétés non-linéaires, en termes de gain et de sensibilité en phase d'un PS-FOPA dégénéré à double pompe pour différentes configurations. Une analyse numérique plus précise est obtenue en utilisant le modèle à 7 ondes qui incorpore les ondes supplémentaires issues de processus de mélange à quatre ondes d’ordre élevé. Ce modèle permet de donner une interprétation physique plus précise des interactions multi-ondes en fonction des conditions d'accord de phase, révélant les relations sous-jacentes entre la dispersion et la sensibilité de phase. De plus, la capacité de régénération simultanée de phase et d'amplitude d'un PS-FOPA basique est évaluée pour l'optimisation globale. Il permet d'exploiter pleinement la capacité potentielle d'un PS-FOPA de base agissant comme un bloc de construction fondamental des futures fonctionnalités tout optiques. L’approche basée sur ce modèle permet une optimisation orientée vers l’application et revêt une importance particulière pour la conception et l'optimisation de tels PS-FOPAs dans divers scénarios.

In distributed interferometric antenna arrays, accurate measurement and long-term stabilization of transmission delay are essential for coherent combining and accurate target localization. This paper presents a fiber-based delay measurement and stabilization system. The employed dual-tone probe enables accurate measurement of delay variations and the absolute delay simultaneously, while a phase-locked loop provides long-term delay stabilization. Experimental results demonstrate a delay measurement accuracy better than 20 fs and a stabilized delay fluctuation of 15.7 fs (standard deviation) over a 45 km fiber link for 3600 s. The proposed system realizes highly stable microwave transmission with accurately measured absolute delay, making it well-suited for coherent combining and localization in distributed antenna arrays.

Secure communication for critical command nodes has emerged as a pivotal challenge in modern warfare, in particular considering the vulnerability of these nodes to electronic reconnaissance. To cope with the severe interference, this paper proposes a robust solution for long-distance secure command and control system leveraging phase-modulated microwave photonic links. Studies that analyze the impairing nonlinear distortions and phase noise stemming from different sources in optical phase demodulation during long-haul transmission has been carried out, unveiling their impairment in coherent transmission systems. To overcome these limitations, a linearized phase demodulation and noise suppression technique based on composite optical phase-locked loop and multi-core fiber is proposed and experimentally validated. Experimental results demonstrate a long-haul transmission over 100 km with an 81 dB suppression for third-order intermodulation distortion and a 27 dB improvement in noise floor at 5 MHz under closed-loop condition, verifying a significant enhancement in the fidelity in long-distance transmission. This method ensures a highly reliable secure communication for command and control systems in contested electromagnetic environments.

Distributed optical fiber sensing systems based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) play a crucial role in infrastructure monitoring. However, event classification in these systems is hindered by limited data availability, high computational complexity, and slow model training. In this paper, we present an event classification method that integrates tabular machine learning with few-shot learning (FSL) through a pre-trained tabular prior-data fitted network (TabPFN) model. This method transforms the comprehensive set of time-domain statistical features (e.g., mean, peak-to-peak) and frequency-domain energy distribution features extracted from a conventional Φ-OTDR into structured tabular data, enabling subsequent machine learning processing. Specifically, the proposed scheme achieves a five-fold cross-validation accuracy of 90.0% ± 6.24% and an independent test accuracy of 89.0% under extremely small sample conditions and with only 10 samples per class available. Furthermore, with an increasing sample size, the performance further improves, reaching 97.78% ± 1.88% (at 60 samples per class) in cross-validation and 100% (at 50 samples per class) in independent testing, while fitting consistently completes within seconds. Comparative analysis against both strong traditional machine learning models and representative meta-learning FSL baselines further validates the superiority of our approach in few-shot settings. These impressive results and low computational overhead during application pave the way for rapid industrial deployment of Φ-OTDR systems in applications such as oil and gas pipeline monitoring.

We report on a phase-sensitive optical frequency domain reflectometry that allows for frequency distortion suppression in distributed sensing by using dual-wavelength digitally enhanced interferometry. Preserved sensing performance for a strain precision of 0.1 με/√Hz with a sensing spatial resolution of 1.25 m along 4.6 km has been verified in the presence of both environmental and artificially induced frequency distortions, testifying the effective frequency distortion suppression.

Distributed fiber-optic sensing based on optical frequency-domain reflectometry (OFDR) can quantitatively obtain the information of environmental parameters along the optical fiber by analyzing the phase changes or spectral shifts of the Rayleigh backward scattering (RBS) signals when a linear frequency-modulated continuous wave signal is used as the probing signal. In view of the differences in the two approaches for the extraction of the optical parameters of the RBS, we have analyzed the mechanisms of two demodulation methods through theoretical modeling, revealing their differences in accuracy, dynamic range, spatial resolution, and sensing spatial resolution. In addition, experimental comparisons for the performances of the two methods under the same OFDR system parameters through distributed temperature sensing have been conducted, testifying the differences in accuracy. These results can be useful for the design and optimization of the distributed sensing systems based on OFDR.

Conventional cross-correlation is commonly used in the estimation of the spectral shift for optical frequency-domain reflectometry (OFDR) based distributed sensing. However, the potential errors in cases of relatively large shifts have severely limited the sensing dynamic range. We report a modified longest common substring algorithm that exploits two-dimensional image processing for more accurate shift estimation. Theoretical study for the conventional cross-correlation is conducted, which reveals quantitatively the underlying mechanism for the emergence of the correlation errors. By adopting the relative error function as the similarity function in connection with the binarization and normalized projection, it allows for a reduction for the possibility of large estimation error in a two-dimensional image processing manner, leading to a substantial improvement in the sensing range. Experimental demonstrations have verified a remarkable enhancement of about 18.7 times in strain sensing range, which accounts for ∼56.2% of the effective sweep range. The proposed algorithm not only permits opportunities in OFDR based distributed sensing, but also promises enhanced fidelity for applications where cross-correlation is concerned.

In distributed fiber-optic sensing based on optical frequency domain reflectometry (OFDR), Doppler frequency shifts due to the changes of disturbances during one sweep period introduce demodulation errors that accumulate along both the distance and time, impairing the sensing performance. Here, we report distributed dynamic strain sensing using Doppler-shift-immune phase-sensitive OFDR based on frequency-tracking and spectrum-zooming with an ultra-weak reflection array. A theoretical study has been carried out with the introduction of the mismatch coefficient, unveiling quantitatively the impact of the Doppler shift. Following a numerical analysis of the proposed method, a retained precision has been experimentally verified regardless of the position mismatch due to the Doppler effect. Doppler-shift-immune sensing for dynamic strains covering continuous spatial resolution over a distance of 1000 m with a 2.5 cm sensing spatial resolution has been demonstrated, verifying the high fidelity promised by the proposed method.

In distributed fiber-optic sensing based on optical frequency domain reflectometry (OFDR), Doppler frequency shifts due to the changes of disturbances during one sweep period introduce demodulation errors that accumulate along both the distance and time, impairing the sensing performance. Here, we report distributed dynamic strain sensing using Doppler-shift-immune phase-sensitive OFDR based on frequency-tracking and spectrum-zooming with ultra-weak reflection array. Theoretical study has been carried out with the introduction of mismatch coefficient, unveiling quantitatively the impact of Doppler shift. Following a numerical analysis of the proposed method, a retained precision has been experimentally verified regardless of the position mismatch due to the Doppler effect. Doppler-shift-immune sensing for dynamic strains covering continuous spatial resolution over a distance of 1000 m with a 2.5 cm sensing spatial resolution has been demonstrated, verifying the high fidelity promised by the proposed method.