【摘要】 飞秒紫外激光在自由电子激光器的相位调制与反调制、极紫外光刻及超快光学测量等领域中应用广泛。为实现高相干、短波长光源的精密相位控制，构建了基于共线三倍频与近场干涉技术相结合的飞秒紫外脉冲产生与相位同步系统。该系统采用两级偏硼酸钡晶体实现二次谐波与三次谐波产生，最终获得中心波长为343 nm、单脉冲能量约为45μJ的稳定紫外输出，三倍频效率达35%。两路飞秒紫外脉冲经分束、延迟、合束后形成高对比度干涉条纹。利用电荷耦合元件实时采集干涉图样，并通过频域滤波与相位解包算法提取相位误差信号，进而实现闭环反馈校正。实验结果表明，系统稳态运行时，时间抖动的均方根误差可由154 as降至88 as。该飞秒紫外激光同步方案兼具相位同步稳定性与长期运行可靠性。研究结果为紫外波段超快光学系统实现阿秒级同步控制提供了新方法，也为实现自由电子激光向短波区扩展提供了关键技术支撑。更多还原

【Abstract】 Objective Femtosecond ultraviolet（UV） pulses are indispensable for phase modulation and demodulation in free-electron lasers（FELs）, extreme ultraviolet lithography, and precision ultrafast metrology. However, achieving attosecond-scale timing stability at UV wavelengths remains challenging: synchronization methods based on balanced optical cross-correlation rely on efficient sum-frequency generation in nonlinear crystals, yet such crystals are scarce or impractical for the UV and vacuum-ultraviolet（VUV）spectral ranges. Herein, we develop and validate a compact, crystal-independent phase-locking strategy tailored specifically to the UV regime. Stable 343 nm pulses are generated via collinear third-harmonic generation（CTHG）, and attosecond-precision synchronization is realized using near-field interferometry. This work pursues two core objectives:（i） to provide a high-efficiency UV source suitable for seeding and timing applications, and（ii） to demonstrate a closed-loop phase-stabilization scheme that eliminates the need for additional nonlinear conversion in the synchronization path—thereby enhancing reliability and maintainability during long-term operation in both laboratory setups and FEL facilities.Methods An ytterbium-doped yttrium aluminum garnet（Yb∶YAG） laser（center wavelength of 1030 nm, pulse duration of ～487 fs, repetition rate of 1 kHz, single-pulse energy of 130 μJ） drives a two-stage frequency-conversion chain based on beta barium borate（BBO） crystals. First, Type-I second-harmonic generation（SHG） in a 3.5 mm-thick BBO crystal yields 515 nm pulses with ～70 μJ energy and ～54% conversion efficiency, and the SHG stage introduces a calculated group-velocity mismatch of ～88.6 fs·mm^-1, corresponding to a temporal walk-off of ～310 fs, which is compensated by a 1 mm-thick calcite plate（cut at 43.5°） placed downstream of the SHG crystal, followed by a half-wave plate（HWP） that aligns the polarization state prior to a second Type-I BBO crystal enabling collinear third-harmonic generation（THG）, which produces 343 nm UV pulses with ～45 μJ single-pulse energy and ～35% THG efficiency. The UV beam is split into two arms by a beamsplitter: the transmission arm incorporates a motorized optical delay line（ODL） mounted on an electrically driven translation stage with a nominal step size of 10 nm, while the reflection arm serves as the reference, and after recombination at a small crossing angle of ～0.06°, the two beams form high-visibility interference fringes on a charge-coupled-device（CCD） camera; a Fourier-domain filter isolates the spatial-carrier band, wrapped phase is extracted from the filtered interferograms, robust temporal unwrapping is performed to retrieve the instantaneous inter-pulse phase error, and a proportional-integral-derivative（PID） controller drives the ODL to null this error, thereby closing the feedback loop, with the overall feedback bandwidth limited by the camera frame rate and actuator response and reaching approximately 2 Hz in the current setup. Prior to the experiments, numerical simulations of near-field interference for Gaussian beams are conducted to optimize the crossing angle, pixel sampling rate, and unwrapping range.Results and Discussions The near-field interference readout yields a single-valued phase observable over ±0.5T₀（single period T₀≈1.14 fs）, which is validated by our numerical model: for Gaussian beams with a small crossing angle（～0.06°）, the simulated fringe pitch is ～0.33 mm, with the pattern translating linearly with the applied relative delay while the transverse envelope along the y-axis remains Gaussian（Fig. 2）. This behavior defines the operating range necessary for unambiguous tracking and validates the use of the unwrapped fringe phase as the error signal. Experimentally, the imaging-based feedback maintains stable locking over an observation window of ～118 min: when the loop is open, the in-loop timing trace exhibits a broader excursion and frequent spikes, whereas closed-loop operation contracts the fluctuation band and results in a notably more stationary trajectory [Fig. 4（a）]. Quantitatively, the root-mean-square（RMS） timing jitter is reduced from 154 as to 88 as—corresponding to approximately one-thirteenth of an optical cycle at 343 nm. The probability-density curves further confirm this improvement, showing a taller, narrower peak near zero and significant suppression of long tails under feedback [Fig. 4（b）]. Collectively, the time-domain and statistical results indicate that the loop primarily suppresses broadband random phase noise and sporadic disturbances. A key advantage of this approach is its reliance solely on linear imaging and a calibrated delay line, thus eliminating the need for ultraviolet sum-frequency or cross-correlation diagnostics as well as the associated constraints of nonlinear crystals and dispersion management. This simplicity enables a compact layout, easier alignment, and enhanced maintainability. The same sensing principle can be extended to synchronize multiple ultraviolet arms to a common reference, making it well suited for seeded FEL beamlines and multi-branch pump–probe experiments that demand attosecond-level timing margins.Conclusions We report a synchronization strategy that integrates an efficient collinear third-harmonic UV source（343 nm center wavelength, ～45 μJ single-pulse energy, ～35% efficiency） with a crystal-independent near-field interferometric phase-locking loop. The closed-loop system combines Fourier-domain fringe filtering, phase unwrapping, and motorized delay control to achieve longduration attosecond-level stability: the in-loop root-mean-square（RMS） timing jitter is reduced from 154 as to 88 as（～43% reduction）, corresponding to a phase stability of approximately λ/13（λ is the wavelength） at 343 nm. By eliminating the need for additional nonlinear conversion in the synchronization arm, this method delivers a compact, stable, and maintainable solution—well suited for UV-band ultrafast experiments and FEL seeding, where high longitudinal coherence and repeatability are critical requirements. Future work will focus on three key directions: enhancing loop bandwidth via faster imaging and piezoelectric actuation, enabling real-time anomaly detection and phase-trend prediction using data-driven estimators, and extending the strategy to longer beamlines and multi-terminal networks relevant to extreme ultraviolet and soft X-ray FELs.更多还原