【摘要】 集成光频梳在精密测量、高速光通信等领域具有重要应用价值。本研究基于主动锁模机制，利用薄膜铌酸锂与Ⅲ-Ⅴ族反射型半导体光放大器混合集成平台，旨在实现宽带锁模光频梳的产生。在理论层面，基于Haus主方程系统分析了调制深度、群延迟色散及增益带宽对锁模光谱宽度的影响。在实验环节，针对芯片间耦合进行了优化设计与对准误差分析。实验结果表明：所构建的异质集成激光器实现了3 dB光谱带宽达5.5 nm的锁模输出，包含105根梳齿，噪底以上光谱覆盖范围为1589～1604 nm；测得激光拍频信号信噪比达62 dB，有效验证了梳齿之间的高相干性。本研究充分证实了该异质集成平台在宽带、GHz重复频率光频梳生成领域的潜力，为片上光频梳光源在精密测量、微波光子学等前沿领域的实用化发展奠定基础。更多还原

【Abstract】 Objective With the increasing demand for integrated optical frequency comb(OFC) sources, research into integrated mode-locked lasers(MLLs) has intensified. Early integration efforts primarily focused on passively mode-locked lasers employing saturable absorbers. However, the stable operating range of such devices is typically confined to narrow windows of driving current and bias voltage. In contrast, actively mode-locked operation achieved through the hybrid integration of reflective semiconductor optical amplifiers(RSOAs) and thin-film lithium niobate(TFLN) external cavities offers significantly higher tunability in terms of both output power and repetition rate. An early study of RSOA-TFLN integrated platforms demonstrates mode-locked operation, but the achievable spectral bandwidth remains in the sub-nanometer regime. Although subsequent research attempt to broaden the spectrum by introducing the Kerr effect within a microresonator, such methods necessitate dispersion engineering and partially compromise the tuning flexibility associated with harmonic mode locking. To address the spectral bandwidth limitations of previous actively modelocked lasers, this work exploits the broadband gain of the RSOA to broaden the mode-locked spectrum and demonstrates a broadband microcomb mode-locked laser based on a simple integrated configuration, providing an effective pathway toward chip-scale optical frequency combs.Methods Active mode locking relies on periodic phase modulation to establish phase synchronization among longitudinal modes within the resonator. When the modulation frequency matches the cavity free spectral range(FSR), equally spaced sidebands enable effective coupling between adjacent modes, leading to stable mode-locked operation. The laser is described by the Haus master equation(HME), which accounts for the combined influences of gain, loss, cavity dispersion, and modulation depth. By assuming a steady-state solution in the form of a chirped Gaussian pulse, an analytical expression for the spectral bandwidth is derived, revealing the combined influence of gain bandwidth, dispersion, and modulation depth on the spectral performance of the mode-locked laser. Experimentally, an actively mode-locked laser is realized through hybrid integration of an RSOA chip and a TFLN chip. The TFLN chip is fabricated on a 600-nm-thick X-cut MgO-doped lithium niobate-on-insulator wafer, with ridge waveguides of 300 nm depth defined by inductively coupled plasma dry etching. Gold traveling-wave electrodes with a thickness of 800 nm are fabricated by electron-beam evaporation, forming a 6.5-mm-long phase modulator. To minimize intracavity loss, a spot-size converter(SSC) with a tapered structure is integrated on the TFLN chip to expand the waveguide width from 1.4 μm in the modulation region to 5 μm at the coupling facet, ensuring optical mode matching with the RSOA. Finite-difference time-domain simulations indicate a theoretical coupling loss of 1.33 dB. Further analysis of the coupling loss as a function of waveguide width and alignment offsets reveals that, owing to the small mode-field height, vertical alignment precision is critical for maintaining high coupling efficiency. The mode-locked laser cavity is formed by the high-reflection-coated facet of the RSOA and an integrated Sagnac loop mirror on the TFLN chip.Results and Discussions To characterize the gain properties of the RSOA, its amplified spontaneous emission(ASE) spectrum is measured in the absence of external cavity feedback. At a driving current of 100 mA, the RSOA exhibits a broad ASE spectrum with a 3 dB bandwidth of approximately 70 nm, covering the wavelength range from 1450 to 1650 nm. Periodic oscillations observed at the top of the ASE spectrum originate from residual reflections at the facet of the tapered optical fiber. After coupling the RSOA to the TFLN chip, the laser output without a raio frequency(RF) driving signal reveals a random multi-longitudinal mode distribution characterized by large intensity fluctuations and the absence of a stable spectral envelope. When an RF driving signal with a frequency matched to the cavity FSR is applied(6.17 GHz, 33 dBm), the laser transitions into a stable actively mode-locked state. The resulting output spectrum exhibits a remarkably flat and broad envelope with a 3-dB bandwidth of 5.5 nm. The optical spectrum above the noise floor spans from 1589 to 1604 nm, corresponding to a total of 283 comb lines, among which 105 lines fall within the 3 dB power window. A magnified spectral view confirms that the comb-line spacing is strictly equal to the applied RF modulation frequency of 6.17 GHz. The coherence of the generated frequency comb is further evaluated by measuring the beat-note signal using an electrical spectrum analyzer. With a resolution bandwidth of 100 Hz, the beat note exhibits a signal-to-noise ratio of 62 dB, confirming strong phase coherence among the comb lines and stable phase synchronization of the longitudinal modes.Conclusions In conclusion, this research provides a systematic theoretical analysis and experimental demonstration of an actively mode-locked laser based on a hybrid Ⅲ-Ⅴ/TFLN platform. Analysis based on the HME indicates that broadband gain is the fundamental prerequisite for broadband spectral expansion. By integrating a broadband RSOA chip with a high-speed TFLN external cavity, a mode-locked microcomb laser with a 6.17 GHz repetition rate and a 5.5 nm 3 dB spectral bandwidth is demonstrated. These results underscore the immense potential of the integrated Ⅲ-Ⅴ/TFLN platform for providing high-performance on-chip frequency combs for optical communications, LiDAR, and precision metrology.更多还原