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波长可快速调谐的光源是多种高速光学测量系统的核心器件[1-2]。在扫频光学相干层析成像（SS-OCT）应用[3-4]中，扫频速率与光学带宽共同决定了A扫线的采集速度与轴向分辨率，而光源的相干长度则制约着有效成像深度。在相干激光测距与测速领域[5-6]，频率调制的线性度与可重复性直接影响拍频信号的解算精度。当前应用端对光源的需求正从单纯的“更高扫速”与“更宽范围”向“可编程能力”延伸，例如实现非线性扫频、分段扫频或与外部系统时钟同步的自定义轨迹。
现有扫频光纤光源主要分为两类：一类基于机械或微机电系统（MEMS）调谐元件[7]，其调谐惯性与机械扫描速率之间存在固有矛盾；另一类通过腔内非线性动力学实现高速扫频，典型方案包括傅里叶域锁模（FDML）[8]、色散拉伸脉冲扫频[9]及孤子自频移[10-11]等。然而，后者的扫频轨迹往往由腔体结构与动力学过程预先决定，缺乏灵活的可重构性。声光光栅（AOG）兼具全光纤集成与纯电控快速调谐的优势，已被证明可实现纳米量级带宽的可调滤波，且调谐范围较宽[12-16]。然而，当其作为腔内动态选模元件参与锁模扫频时，快速调谐条件下的实时光谱演化特性尚缺乏单发尺度的直接实验证据。
色散傅里叶变换（DFT）技术[17-18]为解决这一难题提供了有力工具。该技术通过色散光纤将脉冲光谱映射至时域波形，可实现逐往返周期的单发光谱获取，时间分辨率可达腔周期量级。本文的“逐往返测量”是指以相邻腔内脉冲为单位进行单发光谱记录，即激光脉冲每完成一次腔内往返即可获得一帧光谱；相邻两帧光谱之间的时间间隔为腔内往返时间
$ {T}_{\text{RT}} $ ，满足$ {T}_{\text{RT}}=1/{f}_{\text{rep}} $ ，其中$ {f}_{\text{rep}} $ 为激光器的基频重复频率。基于此，本文采用声光光栅构建可编程扫频锁模超快光纤激光器，并利用DFT技术实现逐往返光谱观测，系统研究2 000 nm/s与10 000 nm/s两种扫速下的实时光谱演化与统计重复性，旨在揭示扫频过程中锁模相干性的维持与受扰动力学机制，为构建可编程高速扫频超快种子源提供实验依据。 -
如图1所示，实验搭建的全光纤环形激光器采用双端980 nm激光二极管泵浦，增益介质为1 m掺铒光纤。腔内包含约6.4 m标准单模光纤及声光光栅模块中的13.5 cm色散补偿光纤（DCF），总腔长约为7.54 m，对应的基频重复频率为27.1 MHz（往返时间36.9 ns）。根据光纤标称色散参数估算，在1 550 nm波段腔内净色散处于反常色散区，二阶色散
$ \;{ \beta }_{2,\text{total}} $ 约为−0.115 ps2。锁模机制由单壁碳纳米管可饱和吸收体配合偏振控制器共同实现。声光光栅模块由偏振相关器件与偏振控制器构成偏振选择结构：在满足相位匹配条件时，声光效应诱导的偏振旋转被转换为透射率变化，从而形成可调谐的带通滤波窗口。优化条件下模块插入损耗约为5 dB。声光光栅通带建立时间取文献典型值，约为150 μs量级[14]，该参数在后文中作为衡量系统动态响应特征时间的参考依据。
动态测量系统包含两路同步采集通道：一路用于记录脉冲序列强度，另一路通过4 km色散补偿光纤构成的色散傅里叶变换链路实现单发光谱的实时测量。如图2所示，扫频驱动信号采用分段线性、非对称三角形射频波形：驱动频率在1.028～1.030 MHz范围内变化，对应中心波长调谐范围约为1 550～1 552 nm。慢扫段固定为1 ms（由1 552 nm至1 550 nm，等效扫速2 000 nm/s）；回扫段设置为0.2 ms（扫速10 000 nm/s），形成完整的扫频周期。
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如图4所示，在2 000 nm/s慢扫段与10 000 nm/s回扫段构成的完整扫频周期内，色散傅里叶变换光谱图显示中心波长随时间连续平移，且光谱轮廓在扫频全程保持高度一致，并且，Kelly边带在扫频过程中始终可辨，表明孤子态并未因动态滤波作用而被破坏。时域观测通道同时显示稳定的连续脉冲输出。
为定量评估扫频过程的确定性与可重复性，首先提取了中心波长轨迹。对每个单发光谱利用质心法计算瞬时中心波长。对于离散采样点
$ {\lambda }_{i} $ 及其对应的光谱强度$ I({\lambda }_{i}) $ ，中心波长$ {\lambda }_{\mathrm{c}} $ 定义为$ {\lambda }_{{\mathrm{c}}}=\dfrac{{\displaystyle\sum}_{i}{\lambda }_{i} I\left({\lambda }_{i}\right)}{{\displaystyle\sum}_{i}I({\lambda }_{i})} $ ，对所有光谱重复上述计算，得到的中心波长序列$ \left\{\lambda _{{\mathrm{c}}}^{\left(rt\right)}\right\} $ ，其中$ rt $ 为往返周期序号。质心计算结果显示，波长移动平滑无突跳，严格遵循射频波形设定的确定性轨迹。进一步，为评估单发光谱的波形可重复性，计算了相邻光谱间的余弦相似度。两种扫速下相似度均接近1，仅在10 000 nm/s回扫段偶见轻微下降，表明该状态下锁模相干性仅受到微弱扰动但仍得以良好维持。值得关注的是，逐往返测量的时间分辨率为腔周期36.9 ns。在此时间尺度上观测到的连续光谱平移表明：输出中心波长的更新可在亚微秒量级内完成，显著快于声光光栅通带建立时间（百微秒量级）。这一对比揭示了一个重要现象：扫频锁模过程中“输出波长的连续演化”并不等价于滤波器件通带完全建立后的准静态平移，而是腔内增益-损耗动态平衡与光谱约束共同作用的结果。
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本文报道了一种基于偏振依赖声光光栅的可编程扫频超快光纤激光器，并借助色散傅里叶变换技术实现了逐往返周期的实时光谱观测。该系统通过射频驱动实现中心波长的确定性控制，在2 000 nm/s与10 000 nm/s扫速下均能维持稳定的相干锁模扫频，光谱连续平移且Kelly边带清晰可辨。逐往返测量进一步揭示，输出中心波长的更新可在亚微秒量级内完成，显著快于AOG通带建立时间，表明扫频锁模过程中的波长演化是腔内增益-损耗动态平衡与光谱约束共同作用的结果。本研究为可编程高速扫频超快光源的动力学机制提供了直接实验依据，也为后续通过优化腔参数以进一步提升扫频性能奠定了基础。
可编程扫频超快光纤激光及其实时动力学分析
Programmable frequency-swept ultrafast fiber laser and its real-time dynamic analysis
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摘要: 面向扫频光学相干层析成像（SS-OCT）及高速相干探测等应用，扫频光源需兼顾高扫速、宽调谐范围、动态相干性维持及可编程扫频轨迹。本文报道一种基于偏振依赖声光光栅（AOG）的可编程扫频超快光纤激光器，AOG作为可调谐滤波窗口，其中心波长由射频驱动频率唯一确定。借助色散傅里叶变换（DFT）实现逐往返单发光谱观测，研究了
2000 nm/s与10000 nm/s两种扫速下的实时演化与统计重复性。结果表明：两种扫速下光谱均连续平移，Kelly边带清晰可辨，相邻光谱间相似度保持较高水平，相干锁模得以良好维持。逐往返测量进一步揭示，输出中心波长的更新可在亚微秒量级内完成，显著快于AOG通带建立时间，表明扫频锁模过程中的波长演化是腔内增益-损耗动态平衡与光谱约束共同作用的结果。本研究为可编程高速扫频超快光源的动力学机制提供了直接实验依据。Abstract:BackgroundWavelength-swept sources for swept-source optical coherence tomography (SS-OCT) and high-speed coherent detection must simultaneously provide rapid scanning, a wide tuning span, stable coherence, and flexible sweep trajectories. However, maintaining coherent mode locking during fast, user-defined wavelength sweeping remains challenging because the intracavity spectral filter, gain dynamics, and soliton spectral confinement evolve on different time scales. PurposeThis work demonstrates an all-fiber, passively mode-locked ultrafast ring oscillator whose wavelength-sweep trajectory is programmed directly in the RF domain. MethodsAn intracavity polarization-selective acousto-optic grating (AOG) is employed as a dynamically tunable spectral-control element. The AOG forms a translating low-loss transmission window, with its center wavelength uniquely determined by the RF drive frequency, enabling deterministic wavelength scanning through arbitrary waveform modulation. To resolve the transient spectral evolution during swept operation, round-trip-resolved single-shot spectra are recorded using dispersive Fourier transform (DFT). The sweep dynamics and spectral reproducibility are examined at sweep rates of 2000 and 10000 nm/s.ResultsAt both sweep rates, the spectral envelope drifts smoothly along the programmed trajectory, while the Kelly sidebands remain distinct throughout the sweep. The adjacent-shot spectral cosine similarity remains high, confirming that coherent mode locking is sustained during rapid wavelength scanning. The round-trip-resolved measurements further indicate sub-microsecond updates of the output center wavelength, far faster than the AOG passband build-up time. This behavior suggests that the observed wavelength evolution arises from the combined effect of intracavity gain–loss dynamics and spectral confinement of the mode-locked pulse. ConclusionsThese measurements provide direct real-time evidence of the dynamical mechanism of trajectory-programmable high-speed swept ultrafast fiber sources and verify that RF-domain programming can sustain coherent swept mode locking under high-speed wavelength scanning. -
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图 2 用于波长扫描的可编程射频驱动波形
Figure 2. Programmable RF drive waveforms for wavelength sweeping
图 3 超快光纤扫频激光器的步进调谐基线性能
Figure 3. Step-tuned baseline performance of the wavelength-swept ultrafast fiber laser
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