新型离散傅里叶锁模光纤激光器理论与实验研究

We propose to develop a theoretical model for discrete Fourier domain mode locked (DFDML) fiber lasers. We will investigate the mechanism of pulse formation and shaping in the DFDML laser cavity. The principle and parameters that affect pulse quality, particularly stability, relative intensity noise (RIN), and coherence length will be systematically investigated. We will also explore the methods to improve pulse quality. We will theoretically investigate the pulse shaping and compression mechanism which are achieved by incorporating a segment special nonlinear fibers such as dispersion decreasing fiber or a nonlinear fiber Bragg grating in the DFDML laser cavity. Methods and laser configurations to reduce the time-bandwidth product of the discrete chirped sub-pulse in the cavity will be explored. Guided by the results of the theoretical investigation, DFDML fiber lasers with improved output pulse quality will be built experimentally. We propose two novel DFML fiber laser configurations. The first one is based on time domain intensity modulation. The scheme uses an intensity modulator to discretize the frequency sweep signal into multiple sub-pulses. The flexibility of the laser system is expected to be significantly improved without using a comb filter such as Fabry-Perot etalon in the cavity. The second one is based on a novel mode-locking technique we invented, called time-frequency mode locking (TFML), which can improve the linearity of the frequency sweeping in the output signal of DFDML fiber laser. A fiber based Mach-Zehnder interferometer is used to slice the spectrum and generate the corresponding clock signal in time domain. The clock signal is then used to trigger a series of ultra-short electrical pulses to drive an intensity modulator, which will then chop the frequency sweeping optical signal into short sub-pulses. A high-quality DFDML fiber laser output is expected using the proposed scheme.

傅里叶锁模激光器是下一代扫频光学相干断层扫描(OCT)技术最具潜力的光源之一。新型离散傅里叶锁模激光器的研究将能极大地提高激光器的输出信号质量，提高OCT系统的灵敏度，信噪比和探测深度。 本项目将建立离散傅里叶锁模激光器的理论模型，对腔内脉冲形成机制进行深入的分析，研究影响信号稳定性、信噪比、相干性等指标的关键机理和参数，探索提高输出脉冲质量的方法；研究基于特种非线性光纤和光纤光栅的腔内脉冲压缩整形技术，探索降低离散子脉冲信号带宽脉宽积的方案以提高脉冲信号质量，并研制基于此方案的离散傅里叶锁模激光器；提出基于幅度调制器实现时域离散傅里叶锁模激光器的方案，采用时域调制的方法对扫频信号进行离散化，极大地提高系统灵活性；提出结合梳状滤波器和时域调制的时频域双锁模激光器方案，采用光纤马赫曾德滤波器进行频域离散化，并触发调制器对光信号进行斩波压缩，从而实现频域上均匀分布的高性能离散傅里叶锁模激光器。

傅里叶锁模激光器是下一代扫频光学相干断层扫描(OCT)技术最具潜力的光源之一。新型离散傅里叶锁模激光器的研究将能极大地提高激光器的输出信号质量，提高OCT 系统的灵敏度，信噪比和探测深度。.理论方面我们基于参考系变换，建立了能够分析频域梳状滤波和时域脉冲调制的离散傅里叶锁模激光器理论模型，通过对系统中信号建立、脉冲成形过程的详细研究，明确了激光腔内输出脉冲的成形机制，并明确了离散傅里叶锁模激光器输出稳定的高相干性脉冲的参数范围。我们还通过对理论模型的分析，研究发现了傅里叶锁模激光器中高频噪声的深层起因为累积的相对频移导致的Eckhaus不稳定。我们详细研究了基于光子晶体光纤、波导上的脉冲压缩机制来对脉冲性能优化探索新的途径。基于理论分析，我们阐明了采用时域调制以及时频域双锁模激光器的运行机制及其在性能方面的差异，明确指出时频域双锁模激光器能够提供更佳的信号质量，为实验设计提供了依据。基于理论指导，我们在实验上研制出了基于法布里-珀罗和微环梳状滤波器的离散傅里叶锁模激光器。其输出通道间隔为50 GHz，相比传统傅里叶锁模激光器，基于微环梳状滤波器的离散傅里叶锁模激光器将扫频脉冲的相干长度从数毫米提高到了数十厘米，提高幅度达到百倍以上。我们还研制出基于时域调制的离散傅里叶锁模激光器。通过精确控制调制脉冲的时间间隔分布，我们可以通过时域调制获得频域上等间隔的扫频脉冲串。我们在实验上研制出了基于时频域双约束的时频域双锁模激光器。通过法布里-珀罗和时域调制器的协同工作，获得了获得均匀频率间隔50 GHz的稳定波长扫描脉冲序列。.本项目研制出的离散傅里叶锁模激光器可以提高扫频光源的性能，对于新一代OCT系统的研发具有重要意义，研究期间建立的理论模型和对新器件新机制的研究，将为本领域后续的研究工作提供坚实的基础。