Modeling of ultra-low noise operation in Fourier domain mode-locked (FDML) lasers

本课题的目标是模拟傅立叶域锁模光纤腔激光系统的超低噪声运行。根据各种医学成像和传感应用的需要，这些在宽光谱范围内产生快速的频率扫描。一个重要的例子是光学相干断层扫描（OCT），它产生人体组织的深度分辨信息。通常，FDML激光器表现出强烈的高频强度波动，瞬时相干长度被限制在几毫米。对于高度色散补偿的FDML激光器，已经发现了超低噪声的工作状态，其特点是强度噪声几乎消失，并提供了显着改善的相干特性。因此，获得了更好的OCT性能，并使基于fdml的超短光脉冲产生等新应用成为可能。从科学上讲，这种运作机制非常有趣，因为它依赖于一种迄今尚未发现的自稳定机制。在本提案中，我们的目标是建立一个超低噪声FDML操作的理论模型，然后用于研究和推进相关应用。在这里，我们建立在DFG项目JI 115-3/1中开发的模拟方法的基础上，已经考虑了极化动力学，极化动力学已被证明会显著影响FDML激光器的噪声特性。这种方法将被扩展到包括半导体光放大器的非线性动力学，这在这种情况下也起着重要的作用，以及其他相关的影响。我们将利用所得到的模型来确定实现超低噪声运行的自稳定机制，并与实验伙伴密切合作，系统地提高激光器的性能。该项目还将包括开发和研究由该操作机制实现的特殊应用，特别是将FDML频谱压缩为超短脉冲和频谱中频率梳结构的出现。此外，我们将从理论上研究基于拉曼增益介质的FDML激光器，其具有比通常使用的半导体光放大器更快的增益恢复，以及基于具有准静态行为的掺杂光纤放大器的配置。因此，我们将阐明增益恢复时间如何影响FDML激光器的相干特性，特别是超低噪声工作。

The goal of this project is to model ultra-low noise operation in Fourier domain mode-locked (FDML) fiber cavity laser systems. These generate fast frequency sweeps over a wide spectral range, as needed for various medical imaging and sensing applications. An important example is optical coherence tomography (OCT), which yields depth-resolved information of human tissue. Typically, the FDML laser exhibits strong high-frequency intensity fluctuations, and the instantaneous coherence length is limited to a few mm. For highly dispersion compensated FDML lasers, an ultra-low noise operating regime has been discovered which is distinguished by nearly vanishing intensity noise and offers considerably improved coherence properties. Consequently, significantly better OCT performance is obtained, and novel applications such as FDML-based ultrashort optical pulse generation become possible. Scientifically, this operating regime is highly interesting because it relies on a hitherto unidentified self-stabilization mechanism. In this proposal, we aim at developing a theoretical model for ultra-low noise FDML operation, which will then be employed to investigate and advance related applications. Here we build on our simulation approach developed in the DFG project JI 115-3/1, already accounting for the polarization dynamics which has been shown to significantly affect the noise properties of the FDML laser. This approach will be extended to include the nonlinear dynamics of the semiconductor optical amplifier which also plays an important role in this context, as well as other relevant effects. We will use the resulting model to identify the self-stabilization mechanism enabling ultra-low noise operation, and to systematically improve the laser performance in close collaboration with our experimental partner. This project will also include the development and investigation of special applications enabled by this operating regime, in particular the compression of the FDML spectrum to ultrashort pulses and the emergence of frequency comb structures in the spectrum. Furthermore, we will theoretically investigate FDML lasers based on Raman gain media, featuring a much faster gain recovery than the typically used semiconductor optical amplifiers, as well as configurations based on doped fiber amplifiers with a quasi-static behavior. Hereby we will clarify how the gain recovery time affects the coherence properties of FDML lasers, and in particular ultra-low noise operation.