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

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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.