Modeling of polarization effects in Fourier domain mode-locked (FDML) lasers
Final Report Abstract
The goal of this project has been to model polarization effects in Fourier domain mode-locked (FDML) fiber cavity laser systems, motivated by the fact that non-polarization-maintaining lasers constitute the most widely used type of FDML source. In this context, we have extended our previously developed theoretical description of FDML operation by including the polarization dynamics, and have implemented an efficient simulation algorithm based on the split-step Fourier method. More specifically, we have considered bending birefringence, polarization mode dispersion, as well as cross-phase modulation in the fiber spool, and have implemented other polarization sensitive optical elements such as the polarization controller. The developed approach has been applied for the investigation of a widely wavelength-swept FDML setup, as used for picosecond pulse generation by temporal compression of the sweeps. By extensive comparison to experimental results from Prof. Robert Huber’s group (Universität zu Lübeck), we could validate our modeling approach. Furthermore, we have demonstrated that polarization effects can play an important role. For non-polarization-maintaining (non-PM) configurations with polarization-insensitive semiconductor optical amplifier (SOA), the polarization dynamics has been shown to have a beneficial effect on the coherence properties. These findings are consistent with experimental observations, where non-PM lasers with a single-polarization SOA have been found to have a lower coherence length than configurations with polarization-insensitive SOA. These results have direct implications on the design of FDML-based sensing and imaging systems. Furthermore, we have developed an exact analytical model for a Fabry-Pérot filter with a moving end mirror, which is used as rapidly wavelength-tuned bandpass filter in FDML lasers. Based on this model, we could confirm that in the swept-filter reference frame moving along with the sweep filter center wavelength, the sweep filter characteristics can be described by a stationary transmission for arbitrarily fast wavelength sweeps. Furthermore, our model shows that for rapid wavelength sweeps, the Doppler shift caused by the moving end mirror becomes essential for the sweep filter operation. It has been experimentally demonstrated that the polarization dynamics in non-PM FDML lasers with polarization insensitive SOAs, together with near-perfect intracavity dispersion compensation, enables a novel operating regime distinguished by nearly vanishing intensity noise and considerably improved coherence properties. There is evidence that this ultra-low noise operation relies on a hitherto unidentified selfstabilization mechanism, enabled by the polarization degree of freedom which allows the laser to adjust the roundtrip time and thus compensate small timing errors caused by detuning or chromatic aberrations. Consequently, significantly better OCT performance is obtained, and novel applications such frequency comb operation of the FDML laser might become possible. A primary goal for the future will be to develop a theoretical model for ultra-low noise FDML operation, which will then be employed to investigate and advance related applications. To this end, the approach developed in this proposal will have to be extended to include the nonlinear dynamics of the semiconductor optical amplifier which also appears to play an important role in this context, as well as other relevant effects. An important point will be to identify the self-stabilization mechanism enabling ultra-low noise operation, and to systematically improve the laser performance in collaboration with experiment. In this context, it will also be highly interesting to model 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 quasistatic behavior. This will help to clarify how the gain recovery time affects the coherence properties of FDML lasers, and in particular ultra-low noise operation.
Publications
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“Modeling and analysis of polarization effects in Fourier domain mode-locked lasers,” Opt. Lett. 40, 2385–2388 (2015)
C. Jirauschek and R. Huber
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“Wavelength shifting of intra-cavity photons: Adiabatic wavelength tuning in rapidly wavelength-swept lasers,” Biomed. Opt. Express 6, 2448–2465 (2015)
C. Jirauschek and R. Huber
