Modeling of ultra-low noise operation in Fourier domain mode-locked (FDML) lasers
Final Report Abstract
The goal of this project was 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 detail, we aimed at developing a theoretical model for ultra-low noise FDML operation, which was subsequently employed to investigate associated phenomena and pave the way towards related applications. In order to enable realistic simulations of ultra-low noise operation and other operating regimes, our FDML simulation approach was extended to include the nonlinear dynamics of the semiconductor optical amplifier (SOA), which plays an important role in this context. Together with our experimental collaboration partner, ultra-low noise operation over a sweep bandwidth exceeding 100 nm has been realized by minimizing timing errors due to residual intracavity dispersion. Based on numerical simulations as well as a simplified analytical model, we could show that above mentioned self-stabilization mechanism is enabled by the frequency dependent group delay of the tunable bandpass filter, allowing each wavelength present in the swept laser field to pick a position in the filter window such that still perfect synchronization is obtained even for non-perfect dispersion compensation. When the timing errors exceed a certain threshold, the intensity trace suffers from high-frequency fluctuations, which have a negative impact on the imaging quality in OCT applications. Our simulations revealed the conditions under which different types of emerging intensity patterns, such as dips in the intensity trace as well as fringe-type and quasi-periodic patterns (co-)exist, and our theoretical results were found to be in good agreement with experimental observations. Furthermore, we have investigated the presence of frequency comb features in the spectral output of ultra-low noise FDML lasers, indicating remarkable coherence properties of ultra-low noise operation given the cavity length of up to a few km. Notably, the coherent relationship of the optical field over a few hundred roundtrips, and related emergence of frequency comb features, indicates true modelocking in ultra-low noise FDML lasers. Finally, our simulations indicated that replacing the SOA by a doped fiber amplifier will not lead to improved FDML operation, but will rather narrow down the accessible sweep range, which agrees with experimental observations. From a practical point of view, we employed our model for investigating the robustness of ultra-low noise operation, e.g., the maximum amount of tolerable residual dispersion, which is also highly relevant with regard to commercial applications.
Publications
- “Ultra low noise Fourier domain mode locked laser for high quality megahertz optical coherence tomography,” Biomed. Opt. Express 9, 4130–4148 (2018)
T. Pfeiffer, M. Petermann, W. Draxinger, C. Jirauschek, and R. Huber
(See online at https://doi.org/10.1364/BOE.9.004130) - “Coexistence of intensity pattern types in broadband Fourier domain mode locked (FDML) lasers,” 2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference (CLEO EUROPE/EQEC), Poster EF-P.5, Munich, Germany, June 23–27 2019 (IEEE Conference Proceedings, ISBN 978-1-7281-0469-0
M. Schmidt, T. Pfeiffer, C. Grill, R. Huber, and C. Jirauschek
(See online at https://doi.org/10.1109/CLEOE-EQEC.2019.8872381) - “Modeling of the ultra-stable operating regime in Fourier domain mode locked (FDML) lasers,” 2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference (CLEO EUROPE/EQEC), Poster CJ-P.46, Munich, Germany, June 23–27 2019 (IEEE Conference Proceedings, ISBN 978-1-7281-0469-0
M. Schmidt, T. Pfeiffer, C. Grill, R. Huber, and C. Jirauschek
(See online at https://doi.org/10.1109/CLEOE-EQEC.2019.8873213) - “Coherence of Fourier domain modelocked (FDML) lasers in the ultra-stable regime,” XIX International Conference on Laser Optics (ICLO 2020), Talk TuR1-03 (online), St. Petersburg, Russia, November 2–6 2020 (IEEE Conference Proceedings, ISBN 978-1-7281-5233-2
M. Schmidt, C. Grill, R. Huber, and C. Jirauschek
(See online at https://doi.org/10.1109/ICLO48556.2020.9285488) - “Self-stabilization mechanism in ultra-stable Fourier domain mode-locked (FDML) lasers,” OSA Continuum 3, 1589–1607 (2020)
M. Schmidt, T. Pfeiffer, C. Grill, R. Huber, and C. Jirauschek
(See online at https://doi.org/10.1364/OSAC.389972) - “Intensity pattern types in broadband Fourier domain mode‑locked (FDML) lasers operating beyond the ultra‑stable regime,” Appl. Phys. B 127, 60 (2021).
M. Schmidt, C. Grill, S. Lotz, T. Pfeiffer, R. Huber, and C. Jirauschek
(See online at https://doi.org/10.1007/s00340-021-07600-1)
