Custom designed and unstrained SESAMs for fiber based shortpulse lasers

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Ultrashort pulse lasers with pulse lengths in the femtosecond range are used in industry and research, e.g. as seed lasers in materials processing, biophotonics and quantum sensing. To generate ultrashort pulses from solid-state and fiber lasers, saturable absorber mirrors (SESAMs) are typically used for passive mode coupling. The advantages of SESAMs are their versatile adaptability to a wide variety of laser systems and their ease of integration into the laser cavity, since as passive components they do not require any further control. However, for fiber-based ultrashort pulse lasers operating in the wavelength range around 1550 nm, there are two key challenges that have so far hampered the use of SESAMs or led to premature laser failure in the field. First, the dynamic behavior of mode-locked ultrashort pulse lasers is complex, i.e., in principle very close to a chaotic state. To keep the laser robust and stable in a desired well-defined operating state, the mode-locking SESAM must be tuned to the specific laser. To do this, the entire laser system must be fully understood not only in the stable state of mode-locked pulse generation, but especially in the startup phase. This stage of maturity has not yet been reached for fiber-coupled pulse lasers with wavelengths around 1550 nm. Instead, one currently works with empirically determined requirements and assigns specially selected SESAMs to a particular laser.Second, on the standard GaAs substrate used for SESAMs, there is no strain-free and long-term stable absorber material for 1550 nm. For this wavelength, the required fast absorber consists of In0.47Ga0.53As with strong crystal bias of +3.5% to the GaAs substrate. Although the bias can be reduced by incorporating small amounts of nitrogen, the resulting dilute nitrides have not been shown to have long-term stability. In commercial systems, the failure rate of 1550 nm SESAMs is increased 40-fold compared to 1030 nm SESAMs. Nevertheless, commercially available 1550 nm SESAMs are based on GaAs. Both limiting aspects are addressed in this project to overcome the hurdles for the use of fiber-based ultrafast lasers at 1550 nm in close collaboration with TOPTICA Photonics AG. There is a DFG precursor project for both. Using a numerical model of the SESAM mode-locked fiber laser, optimized laser and absorber parameters are to be determined for the desired single-pulse operation of the fiber laser. These optimized absorbers will then be grown at the Fraunhofer Institute and tested at TOPTICA and University of Hamburg in designated fiber lasers for startup behavior, dynamics and aging. Several of these cycles will be run to achieve an overall optimized system.