Abstract
We present a detailed study of the carrier-envelope offset (CEO) frequency dynamics of SESAM modelocked thin disk lasers (TDLs) pumped by kW-class highly transverse multimode pump diodes with a typical M(2) value of 200-300, and give guidelines for future frequency stabilization of multi-100-W oscillators. We demonstrate CEO frequency detection with > 30 dB signal-to-noise ratio with a resolution bandwidth of 100 kHz from a SESAM modelocked Yb:YAG TDL delivering 140 W average output power with 748-fs pulses at 7-MHz pulse repetition rate. We compare with a low-power CEO frequency stabilized Yb:CALGO TDL delivering 2.1 W with 77-fs pulses at 65 MHz. For both lasers, we perform a complete noise characterization, measure the relevant transfer functions (TFs) and compare them to theoretical models. The measured TFs are used to determine the propagation of the pump noise step-by-step through the system components. From the noise propagation analysis, we identify the relative intensity noise (RIN) of the pump diode as the main contribution to the CEO frequency noise. The resulting noise levels are not excessive and do not prevent CEO frequency stabilization. More importantly, the laser cavity dynamics are shown to play an essential role in the CEO frequency dynamics. The cavity TFs of the two lasers are very different which explains why at this point a tight CEO frequency lock can be obtained with the Yb:CALGO TDL but not with the Yb:YAG TDL. For CEO stabilization laser cavities should exhibit high damping of the relaxation oscillations by nonlinear intra-cavity elements, for example by operating a SESAM in the roll-over regime. Therefore the optimum SESAM operation point is a trade-off between enough damping and avoiding multiple pulsing instabilities. Additional cavity components could be considered for supplementary damping independent of the SESAM operation point.
Highlights
Optical frequency combs based on modelocked lasers [1,2,3] have attracted a lot of attention over the last decade due to their broad range of applications
We present a detailed study of the carrier-envelope offset (CEO) frequency dynamics of semiconductor saturable absorber mirror (SESAM) modelocked thin disk lasers (TDLs) pumped by kW-class highly transverse multimode pump diodes with a typical M2 value of 200-300, and give guidelines for future frequency stabilization of multi-100-W oscillators
As an important step in this direction, we demonstrate for the first time, to the best of our knowledge, CEO frequency detection of a laser system delivering more than 100 W, based on a SESAM modelocked 140-W Yb:YAG TDL operating at 7 MHz pulse repetition rate
Summary
Optical frequency combs based on modelocked lasers [1,2,3] have attracted a lot of attention over the last decade due to their broad range of applications. Ideal driving sources for HHG with high average photon flux need a challenging combination of short pulse durations (< 100 fs), high pulse repetition rate (> MHz), and high pulse energy (> 100 μJ), which requires high average power (>> 100 W) These goals can be achieved with the recent progress in ultrafast diode-pumped Yb-doped solid-state laser systems with multi-100-W average output power based on either fiber amplifier systems [8], Innoslab amplifiers [9], or semiconductor saturable absorber mirror (SESAM) modelocked thin disk lasers (TDL) [10]. We address another important requirement, which is the carrier envelope offset (CEO) frequency stabilization. Taking into account all these effects at a fixed pulse duration of 77 fs, we calculated ∂q / ∂ES ≈ 450 J−1 for our Yb:CALGO TDL
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