Frequency comb offset dynamics of SESAM modelocked thin disk lasers

Abstract

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 tradeoff between enough damping and avoiding multiple pulsing instabilities. Additional cavity components could be considered for supplementary damping independent of the SESAM operation point. , 23, issue 17, 21836-21856, 2015 which should be used for any reference to this work 1 Published in Optics Express

Carrier-Envelope Offset Stabilized Ultrafast Diode-Pumped Solid-State Lasers

“…The pump current has a direct influence on the pulse parameters of the femtosecond laser, such as pulse duration and energy, which directly translates into a change of fCEO. CEO stabilization by pump current modulation has thus been implemented in various DPSSLs that will be described in more details in Sections (3) and (4) [21,[25][26][27][28][29][30][31]. However, the pump current acts onto fCEO through the gain medium, and the corresponding modulation bandwidth is limited by the upper-state lifetime of the gain material that induces a roll-off in the modulation response of fCEO.…”

“…This achievement was the first evidence that this type of diodes could be used to pump a DPSSL for frequency comb applications, despite the higher intensity noise resulting from its highly multimode operation. This multimode noise can be transferred to the modelocked laser and ultimately to the CEO beat as shown for a TDL [31], thus making the laser more challenging to self-reference. This effect was experimentally shown by Link et al, in an Yb:KGW SESAM-modelocked DPSSL at 1040 nm that was alternately pumped by a spatially-multimode low-brightness ( ≈ 32) high-power diode array or by a single-transverse-mode high-brightness ( < 1.2) beam from a semiconductor vertical external cavity surface-emitting laser (VECSEL) [66].…”

“…For this reason, they are also commonly used as pump sources in high-power TDLs [30,31] as will be discussed in Section 4. However, their multimode nature results in a lower brightness in comparison to tapered diodes.…”

“…A detailed study of the frequency dynamics of fCEO in SESAM-modelocked TDLs was recently reported by Emaury et al, where the behavior of the aforementioned Yb:CALGO laser with 65 MHz repetition rate and high-power Yb:YAG laser with sub-10 MHz repetition rate was compared [31]. A complete noise characterization of these two laser systems including the measurements of various relevant transfer functions combined with comparison with theoretical models evidenced a fundamentally different behavior of the two TDL resonators.…”

“…This is the most common method to CEO-stabilize fiber laser frequency combs using the self-referencing f-to-2f interferometry method [24]. This method is applicable in a similar way in DPSSLs and constitutes the most common means to CEO-stabilize this type of femtosecond lasers [21,[25][26][27][28][29][30][31].…”