## 3 Self-Referenced DPSSLs with Bulk Gain Material
### One-Micrometer Ytterbium-Doped Lasers
This non-commercial high-brightness source was fairly sensitive to optical feedback, which can potentially lead to a catastrophic damage of its facets [65] and resulted in a poor long-term reliability. The next breakthrough occurred when [PERSON] _et al._, demonstrated a strong CEO beat (30 dB in 100 kHz RBW) in a gigahertz DPSSL pumped by a commercial multi-transverse-mode pump diode [20]. 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 [PERSON] _et al._, in an Yb:KGW SESAM-modelocked DPSSL at 1040 nm that was alternately pumped by a spatially-multimode low-brightness (\(M^{2}\approx 32\)) high-power diode array or by a single-transverse-mode high-brightness (\(M^{2}\ <\ 1.2\)) beam from a semiconductor vertical external cavity surface-emitting laser (VECSEL) [66]. A flipping mirror provided a simple interchange between these two sources to pump the exactly same modelocked laser with 133 MHz repetition rate. Both pumping schemes enabled an average output power of 450 mW in 130 fs pulse duration to be generated for 2 W of pump power. This led to a generated CEO beat with a similar SNR of more than 30 dB after SC generation and _f_-to-2\(f\) interferometry. However, a significantly broader linewidth of the CEO beat was observed with the low-brightness multimode pump diode, which mainly resulted from a higher noise in the frequency range of 1 kHz to 100 kHz. According to the concept of the \(\beta\)-separation line introduced in Section 2.3, this higher noise occurring at high frequency implies a larger loop bandwidth requirement to achieve a tight \(f_{\mathrm{CEO}}\) lock. Despite the higher CEO noise resulting from the use of a low brightness multimode pump diode, [PERSON] _et al._, demonstrated that a tight CEO lock could be achieved by feedback to the current of the multi-spatial-mode pump diode [56]. An in-loop residual integrated phase noise of 407 mrad [1 Hz, 5 MHz] was achieved in this case with a loop bandwidth of \(\sim\)300 kHz. Despite their higher noise, multi-transverse-mode pump diodes offer various advantages such as the robustness and reliability of industrial-grade diodes, and power scalability achieved by increasing the number of emitters coupled to the multimode fiber. 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. In the work of [PERSON] _et al_. [20], a pump power of 13 W at 978 nm with an \(M^{2}\) value of \(\sim\)25 was successfully used to produce 3.4 W of average power at 1047 nm in an Yb:KGW SESAM-modelocked laser with 125 fs pulse duration and 1.06 GHz repetition rate. The use of a \(\Sigma\)-shape cavity instead of the simpler Z-shape that was used in a former Yb:KGW gigahertz laser pumped by a tapered diode [19] offered more flexibility for cavity dispersion management. A coherent octave-spanning SC spectrum was generated directly in a PCF from the output of the modelocked oscillator, without any additional pulse compression or amplification, enabling the detection of a strong CEO beat (30 dB SNR in 100 kHz RBW) in an _f_-to-2\(f\) interferometer. However, no CEO stabilization was realized in this laser. The first self-referenced gigahertz DPSSL was demonstrated by [PERSON] _et al._, in 2014 [21]. The gain material was changed to Yb:CALGO here, placed in a Z-shape cavity. Yb:CALGO has a broader emission bandwidth than Yb:KGW and thus has the potential to produce shorter pulses.