## 5 Conclusions and Outlook  
In the first years after their invention, CEO-stable laser systems were either based on Ti:Sapphire or fiber lasers. However, in the last years, CEO-stabilized DPSSLs have become a promising alternative for frequency comb generation with state-of-the-art performance. The DPSSL technology combines the advantages of fiber lasers in terms of simplicity and reliable diode pumping with the low-noise operation of Ti:Sapphire lasers resulting from their high Q-factor resonator. SESAM-modelocked DPSSLs provide self-starting and reliable operation. Their simple pumping using laser diodes provides a reliable scheme for frequency comb CEO-stabilization using the common method of feedback to the current of the pump diode. The lower noise of DPSSLs arising from their low cavity losses results in less demanding requirements in terms of feedback bandwidth for CEO stabilization compared to fiber lasers. A tight CEO lock has been demonstrated in various DPSSL systems using this simple feedback scheme, despite the limited bandwidth achievable with this approach resulting from the relatively long upper-state lifetime of the gain material. Faster actuators for improved CEO stabilization have also been demonstrated to control and stabilize _f_CEO with a higher bandwidth, such as by opto-optical modulation of an intra-cavity SESAM [44] or using an intra-cavity AOM in TDLs [61].  
The superior emission properties of DPSSLs in terms of average output power and pulses duration also enable CEO detection directly from the output of the laser oscillator in various systems, without the need for any external amplification or pulse compression, which keeps the system more simple and cost-efficient. DPSSL frequency combs will certainly continue their expansion in the near future, especially for the realization of frequency combs with multi-gigahertz repetition rates, a range that is difficult to achieve for fiber lasers. High repetition rate combs are of particular interest for applications such as ultra-low-noise microwave generation or astronomical spectrographs calibration. First experiments of ultra-low-noise microwave generation using DPSSL combs with repetition rates in the 100 MHz range were reported with very promising performance [73; 75; 26], and it is likely that further applications will develop in the future that will benefit from the low-noise properties and high repetition rate capabilities of DPSSLs. Such gigahertz DPSSL combs can be pumped using commercial multi-transverse-mode diodes that make these laser systems cost-efficient and reliable, without compromising the achievement of a tight CEO lock [21]. DPSSLs have also shown their hardness to radiation [58], which makes them an attractive alternative to fiber lasers for the implementation of frequency combs in a space environment, for instance for a future master optical clock in space.  
Another field where DPSSLs are already in a leading position is high-power ultrafast oscillators. The TDL geometry has currently demonstrated the highest average power and pulse energy emitted from any oscillator [36; 78]. New breakthroughs in this domain are expected in the near future for the generation of stabilized frequency combs in the high average power regime of \(>\)100 W, which will be of paramount importance for high field physics such as high harmonic generation to produce extreme ultra-violet frequency combs.  
We acknowledge support by the Swiss National Science Foundation (SNF) project 146738, the European Research Council (ERC Starting Grant 2011 #279545), and Nano-Tera.ch MIXSEL-2 (20 NA21_145932).