## 3 Self-Referenced DPSSLs with Bulk Gain Material
### One-Micrometer Ytterbium-Doped Lasers  
Some years later, the first self-referenced Yb DPSSL in the 1 \(\upmu\)m spectral range was reported by [PERSON] _et al._[25]. The Yb:KYW laser was pumped by two 600 mW fiber-coupled laser diodes at 980 nm stabilized with a fiber Bragg grating to produce 250 mW output power in \(\sim\)290 fs at a repetition rate adjustable between 160 MHz and 190 MHz. As a result of the quite long pulses emitted by this laser, coherent octave-spanning SC generation for CEO beat detection could not be directly achieved from the laser output, but required external amplification using an Yb:fiber amplifier and pulse compression. An average power of 300 mW compressed down to 113 fs was launched into a microstructured fiber to generate a coherent spectrum extending from 650 nm to 1450 nm that allowed \(f\) CEO to be detected in an _f_-to-_2\(f\) interferometer with a SNR of 34 dB (in 100 kHz RBW). The CEO frequency was stabilized by feedback to the current of one of the pump diodes with a loop bandwidth of about 20 kHz limited by the roll-off response of \(f\) CEO to the pump current modulation. In these conditions, an integrated phase noise of 1.7 radians [5 Hz-4 MHz] was obtained from the measured CEO frequency noise PSD [25]. Later on, improvements of the CEO lock were reported for the use of this frequency comb as an optical-to-microwave frequency divider for low-noise microwave generation [26]. With a total pump power of 830 mW and a repetition rate of 186 MHz, the CEO servo bandwidth was extended to \(\sim\)100 kHz by the use of a phase-lead filter, which enabled overcoming the \(\sim\)45 kHz limitation of the upper state lifetime of the Yb:KYW crystal. In this condition, a residual integrated phase noise of 300 mrad was achieved [0.1 Hz-1 MHz]. Frequency combs based on Yb DPSSLs in the 1 \(\upmu\)m range have become fairly popular in the last five years and have been the subject of several studies and developments. A research direction of particular interest has been the extension of combs operation to the range of gigahertz or multi-gigahertz repetition rate. High repetition rate lasers enable an easier access to the individual optical comb lines and an increased power per comb mode for a given average power that leads to an improved SNR in various applications. However, SESAM-modelocked DPSSLs have to overcome some challenges when increasing the repetition rate, as the decreased pulse energy may result in Q-switching instabilities [64]. A key achievement towards the self-referenced stabilization of gigahertz DPSSL combs was the first CEO detection realized by [PERSON] and co-workers in an Yb:KGW laser [19]. Using 6 W of pump power from a distributed Bragg reflector (DBR) tapered laser diode at 980 nm, more than 2 W of average power was emitted at 1042 nm in modelocked operation with a repetition rate of 1.0 GHz and a pulse duration of \(\sim\)290 fs based on SESAM-modelocking. The high output power of this oscillator enabled an octave-spanning SC to be generated in a PCF directly from the oscillator output, without any external amplification, which kept the system simple. However, the coherence of the generated SC spectrum was too low to detect a CEO beat, especially at the wavelengths used in the _f_-to-_2\(f\) interferometer. This was confirmed by numerical simulations that showed that a soliton order of around 13 was achieved for the pulses launched into a PCF from this oscillator. By reducing the pulse duration to around 100 fs using a passive fiber-compressor and using a lower average power of 1.1 W, the soliton order was reduced to five, enabling the detection of a CEO beat at 680 nm with an SNR of 27 dB (in 1 MHz RBW). The CEO frequency could be tuned by changing the current of the pump diode, but no stabilization was realized at this stage. A weakness of this first self-referenceable gigahertz DPSSL was the tapered pump diode.