## 3 Self-Referenced DPSSLs with Bulk Gain Material
### One-Micrometer Ytterbium-Doped Lasers
In addition to the higher RIN of these pump diodes compared to a single-transverse-mode pumping scheme that was previously discussed, spectral fluctuations of the pump source can also induce strong _f_CEO noise for pump lasers that do not incorporate any wavelength stabilizing element. For instance, the Yb:CALGO gain medium exhibits a quite narrow absorption peak at 980 nm [70]. Any wavelength fluctuation of the pump source is thus converted in fluctuations of the absorbed pump power that in turn result in _f_CEO frequency noise. Hence, a drastic improvement of the CEO stabilization was demonstrated in the aforementioned Yb:CALGO gigahertz femtosecond laser with the use of a passively wavelength-stabilized pump diode [57]. Using two different versions of the same spatially-multimode laser diode (LIMO 60-F100-DL980 from Lissotschenko Mikrooptik GmbH, Dortmund, Germany), a strong difference in the free-running CEO beat linewidth was observed. The early results previously mentioned were obtained with the simplest version of the pump diode without any wavelength-stabilizing element. The resulting pump spectrum was fluctuating in a range of about 7 nm, much larger than the absorption peak of Yb:CALGO. On the other hand, a more evolved version of the pump diode that stabilizes the wavelength using a volume holographic grating (VHG) resulted in a stable pump spectrum with a narrow width of only 0.1 nm centered at 980.4 nm, well matched to the Yb:CALGO absorption peak. In addition to its direct effect on the pump spectrum, the VHG stabilization also showed a strong improvement of the RIN of the pump source, with a reduction of 20 dB at all frequencies above 100 Hz. All in all, these two effects resulted in a drastic improvement of the free-running CEO beat linewidth, with a reduction from 1.7 MHz FWHM with the unstabilized pump source down to 170 kHz with the VHG-stabilized pump in an observation time necessary to average 100 spectra recorded on a microwave spectrum analyzer. As a result, a tight CEO lock was achieved by feedback to the pump current with a loop bandwidth slightly lower than 200 kHz. The residual integrated phase noise of only 339 mrad is the lowest value obtained from a gigahertz DPSSL. These results clearly showed the benefit of a wavelength-stabilized pump source, but also demonstrate that a tight CEO lock can be achieved in DPSSLs with repetition rates in the gigahertz range using cost-efficient commercial multi-transverse-mode pump diodes. Beside their potential for frequency combs with large mode spacing, Yb-DPSSLs are also attractive for very low-noise operation at lower repetition rates. A fully-stabilized frequency comb with a low residual integrated phase noise of the CEO beat of 83 mrad [1 Hz-1 MHz] has been reported by [PERSON] _et al._[29]. A commercial Origami-10 modelocked laser (Onefive GmbH, Zurich, Switzerland) emitting short pulses of 111 fs duration at a center wavelength of 1047 nm and at a repetition rate of 100 MHz was used to generate a SC spectrum in a 50 cm long PCF. A CEO beat with 40 dB SNR in a RBW of 91 kHz and a narrow free-running linewidth of \(\sim\)15 kHz (observed in a sweep time of 461 ms) was detected in an _f_-to-_2\(f\) interferometer. These noise figures for a free-running comb resemble more the properties of a modelocked DPSSL than of a fiber laser, but the exact configuration of the Origami-10 femtosecond laser is unknown and was not disclosed in [29]. Nevertheless, this laser has been included in the present discussion of Yb-doped DPSSLs and its properties are also listed in Table 1. A stabilization loop with feedback applied to the current of the pump diode with a loop bandwidth of about 130 kHz was realized, and the contribution of the locked _f_CEO to the instability of the optical carrier corresponds to a fractional frequency stability of \(<\)10\({}^{-17}\) at 1 s averaging time (with 500 Hz noise equivalent bandwidth). A demanding application for frequency combs in the future is their operation in a space environment.