## 3 Self-Referenced DPSSLs with Bulk Gain Material
### 1.5-\(\mathrm{\SIUnitSymbolMicro m}\) Erbium-Doped Lasers  
Commercial and laboratory-grade Er:fiber frequency combs are used nowadays in a variety of applications in the 1.5 \(\mathrm{\SIUnitSymbolMicro m}\) spectral range, such as low-noise microwave generation, high-resolution spectroscopy or distant comparison of optical atomic clocks via the optical fiber network. DPSSL combs are attractive for all these applications as a result of their low-noise potential, as demonstrated in the previous sub-section for Yb-based DPSSLs at 1 \(\mathrm{\SIUnitSymbolMicro m}\). However, very few DPSSL combs have been demonstrated so far in the important 1.5 \(\mathrm{\SIUnitSymbolMicro m}\) telecom range. The first self-referenceable modelocked DPSSL was reported by [PERSON] _et al._[72] based on an Er:Yb:glass oscillator (ERGO) pumped by a single-transverse-mode wavelength-stabilized telecom-grade diode at 976 nm. The SESAM-modelocked laser emitted 110 mW of average power in 170 fs at a repetition rate of 75 MHz. A strong CEO beat with an SNR close to 50 dB (100 kHz RBW) was detected after SC generation in a polarization-maintaining HNLF and a standard _f_-to-2\(f\) interferometer. Owing to the small overall optical losses in the cavity (less than 3%) that resulted in a high Q-factor resonator, this laser demonstrated a record narrow-linewidth CEO beat with only 4 kHz FWHM. A tight phase lock of _f_\({}_{\mathrm{CEO}}\) was achieved by feedback to the pump current with a moderate feedback bandwidth of around 5.5 kHz and a residual integrated phase noise of 0.72 rad (0.1 Hz-100 kHz) [27]. Full stabilization of this frequency comb to an H-maser was demonstrated with a fractional frequency stability of a comb line at 1558 nm of \(\sim\)4\(\times\)10\({}^{-13}\) at 1 s, assessed from the beat signal between the comb line and a cavity-stabilized laser and limited by the stability of the RF reference used to lock _f_\({}_{\mathrm{rep}}\)[28]. This DPSSL comb was also stabilized to an optical reference made of an ultra-stable laser for low-noise microwave generation and showed its benefit compared to a commercial Er:fiber comb in terms of CEO contribution to the phase noise [73]. The CEO stabilization in this ERGO laser was improved by the use of a faster actuator for _f_CEO control that enabled extending the feedback bandwidth beyond the limitation imposed by the upper-state lifetime of the gain medium. [PERSON] _et al._, demonstrated a new tuning and stabilization mechanism of _f_CEO by opto-optical modulation (OOM) of the same SESAM as already used to initiate self-starting modelocking in this laser using a continuous wave pump laser at 812 nm [44]. The method combines high feedback bandwidth with low loss, low nonlinearity, and low dispersion, which makes it power-scalable and very attractive for high power oscillators such as TDLs. A drastic improvement of the stabilized CEO beat in the ERGO comb was achieved compared to pump current modulation, in particular a fractional frequency instability of _f_CEO reduced by a factor of four and a residual integrated phase noise of only 63 mrad (1 Hz-100 kHz), or even less than 30 mrad when technical noise at 50 Hz and harmonics was removed. This integrated phase noise is the lowest value reported so far for any DPSSL comb to the best of our knowledge and clearly proves the capability of DPSSL combs for low-noise operation. A comparison between different approaches for _f_CEO stabilization in a frequency comb generated from a similar Er:Yb:glass DPSSL was reported by [PERSON] and [PERSON] [59]. The output spectrum of a commercial Origami-15 Er:Yb:glass DPSSL (Onefive GmbH, Zurich, Switzerland) with 190 fs transform-limited pulses at 1560 nm and 100 MHz repetition rate with an average output power of 115 mW was spectrally broadened in a HNLF. A CEO beat was then detected in an _f_-to-_2\(f\) interferometer.