# SESAM Mode-Locked Yb:Ca3 Gd2(BO3)4 Femtosecond Laser
###### Abstract  
We report on the first passively mode-locked femtosecond-laser operation of a disordered Yb:Ca3 Gd2(BO3)4 crystal using a SEmiconductor Saturable Absorber Mirror (SESAM). Pumping with a single-transverse mode fiber-coupled laser diode at 976 nm, nearly Fourier-transform-limited pulses as short as 96 fs are generated at 1045 nm with an average output power of 205 mW and a pulse repetition rate of ~67.3 MHz. In the continuous-wave regime, high slope efficiency up to 59.2% and low laser thresholds down to 25 mW are obtained. Continuous wavelength tuning between 1006-1074 nm (a tuning range of 68 nm) is demonstrated. Yb:Ca3 Gd2(BO3)4 crystals are promising for the development of ultrafast lasers at ~1 \(\upmu\)m.  
ultrafast laser; solid-state laser; ytterbium laser; mode-locking +
Footnote †: journal: Applied  
and attractive nonlinear optical properties making them suitable for, e.g., self-frequency doubling. Monoclinic calcium RE oxoborates Yb:YCa\({}_{4}\)O(BO\({}_{3}\))\({}_{3}\) are a good example of such crystals that exhibit local structure disorder leading to smooth and broad gain profiles extending up to 1.1 \(\upmu\)m. Such advantages make them very suitable for high-power operation in the continuous-wave (CW) regime [6; 7; 8], as well as for the design of broadband, widely tunable and mode-locked (ML) lasers operating at \(\sim\)1 \(\upmu\)m [9; 10]. A diode-pumped Yb:GdCa\({}_{4}\)O(BO\({}_{3}\))\({}_{3}\) (abbreviated Yb:GdCOB) laser ML by a SESAM generated 90 fs pulses at \(\sim\)1046 nm, corresponding to an average output power of 40 mW at a pulse repetition rate of 100 MHz [10].  
Recently, another calcium double borate crystal family with the chemical formula Ca\({}_{3}\)RE\({}_{2}\)(BO\({}_{3}\))\({}_{4}\), where RE = Y, Lu, Gd and La, attracted attention for Yb\({}^{3+}\) doping [11; 12; 13; 14]. These crystals belong to the orthorhombic class with the space group _Pnma_ and they are structurally disordered. Thus, a Yb\({}^{3+}\)-doped Ca\({}_{3}\)Gd\({}_{2}\)(BO\({}_{3}\))\({}_{4}\) crystal (abbreviated Yb:GdCB) with high optical quality has been successfully grown with the Czochralski method and laser performance in the CW regime was characterized [11; 15; 16]. In the GdCB structure, the Ca\({}^{2+}\) and Gd\({}^{3+}\) cations statistically occupy three non-equivalent crystallographic sites, forming M-oxygen distorted polyhedrons [17] (the dopant Yb\({}^{3+}\) ions are expected to replace the Gd\({}^{3+}\) ones). This leads to a \"glassy-like\" spectroscopic behaviour of Yb:GdCB crystals at the expense of a relatively low thermal conductivity of 0.92 W/mK at room temperature [18]. It features an extremely broad, flat and smooth gain profile which is very suitable for ultrafast pulse generation from a mode-locked laser. Pumping with a multi-transverse mode fiber-coupled laser diode at 976 nm, a maximum CW output power of 1.4 W was obtained at \(\sim\)1060 nm with a relatively low slope efficiency of 23.7% and high laser threshold of 900 mW [16]. Self-Q switched operation was also reported [19]. However, there are no reports on passively ML operation of a Yb:GdCB laser to date.  
In the present work, we report on the laser performance of the disordered Yb:GdCB crystal in the CW and the passively ML regimes. Pumping with a single-transverse mode fiber-coupled laser diode, nearly Fourier-transform-limited pulses as short as 96 fs were generated using a SESAM. To the best of our knowledge, this is the first report on ML operation of the Yb:GdCB crystal.