# Continuous-Wave and Mode-Locked Operation of an In-Band Pumped Tm,Ho,LuCaGdAlO4 Laser
## 4 Mode-Locked (ML) Laser Results  
To study mode-locking of the Tm,Ho,Lu:CALGO laser, the sample was mounted in the same way as for the CW laser studies, i.e., with its \(c\)-axis vertical, however, in order to utilize the full pump power available (5.5 W unpolarized) the polarizer and attenuator in Figure 2 were removed. The modified experimental set up shown in Figure 6 includes a SEmiconductor Saturable Absorber Mirror (SESAM), used as a rear reflector, and three dispersive mirrors (DM) for realization of a soliton like regime at negative overall cavity group delay dispersion (GDD). All DMs were specified with a GDD of \(-125\) fs\({}^{2}\) per bounce. Four bounces on each of the plane DM\({}_{1}\) and DM\({}_{2}\) and two bounces on the curved (RoC = \(-100\) mm) DM\({}_{3}\) per cavity round trip provide a total negative GDD of \(-1250\) fs\({}^{2}\). DM\({}_{3}\) simultaneously serves to create a second cavity waist with a sufficiently small size for saturation of the SESAM absorption. The estimated radii of this second beam waist on the SESAM were \(w_{\rm L}\) = 114 and 149 \(\upmu\)m in the sagittal and tangential planes, respectively. In this cavity configuration, and using the full pump power, we measured the output power as a reference with the \(T_{\rm OC}\) = 0.2% OC and the SESAM substituted by a plane totally reflecting mirror. The output power was 360 mW for an estimated absorbed power of 3.84 W distributed between the two polarizations. Different GaSb based SESAMs with InGaAsSb quantum wells [25], OCs, and GDD values (by decreasing the number of bounces in Figure 6 or using DMs with higher negative GDD) were studied. The optimum SESAM for this laser in terms of stable ML operation and broad output spectra turned out to be one designed for the central wavelength of 2060 nm with two InGaAsSb quantum wells (thickness: 8.5 nm) and a 50 nm cap layer  
Figure 5: **(a,b)** spectral tuning curves in the CW regime obtained with a Lyot filter at 2.5 W incident pump power for \(T_{\rm OC}\) = 0.2% and 0.5%. The laser is \(\sigma\)-polarized due to the insertion of the Lyot filter. Figure 6: Experimental set up of the ML Tm,Ho,Lu:CALGO laser. Pump laser: Raman shifted Er Fiber Laser (FL); L\({}_{1}\): AR coated plano-convex lens (\(f\) = 75 mm); M\({}_{1}\)-M\({}_{2}\): concave dichroic mirrors (RoC = \(-100\) mm); DM\({}_{1}\) (flat), M\({}_{2}\) (flat) and M\({}_{3}\) (RoC = \(-100\) mm): dispersive mirrors with GDD = \(-125\) fs\({}^{2}\) per bounce; OC: wedged output coupler; SESAM: SEmiconductor Saturable Absorber Mirror. without any AR coating [26]. The recovery measurements of the initial absorption by the pump-probe method at 2040 nm with 180-fs pulses indicated biexponential decay with intraband and interband relaxation times of 0.3 and 20 ps, respectively. The saturation fluence of such SESAMs is of the order of 10 ul/cm\({}^{2}\), the modulation depth is 0.23% and the non-saturable losses are 0.12%. Stable steady-state ML operation, though not self-starting, could be achieved only with the 0.2% OC. Higher \(T\)oc resulted in narrow output spectra at the optimum round trip GDD = \(-\)1250 fs\({}^{2}\). With the \(T\)oc = 0.2% OC the ML laser delivered a maximum average output power of 148 mW at the same absorbed pump power level of 3.84 W as in the CW reference measurement. Although the reduction of the output level compared to the CW regime is more than two times, at this low OC transmission, this confirms the relatively low (non-saturable) insertion losses of this SESAM. The increased losses and possibly the much broader spectral extend in the ML regime resulted in switching the output polarization of this laser to \(\sigma\). This is consistent with our previous observations because the resulting central wavelength was below 2075 nm [23]. The optical spectrum of the ML Tm,Ho,Lu:CALGO laser shown in Figure 7a was centered at 2071.5 nm with a sech\({}^{2}\)-fitted spectral FWHM of 21.5 nm.