## 3 Results  
As a comparison, by replacing the SESAM with a dichroic mirror high-reflection coated in the 2600-3000 nm, lasing characteristics of the CW mode of operation were first evaluated. The threshold was \(\sim\)0.58 W, and 111 mW of 2.7 \(\upmu\)m laser output was achieved at the pump power of 7.5 W, as shown in Figure 2. With the SESAM replacing the cavity end mirror, the laser threshold was reached at the increased pump power of 0.62 W and laser oscillation in the QML mode is realized. At a pump power of 7.5 W, \(\sim\)92 mW of average output power was measured (see Figure 2), which corresponds to a conversion efficiency of 82.9% from the CW operation under the same operation condition. The measured pulse repetition frequency and widths of the Q-switched envelopes versus the incident pump power are shown in Figure 3. As expected, with the incident pump power increasing, the repetition frequency increased and the width of the Q-switched pulse envelope decreased. The repetition frequency of the Q-switched envelope increased from 5.1 to 23.8 kHz and the pulse width reduced from 2.7 to 1.4 \(\upmu\)s as the incident pump power increasing from 1.0 to 5.0 W. Output energies  
Figure 1: Diagram of the Er:Y\({}_{2}\)O\({}_{3}\) Q-switched and mode locked (QML) laser. HWP: half-wave plate; F1–F3: plane-convex lens; R\({}_{1}\)–R\({}_{3}\): concave mirrors with the reflectivity of \(>\)99.8% at 2650–2950 nm and the transmission of \(>\)97% at 965–980 nm; OC: output coupler. Figure 2: The average output power in continuous-wave (CW) and QML mode of operation. of the Q-switched pulse envelops at 5.0 W of incident pump power were estimated to be \(\sim\)3.11 \(\upmu\)J. At the pump power of 5 W, the oscilloscope trace of a typical pulse train and a single Q-switched pulse envelop of the QML laser are shown in Figure 4. It can be seen that the Er:Y\({}_{2}\)O\({}_{3}\) ceramic laser operated at a highly stable state and its pulse-to-pulse amplitude fluctuation for Q-switched envelops was estimated \(<\)5%. Mode-locking of \(>\)90% modulation depth could be observed for a wide range of pump powers from threshold to \(\sim\)5 W.  
QML pulse envelops with 100% modulation depth were obtained with further increasing the launched pump power. Oscilloscope trace of the mode locked pulse train lying underneath a typical Q-switched pulse envelop is shown in Figure 5. When the incident pump power was tuned to be 7.5 W, the repetition rate and pulse width of the Q-switched envelop were 29 kHz and 1.2 \(\upmu\)s, respectively. It is shown in Figure 5 that stable mode-locked pluses lying underneath a Q-switched envelope have the pulse-pulse separation of \(\sim\)7.7 ns, corresponding to the repetition rate of \(\sim\)130 MHz, matching well with the time of the resonator roundtrip. The estimated width (FWHM) of mode locked pulses was less than 1.43 ns limited by the response time of the 3 \(\upmu\)m detector we used (1.5 ns). Figure 6 is the radio frequency spectrum of the QML pulses at 7.5 W of incident pump power, showing a fundamental beat signal of 130 MHz (resolution bandwidth of 1 kHz) and the signal-to-noise ratio of 45 dB, indicating stable Q-switched mode locked operation of the laser. Figure 7 shows the output spectra in both CW and QML mode of operation. It shows that the laser wavelength is centered at 2726.5 nm, and the spectra bandwidths in QML and CW mode of operation are \(\sim\)0.36 nm and \(\sim\)0.2 nm, respectively, which indicates that there are more longitudinal modes oscillate in the QML operation regime. Figure 4: (a) Oscilloscope traces of a stable QML pulses train at 23.8 kHz and (b) expanded temporal shape of a single QML pulse. Figure 3: The repetition frequency and output pulse width (FWHM) of the Q-switched envelopes.