# Self\(-\)Mode\(-\)Locked \(2-\)um GaSb\(-\)Based Optically Pumped Semiconductor Disk Laser
## 3 Results and Discussion  
First, to measure the temporal characteristics of the self\(-\)mode\(-\)locked pulses, a high\(-\)speed 2 \(\upmu\)m InGaAs photodetector (EOT, ET\(-\)5000, 12.5 GHz, Walnut, CA, USA) combined with a 59 GHz digital storage oscilloscope (Tektronix, DPO75902 SX, Beaverton, OR, USA) was used. Figure 3a,b show the typical time trace of our SML GaSb\(-\)based SDLs measured at the cavity length of \(\sim\)365 mm and \(\sim\)900 mm, respectively. Their average output power is 168 mW. The pulse separation of \(\sim\)2.47 ns and \(\sim\)6.03 ns corresponds right to the round \(-\)trip time of their cavity length. The pulse train with \(\sim\)404 MHz and \(\sim\)166 MHz repetition rate can be calculated. A representative 1 us long\(-\)span pulse train is shown in the insertion of Figure 3a,b. The intensity fluctuation of the mode\(-\)locked pulses in Figure 3a is obviously more stable than in Figure 3b. This experimental result is consistent with our previous prediction of the cavity length. Based on the DDE model, we also calculated the pulse train with a 365 mm and a 900 mm cavity length. As shown in Figure 3c,d, the measured pulse repetition frequency is consistent with the calculated result. The smaller error within tolerance may be due to the error in device installation and adjustment. Subsequent experiment results were measured under a relatively stable cavity length of \(\sim\)365 mm. The mode\(-\)locked pulse duration was also directly measured by the high\(-\)speed photodetector and oscilloscope. The pulse duration was \(\sim\)255.48 ps with a near\(-\)Lorentz shape, as shown in Figure 4a. The experimentally measured pulse duration was much larger than our simulation results because we did not consider the nonlinear effects, such  
Figure 3: Output pulse train with (**a**) 365 mm cavity length, 404 MHz repetition rate, (**b**) 900 mm cavity length, and 166 MHz repetition rate. Insets: the pulse train in a 1 μs time segment. The simulated pulse train of the SML SDLs with a cavity length of (**c**) 365 mm, (**d**) 900 mm. as group delay dispersion, in our calculations. A strong nonlinear has been observed in GaSb\(-\)based SESAMs which have a similar multi\(-\)QW epitaxial structure to GaSb SDLs [58]. Positive intracavity roundtrip group delay dispersion of 2030 nm GaSb SDLs is on the order of 1000 fs\({}^{2}\), which has an intense negative impact on the pulse duration and the laser performance [21]. Considering the nonlinear effects introduced by quartz F\(-\)P etalon, SiC heat spreader, and GaSb gain chip, it is difficult to accurately simulate the pulse duration of SML GaSb SDLs. The spectral filtering element in the resonant cavity also affects the pulse duration. Further study is required to analyze these in more detail. The radio frequency (RF) spectrum was measured by a mixed\(-\)domain oscilloscope with 1 GHz bandwidth (Tektronix, MPO3104, Beaverton, OR, USA). At the repetition rate of \(\sim\)404 MHz, the RF spectrum shows a clean peak without side peaks, as shown in Figure 4b. This reveals the GaSb SDLs operating in a stable SML mode and a near\(-\)fundamental transverse mode. As shown in the insert of Figure 4b, the 1 GHz wide\(-\)span RF spectrum measurement indicated the single\(-\)pulse operation of the mode\(-\)locked GaSb SDLs. The laser output power characteristics were measured with a power meter (Thorlabs, S425C\(-\)L, PM100D, Newton, NJ, USA). Without the aperture and the F\(-\)P etalon, the laser operated in CW mode could reach 560 mW with the incident pump power of 10.7 W, as shown in Figure 5a. This value is lower than our previous work [59], mainly because we used a longer cavity and an output coupling mirror with higher transmittance. The laser spectra were measured using a Fourier transform optical spectrum analyzer (Thorlabs, OSA207C, Newton, NJ, USA), as shown in the insert of Figure 5. Without other components in the cavity, the uncoated SiC heat spreader on the chip surface acted as an F\(-\)P etalon.