# Self\(-\)Mode\(-\)Locked \(2-\)um GaSb\(-\)Based Optically Pumped Semiconductor Disk Laser
## 2 Experiment Design and Setup
When the cavity length is extended to 600 mm and 800 mm, a single pulse will be irregularly broadened and the pulse waveform will become irregular too. This instability may be due to the long cavity length increasing the round\(-\)trip time \(\tau_{r}\) and approaching the gain relaxation time, and the long cavity length will also increase the loss of the resonator. If we want to offset the loss fluctuation caused by this cavity length, we can only increase \(g_{0}\) by increasing the pump power continuously. However, it is difficult to achieve SDLs in the GaSb which have an obvious upper heat dissipation limit [8]. Therefore, we believe that it is more reliable to select a cavity length of less than 400 mm for verification of self\(-\)mode\(-\)locking in GaSb\(-\)based SDLs. A Te\(-\)doped 2\({}^{\prime\prime}\) (001) GaSb wafer was grown in a VECCO Gen\(-\)II solid source molecular beam epitaxy system at \(\sim\)430 \({}^{\circ}\)C for the SDLs chip structure. First, a buffer layer of GaSb was grown on the substrate. A Distributed Bragg Reflector (DBR) comprising 21.5 pairs of GaSb/AlAs\({}_{0.08}\)Sb\({}_{0.92}\) quarter wavelength layers follows. The DBR has a broadband reflectance of \(\sim\)99.9%, centered at a wavelength of 2 \(\upmu\)m. The active regions were grown directly on top of DBRs with resonant periodic gains (RPG) and symmetric structures. Ten In\({}_{0.2}\)Ga\({}_{0.8}\)Sb QWs with a thickness of \(\sim\)10 nm, arranged in five groups of two QWs each, make up the active region. The QWs are separated by 20 nm thick layers of GaSb and equally enclosed by \(\lambda\)/2 GaSb barrier layers. In comparison with Al\({}_{0.3}\)Ga\({}_{0.7}\)As\({}_{0.02}\)Sb\({}_{0.98}\), the GaSb barrier layer can provide relatively high thermal conductivity. In addition, the pump source with a wavelength of 1470 nm can be used to reduce quantum defects, therefore reducing heat generation and improving laser performance. An Al\({}_{0.8}\)Ga\({}_{0.2}\)As\({}_{0.03}\)Sb\({}_{0.97}\) confinement layer was grown on top of the gain region to prevent carrier nonradiative recombination. The surface was protected from oxidation with a thin GaSb cap layer. We cleaved the chip into 3 \(\times\) 3 mm\({}^{2}\) pieces, which were then liquid\(-\)capillary\(-\)bonded [54] to an uncoated 400 \(\upmu\)m thick 6H\(-\)SiC intracavity heat spreader of 5 \(\times\) 5 mm\({}^{2}\) dimensions. The SDLs chip package was then mounted on a copper heat sink cooled by Peltier elements. The heat sink temperature was controlled at 10 \(\pm\) 1 \({}^{\circ}\)C.  
The gain chip was set up in a Z\(-\)shaped laser resonator, as shown in Figure 2a. The cavity was formed by the DBR, curved high\(-\) reflectivity mirror M\({}_{1}\) with a radius of curvature \(r\) = 200 mm, plane high\(-\)reflectivity mirror M\({}_{2}\), and output coupler M\({}_{3}\) with a reflectivity \(R\) = 95% at 2 \(\upmu\)m wavelength. The pump laser we used was a DILAS M1F4S22 with an emission wavelength of 1470 nm. The 400 \(\upmu\)m core diameter fiber\(-\)coupled pump beam was focused into a spot on the SDLs chip with a diameter of \(\sim\)400 \(\upmu\)m. The incident angle of the pump beam was \(\sim\)30\({}^{\circ}\). The reflectivity of the incident pump laser was measured to be 48 \(\pm\) 2% on the SiC-air interface. To make the transmission mode in the cavity close to circular, the folding angle of the Z\(-\)shaped cavity should be as small as possible. While ensuring that the resonator is in a steady state, it is also necessary to consider that the spatial positions between mirrors cannot conflict with each other. Here, we set \(L_{1}\) to 110 mm, which is slightly larger than half of the radius curvature of M\({}_{1}\). Additionally, the length of \(L_{2}\) was set as 75 mm. For spectral control, an F\(-\)P etalon was inserted into the cavity. Based on the configuration of the resonator, we used the transmission matrix to calculate the fundamental transverse mode size in the cavity, as shown in Figure 2c.