# Self\(-\)Mode\(-\)Locked \(2-\)um GaSb\(-\)Based Optically Pumped Semiconductor Disk Laser
## 1 Introduction  
Optical pumped semiconductor disk lasers (SDLs), also known as vertical\(-\)external\(-\)cavity surface\(-\)emitting lasers (VECSELs), are versatile lasers in applications requiring both high power and good beam quality at a variety of wavelengths [1; 2; 3; 4]. Gallium\(-\)antmonide (GaSb) material systems can be used to fabricate SDLs that operate at wavelengths of 2-4 um [5; 6; 7; 8]. High\(-\)pulse\(-\)energy lasers with a short pulse at the mid\(-\)infrared wavelength region have particularly important applications for time\(-\)resolved molecular spectroscopy, remote sensing, infrared countermeasures, medical, and optical free\(-\)space communications [9; 10; 11; 12; 13]. At present, the methods of generating short pulses at mid-infrared wavelengths [14] mainly include Kerr\(-\)lens mode\(-\)locking (KML), nonlinear polarization rotation (NPR), and intracavity semiconductor saturable absorption mirror (SESAM). GaSb\(-\)based materials are more used as SESAMs with \(\sim\)2 um wavelength in applications [15; 16; 17; 18]. Studies on mode\(-\)locking of GaSb\(-\)based disk lasers are relatively sparse [19; 20; 21]. The main challenge to achieving mode\(-\)locking operation in the GaSb\(-\)based SDLs is the high quality and reliable epitaxial growth of GaSb\(-\)based SDLs and SESAMs wafers. More details of the passively mode\(-\)locked GaSb\(-\)based SDLs with SESAM can be found in Table 1. Obviously, the lower performance index means that there is still a great possibility of progress in the research of this field.  
One intriguing phenomenon that has been observed in near\(-\)infrared GaAs\(-\)based SDLs is so\(-\)called self\(-\)mode\(-\)locking (SML) [22; 23; 24; 25; 26], which forms a stable mode\(-\)locked pulse laser output without an intracavity SESAM. The origin of self\(-\)mode\(-\)locking is conjectured to result from the nonlinear Kerr lens effect from the semiconductor epitaxial structure itself [22]. In this way, the SML SDLs can more easily realize mode\(-\)locked pulse operation with simplified system design and broader spectrum coverage. The physical mechanism of SML was described as Kerr lens mode\(-\)locking has been widely studied insemiconductor materials such as GaAs and InP [27; 28; 29; 30; 31], and soli\(-\)state crystal lasers such as Ti: Sapphire [32; 33; 34; 35]. The strong self\(-\)focusing of the laser beam combined with either a hard aperture or a soft aperture in Kerr medium is employed to produce a self\(-\)amplitude modulation [36]. In addition to mode\(-\)locked lasers, devices developed based on this mechanism also include wavelength/frequency division multiplexing [37; 38; 39], optical frequency comb [40; 41; 42; 43], supercontinuum generation [44; 45; 46], and so on. However, there are currently no relevant reports on the Kerr effect in GaSb\(-\)based gain materials, nor have there been corresponding reports on self\(-\)mode\(-\)locked GaSb\(-\)based lasers.  
Inspired by the self\(-\)mode\(-\)locking effect observed in GaAs\(-\)based SDLs [22; 23], this paper validated and implemented continuous\(-\)wave (CW) mode\(-\)locked GaSb\(-\)based SDLs employing self\(-\)mode\(-\)locking. Based on the pulse dynamics simulated by delay differential equations (DDE), we designed a Z\(-\)shaped cavity with a long cavity length for SML GaSb\(-\)based SDLs. Then, we systematically measured the pulse characteristics of the SML GaSb\(-\)based SDLs.