The main optical output power limitation in high power laser diodes is the catastrophic optical mirror damage (COMD) initiated by facet heating due to optical absorption, which limits the reliable power and lifetime of a single laser. Facet heating correlated with current injection near laser facets can be reduced by unpumped window structure. However, the high-power laser slope efficiency drops as the length of the window increases. In this work, separately pumped window (SPW) method is proposed and experimentally demonstrated to significantly reduce the facet temperature of the semiconductor lasers without compromising their performance. We used 5-mm long high-power laser diodes and compared its performance and facet temperature to the devices integrated with SPW facet sections, which are electrically isolated from the laser section. The slope efficiencies of the lasers with SPW and that of 5-mm lasers without SPW are comparable when SPW is pumped at its transparency current, illustrating that SPW integrated lasers preserve their slope efficiency. As the window pumping current increases, the threshold current of the laser with SPW decreases when the SPW approaches transparency. The facet temperature rise (AT) of the lasers were measured by the thermoreflectance method. The AT measured at waveguide regions of lasers was shown to be reduced by 42% implementing SPW region to conventional lasers. Therefore, SPW technique was shown to be a promising approach to increase the COMD level of the high-power laser diodes and it opens up a new avenue for reliable semiconductor laser operation at very high output power levels.