A laser diode sub-assembly includes a plurality of laser diode bars, electrically conductive spacers and electrically insulative heat spreaders. The spacers and laser diode bars are arranged by alternating to affix each other via the solder preforms and aligned on the front surfaces, while the back surfaces of the spacers affix the top metalized surfaces of the heat spreaders via the same type of solder preforms. Specially designed fixtures can hold all the components in positions and go through the reflowing. The subassembly can be formed by one heating step. The laser diode stack can be formed by soldering the bottom metalized surfaces of the heat spreaders, via a lower melting temperature solder preform, to a heat sink.