The　availability　of　ultra-short　laser　pulse　sources　such　as　picosecond　lasers　makes　it　possible　to　improve　the　quality　(e.g.　heat　affected　zone　＜10　mu　m)　in　laser　machining　engineering　materials　such　as　metals　and　carbon　fibre　reinforced　polymer　(CFRP)　composites.　However,　the　processing　rate　and　the　machining　depths　of　such　lasers　are　very　low　and　thus　demands　better　understanding　of　process　limitations　and　machining　strategy.　This　paper　identifies　a　self-limiting　effect　in　high　power　picosecond　laser　machining　and　compares　single　and　multiple　parallel　pass　strategies　during　drilling/machining　of　CFRP　composite　and　mild　steel　sheets.　Electromagnetic　(EM)　simulation　was　performed　to　understand　the　taper　formations　and　the　self-limiting　effects　in　ps-pulsed　laser　drilling　and　to　determine　the　maximum　cutting/drilling　depths.　The　work　shows　that　the　machining　depth　and　material　removal　rate　were　significantly　improved　by　increasing　the　number　of　parallel　passes　under　the　same　energy　delivery　and　machining　time.　A　new　machining　mechanism　is　proposed.　This　applies　to　both　the　CFRP　and　the　mild　steel　investigated.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.