It　is　used　to　know　that　the　threshold　current　of　laser　increases　with　respect　to　the　length　of　cavity.　However,　by　undoping　the　active　region　and　adopting　linear　graded　index(GRIN)　quantum　well　laser　structure,　it　was　found　that　the　threshold　current　of　single　quantum　well　laser　stays　constant　for　a　large　range　of　the　length　of　cavity.　The　leakage　current　and　the　nonradiative　recombination　are　reduced　due　to　the　change　of　the　setting;　as　a　result,　the　internal　loss　is　minimized.　The　threshold　gain　is　mainly　depending　on　cavity　facet　loss;　therefore,　the　threshold　current　stays　constant　for　a　large　range　of　the　length　of　cavity.