A single In0.49Ga0.51P quantum well sandwiched by In0.49Ga0.51P/In-0.49(Ga0.6Al0.4)(0.51)P digital alloy structures was investigated in terms of optical modal gain, where gain saturation effects were also considered for both changes in wavelength and stripe length by using a modal gain contour map analysis. We found the gain threshold density is considerably lower by an order of magnitude when compared to the Mott density (similar to 2 x 10(12) cm(-2)), which can be attributed to a carrier-harvesting effect through the mini-band of the digital alloy. (C) 2014 Elsevier B.V. All rights reserved.