We present a semi-analytical model for studying the phase-amplitude coupling (a-factor) in quantum dot (QD) semiconductor lasers, which takes into account the influence of carrier populations in the excited state and in the two-dimensional carrier reservoir on the refractive index change. Calculations of the a-factor based on the amplified spontaneous emission method and on the "FM/AM" technique are both investigated. It is shown that the a-factor of a QD laser strongly depends on the energy sepa-ration between the ground state and the off-resonant states. Through band structure engineering, the a-factor can be reduced by enlarging this energy separation. V C 2014 AIP Publishing LLC.