We present a comprehensive analytical theoretical model for the relative intensity noise (RIN) spectrum of integrated semiconductor quantum-well (QW) lasers under injection-locking. We use a novel setup by employing an integrated electroabsorption modulator-laser (EML) to measure the RIN of the injection-locked distributed feedback (DFB) laser, where the modulator section is used as a photodetector. The EML has an anti-reflection coating on the laser side, so that an injection light from an external master laser can be coupled effectively into the laser section. This scheme simplifies the setup and reduces the alignment loss between discrete optical components. Experimental data indicates that the injection-locking technique can reduce the RIN noise floor and increase the relaxation frequency of the laser. We also compare the RIN spectra of the free-running laser with the injection-locked laser and show an increase of the relaxation frequency from 3.7 GHz (free-running) to 11.3 GHz (injection-locked). By fitting the experimental data using our model, we show very good agreement between our data and theory. Our model considers the optical confinement factor of photons and carriers for quantum-well structure lasers. We also improve the injection-locking RIN model by including the gain saturation from the master laser noise. (C) 2008 Elsevier Ltd. All rights reserved.