Experimental measurements of the injection lock band of an angled-grating distributed feedback laser have been performed systematically as a function of the unlocked slave laser power. Fringe visibility measurements were used to ascertain the quality of the lock between the master and slave laser as a function of master coupling level and frequency detuning. Experimental data show increasing spectral instabilities under optical injection as the slave laser power is increased, resulting in the stable portion of the lock band decreasing in width. (C) 2000 American Institute of Physics. [S0003-6951(00)04623-4].