A novel measurement scheme is presented for injection locking experiments with semiconductor lasers. In the method, the light generated by the master laser travels through an optical fiber before being injected into the cavity of the slave laser. Using the reciprocity principle, it is possible to control accurately the absolute light power injected from the master laser in the cavity of the slave laser once the power coupling coefficient between the slave waveguide mode and the fiber mode is determined. In addition, the polarization can be fully controlled despite the birefringence of the optical fiber. The scheme may be easily adapted for experiments with passive optical feedback.