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Presents a theoretical investigation of the noise behavior of a semiconductor laser operating under relatively strong light injection. Equations have been presented to describe the noise effect through the calculation of the relative intensity noise as well as the carrier-to-noise ratio available at the receiving end. Illustrative examples are given, showing the impact of the master and slave laser bias currents. Also, the injection-locked and free-running operation regimes have been comparatively analyzed. The results show how the noise characteristics are affected by optical injection and, consequently, how the operating conditions must be chosen to reduce this effect. In particular, it is shown, in agreement with previous works, that the master laser emission noise will essentially take the lead. As a result, to improve the noise behavior by injection locking a solitary laser, the use of a low-noise master laser is required. To make it easy to apply the present results to any laser diode under the stable-locking condition, the necessary relations are explicitly given before specifying the parameters of simulation.