Injection locking of a semiconductor laser can significantly improve the broadband modulation characteristics. By adjusting the frequency offset between the master laser and the slave laser, improved modulation bandwidth or flatness of the modulation response can be emphasized. The improved modulation characteristics are accompanied by reduced broadband relative intensity noise. Our calculations predict that the parasitic-free modulation bandwidths can be enhanced above the theoretical limit for the free-running laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>