A directly modulated semiconductor laser whose optical output can be modulated by varying the transmittance of an end reflector of the laser cavity. In an example embodiment, the end reflector can be implemented using a lightwave circuit in which optical waveguides are arranged to form an optical interferometer. At least one of the optical waveguides may include a waveguide section configured to modulate the phase of an optical beam passing therethrough in response to an electrical radio-frequency drive signal in a manner that causes the transmittance and reflectance of the end reflector to be modulated accordingly. Advantageously, relatively high (e.g., >10 GHz) phase and/or amplitude modulation speeds of the optical output can be achieved in this manner to circumvent the inherent modulation-speed limitations of the laser's gain medium.