A new technique for sinusoidal intensity modulation of light at microwave frequencies using a frequency modulated semiconductor laser and an interferometer is described. This technique can be more efficient than direct amplitude modulation and, since sidebands at harmonics of the modulation frequency are generated, is capable of reaching higher frequencies than direct modulation. The highest observed frequency (18 GHz) was generated by direct modulation of a laser diode at 9 GHz, and was limited by the bandwidth of available detectors. Frequencies in excess of 100 GHz can be obtained using suitable detectors.