Direct modulation of long-cavity semiconductor lasers

The application of a high-speed signal via the drive current to a long-cavity semiconductor laser (cavity length >similar to 1 cm) differs from the short-cavity case in that the variation of the optical field within one round-trip time is not negligible. We theoretically investigate the modulation response for a semiconductor laser of arbitrary length and relate it to experiments on the multifrequency waveguide grating router laser, a long-cavity laser. We find that the largest modulation bandwidth Is achieved by having the modulated amplifier and the desired output as far apart as possible, the modulation bandwidth can be further increased by simple electronic precompensation, and because of fast nonlinearities the broadband intensity modulation of long-cavity lasers exhibits substantially less chirp than short-cavity lasers, the chirp essentially equal to that of the amplifiers. Finally, we demonstrate the direct modulation of three channels simultaneously in a multifrequency laser at 1.24 Gb/s each.