Semiconductor laser diode was first developed in the 1970’s,with the advent of optical fiber, such LD radiation properties as brightness, directivity, narrow spectral width and coherence made them the best light sources for long haul fiber optic links. In response to improve the laser diodes quality, quantum-well and distributed-feedback laser diodes with extremely narrow spectral width have been developed. These laser diodes have become the most popular light sources for long distance transmitters. Vertical-cavity surface-emitting lasers (VCSEL) are among the latest development in the technology of optical fiber light sources.

WDM, dense WDM and high-density WDM technologies pose a new challenge for laser diodes designers, the channel separation in this technologies-measurement now in frequency not in wavelength-is today so small that a spectral width of 0.1nm in a laser diode is now unacceptable. Thus, as is true of the entire fiber optic communications systems, laser diode technology continues to evolve dynamically.