Recent progress in long-wavelength light sources and photodetectors for optical fiber communications

InGaAsP/InP double heterostructure laser diodes and light-emitting diodes (LED’s) are primary sources to be used for lightwave communication systems in the 1.3 to 1.6 μm wavelength region. For use in multimode optical fiber systems near 1.3 μm wave-length, where the chromatic dispersion is minimal, LED’s are attractive because they utilize relatively simple drive circuits without need for feedback to control output power and are capable of operating over a wide range of temperatures with extremely good reliability. Significant improvement has been made in LED-to-fiber coupling in recent years. With an integral lens design, the small-area high-radiance surface emitting LED can now launch 50 to 100 μW optical power into a graded-index fiber. Modulation bandwidths of about 100 MHz can be obtained with these surface emitting LED’s. On the other hand, edge-emitting LED’s and superluminescent LED’s have narrower spectral widths and higher coupling efficiency to fibers, but the power output of the devices decreases more rapidly with increasing temperature than does the output of surface emitters. Recent results of accelerated life tests on surface-emitting LED’s show that a mean-time-to-failure greater than 109 hours at room temperature can be expected, whereas the mean- time-to-failure for edge-emitting diodes is about 106 hours.