OPTOELECTRONIC PROPERTIES OF SEMICONDUCTOR-LASERS BUTT-COUPLED TO OPTICAL FIBERS

The authors have theoretically and experimentally examined the electrical noise properties of a semiconductor laser coupled to a single-mode fiber with a plane endface. It is found that the performance of semiconductor lasers butt-coupled to optical fibers critically depends on the axial position of the fiber. The electrical noise of the laser as well as the output power and the relaxation frequency vary periodically with a period of λ/2 as the fiber is translated in the axial direction. This effect is due to the threshold-current dependence on the interference condition among the waves within the external cavity formed by the laser mirror and the fiber endface. Measurements show that the influence of the fiber may change the output power as much as ±40% and the modulation bandwidth, determined by the relaxation frequency, by as much as ±35% (for a bias 15% above the solitary threshold). Moreover, the pulse response and the longitudinal mode spectrum deteriorate if the laser-to-fiber distance departs from its optimal value. The authors propose a nonoptical scheme for maintaining the optimal laser-to-fiber distance so as to avoid these degrading effects.