Abstract

A coupling circuit for a 1.3-μm wave length InGaAsP/InP laser diode (LD) with a hemispherical-end single-mode fiber has been optimized to attain low coupling loss and to suppress influence of optical feedback from fiber input and output endfaces. As a result, a low coupling loss of 2.4 dB was achieved experimentally by using a polished hemispherical fiber end with curvature radius of 10 μm. For conventional Fabry-Perot (F.P.) LDs with external feedback from the hemispheric endface, output power variations of 0.3-0.6 dB and longitudinal mode hopping of 0.8 nm occurred when the external cavity length was varied by a quarter wavelength of the laser light. The output power fluctuation tended to enlarge in automatic power control (APC) conditions. By applying antireflection coating with reflectivity of −30 dB to the hemispherical endfaces, power fluctuation was sufficiently reduced, even in the APC condition. These external cavity effects may also occur in distributed feedback (DFB) LDs. However, mode hopping due to external feedback was not observed experimentally for the DFB LDs. An intensity noise, due to external feedback from the fiber output endface, was also serious for both types of LD when operating above threshold. Peak noise was reduced more than 20 dB by decreasing the reflectivity of the fiber output endface to less than −40 dB by using matching fluid.

© 1989 Optical Society of America