Abstract

For optical fiber communications, the facet reflectivity of traveling wave laser amplifiers has to be extremely low (<10^-3) to obtain wide bandwidth and high output saturation power. Previously, to achieve such a low facet reflectivity by antireflection coating alone required extremely tight control on the refractive indices and the thicknesses of dielectric layers. To relax the antireflection coating requirement, angled-facet^1-3 and window⁴ structures have been proposed and demonstrated. Without in situ monitoring on the device output characteristics during dielectric coatings, <0.1% effective modal reflectivity has been achieved by use of angled facets. Assuming a Gaussian field distribution for a 7° angle and a 2-µm mode width, the effective modal reflectivity without coatings is estimated to be ~0.7% for 1.3 µm and ~2% for 1.55 µm.⁵ To further reduce the reflectivity, one has to increase the angle or the mode width by increasing the waveguide width. However, it has a limitation imposed by high-order lateral modes and the required antireflection coating. We propose and demonstrate a new structure to achieve <0.01% effective modal reflectivity with a buried heterostructure. By flaring the waveguide near the facets as shown in Fig. 1, the mode width of the fundamental lateral mode can be gradually increased from that of the straight waveguide in the middle of the device with very little loss. Moreover, the high- order lateral modes generated at the angled facet will suffer high propagation loss and less optical gain through the straight waveguide.

© 1990 Optical Society of America