Improvements in index alignment method for laser-fiber array packaging

An “index alignment” method, based on the registration of fiducial marks, was previously developed for passive alignment of a laser array to a corresponding fiber array. This method has recently been improved and used to fabricate laser-fiber array transmitter modules for single-mode operation at 1300 nm. An improved computer-controlled alignment-stage system with machine-vision features has been installed in order to render the alignment procedure faster, more precise, and more reliable; this system was particularly effective in quick achievement of the difficult but necessary angular alignment of the components. Special “self-registration” component-fabrication techniques were also developed to avoid mask-registration errors associated with the fabrication of the laser chip and the fiber carrier, so that the fiducial marks were automatically keyed to the positions of the laser ridges and V grooves, respectively. Measures were also taken to improve the accuracy of alignment of the etch mask to the silicon crystallographic axes during fiber-carrier fabrication, and to improve the etch-stop indication process. These techniques permitted an estimated accuracy and wafer-scale uniformity of ~±0.5 μm of the fiber-carrier V-groove widths. In addition, the essential elements of of a practical technique for separation of a large-array laser-fiber module into a multiplicity of small-array submodules was demonstrated; in this way the cost of the alignment procedure could be amortized over a large number of transmitter modules. Several connectorized laser-fiber array modules were fabricated with the improved alignment apparatus and components. Test results showed that for single-mode operation at 1300 nm, coupling efficiencies greater than 8% could be achieved for a laser-fiber spacing of about 35 μm. Such values closely approach the 9% coupling efficiency observed at this spacing with active alignment. Tests of the completed module at 1 Gb/s showed values of RIN low enough to permit operation at distances of about 1 km