Characteristics of VCSEL arrays for parallel optical interconnects

The use of vertical cavity surface emitting lasers (VCSELs)in a parallel optical interconnect for Motorola´s OPTOBUS^TM interconnect was made public over 1 year ago. This was the first time VCSELs were introduced into a product which took advantage of the excellent qualities of VCSELs over edge-emitting lasers. Motorola´s OPTOBUS^TM interconnect is a ten channel parallel bi-directional data link based on two 10 channel multimode fiber ribbons. One of the key differences in this type of interconnect compared with previous data link designs is the use of the VCSELs as the optical source for the link´s fiber optic transmitter. A single 1×10 VCSEL array from a GaAs wafer is die attached to a 10 channel GUIDECAST^TM optical interface unit which couples the emission from each laser device to its corresponding fiber ribbon channel and thus negates the use of expensive manufacturing techniques such as active alignment and pigtailing. The OPTOBUS^TM interconnect achieves its performance goals (which include low cost) via the unique characteristics of the GaAs VCSELs arrays. For example, the 850 nm devices produce a circular symmetric beam with a half angle of about 10 degrees allowing the coupling loss into the waveguide to be less than 3 dB. In addition, to maintain low manufacturing costs, each VCSEL array is individually and automatically probe tested (just as in the silicon industry) to verify that each VCSEL achieves the OPTOBUS^TM interconnect´s stringent electrical, optical, thermal and mechanical specifications. Typical computer generated wafer maps from automated production tooling and statistical parametric results are discussed. The combination of low threshold currents with superior thermal and optical performance allow the devices to be modulated under fixed bias conditions. Typical drive currents of 3X threshold are used to obtain nominal FDA Class 1 safety optical power levels from the GUIDECAST^TM optical interface unit