High Power Digital VCSEL Transmitter

The need for high-speed data interconnects within modern fighter aircraft is clear, and development of optical components for speeds up to 100Gbps has been ongoing. One of the principal challenges of military grade optical interconnects is the total passive loss budget resultant from in line connector loss. The loss of link budget is driven by the reduction in speed of the fundamental optical components, the bandwidth of the optical fiber, and the reduced responsivity of detectors as the speed has increased. The total transmitter optical power is also generally limited by eye safety concerns in commercial applications. Taken together, this has forced communications standards like Ethernet and Fibre Channel to adopt more advanced signaling techniques, limit the temperature range of operation, and include forward error correction for the links operate reliably. This has led to a divergence in the capability of current commercial transmitters in the commercial and military environments. As a result, the Navy has funded development of higher speed transmitters, receivers and optical fibers, but has not addressed the passive optical loss problem. Modern VCSELs typically have power output in the +3dBm range over temperature, and scaling the power to +10dBm would add margin to the link without degrading the optical performance. This proposal develops VCSELs capable of achieving +10dBm and maintaining the required bandwidth by introducing a new type of VCSEL structure. In phase I, design of stacked tunnel junction VCSEL and multi-element tunnel junction isolated VCSELs will be considered. Stacked tunnel junction VCSELs can achieve a higher power conversion efficiency by recycling the electrical carriers to create more than one photon per carrier pair, thereby increasing the slope efficiency of the VCSEL by the number of active regions used. These VCSELs are now widely deployed in LiDAR systems but have not been optimized for high-speed operation. Multi-element tunnel junction isolated VCSELs allow several emission areas to be spaced in parallel and in close proximity and are driven from common  electrodes. Recent advances have shown modulation speeds that are compatible with 50Gb/s operation, and scaling the structure can provide the needed power. The close spacing of the active regions ensures good coupling of the emission to a multimode optical fiber. At the end of the phase I base period, a design will be selected and epitaxial growth will be initiated. In the phase I option period, the devices will be fabricated and undergo extensive on wafer DC and AC characterization that will be used to inform the direction of a phase II proposal. In phase II, the design will be iterated to meet the final objectives of +10dBm optical output power and bandwidth required to operate at 50Gb/s using NRZ signaling. Initial reliability studies will also be done in phase II to ensure the VCSELs are capable of meeting the environmental requirements of the Navy.