Cryogenic Optical Links for Digital Sensor Transmission and Readout with Energy Efficient Modulators

Digital sensor data received by infrared focal plane arrays (IRFPA) faces a readout bottleneck before exiting the cooled environment to room temperature processing. This is the very same bottleneck faced in other applications with harsh environments, such as quantum and classical supercomputing. Electrical links are lossy and bandwidth limited, and coaxial cables have a high thermal conductivity and electromagnetic interference as compared to optical fibers. For these reasons optical links have revolutionized data center communication, but further development is still required to adapt the same technologies to cryogenic temperatures. Datacenter silicon photonics rely upon thermal phase shifters to maintain proper biasing despite fabrication variance and drifting environmental conditions. Critically, any heat dissipation inside the cooled environment must be compensated by a 500X cooling overhead. Additionally, the thermo-optic coefficient in silicon is orders of magnitude lower at 77K as opposed to 300K. There are various monolithically-integrated control methods for silicon photonic modulators, but to be implemented at low temperatures, wide athermal tuning may be needed. A possible alternative may be electro absorption modulators (EAMs), which offer a compromise between the stability of MZMs and the compactness of RRMs. The strength and speed of modulation is insensitive to temperature, but the operation wavelength can shift by 100 nm from 300K to 77K. Furthermore, light is absorbed into photo-current, so low optical power is required to minimize heat dissipation. An efficient EAM may outperform thermal tuning-based modulators, but deliberate design decisions must be made for cryogenic operation. The scope of this Phase I effort is to identify candidate modulators capable of high-speed error-free operation at cryogenic temperatures, and to outline plans for fabrication in existing foundry offerings. In addition, full link analysis or power consumption, potential roadblocks to scale, and promise for monolithic integration will be explored. Lucidean and UCSB will provide a final report detailing the analysis and measurement results.