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

Digital sensor data produced by infrared focal plane arrays (IRFPA) faces a readout bottleneck in exiting the cooled environment for 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. Silicon photonics has emerged as a compelling platform for optical links due to the ability to support large-scale integration at low cost.  However, the most widely used modulators, ring resonator modulators (RRMs) and Mach-Zehnder modulators (MZMs), require bias control relying on inefficient thermal phase shifters to maintain proper biasing to overcome fabrication variance and compensate for drifting environmental conditions. To make matters worse,  the thermo-optic coefficient in silicon is orders of magnitude lower at 77K as opposed to 300K making thermal tuners highly inefficient.  Compounded by the fact that heat generation  inside the cooled environment must be compensated by a 500X cooling overhead, thermal tuning of any kind is a non-starter for cryogenic applications.   Our Phase I has clearly demonstrated that electro absorption modulators (EAMs), devices that offer a compromise between the stability of MZMs and the compactness of RRMs have the potential to realize highly efficient optical links at cryogenic temperatures. Most importantly, the strength and speed of modulation in EAMs is insensitive to temperature. The operational wavelength of the EAM can shift by ~100 nm from 300K to 77K, but can be set by tuning the bandgap of the device. EAMs do not require thermal tuning and their operating point is set by a simple voltage adjustment. While Phase I focused on a SiGe-based EAM, in Phase II we will develop III-V-on-Si EAMs offering substantial benefits in efficiency and density that are capable of being directly driven by current ROIC output stages.  To minimize the impact of absorbed EAM photocurrent, we have developed optimized link architectures that minimize transmitted optical power, reducing the effects of power generated by photocurrent as well as coupling and propagation losses. In Phase II, we will  build on the Phase I results and build modulators in a commercial foundry that will underpin demonstrations of high-speed error free link operation at cryogenic temperatures. Modulator performance will be characterized at room temperature and cryogenic temperatures, and full link transmission with error free operation below 600 fJ/bit will be demonstrated with the modulators at 77K. Lucidean and UCSB will provide a final report detailing the design, analysis, and measurement results.