Enhanced Coupled Strength (ECS) gratings fabricated into III-V based devices offer high reflected power per unit length and broad band reflectivity as compared to conventional 1st order gratings, desired qualities for short-haul high speed transmitters that can be implemented without the need for chip-level temperature control, contributing to the low power per transmitted bit. For commercial DBR lasers, the grating reflectivity results in an extremely narrow reflectivity spectrum, which is highly desired for most/many applications, but requires a power hungry thermo-electric cooler to maintain a fixed frequency. The proposed LEAM (laser electro-absorption modulator) requires a broad reflectivity spectrum, which, by Fourier Transform theory, means the gratings have to be very short, which only ECS gratings allow. By integrating the ECS gratings, laser, and electro-absorption modulator (EAM) into the same III/V epitaxial layers, the shift in wavelength due to temperature changes of the laser region will be identical to that of the electro-absorption modulator section, therefore eliminating temperature control requirements, contributing significantly to a low power per transmitted bit for data communications systems. A novel laser and integrated electro-optic modulator will be described, that when coupled with CWDM and 12 channel ribbon fiber, can enable a total of 7.2 Tbps connectivity, with calculated operating speeds of >100 Gbps per channel non-return-to-zero (NRZ or PAM2). VCSEL based systems have shown signs of speed limits to significantly exceed 28 GBd (Gigabaud) PAM (4-level pulse amplitude modulation). Additionally, short 2nd order outcoupling gratings can be designed into the transmitter device to efficiently outcouple power into a single mode fiber. From a manufacturability point of view, fabricating the devices with both p and n metal pads on the same side of the wafer as the out-coupler allows wafer level testing, which has been a key advantage of VCSELs (Vertical Cavity Surface Emitting Lasers). VCSEL epitaxial growth can contain on the order of thousands of discrete layers with a thickness of ~10 microns. By contrast, the epitaxy grown for this thesis work contains ~ 30 layers not including doping changes.The transmitter is designed to leverage mostly existing mature semiconductor process methods with a few exceptions. Challenging process development work included methods to fill and planarize a low optical index material in the grating teeth spaces such that a higher index material can be deposited on top of the planarized low index layer and precisely coincide with the top of the grating teeth.
Electrical and Computer Engineering
Dr. Gary Evans
Electrical, Electronics Engineering
Number of Pages
SMcWilliams Thesis_Jan 2024 Final.pdf
McWilliams, Scott and McWilliams, Scott, "Design and Fabrication of a Low Power 7.2 Terabit Transmitter for Exascale Computing" (2023). Electrical Engineering Theses and Dissertations. 66.