VCSEL Techniques for Wavelength-Multiplexed Optical Interconnects

Licentiate thesis, 2019

The majority of global data communication is taking place within data centers where data is stored and processed and where the largest part of the power used for global networking is consumed. With the rapidly increasing use of Internet-based applications and services, data centers are equipped with a larger number of servers and switches requiring higher bandwidth connectivity. Optical interconnects (OIs) are used to provide the connectivity needed. Short-reach OIs are dominated by 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) due to their low fabrication cost, low power consumption, high modulation speed, and circular output beam. With the need for even higher bandwidth connectivity, large efforts have been invested in the development of VCSEL-based OIs offering higher aggregate capacity. Until now, higher capacity has been achieved mostly through an increase of the lane rate by higher speed VCSELs and higher order modulation formats. Furthermore, spatial division multiplexing (SDM), using parallel fibers or multicore fibers, has proven effective for increasing the aggregate capacity. With these techniques, it is expected that the OI capacity will saturate at the  1 Tbit/s level.

Capacity beyond the limits of current technologies is expected by also exploring the wavelength dimension, referred to as wavelength division multiplexing (WDM). This calls for the development of high-speed VCSELs at multiple wavelengths. To also enable the very small footprint transceivers and high bandwidth density needed as transceivers move closer to the switch AISC, the multiple wavelength VCSELs should be in a monolithic array. This requires a VCSEL technology where the wavelength of individual VCSELs can be precisely set in a post-growth fabrication process. As an integration platform for multiplexing and fiber coupling we envision a photonic circuit on Si with Si3N4 waveguides and grating couplers for VCSEL integration. With such waveguides being single mode and the grating couplers being polarization sensitive, the VCSELs in the array should be single transverse and polarization mode, in addition to having a high modulation bandwidth.

In this thesis, an intra-cavity phase tuning technique, based on an Ar ion-beam etching process with sub-nm precision, is demonstrated for setting the resonance wavelength of VCSEL resonators with <2 nm precision in the wavelength range 1040-1070 nm. Single transverse and polarization mode VCSELs with a record output power of 6 mW are also demonstrated. Suppression of higher order transverse modes and the orthogonal polarization state is achieved by etching a shallow mode filter in the surface of the VCSEL.