Birefringence characterization of coupled-cavity VCSELs

The growing demand for high-speed optical data transmission in data centers necessitates VCSELs with higher modulation bandwidths. Conventional VCSELs are limited by the relaxation oscillation frequency, restricting performance to about 35 GHz. Coupled cavity VCSELs (CC-VCSELs) overcome this limitation by forming optical supermodes that introduce additional resonances and extend the modulation bandwidth beyond conventional designs. Another promising approach is the spin-polarized VCSEL (Spin-VCSEL), where the emitted polarization is governed by carrier spin dynamics, enabling operation frequencies exceeding 200 GHz and potential data rates beyond 240 Gbit/s per channel. Combining both CC-VCSEL and Spin-VCSEL concepts offers a highly promising route toward further bandwidth enhancement. This work presents birefringence measurements of circular and bow-tie CC-VCSELs using a rotating half-wave plate, linear polarizer, and high-resolution optical spectrum analyzer, providing a foundation for future studies on spin injection and polarization dynamics in coupled cavity structures.