Fabrication and Characterization of Low-Threshold Polarization-Stable VCSELs for Cs-Based Miniaturized Atomic Clocks

Vertical-cavity surface-emitting lasers (VCSELs) with single-mode, single-polarization emission at a wavelength of 894.6 nm have been fabricated for Cs-based atomic clock appli- cations. For polarization control, monolithically integrated sur- face gratings are employed. Simulated and experimental results show that the longitudinal position of the surface grating has a significant influence on the threshold current. With a grating in the topmost in-phase layer, the threshold currents are reduced to 40% compared to earlier atomic clock grating VCSELs with in- verted structure. The output polarization is parallel to the grating lines with a peak-to-peak difference between the dominant and the suppressed polarization modes of 25 dB even at substrate tem- peratures up to 80oC. Small-signal modulation characteristics of grating VCSELs are presented. The modulation bandwidth ex- ceeds the required 5 GHz at a bias current of only 0.9 mA above thresholdatroomtemperature.Arevisedchipdesignwithsmaller mesa size and thinner passivation layer has been implemented. The VCSELs show similar electrical parasitic bandwidths but require fewer fabrication steps and hence have reduced processing com- plexity. K-factors of less than 0.4 ns corresponding to a maximum 3-dB bandwidth exceeding 25 GHz are obtained at 80oC ambient temperature.