Surface Grating Optimization for Low-Threshold Atomic Clock VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) with emission in a single-mode, singlepolarization beam at a wavelength of 894.6 nm have been fabricated for use in Cs-based atomic clocks. For polarization control, monolithically integrated surface 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 inverted 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 temperatures up to 80 ◦C. Small-signal modulation characteristics of grating VCSELs are presented. The modulation bandwidth exceeds the required 5GHz at a bias current of only 0.9mA above threshold at room temperature.