Electro-Absorption Modulator Vertically Integrated on a VCSEL: Microstrip-Based High-Speed Electrical Injection on Top of a BCB Layer

Vertical-cavity surface-emitting lasers with vertically integrated electro-absorption modulators (EAM-VCSELs) potentially can reach higher modulation bandwidths than directly current-modulated VCSELs. The aforementioned device modulation capabilities are, however, currently restricted by their electrical contact parasitics. It, thus, becomes critical to optimize their access line to improve performance. In this paper, we numerically and experimentally demonstrate that a microstrip (MS) access line using a planarized benzocyclobutene (BCB) layer exhibits improved high-frequency characteristics compared to the coplanar waveguide (CPW) access line used to-date, since it reduces the losses induced by the doped substrates. We also use this opportunity to introduce an innovative technique for BCB layer planarization, which is not only compatible with the EAM-VCSEL double-mesa structure, but is also independent of the device pitch. The resulting BCB layer dielectric permittivity and losses are measured up to 100 GHz, and a side-by-side comparison of the electrical response of three-electrodes MS and CPW access lines is subsequently carried out. Finally, using the measured pad and access line RF responses and the EAM modulation characteristics, the devices with MS access are shown to be no longer limited by their electrode parasitics, but by the modulator internal impedance.