High-Density Electrical and Optical Assembly for Subminiature VCSEL-Based Optical Engine

In this article, we propose a subminiature (10.9 mm
$\times56.6$
mm) vertical cavity surface-emitting laser (VCSEL)-based optical engine with a low crosstalk penalty for onboard applications. When applying optical engines to onboard interconnects, ICs [laser drivers and transimpedance amplifiers (TIAs)] and active optical devices (light sources and photodetectors) must be mounted densely to make the footprint as small as possible. It is a concern that such a high-density integration could increase the crosstalk between transmitter (Tx) and receiver (Rx) devices, which could be caused by the supply current difference between the circuit from laser drivers to light source and the circuit from photodetectors to TIAs. In this article, by inserting a gap in the ground electrode, a compact optical engine (less than half of the footprint of quad small form-factor pluggable-double density (QSFP-DD) compliant engines) enabling a 25.78-Gb/s error-free optical transmission is successfully fabricated. We optimize the gap width to decrease the crosstalk while maintaining efficient heat dissipation via the electrode. We compare the characteristics of the fabricated optical engine to the engine with the gap-less ground electrode structure formed in the same compact size. Then, we both theoretically and experimentally confirm a link power budget savings of about 1.8 dB, which is sustained even under high-temperature (
$T_{c} = 70\,\,^{\circ }\text{C}$
) operation. In addition, to realize further high-density assembly, we also represent a lens-less optical coupling by inserting a 90
^o
-bent graded-index (GI) core polymer waveguide between the optical transmitter and multimode fiber. The transmission performance of the 90
^o
-bent GI-core waveguide is preliminarily evaluated, and we successfully transmit 53.125-Gb/s PAM4 optical signals experimentally.