Flip-chip and bond wire/airbridge transitions between passive microwave transmission lines and laser diodes

Flip-chip technology and bond wires or airbridges are two alternative methods to connect lasers or photo diodes to the RF circuitry. With bandwidth requirements exceeding the 20GHz range, flip-chip transitions are considered to be more suitable and potentially applicable for frequencies as high as 100GHz. In this paper a study is presented comparing the electrical performance of wire bond/airbridges with that of flip-chip transitions considering the case of interconnecting a 50 Ohm microstrip or coplanar waveguide to a low impedance laser diode. Because the electrical characteristic of the flip-chip transition is dominated by the mismatch between the laser and the transmission line, it was found that in the case investigated here its performance is closer to that of an airbridge with a return loss only 6-8dB better on the average.