| Bandwidth has been one of the drivers in photonic sources and will very likely remain one of them. Higher bandwidth sources would not only be useful in data and telecommunication but also in other fields such as gas sensing and optical coherence tomography, a microscopic technique. As silicon photonics offers a great prospect with the supreme CMOS technology, we focused on a broadband source on silicon. To circumvent the indirect silicon band gap, InP epitaxial stacks were bonded. More precisely, two stack were bonded, each of which were subjected to quantum well intermixing (QWI). This way, four different band gaps were created. The fabrication process consists of three stages: QWI, multiple die bonding and the post bond process. Problems were identified and addressed in each of them. Most prominent were phosphorus evaporation during the thermal anneal in QWI, for which a dielectric cap layer should be used and the size and protection of the gap region in between the two bonded dies, for which a protective oxide was introduced. The laser structures on the die at 1540nm were comparable to other devices from our group. The intermixed devices had a higher threshold, indicating a limited but noticeable damage to the quantum wells. The other die did not produce lasers because of a bad P contact. By combining the different band gaps - 1540nm, 1460nm, 1380nm and 1300nm - in a series manner approximately 300nm of 3dB bandwidth was achieved. When organizing the band gaps in the order given, a one directional devices is formed which output at the right. Note that the light at 1540nm will travel through all the other transparent regions. We noticed that the longest wavelengths were not measured, probably because of highly asymmetric below band gap gain from the following section. Nonetheless, 300nm of 3dB bandwidth is a result most other broadbanding techniques are only able to achieve after thorough optimizing of all parameters. |
