One of the technical challenges the current data revolution faces is finding an efficient way to route the data. This task is usually performed by electronic switches, while the data itself is trans­ferred using light confined in optical waveguides. For this reason, conversion from an optical to an electronic signal and back-conversion are required, which costs energy and limits the amount of trans­ferable information. These drawbacks are avoidable with a full optical switch operation. One of the most promising approaches is based on micro­electro­mechanical systems (MEMS), thanks to decisive advantages such as low optical loss and energy consump­tion, monolithic inte­gration, and high scala­bility. Indeed, the largest photonic switch ever demonstrated uses this approach.

Until now, those MEMS photonic switches have been fabricated using nonstandard and complex processes in labora­tory environ­ments, which has made their commerciali­zation difficult. But University of California Berkeley researchers initiated a colla­boration that gathered engineers from different universities worldwide to demonstrate that the diffi­culties could be overcome. They created a photonic MEMS switch using a commercially available complementary metal-oxide-semi­conductor (CMOS) fabrication process without modification. The use of this well-known micro­fabrication platform represents a huge step toward industriali­zation because it is compatible with most current technologies, cost-effective, and suited for high-volume production.

In their research, the photonic switch was fabricated on silicon-on-insulator (SOI) 200-mm wafers using regular photolithographic and dry-etching processes in a commercial foundry. The whole photonic integrated circuit is included in the silicon top layer, which has the advantage of limiting the number of fabri­cation steps: There are two different dry-etching processes, one lift-off to create metal inter­connects, and the final release of the MEMS by oxide etching. The switch design includes 32 input ports and 32 output ports, representing a 32 x 32 matrix of the same replicated element. In each of the single elements, the light transfer from one channel to the other is produced by decreasing the distance between two waveguides to couple their modes, an operation achieved by an electrostatic comb drive also included in the silicon top layer.

“For the first time, large-scale and integrated MEMS photonic switches have been fabricated in a commercial foundry on 200-mm SOI wafers. In my opinion, this is a convincing demons­tration that this technology is suited for commerciali­zation and mass production. They could be incorporated in data communi­cation systems in the near future,” said Jeremy Béguelin, one of the Berkeley researchers. The researchers evaluated the performance of the photonic switches by measuring several important parameters: the light power loss through the entire switch of 7.7 dB, the optical bandwidth of about 30 nm at the 1550 nm wavelength, and the speed of the switching operation of 50 μs. These values are already excellent in comparison with other photonic switch approaches, and ways to improve them have already been identified.

By using a CMOS-compatible fabri­cation process and SOI wafers, the research team created a robust and efficient photonic switch based on MEMS technology. Such work opens a promising path toward the commerciali­zation and mass production of large and inte­grated photonic switches, a future key component of data communi­cation networks. (Source: SPIE)

Reference: S. Han et al.: 32 × 32 silicon photonic MEMS switch with gap-adjustable directional couplers fabricated in commercial CMOS foundry, J. Opt. Microsys. 1, 024003 (2021); DOI: 10.1117/1.JOM.1.2.024003

Link: Integrated Photonics Laboratory, University of California Berkeley, Berkeley, USA