Photonic MEMS Switches Show Promise for Datacom

Members of an international collaboration initiated by researchers at the University of California, Berkeley used a commercially available CMOS fabrication process to develop a photonic switch based on MEMS technology. The ability to microfabricate a MEMS switch using an unmodified CMOS process moves the MEMS technology toward industrialization, as the platform is compatible with most current technologies.

It is also a cost-effective method, making it well suited for high-volume production. Currently, engineers rely on nonstandard and complex processes in laboratory environments to fabricate most MEMS photonic switches, making mass production and commercialization difficult.

The collaborators fabricated the switch on silicon-on-insulator (SOI) 200-mm wafers, using regular photolithographic and dry etching processes in a commercial foundry setting. The silicon top layer includes the whole photonic integrated circuit (PIC) — a quality that limits the number of fabrication steps. Two distinct processes facilitate dry etching (one to create metal interconnects and another that serves as the final release of the MEMS by oxide etching), and the physical switch includes 32 input ports and 32 output ports, representing a 32 × 32 matrix of the same replicated element; the full size of the switch is 5.9 × 5.9 mm. Decreasing the distance between the two waveguides to couple their modes, which the engineers achieved by also including an electrostatic comb drive in the silicon top layer, produces the light transfer from one channel to the other.

Partial SEM image of the switch matrix: The whole structure patterned in the top silicon layer by dry etching seems to 'float' as the oxide is removed. Each matrix unit contains an electrostatic comb drive that can selectively move portions of the waveguides to establish a desired light path from one of the 32 input ports to one of the 32 output ports. Courtesy of SPIE via Han et al.

The technology could be incorporated in data communication systems in the near future, said Jeremy Beguelin, a participating researcher from the University of California, Berkeley. Data-routing commonly requires using one or more electronic switches, and the transfer of data often occurs in light-confining optical waveguides. For this reason, conversion from an optical to an electronic signal (as well as back conversion) is required, which costs energy and limits the amount of transferable information. All-optical switching effectively resolves those problems, and MEMS-based approaches offer low optical loss and energy consumption with high scalability and monolithic integration.

A widely compatible and cost-effective MEMS solution further increases the technology’s benefits and increases its potential for deployment in the near term.

The architecture of the silicon photonic MEMS switch with gap-adjustable directional couplers. Light is coupled to the chip using the grating couplers. There are two pairs of the directional couplers and one comb-drive actuator per unit cell. The light paths on the chip are controlled by changing the gap spacing of each directional coupler. Courtesy of SPIE via Han et al.

The researchers evaluated the performance of the photonic switches by evaluating the light power loss of the entire switch (7.7 dB), the optical bandwidth (30 nm at the 1550-nm wavelength), and the speed of the switching operation (50 µs).

Those values compared favorably to existing approaches. The researchers were also able to identify areas and pathways for improvement.

The research was published in the Journal of Optical Microsystems (www.doi.org/10.1117/1.JOM.1.2.024003).