Tiny silicon photonic switches go large

Researchers at the University of California, Berkeley have developed a novel silicon photonic switch, which has the largest scale and the lowest optical losses reported to date for such a highly integrated device. Their research is considered one of the top-rated papers at OFC 2015.

Optical switches reconfigure the path taken by optical signals, selectively directing them from one route to another – in this case from one waveguide to another on a silicon chip. The research is significant because increasing the number of input and output ports on such an optical switch provides larger processing capacity in a small device.

“Our photonic switch has 50 input and 50 output channels, for a total of 2,500 switching elements located on the cross points of these channels, which is the largest-scale silicon photonic switch ever reported," said Tae Joon Seok, a postdoctoral researcher at the Integrated Photonics Laboratory at UC Berkeley.

Seok said that the largest silicon photonic switch previously reported by other groups had eight input and eight output channels. These devices consist of a number of smaller optical switch elements with one or two input and output channels (1x2 or 2x2), which are cascaded to create larger switch sizes.

In this architecture, the power of the optical signal is attenuated each time it passes through a switch element and the losses soon start to add up. The cumulative value of the optical insertion loss limits the useful size of the switch that can be created. Optical amplifiers can be added to compensate for losses, but this increases the power consumption and overall cost of the switch.

To address this problem, Seok and his co-workers designed a new, more-scalable switch architecture. Rather than connecting smaller optical switches in succession to form a switching network, the new architecture is a single-stage switch with a MEMS switching mechanism, which eliminates the problem of cumulative loss.

Seok and his colleagues first patterned a bottom silicon layer to create the east-to-west input and north-to-south output channels. Then, with the aid of a sacrificial spacer layer, a silicon layer was built on and patterned to form a specific type of switching element, called an adiabatic coupler. An adiabatic coupler is a gradually tapered waveguide, which transfers light from one channel to another channel without wavelength dependency.

When the switch is in the ‘on’ mode, adiabatic couplers on the top layer are pulled down to bottom channels, transferring light and making 90 degree turns from the input channel to the output channel. When the switch is in the ‘off’ mode, the light stays in the input channel.

“In this architecture, light from each input port would only pass one switching element in their light paths rather than passing multiple ones like in conventional silicon switch, which greatly reduces the [optical] energy losses,” Seok explained.

Moreover, in the ‘off’ mode, light propagating in the bottom layer channels stays in the channel without being disturbed by the presence of the switching elements above. As a result, the optical loss per port of the new silicon photonic switches is three times lower than in previous designs.

The new switch design also features hundreds of nanometers of bandwidth, which is about 10 times broader than that of typical silicon switches and allows the switch to operate across a wider wavelength range.

"Although commercially available [devices] like 3D-MEMS optical switches can also have up to hundreds of input/output ports and low energy loss, the switch speed is slow, around millisecond level, and the physical size is large," Seok said. "Our switch features sub-microsecond switching time, which is three orders of magnitude faster than 3D-MEMS switches.  Also, the new switch is based on silicon photonics and can be implemented on a tiny silicon chip less than 1 cm x 1 cm, which may reduce the manufacturing costs and enable a low-cost mass production."

Nick Parsons, chief technology officer of switch developer Polatis, commented: “It’s an impressive bit of academic research to get to that level of integration, but there’s quite a long way to go to get to a product.” He points to the 9.6dB insertion loss of the silicon photonic device, which is about 10 times higher than in commercial products.

Parsons also notes that the design, while representing a significant improvement over previous devices, is still difficult to scale because the number of cross points in the switch scales as the square of the number of ports. The technology used inside Polatis’ switch, which is based on beam steering, scales as 2n where n is the number of ports.

However, a silicon photonic switch probably occupies a different application space to the macro optical circuit switches available from companies such as Polatis and Calient. Parsons says the technology would be more suitable as an element on a motherboard for chip-to-chip or board-to-board communications, where space is at a premium.

The UC Berkeley researchers say their next steps are to further reduce the optical losses of their switch and to integrate the device with its electronic driver circuitry.

The presentation, ‘50x50 Digital Silicon Photonic Switches with MEM-Actuated Adiabatic Couplers’" by Tae Joon Seok and colleagues, will take place at 14:30, Monday, 23 March 2015, in Room 403A at the Los Angeles Convention Center.

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