[mstresh]

I did my PhD work in silicon photonics (in a different lab group and not associated with the authors in the paper) and thought I could chime in with some extra background and why this result is interesting to the silicon photonics community.

First off, silicon photonics has already made its way into several products, mostly active optical cables (a device that directly converts an electrical signal to an optical signal). See, for example, Luxtera/Molex, Acacia, and Kotura/Mellanox. Additionally, many other companies have demoed interesting things at trade shows (e.g. Cisco, Intel, Fujitsu, and others).

In general, the appeal of silicon photonics is that we can fabricate almost all of the components of an optical link on a single chip using the same fabrication tools as what you might find in a standard CMOS fab. Modulators, detectors, switches, filters, and other devices have been demonstrated on a single wafer. Many organizations (ePIXfab, IBM, IME A*STAR, Intel, Freescale, and others) have fabrication processes that have all of these devices right next to each other on a wafer and are capable of 25+ Gb/s data transmit and receive.

Others in the comments have mentioned the lack of switches in the article. Making optical switches in silicon has been demonstrated before, usually with either a Mach-Zehnder interferometer or resonant structure. The phase of light or resonance are most commonly adjusted through the thermo-optic or plasma dispersion effect. I'm at work now, but I can dig up references if anyone is interested later.

This result by Piggot, et. al., is most interesting because it is a unique device geometry for performing a wavelength splitting function. The performance of the device itself isn't particularly impressive relative to other devices with similar functionality that have already been demonstrated [1]. Additionally, the use of an MMI structure for wavelength multiplexing is also not novel [2].

So how does this relate to "light-based computers?" The vision that places like IBM research try to sell is that we will eventually integrate photonics (either monolithically, or flipped in some form) onto our processors and memory chips to enable high-throughput on- and off-chip I/O. This is still likely 10 years away from commercial products. Near-term, look for silicon photonics in your data centers and fiber-optic regional, metro, and long-haul networks. (FTTx one day, but silicon photonics currently can't compete in economics with a DML shoved into a TO can.)

[1] See http://www.nature.com/lsa/journal/v1/n3/full/lsa20121a.html for a review article on silicon passive optical devices [2] http://dx.doi.org/10.1063/1.4812746

[sounds]

Could you clarify why splitting based on wavelength makes this useful in an optical switch? I assume it's useful in making an interferometer?

[mstresh]

Generally speaking, being able to split based on wavelength lets you transmit data on multiple wavelengths to increase your bandwidth. The flow is to modulate each wavelength individually, mux them together, send them through a single fiber or waveguide, and then demux them on the other side. In a switch, you could imagine switching each wavelength individually and optionally combining them into a single waveguide out of each port of the switch.

This particular device could not be used to make an interferometer. The device has 1 input (call it port 1) and 2 outputs (call them ports 2 and 3). If you input 1550 nm light to port 1, most of it goes to port 2. If you input 1310 nm light to port 1, most of it goes to port 3. This also works backwards: if you input 1550 nm light to port 2 most of it goes to port 1. If you input 1550 nm light to port 3, 10% of it goes to port 1 and the other 90% gets radiated outward as loss (crosstalk is -10 dB). So if you tried to input 1550 nm light to both ports 2 and 3 there won't be much interference at port 1 unless there is a large power imbalance between your two input beams.

[sounds]

Ok, poor question on my part, but a great answer. Thanks! Yes, wavelength-division multiplexing can expand the capacity of a single strand.

I meant, packet switching.

Switching packets of photons would be done by transitioning from transparent to opaque/inverted polarity/shifted wavelength/etc.

I'm way out of my depth here but an interferometer seems like one method that current research is looking at to accomplish that. What do you think looks the most promising?

[mstresh]

Yes, an interferometer is certainly a very common method to perform switching. What ultimately gets used will depend on the technology/material system. Silica-on-silicon and silicon photonics-based switches will likely use Mach-Zehnder interferometers. MEMS switches currently use movable mirrors or gratings.