The Guiding Light & Other Photonic Soaps

The Guiding Light & Other Photonic Soaps
by Mitch Heins on 06-07-2016 at 7:00 am

 I’m a child of the sixties and seventies and on the occasion when I was sick and couldn’t go to school I got to experience the world of daytime TV soap-operas. Back then we only got 3 channels and it wasn’t until the late 60’s that we got color TV! I remember titles like “The Guiding Light”, “Secret Storm”, and “As The World Turns”. Forty plus years later, I am again re-living “The Guiding Light” except now it’s in the form of reading about “guiding-the-light” on silicon photonic integrated circuits (PICs). Like the daytime soaps, there seems to be a never ending cast of characters (photonic components) that are being presented. I thought it would be instructional to review one of the characters used to “guide-the-light” on a PIC.

 Waveguides are the primary components used in photonic design to guide light. More than a conduit for light, they are building blocks from which other components are created to modulate, filter and switch light. From the book Silicon Photonics Design, section 1.4, silicon PIC waveguides are most commonly made from the active device layer of silicon-on-insulator (SOI) wafers (see fig 3.1– cross section of SOI wafer). Much research has gone into engineering the waveguide geometries. There are several types of waveguides, but the most commonly used are the strip (or rectangular) and rib waveguides (see fig 3.4). Strip waveguides are typically used for routing and low-loss tight curves while rib waveguides are often used to create electro-optic devices such as modulators as the rib allows for electrical connections to be made to the waveguide. And yes, you read that right. Photonics routing uses curves, not Manhattan style shapes and waveguides are typically single-layer designs as the silicon crystal of the waveguide core is grown, not deposited on the wafer.

 One of the key metrics for photonics is signal loss. The signal intensity must be great enough to be sensed at the end of the signal’s journey and every piece of waveguide it traverses takes its toll. There are several contributors to waveguide loss including absorption due to metal in proximity, scattering and reflections due to sidewall roughness, material loss in active doped structures and surface-state absorption from improperly or un-passivated waveguides. One might think that the best waveguide would be one in which no loss is allowed at all. However, it is these loss mechanisms that actually enables the manipulation of light in the waveguide. Without them, waveguides would just be simple conduits or light wires. Fortunately for us this is not the case. Before going further, a couple of keys points should be noted about photonics and waveguides.

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  • Light can be made of different wavelengths (colors) and we can encode different data or messages on these different wavelengths.
  • Light of different wavelengths (and therefore the messages encoded on those wavelengths) can simultaneously occupy the same space without interfering with each other.

     The implications of these two points are far reaching. In some cases, it is signal absorption in un-passivated waveguides that is used to make detectors. But that’s a topic for yet another article. For waveguide routing these points enable us to use what is referred to as wavelength division multiplexing (WDM). WDM lets us convert from the spatially parallel electric domain to a wavelength-parallel optical domain, significantly reducing the number of waveguides needed to transmit large amounts of data (see Fig 2.2 from the book Photonic Network-on-Chip Design). Imagine a 64-bit bus worth of data all traveling down 1 waveguide! Secondly, it is the radiation of light outside of the waveguide that lets us couple the light in one waveguide to other waveguides. By controlling the resonance points of these specialized resonant waveguides (micro-rings) we can switch one or multiple signals simultaneously.  Moreover, we can also use this capability to filter one or more wavelengths from the main signal for additional processing or routing to specific sensors. This is the basis for building high performance, low power switching matrices suitable for switching wide parallel data such as required for CPU-to-memory applications.

    More recently another waveguide character has come onto the scene, that being Silicon Nitride (Si[SUB]3[/SUB]N[SUB]4[/SUB]).  Si3N4 can also be used as a waveguide material and unlike crystalline Si, Si[SUB]3[/SUB]N[SUB]4[/SUB] can be grown onto the wafer which means it can be stacked, much like metal systems in electrical ICs. While photonics allows us to cross waveguides on the same layer without interference (see point 2 above), there is a loss penalty that is incurred at each crossing. Having the ability to stack waveguides with less lossy crossings enables switch-matrices that look like figure 5.53.

    Just like in the soaps of the 60’s, this new character, Si[SUB]3[/SUB]N[SUB]4,[/SUB]adds an entirely new twist to the photonics story line. It has already prompted an entirely new dialog around subjects such as photonic networks-on-chip, packet-switching vs circuit-switching networks and new terms like selective transmission, an interesting combination of electronic and photonic networking that uses some of both technologies. We are at the beginning of a truly interesting time for silicon photonics. The plot is starting to evolve and become richer as more new characters are added into the story line … and just like the daily viewers of those early TV soaps, I can hardly wait to see what will happen in the next episodes.