What is Silicon Photonics?

Version 12

    This post is basic and meant for users who wish to understand the value of using Silicon Photonics chips in the data center.




    Ever since the invention of the transistor more than 60 years ago, semiconductor chips have used electrons for communications.  Each new generation of devices offered more transistors in a smaller area, operating at faster speeds. Today, the semiconductor industry exceeds $300B per year with a single CMOS chip containing as many as a billion or more transistors.  These complex circuits are still 100% electrical.


    Using photons instead of electrons, the first optical communication systems, based upon lasers and optical fiber, were introduced in the 1980s for long distance telecommunication. At that time optical components required exotic material systems for lasers, detectors, filters, isolators, modulators, and switches. Optical transceivers and optical receivers were hand assembled from 100s of piece parts. Thirty five years later, most optical transceivers are assembled by hand. Volumes have increased dramatically; transmission speeds are much faster; some automation has been introduced; but the optical assembly process is still far too complex when compared to electronics applications.


    Silicon photonics brings optical communications into the fabrication space of the semiconductor industry, enabling low-cost, high-volume assembly. The opto-electronic functions are fabricated on the same CMOS wafers using the same equipment and methods as electronic chips.  The wafers are process in the same fabs as those running electronics chips. The wafers are diced into chips just like electrical ones. Optical chips can be just as inexpensive as their electrical cousins.  When mass volumes are needed, the wafer fab simply runs more wafers of the same recipe. In other words, scaling to mass production is easy.


    The following figure illustrates wafer fabrication of silicon photonics optical engine.



    Not only does silicon photonics eliminate the need for hand assembly of 100s of piece parts, silicon photonics chips are much, much smaller than the optical subassemblies they replace. Silicon photonics can support 100 Gigabits per second transmission on chips less than half the size of a postage stamp. When incorporated in small, pluggable transceivers, these tiny chips enables high speed routers and switches in data centers to communicate with pipes of 100 Gb/s at distances up to 2km.

    The following figure is an actual figure of 100Gb/s transceiver with silicon photonics in non-hermetic package.

    Previously, optical solutions assembled from discrete components had to be packaged in expensive, hermetically sealed packages. A speck of dust between any of the components would inhibit the light path and render the product useless. By contrast, silicon photonics devices are totally self-contained within the layers of the chip. With no need for hermiticity, they can reuse low-cost industry standard electronics packaging.


    The following figure illustrates Wavelength division multiplexing (WDM) combines multiple frequencies of light onto single strand of fiber:

    Another huge advantage of silicon photonics is the capability to incorporate is Wavelength Division Multiplexing (WDM). With WDM, four, eight or even 40 channels of light, each at a different frequency, can operate in parallel over a single waveguide on the chip, coupling to a single strand of optical fiber, reducing the cost of the networking fabric in data centers.

    The capability to bring faster, smaller, interconnects, which consume less power, offers the entire semiconductor industry a whole new world of opportunities.  Next generation data centers, high performance computers, and eventually consumer video products will all benefit from optical interconnects built from silicon photonics.


    In summary silicon photonics brings the following advantages to high speed networking:

    1. Integration of 100s of piece parts into a single silicon chip, making assembly easy
    2. Low power, low enough, for example, that a 2km 100Gb/s transceiver consumes less than 3.5W,
    3. Density, small enough, for example, that a 100Gb/s WDM transceiver can fit inside the popular QSFP transceiver
    4. Low cost due to simplified, electronics-style, nonhermetic packaging.


    To learn deeper about the various components inside the transceiver refer to Inside the Silicon Photonics Transceiver.