This post is meant for IT architects that wishes to understand the benefit of using silicon photonics with WDM in the data center.
We have already discussed that the innovative approach for 100 Gb/s transceivers is to use silicon photonics chips inside the optical transceiver. Because silicon photonics chips are fabricated using the same CMOS wafers as electronics chips, they are low cost. Silicon photonics chips are processed using mask layers in the same foundry as electronics wafers. Just like traditional wafers, the silicon photonics wafers are diced into chips and packaged. Optical chips fabricated in this manner 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.
A huge advantage of optical communication is the ability to provide parallel channels over the same optical fiber using different wavelengths of light. This technique is called Wavelength Division Multiplexing (WDM), and there is no equivalent in the electrical domain. With WDM, four, eight or even 40 channels of light, each at a different frequency, can use a single strand of optical fiber. In stead of parallel transceivers, (and parallel fibers), WDM provides a cost effective way to increase the bandwidth of data center fabrics.
The following figure demonstrates the receiver side. You can see one signal that splits to several different channels or wavelengths of light.
Optical networks typically use some sort of grating (think of a grating as a "super prism") to combine or separate wavelength channels. In Telecom networks, arrayed waveguide gratings (AWG) are often used for WDM networks, especially those consisting of 40 to 80 or more wavelength channels. AWGs provide good channel separation, with low crosstalk, and low insertion loss. However, the chip size of these is quite large so they are not the best solution for data center fabrics.
A reflector grating, often called an Echelle grating, consists of waveguides and a small, curved mirror structure, fabricated in the silicon chip. It is the reflector mirror that provides the optical multiplexing function. Channel crosstalk, insertion loss, and channel separation are almost as good as the AWG, and the much, much smaller size makes them the ideal choice for an optical engine.
Below are two silicon photonics chips with integrated Echelle Gratings for wavelength multiplexing and demultiplexing functions.
- Transmitter side: Four lasers, each transmitting at 25 Gb/s (each one of them is using a different wavelength) are modulated using on-chip modulators. Then the signals are combined using a Echelle grating to a single output fiber, containing an entire 100 Gb/s.
- Receiver side: One input channel (also carrying four wavelengths of light and 100 Gb/s) with four parallel channels on different wavelengths of light is separated by an Echelle grating (demultiplexed) before conversion to electrical signals.
The above optical engine chips for 100Gb/s (4x25Gb/s) for 2km of operation over a single strand of fiber. The Echelle gratings, used for both the WDM Mux and DeMux are fully integrated into the silicon chips. Other WDM techniques can be used, but Echelle gratings have strong advantages.
The advantages of Echelle gratings
- Size: the chips for WDM version of the optical engine are the same size as the chips for parallel transmission. I.e., adding WDM does not increase the size of the chips. Arrayed waveguide gratings (AWG), popular in Telecom applications, would take the entire chip area for just the grating.
- Low loss: published results from Mellanox show the WDM loss to be around 2 dB; a very good number for WDM filters.
- Scalability: more channels can be easily added. Mellanox has demonstrated devices to 40 channels.
The above diagram is the measured spectra of a 12 channel WDM multiplexer silicon photonics chip from devices manufactured by Mellanox. Some first generation 100Gb/s solutions for data centers used 10 wavelengths at 10Gb/s per channel. Mellanox silicon photonics chips were used for the WDM functions.
The figure above shows a 1.6 Tb/s 40 channel WDM receiver chip with integrated 40Gb/s germanium detectors and its measured spectrum. This demonstrates the scalability of the Echelle grating in silicon photonics. The 40 channel chip connects to a single fiber at the left side of the chip. The waveguide transmits the light to the reflector grating which separates the spectrum into 40 independent channels. Additional waveguides transmit each channel to a germanium detector where it is converted to 40 electrical channels.
With WDM and silicon photonics, the capability to bring faster, smaller interconnects, which consume less power, already provide data centers with low-cost 100Gb/s QSFP28 transceivers. As we look to the future, next generation data centers will scale from 4 to 8, 16, 32 or more channels on a single, low-cost strand of optical fiber.