Coherent Thoughts on Coherent Optics

If you work anywhere in or near the field of optical networking, chances are you’ve heard of coherent optics. We know what a coherent does — transfer data really fast and really far — but what does “coherent” really mean here?

Let’s start with the “really fast” part.

The bandwidth that can currently be moved over a single DWDM wavelength by conventional means is 25Gb of data per second. That seems like a lot of bandwidth, but speed is relative and the need for it is always growing in our industry. Coherent transceivers use phase and polarization to increase the amount of data conveyed per clock cycle.

Start with a single DWDM channel using polarized signals. It’s possible to have two light signals perpendicular to each other, doubling it’s capacity.  Now we’re talking about 50Gbps!

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We all know that data is 1’s and 0’s, but what if there were more options? Instead of just these 2 states, extra data could be stored in the waveform by using amplitude modulation and phase shift keying.

Using these schemes in different implementations the 50Gbps is doubled to 100Gbps or even 200Gbs! To put that in perspective, a 40 channel DWDM filter can carry 400Gbps with 10Gb signals, but using coherent optics we get 40 200G channels, or 800G! That’s a LOT of bandwidth!

And now for the “really far” piece.

There are a few factors that determine how far a signal will travel. The top two are attenuation and dispersion (polarization and chromatic).

Attenuation is the reduction of amplitude of a signal. For fiber optics, this can include connection points, passive equipment and the inherent loss of fiber. To overcome fiber attenuation, coherent transceivers are able to tune their output wavelength, as well as their output power.

Chromatic dispersion results from different wavelengths, traveling at different speeds over the fiber, resulting in distortion. Polarization dispersion is a drift in the orientation of light signals as it traverses your path. Both chromatic and polarization dispersion are addressed by integrated Digital Signal Processors (DSPs) that compensate for these factors and reconstruct the signal. What used to occupy a good portion of a rack now fits inside a pluggable transceiver.

The combined result of these techniques is that 100G/200G and soon even 400G is attainable on a single DWDM channel over hundreds and even thousands of kilometers! Currently, coherent transceiver form factors are limited to CFP and CFP2, but QSFP-DD and QSFP28 are in development.

You’ll see both ACO and DCO used when describing Coherent optics and while the Analog Coherent Optics and Digital Coherent Optics both deliver exceptional speed and distance, the Analog Coherent Optics leave a lot more to the platform to perform.

The adoption of coherent optics is tempered by nascent platform support and higher price, but the need for bandwidth and long distance transit is applying constant pressure so it won’t be long until you’re contemplating coherent optics needs, if you aren’t already.

Interested in learning more about coherent optics? Chat with one one of our optics experts!