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Looking Ahead: An Introduction to Coherent Optical Transceivers

Looking Ahead: An Introduction to Coherent Optical Transceivers

In addition to 400G transceivers, we are also excited about the future potential of coherent optical transmission at 100G and 200G data rates. This technology uses multiple characteristics of transmitted light to drastically increase the amount of data sent and received over a single wavelength, and its applications range from ultra-long haul (over 2000km with amplification and dispersion compensation) to intra-data center connectivity. In this article, we’ll introduce what makes the technology unique, and the form factors that use it. 

How It Works

Traditional transceivers send and receive data by assigning binary values to high- and low-intensity light from their lasers (intensity modulation). While this technology has become reliable and inexpensive to produce, it has its limitations. Over long-haul distances, the data rate is limited to 25G. Most 100G transceivers use NRZ (non-return-to-zero) modulation, another simple binary scheme.

Coherent data transmission increases efficiency by using dimensions of the signal beyond simple amplitude. Intensity and other characteristics (e.g., phase, polarization, etc.) are encoded into a two-bit “symbol,” thereby multiplying the available bandwidth at that particular wavelength. Currently, the most common coherent modulation scheme is the Dual Polarization Quadrature Phase Shift Keying (DP-QPSK).

While there are many complex modulation schemes like DP-QSPK available, data center applications required something simple, power-efficient, and relatively affordable. PAM-4, a modulation scheme built around four levels of pulse amplitude, satisfies all of these needs, and has become an integral part of the many 400G standards for intra- and inter-data center connectivity applications.

ACO vs DCO

At this time, coherent transmission’s complex internal components require a larger form factor, like the CFP or CFP2, and come in two unique classes: analog (ACO) and digital (DCO). While most of the internal components and functionality are the same, they differ in the location of the digital signal processor (DSP) that codes and decodes the coherent signal. In a DCO optic, the DSP is located on the module itself, while an ACO optic’s DSP is in the switch.

How do they compare?

Look for coherent optical solutions from Champion ONE in early 2019.

Want to learn more about high-data rate solutions? Contact us today!



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