Optical Component Requirements for 400Gbps and 600Gbps Coherent Systems

By Ferris Lipscomb, Ph.D. on January 20, 2017 | Leave a Comment

Coherent transmission systems operating at 100 billion bits per second, or Gbps, have quickly come to dominate all new fiber optic transmission systems with distance reaches of 80 kilometers to 3000 kilometers. These systems use a fundamental modulation, or “on-off” rate, of 32 Giga-baud and use a modulation scheme called Quadrature Phase Shift Keying, or QPSK, which uses phase modulation to transmit 2 bits in each “on-off” cycle, or baud. By using the two independent polarizations in the fiber the amount of information carried can be doubled again to achieve a final data rate of 4 (bits/baud) times 32 Gbaud/sec equals 128 Gbps minus system overhead for an effective data rate of 100 Gbps.

It was a great success.

But, of course, no one was satisfied. Everyone wants to get more data through the same fiber, using similar equipment, and thus reducing the cost of transmitting a bit over the link. There are two ways to increase the data transmission rate: 1) increase the baud rate so that there are more “on-off” cycles and 2) use modulation schemes that pack more bits into each “on-off” cycle. Both of these approaches are being used, often together, and systems offering 200Gbps on a single wavelength are already shipping and single lambda 400Gbps and 600Gbps are just around the corner. But nothing comes for free, and there are degradations in performance associated with the higher data-rates. First, increasing the baud rate means that chromatic dispersion effects in the optical fiber will be higher, increasing as the square of the baud rate. Second, increasing the number of bits encoded in one cycle forces the different symbols closer together acerbating the effects of noise. The figure below shows constellations for QPSK (2 bits), 16 QAM (4 bits) and 64 QAM (6 bits).

For higher order constellations it becomes more difficult to distinguish the different symbols. These effects mean that the more information that is encoded onto a single wavelength, the shorter the distance, or reach, over which that information can be transmitted.

Fortunately, improved optical component performance, along with increasingly powerful digital signal processors, can mitigate some of these negative effects. Equally important, many applications do not require the longest reach possible, and can significantly reduce the cost per bit without sacrificing anything in that application. For example, many interconnections between Datacenters in a metropolitan area are point to point links of around 80 km, and it makes sense to maximize the datarate at the expense of distance. Similarly, many metro links are around 500 km and can tolerate some reduction in the optical reach in exchange for increasing the data-rate.

The same number and type of optical components are required for these higher single lambda datarates as are required for 100G QPSK links, but higher performance is required at each step. For example, to increase the baud rate from 32 Gbaud to 45 baud to 64 Gbaud requires that both the coherent modulator and coherent receiver are able to modulate and demodulate the signals at the higher baud rate. Such components are now on the market from NeoPhotonics and other vendors. Similarly, higher order modulation formats like 16 QAM and 64 QAM are more sensitive to phase noise in the laser and local oscillator, which causes the different symbols to spread and run into each other, effectively closing the eye. Phase noise manifests itself in the linewidth of the laser. Typically, linewidths of 300kHz are sufficient for QPSK systems, but linewidths well under 100 kHz are required for 64 QAM. Ultra-narrow linewidth can be achieved by lengthening the cavity of the laser. For example, NeoPhotonics uses an external cavity to narrow the linewidth of its tunable laser and achieved typical linewidths of well under 100 kHz. This results in a significantly higher signal to noise performance when using higher order modulation schemes like 32 QAM and 64 QAM.

In addition to the optics, the electronics must also keep pace. For higher baud rate systems, the DSP must also operate at higher frequency and must work harder to undo the increased impairments, like chromatic dispersion, at the higher frequencies. For higher order modulation constellations, the analog to digital and digital to analog converters must have higher resolution to resolve the additional symbols, and the DSP must work harder to decode the additional bits. Fortunately, the performance of electronic integrated circuits is continually being improved riding the immense wave of CMOS developments. Each new process node brings increased speeds and lower powers.

The key to decreasing the cost per bit is that the extra cost of the high performance components and electronics is much less than the cost of duplicating all of the hardware. For example, a single wavelength system operating at 64 Gbaud and 64 QAM can achieve 600Gbps, with allowance for overhead, over distances of 80 km suitable for datacenter interconnects (DCI). A modest premium of 25% in the cost of the high performance components and electronics gives a five-fold reduction in the cost per transmitted bit. The shorter reach does not have any negative affect in the DCI application, resulting in a win-win proposition.

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Optical Component Requirements for 400Gbps and 600Gbps Coherent Systems

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