Coherent CFP2-ACO / CFP2-DCO WDM optical modules (100G / 200G) and their application in DWDM systems

    The first press releases about the appearance of coherent CFP modules (coherent CFP optical pluggable modules) began to appear about 5-6 years ago. Then their use in optical sealing systems was new and was essentially a niche solution. Now, after six years, these modules have tightly entered the world of telecom and continue to gain popularity. What they are, how they differ and what solutions based on them offer (and of course the pictures under the spoilers) - all this under the cut. To read the article, you will need an understanding of the basic principles of DWDM systems.

    A brief digression into the past

    Historically, for optical pluggable modules with a transmission speed of 100G, the CFP form factor became the first form factor, it also became the first form factor for CFP-WDM solutions. At that time, there were two solutions on the market:

    1. CFP from Menara (now part of IPG photonics) that allows transmitting 4 separate 28Gbps channels in a standard DWDM 50GHz frequency grid using pulse modulation. I did not get a strong distribution, although in principle I had an interesting potential for building metro networks. Further in the article we do not consider such modules.

    2. CFP from the pioneers - Acacia press release , built on the most advanced at that time coherent detection technology using DP-QPSK modulation.

    What was the breakthrough of modules from Acacia:
    - this was the first module in the industry that offered a separate 50GHz 100Gbit DP-QPSK coherent channel
    - completely tunable in the C-band

    Until then, such solutions always looked something like this: a linear laser was a non-removable element of the board, on which there was only one connector for a client optical module. It looked something like this:


    Let me remind you, at that time it was 2013.

    Such a module replaced the classic linear DWDM interface on a classic transponder operating in the C-band, which can be amplified, multiplexed, etc.

    Now the principles of building coherent networks have become the de facto standard for building in the industry and this will not surprise anyone, and the density and range of optical compression systems has grown many times over.

    Module components

    Their first (Acacia) module was a type of CFP-ACO. A brief reference is given below on how the coherent CFP modules actually differ. But for this, you first need to make a small offtopic and tell a little about what DSP is, which is largely the heart of this technology.

    a little about the module and DSP
    A module generally consists of several components

    1. Narrow Band Tunable Laser
    2. Dual Polar Coherent Modulator
    3. Digital to analog сonverter (DAC / ADC) - A DAC that converts a digital signal into an optical signal and vice versa.
    4. Digital signal processor (DSP) - restores useful information from a signal, removing from it the effects exerted on the useful signal during transmission. In particular:

    • Chromatic dispersion compensation (CMD). Moreover, its stock of mathematical compensation is practically unlimited. And this is remarkable, since the physical compensation of CMD always caused a lot of problems, since this caused an increase in nonlinear effects in the fiber. You can read more about non-linear effects on the Internet or in a book.
    • Compensation of polarization-mode dispersion (PMD). Compensation also occurs in a mathematical way, but due to the complexity of the nature of PMD, this is a more complex process and PMD is now one of the main reasons for limiting the range of optical systems (in addition to attenuation and non-linear effects).

    DSP Works at very high symbol speeds, in the latest systems these are speeds of about 69 Gbaud.

    So how do they differ?

    Coherent optical modules are distinguished from each other by the location of the DSP:

    • СFP-ACO - Only the optical part is located on the module. All electronics are located on the card (board; board) of the equipment where this module is inserted. At that time, there simply were no technologies to place the DSP inside the optical module. In fact, these are first-generation modules.
    • CFP-DCO - in this case, the DSP is located in the optical module itself. The module is a complete “boxed solution”. These are second generation modules.

    Externally, the modules have the exact same form factor. But they have different fillings, consumption (DCO is approximately twice as large) and heat generation. Accordingly, solution manufacturers have a certain flexibility - ACO gives a deeper integration of solutions, DCO allows you to get a solution "out of the box", using such an optical module as a lego cube to build your solution. A separate point - in the vast majority of cases, the work of a pair of DSP is possible exclusively from the same manufacturer. This imposes certain limitations and potentially makes DCO modules much more attractive for interopability tasks .

    Solution Evolution

    As progress does not stand still and MSA is constantly developing new standards, the last form factor in which DSP managed to be located is CFP2.

    In fact, they are, I suppose, close to the next step. Here is the CFP4-ACO
    quite by accident, I stumbled upon this miracle : But I do not know any commercial products on such modules yet.


    Form factor (CFP2) now prevails on all finished commercial products. You probably saw these connectors on telecom equipment and many are confused that these connectors are much more familiar to most QSFP28. Now you know one of the ways to use them (but it is better to make sure that the equipment can work with CFP2-ACO / DCO).

    comparison of connectors QSFP28 and CFP2 on the example of Juniper AXC6160

    In addition to compact sizes, modulation methods are also being improved. In all CFP2-ACO / DCO products I know, not only DP-QPSK modulation is supported, but also QAM-8 / QAM-16. That is why these modules are called 100G / 200G. The customer himself can choose the modulation that suits him based on the tasks. In the near future, modules supporting speeds of up to 400G per optical channel should appear.

    Acacia Solution Evolution

    However, in the vast majority of cases, Ultra long haul (ULH) solutions use classic non-modular linear interfaces, which allow for greater range, better OSNR and higher modulation levels. Therefore, the main field of application of coherent modules are mainly merto / regional networks. If you look here , you can see that they probably

    good prospects:

    DSP Manufacturers

    The global manufacturers of coherent DSPs that sell them to third-party companies are:

    Manufacturers CFP2-ACO / DCO

    Manufacturers of coherent ACO / DCO modules:

    Given that some of these companies are in a state of appraisal of the proposed mergers and acquisitions, it seems to me that the market for suppliers of such solutions will narrow. The production of such modules is a complex technological production, so for now I think it will not be possible to buy them from China suppliers for a long time.

    Impact on the industry

    The appearance of such modules has led to a small transformation of the ecosystem of the solutions offered on the market.

    • At first
    Manufacturers began to use them in classic (transponder) DWDM solutions, as conventional linear interfaces. Having received a bonus modularity, flexibility and cost reduction (by the way, such solutions are often chosen as Alien Wavelength).

    For instance:

    • Secondly
    manufacturers already supplying telecom equipment - switches and routers, expanded the range and added support for such modules, further bringing us closer to the so-called IPoDWDM systems . For instance:

    • Juniper (MX / QFX / ACX)
    • Cisco (NCS / ASR)
    • Nokia (SR)
    • Arista (7500R)
    • Edge-Core (Cassini AS7716-24SC)

    All of the manufacturers listed in the equipment line already have boards for routers or switches that support coherent CFP2 modules.

    • Separately
    It is worth mentioning interesting trends in the global community, for example, the TIP project, one of the focuses of which is the development of open optical networks . The construction of such networks will allow integrating equipment into open source control systems, making the interaction between manufacturers of optical systems more transparent and open. In addition, on the devices themselves (both on transponders using DCO modules, and on ROADM / EDFA), it is planned to use software from various suppliers (for example, Ipinfusion ). Therefore, the trend of recent years remains the unification of the component base of solutions and the uniqueness of software development, in which a fairly large stake has been made on opensource.

    Thank you for your attention, I hope this article turned out to be interesting and useful to you. You can ask additional questions in the comments or in person. If you have something to add on this topic, I will be very happy.

    the main picture for the article is taken
    From the site , I hope they do not mind.

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