My original blog post on August 7th regarding the limitations of using 24-fiber MPO solutions seems to have raised some questions in the industry. The more information we can offer our readers on the differences between 12-fiber and 24-fiber MPO solutions, the more informed decisions they can make. So let me elaborate on some of the concepts I touched on in my original post.
In my original blog, we discussed the fact that when there are more fibers in a connector, loss is higher. The fact remains that loss is directly related to the amount of surface area that ferrules and arrays cover. Why?
Alignment of fiber cores for MPOs is achieved through guide pins, and when misalignment occurs, we get additional loss. Connector rotation that causes misalignment has a larger impact on a larger array—the further a single fiber is from the center of the array, the worse the insertion loss will be. On higher count fiber MPOs, we therefore see a higher loss on the outside fibers that are farthest away from the center.
In a quality fiber connector, the fibers protrude slightly beyond the ferrule. When two fibers are mated using the right pressure, the fibers will deform slightly to fill in any gaps and provide a solid mating. Any variance in the pressure can impact the insertion loss and return loss on a fiber-to-fiber basis. To achieve consistent pressure (and good insertion loss and return loss), it is important to have a very flat ferrule after polishing with all the fibers protruding equally.
With higher fiber count arrays like 24-fiber MPOs, there are more fibers to control, which can significantly increase the odds for height variance. In addition, it is more difficult to achieve a flat surface after polishing on a larger array area. Outermost fibers tend to be recessed compared to fibers in the middle of the array. This is another reason we tend to see higher insertion loss on the outermost fibers of the 24-fiber MPO.
While we have seen significant improvements in polishing techniques, achieving a flat end-face and equal pressure over the array still has its limitations. That’s just the law of physics. If we look at this graphic of the middle two rows of fibers on a 72-fiber array for reference, we can see the variance in the height profiles. The height variance becomes even more pronounced across more rows of fibers.
Belden has already achieved a 0.2dB loss on the 12-fiber MPO and there is a trend in the industry for all 12-fiber MPOs to move to 0.25dB or less. However, due to the difficulty in achieving optimum end-face geometry, we can’t say the same for the 24-fiber MPO. It remains at 0.35dB and 0.4dB for the two leading vendors of MPO ferrules. And it’s easy to see the trend when you consider the fact that the 72-fiber MPO has a loss of 0.6dB.
As stated in my previous blog, it is important to remember that the future 100GBASE-SR4 standard will be a reality and that we will be running 100 Gig over just 8 fibers, rather than 20. Most believe that this new standard will render the 24-fiber MPO a dated interface for 100 gigabit Ethernet. And let’s not forget that the industry is still lacking standard tools for field testing and troubleshooting 24-fiber MPOs.
Some in the industry propose the use of cassettes that break out a 24-fiber MPO into three 8-fiber connectors as an ideal solution. While I cannot disagree that it saves 33% of the fiber, the glass itself accounts for a very small percentage of the total cost of ownership. The increased loss of the 24-fiber MPO is a much bigger issue, especially when we move from simple 2-point channels to 3- or even 4-point channels that provide much more architectural flexibility through the use of zone distribution areas (ZDAs). In fact, the 24-fiber MPO exceeds the 1dB connector loss limit for 40 Gig over OM4 fiber in a 2-point channel. Furthermore, in looking at the proposed 100GBase-SR4 standard, a 16.7m total reach for a 3-point configuration (ZDA) over OM3 fiber is going to be a challenge for most data centers.
This topic was covered in greater detail in my recent Cabling Installation & Maintenance webinar titled, "Fiber Connectivity for High-Speed Applications". I will also expand more on the issue of multi-point architectures using 12- and 24-fiber MPOs in my next blog.
In the meantime, Belden continues to provide its customers with superior products and support on LC, 12-fiber MPOs and 24-fiber MPOs—while providing them with a clearer picture on the factors that can impact their business.
Dwayne Crawford has more than 20 years of experience in the datacomm industry. He has served on several international standards committees to advance high-performance/low-latency protocols (such as IEEE-1394, GigE Vision and CameraLink) used in real-time image processing and utilizing high-performance computing platforms.