Fiber Certification Testing: All You Really Need Is Loss, Length, Polarity (and Sometimes Reflectance)
As data rates increase to 400 Gig and beyond, and new fiber applications emerge, it’s easy to be confused about which fiber testing parameters are enough to guarantee support for high-speed applications. Other than for short-reach single-mode applications that are more susceptible to reflections and take connector reflectance into consideration, insertion loss testing, length, and polarity are really all you need for Tier 1 certification testing.
Insertion Loss Is Critical in Certification Testing
Measured in decibels (dB), insertion loss is the reduction in signal power that happens along any length of cable for any type of transmission. The longer the cable, the more a signal is reduced (or attenuated) by the time it reaches the far end. In addition to length, events that cause reflections also contribute to overall loss, including connectors, splices, splitters, and bends.
The reason we care so much about insertion loss for fiber links is that to adequately support an application, the signal must have enough power for the receiver to interpret it. In fact, all IEEE fiber applications specify overall channel and connector loss limits — it is the single most important parameter that determines the performance of practically every fiber application, and it’s the critical parameter you need when conducting Tier 1 certification testing with your CertiFiber™ Pro Optical Loss Test Set.
It’s important to note that maximum allowable insertion loss varies based on the application, with higher-speed and multimode applications having more stringent insertion loss requirements. Because insertion loss is directly related to length, higher-speed multimode applications also have reduced distance limitations — the IEEE essentially balances loss and distance requirements to meet the majority of installations. For example, 10 Gb/s multimode (10GBASE-SR) applications have a maximum channel insertion loss of 2.9 dB over 400 meters of OM4 multimode fiber, while 400 Gb/s multimode (400GBASE-SR4) applications have a maximum channel insertion loss of 1.8 dB over just 100 meters of OM4. In contrast, single-mode LR applications have maximum channel insertion losses of about 6.0 dB over 8 kilometers.
Length Plays a Key Role
If the insertion loss is low enough, it means that the signal can be detected at the far end of the link. So why does length matter? Two reasons:
The proper operation of the communications protocol is based on an expectation that signals will be received at the far end within a specified time. Longer lengths mean greater delays.
Dispersion of the waveform as it travels through the fiber can distort it to the point that the receiver can’t tell the difference between a one and a zero. This is tied to the fiber’s modal bandwidth (more on that below).
Standards developers therefore limit the length of the link based on the dispersion characteristics for the type of fiber. That’s why 400GBASE-SR4 is limited to 60 meters on OM3 and 100 meters on OM4 or OM5.
Polarity Always Matters
For every fiber link, the transmit fiber (Tx) must match the corresponding receiver fiber (Rx) at the other end. Proper polarity ensures that this correspondence is maintained.
For parallel optic applications that use multi-fiber MPO connectivity, polarity can be tricky since multiple fibers need to correspond. When polarity isn’t maintained, the link simply won’t work. That’s why Fluke Networks fiber certification testers verify correct polarity for patch cords, permanent links, and channels.
Sometimes Reflectance Is Required, Too
While fiber connectors require a certain reflectance performance to comply with industry component standards, it’s not something you typically need to test for — unless the link needs to support short-reach single-mode applications.
Multimode transceivers are extremely tolerant of reflection, but single-mode transceivers are not. And the low-cost, low-power single-mode transceivers used in short-reach applications like 100GBASE-DR, 200GBASE-DR4, 400GBASE-DR4, and 800GBASE-DR8 are even more susceptible to reflectance.
As a result, IEEE actually specifies insertion loss limits for short-reach single-mode DR applications based on the number and reflectance of connections in the channel. As shown in Table 1, in a 400GBASE-DR4 application with four connectors that have a reflectance value between -45 and -55dB and no connections between -35 and -45dB, the insertion loss is 3.0dB (highlighted in red). If the reflectance of those same four connectors is between -35 and -45dB, the maximum insertion loss goes down to 2.7dB (highlighted in yellow). While you can design your short-reach single-mode system based on manufacturers’ reflectance values, reflectance can worsen over time, so it’s best to build in some margin.
The only way to accurately measure the reflectance of connections for short-reach single-mode applications is to use an Optical Time Domain Reflectometer (OTDR) like Fluke Networks OptiFiber™ Pro OTDR that characterizes the loss and reflections of individual splices and connectors. Note that an OTDR is also required for Tier 2 (or extended) testing.
Table 1. Maximum channel insertion loss for 400GBASE-DR4 applications
What About Bandwidth Testing?
The bandwidth of fiber is specified as modal bandwidth or effective modal bandwidth (EMB), which refers to how much data a specific fiber can transmit at a given wavelength. Fiber applications are specified for use with a minimum bandwidth fiber. Bandwidth testing is done by fiber manufacturers and involves a complex laboratory test with specialized analyzers to send and measure high-powered laser pulses — it’s costly to test accurately in the field, and you don’t need to worry about it if you stick to the standard-defined length and insertion loss limitations.
That’s not to say that once the fiber network is certified and up and running, you won’t test the throughput capability of a channel, but that’s a test of actual connection speed — not the bandwidth of the cable itself. If your insertion loss is still in compliance yet your network isn’t performing, it’s probably worth breaking out the OTDR to check the reflectance of events on the link. If both loss and reflectance are OK, the problem is most likely with the active equipment rather than the cabling. If you do have a loss problem, then it’s time to troubleshoot.
In short, insertion loss plus length and polarity (Tier 1 testing) are almost always what will determine application support — but there may be occasions when you need to add reflectance (short-reach single mode applications and Tier 2 testing).