How to Test Fiber Optic Link Loss

As IoT and big data drives need to increase bandwidth and processing speed to access, transport and store more data than ever before, the growth of high-speed fiber connectivity in LANs and data centers continues to grow. Today, average data centers can contain thousands of fiber links, with hundreds of thousands of high-speed computing environments.

Fortunately, the Ethernet application standard has been synchronized with the established 40 and 100 Gigabit speeds and is developing 400 GbE. However, as transmission speeds increase, fiber insertion loss (eg, attenuation) requirements become more stringent than ever. The insertion loss budget is now one of the top concerns for network and data center managers; maintaining a plug-in budget for a particular application ensures that the transmitted signal arrives correctly at its final destination.

Optical Loss Test Sets (OLTS) help network and data center administrators ensure fiber links stay within budget and provide expected performance

The following are best practices for OLTS testing that is critical to getting the most accurate loss measurements. The loss budget for 40 and 100 gig applications is about half of 10 gig, and every 0.1 dB loss is important.

Reference Cords

One end of the OLTS uses a stable light source and the other end uses a power meter to measure link loss. A reference wire is required to connect to the fiber cable under test – one end is called the “emission” reference line, from the light source to the terminal block, and from the terminal block to the “tail” reference line of the power meter. remote.

So what makes the reference line different from the typical jumper? The reference line is a high-quality test line with reference level connector termination, optical core optical alignment, single mode low loss less than 0.2 dB, multimode less than 0.1 dB. Typical fiber jumpers for normal daily repairs range between 0.3 dB and 0.5 dB and should not be used.

Setting reference

The OLTS must be set to zero dB loss before performing the insertion loss test. This is done by setting a reference that takes into account the loss of the reference line. Consider setting a reference on the OLTS to achieve zero loss, just like recalibrating the scale to zero to get the exact weight.

There are several ways to set up references when using OLTS. The industry standard specifies 1 fiber jumper, 2 fiber jumper, and 3 fiber jumper reference methods. The recommended 1 jumper method provides the best accuracy. Let us see why.

As shown in Figure 1, the 1 jumper reference leads to the point where the emission line is connected from the source to the point where the power meter is connected. If you use a double jumper reference, the mating connections between the two jumpers are also referenced. A 3 jumper reference will add another referenced paired connection.


So how does this translate into testing? Depending on the method you choose, the losses associated with the end of the fiber being tested will be included or excluded in the final loss measurement.

When using a double jump method that references a connection, the final measurement will only include one end connection. Therefore, the 2 jumper reference only provides a partial description of the total loss. And since all connections play an important role in overall channel loss, the dual jumper reference method is not recommended; it provides the highest uncertainty for all methods. The 3 jumper method references two connectors, thus eliminating the loss of connection to both ends of the cable under test.

The 1 jumper method is the only method that includes loss of connection at both ends, actually simulating the use of cable equipment and providing the lowest uncertainty of all measurement methods. However, it is important to remember that after using a single emitter to set the reference, the proper loss of the tail should still be measured – multimode maximum 0.1dB and single mode maximum 0.2dB. And don’t remove the startup guide after setting the reference, otherwise, you will have to restart and reset.

Exceptions to each rule

The single jumper reference assumes that the connector on the power meter is compatible with the cable under test and the test cable is supported by an interchangeable adapter. Although rare, in some cases, people may choose 2 jump shots or 3 jump bids. The double jumper method can be used when there are plugs on both ends of the cable, or if there is a plug at the end of one cable and an adapter at the end of the other cable.

When a better method is not practical, a 3-jumper method can be used, such as when the fiber connector of the test reference line does not match the fiber type of the link, or when the connectors at both ends of the wire are different from each other.

For example, a 3-jumper reference line is used to test an MPO link with an LC tester interface using an MPO to LC fanout cable, or when a pigtail connector is connected to two cable ends and directly connected to a transmission device.

Regardless of the exception, use the 1 jumper reference method whenever possible when testing fiber link loss – this is the method used by professionals.

Original Article Source from, Author: Mark Mullins, Manager of Americas Field Marketing, Fluke Networks