By: Scott Greco
Professional Services Manager
Optical testing results are under more scrutiny today than ever as contractors are required to submit final reports at the end of most projects. The increased occurrence in disputed test results starts with end users understanding some or all of the test procedures and the remainder of disputes come from contractors not understanding some or all of the test procedures. When we read job specifications for customers one of the most inconsistent testing requirements is the use of launch cables for OTDR testing. It is our opinion and that of most of the industry that launch cables should be used anytime OTDR testing is being performed.
If you are unfamiliar with a launch cable it can be simplified as a length of fiber that is connected to the OTDR test port then connected to the Fiber Under Test (FUT). The launch cable is used to allow the OTDR to make a more precise measurement which will be discussed further in a minute. Launch cables can vary depending on applications but some standard lengths include 500 meter and 1km for Singlemode and 100 meters for Multimode fiber. These launch cable lengths have gained popularity as these distances are perfect for the majority of testing applications.
How does a launch cable create a more accurate measurement? To answer this questions we must first understand a term known as “The Launch Dead Zone”. This launch dead zone is created when an OTDR fires the laser through the first connector attached to the machine. A basic principle of light is that when light changes speeds it creates a reflection. In this case a back reflection is being created at the OTDR port which is being returned to the Avalanche detector, which measures faint back reflections and Raleigh scattering. When this strong back reflection hits the detector it is blinded for a brief moment. In this time the machine is not able to measure light being returned from the fiber. This dead zone can hide events that are close to the OTDR and not provide a measurement of the first connector in the FUT.
When we discuss OTDR testing we must differentiate between short and long distance testing. Short fiber testing would typically be any distance under 20km. True long haul technicians work on spans of fiber that are 25 to 80km in length. Understanding this concept is very important as short fiber testing and long haul testing are completely different animals, but both require launch cables.
Short distance fiber optic networks are being taken to the extremes today as Singlemode fiber (typically used on longer distances) is being implemented in very short runs. The reason for this trend is the significant bandwidth that Singlemode offers for applications such as video surveillance and DAS. One project we recently consulted on had Singlemode runs of 275 feet. This distance is difficult to test with an OTDR due to the lack of fiber length. In this situation it is important to understand that a launch cable must be used to test this fiber. Even if the technician has set the pulse width to the lowest setting of 3ns (for example), a launch cable should still be attached to the OTDR and FUT. Even a 500 meter launch cable will give the OTDR enough fiber to recover before making an accurate measurement on the first connector of the FUT and all fiber after the connector. Without a launch cable on a cable of this length, the odds of getting an accurate measurement anywhere on the FUT are unlikely.
Long Haul testing presents its own challenge to the OTDR technician as spans of fiber are very long and will require a technician to use longer pulse widths. As pulse widths are increased the Launch Dead Zone is increased as well as light is injected into the fiber for a longer duration. An example of pulse width dead zones that will shed light on the point is a 500ns pulse width used to test a 50km run generated a dead zone of approximately 91 meters. That is 91 meters of fiber that would not have been accurately measured and could be hiding events. When pulse widths and dead zones increase the opportunity to miss events that are closely spaced together also increases. These events may be reported as grouped events with potentially inaccurate values or hidden and not reported at all. By using a launch cable of 1km the OTDR is able to recover from the large Launch Dead Zone and measure the FUT more accurately.
Another benefit of using a launch cable is the OTDR’s ability to measure the reflectance value of the first connector in the FUT. Reflectance is the back reflection property of a mated pair of connectors which can be greatly affected by damaged or contaminated ferrules. By using a launch cable the OTDR is able to measure the first connector in the FUT very accurately and confirm the connector is performing to industry standard thresholds. Not using a launch cable will miss this measurement, as the first connector would be contained within the dead zone. The practice of not using a launch cable can also increase the risk of poor launch conditions and spreading contamination or damage to other connectors.
Another great practice to implement into your testing procedures is the use of receiver cables. A receiver cable is a launch cable that is used at the far end of the FUT. By using a launch cable at the OTDR end and the far end, a test can accurately measure all connectors in the system. Receiver cables are also quite handy when testing short runs of fiber. By using a launch cable and a receiver cable technicians can easily isolate the FUT by locating OTDR markers at the end of the launch cable and beginning of the receiver cable. Any fiber between these two points is the FUT.
The final reason for using a launch cable is to reduce the wear and tear on the OTDR ferrule. Industry standards indicate that ferrules should be built to withstand 400 mating cycles. One mating cycle consists of one connect and one disconnect with another ferrule. If a technician is testing a 144 cable from both directions, not using a launch cable the OTDR ferrule will receive a minimum of 288 mating cycles on one cable. This ferrule will certainly need to be serviced after only a couple jobs. Instead, if we use a launch cable, the OTDR is connected to the launch cable and remains connected until the first 144 fibers are tested. If our example has optimum conditions the OTDR ferrule should experience only 2 mating cycles for the entire project. This will allow the OTDR ferrule to last much longer. This habit will also place a high number of mating cycles on the launch cable connectors which are much easier and cheaper to replace. We have seen customers replace damaged launch cabled connectors onsite using a fusion splicer with either pigtails or splice on connectors. This is a major advantage to contractors in the field as a damaged OTDR ferrule would require the unit to be taken out of the field and sent in for service.
In all of our experiences in the fiber optic industry, we have not found one good reason why you should not use a launch. Using a launch cable is an important practice that should be a staple of your OTDR testing procedures. Any project manager that knows fiber optics will require you to use a launch cable. If a customer ever questions you on why you use the launch cable, be sure to educate your customer on the points mentioned in the article. Taking time to educate customers can do a lot in the way of securing future work. If a customer ever asks you why you don’t use a launch cable, get ready to be questioned on every step of the project after that.