Loopback testing of a medium length span. Loopback testing of a medium length span.
Aug 20

Launch Cables, Part 2: Using Launch and Receive Cables with an OTDR

Launch Cables, Part 2 discusses the practical use of launch and receive cables in specific testing situations with an OTDR. Part 1 (last month) dispelled some of the myths around launch cables, and explained the basic theory behind their use in testing the near end and far end connectors of a fiber under test (FUT).

Key Definitions

Launch and receive cables are relatively long jumpers or lengths of terminated fiber that are connected to one end of the FUT to aid in OTDR measurement. This is distinct from an “access jumper”, which is generally very short (less than 3 meters) and simply serves as a transition between the OTDR connector and a launch cable or FUT. The OTDR can be connected either directly to a launch cable, or through an access jumper to a launch cable. This helps to reduce repeated direct connections to the OTDR port.


Medium Length Span Testing

With a Launch Cable Only

A single user might carry a launch cable of 250 meters to 1 km to test a medium length span, such as an interoffice fiber span of approximately 10 to 80 km. In this situation, the technician would likely use a pulsewidth of 10 to 30 meters, so even a 100-meter launch cable would be appropriate. The launch cable is attached to the near end connector and an OTDR trace is taken for all of the near end fibers.

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Figure 1. Testing a medium length span with a launch cable. Note that the loss of the near end connector can be measured.

 If bidirectional testing is desired, the technician moves to the far end, attaches the launch cable to the far end connectors, and tests them with the OTDR. All of the mid-span splices would be measured bidirectionally, and the unidirectional measurements of the near and far end connectors are accepted.

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Figure 2. Bidirectional traces with launch cable only. Near end and far end connectors are only measured unidirectionally.


With Launch and Receive Cables

The tester attaches a launch cable at the near end, while a partner at the far end moves a receive cable from port to port as testing is conducted. This method measures both the near and far end connectors from the near end.

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Figure 3. Testing a span with a launch and receive cable.  Both near and far end connectors are measured.

 If bidirectional testing is required, the OTDR can be carried to the far end and the same procedure followed. However, note that both the launch cable and the receive cable should remain in their original locations for consistency in bidirectional analysis. The ideal way to do this is for both technicians to have their own OTDRs and for the two OTDR shots to be taken sequentially. There are OTDR sets that will communicate with each other and do this automatically.

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Figure 4. Bidirectional overlay of traces with launch and receive cables. All events are measured bidirectionally.


Long Length Span Testing

While the techniques above are appropriate for very long spans, sometimes the pulsewidth required to test a span is so long (more than 1 km) that acquiring a launch cable of sufficient length becomes impractical.

Multiple Pulse Testing

In this case, multiple pulse testing can be conducted. The span is first tested with a 1-km pulse to measure the performance of the full span, and then tested again with a shorter pulse (e.g., 10 km) and a launch cable to measure the performance of the near end connector.

Note: Due to the shorter pulse, conducting multiple pulse testing on short and medium spans can provide more detail about the near end connections in the central office, headend, or vault splices.

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Figure 5. Measuring a long span with a launch cable. A long pulse displays entire span; a shorter pulse will not, but allows measurement of events close to the near end connector.


Loopback Testing

Medium Length Spans

This test technique is more common where larger interoffice switches are closer to each other (such as in many European cities) and the testing is done internally rather than by a contractor. The launch cable is attached to Fiber 1 at near end. A loopback cable of receive cable length is attached at the far end between Fiber 1 and Fiber 2. A receive cable is attached to Fiber 2 at the near end. The operator tests through the launch cable from Fiber 1 to Fiber 2, and then tests through the receive cable from Fiber 2 to Fiber 1. Note that in this arrangement, the launch cable acts as the receive cable on the second test.

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Figure 6. Loopback testing of a medium length span. The OTDR can be moved to fiber 2, and the traces overlaid for full bidirectional measurement (not shown).

There are software packages that will analyze the OTDR traces bidirectionally for each fiber. Its advantage over conventional testing is that it only requires a single OTDR and one trace per fiber ; the traces for Fiber 1 and Fiber 2 combine to effectively test one another bidirectionally. The only disadvantage is that the additional length may require a longer pulsewidth, so the far end events will not be able to be resolved as finely as when testing from each end.


Fiber to the Antenna Testing

Surprisingly, some of shortest spans can benefit from the use of launch and receive cables. When fiber is run to the top of cellular towers, a single fiber span might be less than 25 meters (for a 75-foot tower). Rather than testing between the top and bottom of the tower, the tower “top hand” carries a 10- or 20-meter jumper and connects Fibers 1 and 2 while the tower bottom crew does a loopback test with a 20-meter launch cable and a 50-meter receive jumper. This is just another example of the loopback technique discussed above. Because the total distance and loss is so low, an ultra-short pulsewidth (e.g., one meter) is used. This is usually enough to verify that the connector reflectance performance is acceptable and the combined loss of the connector pairs (where installed 1- or 2-meter jumpers prevent resolving connectors individually) is acceptable. It can also measure total end-to-end loss up and down the tower by doing a 2-point loss measurement from the launch cable backscatter to the receive cable backscatter.

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Figure 7. Loopback testing of fibers 1 and 2 from tower bottom to tower top for an FTTA application. (Courtesy VIAVI Solutions)

It is important that the launch and receive cable have similar fiber types. A low-reflection terminator is often installed on the receive cable to prevent a very high reflection from an “open” connection so near the OTDR launch.



Launch and receive cables can be invaluable tools for analyzing a fiber span with an OTDR. It is important that the operator understands their uses and limitations, applies them appropriately, and uses high-quality, properly-maintained cables.


Steve Wolszczak
Fiber Technology Manager
Light Brigade