Fiber Optic OTDR Launch Cable Box
In fact, it is a long piece of fiber, usually exactly 1 kilometer long, which is compactly wound in a coil and terminated with two connectors. All this is placed in a neat box. Why do we need it?
We need a launch cable in order to better see the beginning of the FO line. More precisely – in order to assess signal loss on the patch cord, socket and cross-connect of the near end of the FO line.
As we remember, we cannot see the beginning of the FO line because of the dead zone. But we can connect a launch cable between the FO line and the OTDR, and then the fiber section, previously not visible because of the dead zone, will be located somewhere in the middle of the FO line, and the dead zone will now be on the kilometer-long coil! Have we hit the jackpot here? Not exactly: there is a cross connect between the launch cable box and the FO line, and the trail on the OTDR trace resulting from its peak will be the same width as the dead zone, as well as other mechanical connections. So we still will not see the beginning of our FO line - the trail from the peak in the mechanical connection between the launch cable and the cross-connect of the FO line will cover everything. Then why do we need a launch cable? Let us try to explain it. Measuring without a launch cable, we cannot learn anything definite about attenuation at the beginning of our FO line. And when using a launch cable, we now get a flat section 1 km long on the OTDR trace before the FO line starts (well, maybe a bit less than 1 km – a part of it will be taken by the dead zone), and there is a section of some length (before the first splice / cross connect) at the beginning of the FO line!
Now we can measure signal loss on the initial cross connect using the method of four markers! And it is great. We will see a typical picture of cross connection, where attenuation can be estimated. Yes, we will not determine exactly where signal loss begins, and we will not figure out exactly if there is attenuation (either it is signal loss in the connector between the launch cable and the cross connect, or there is a defective socket, or there is something wrong with the cross connect, or there is something wrong with the segment of first meters of cable when leaving the cross connect). However, if we establish that there is a mess at the beginning of the FO line, we can check all these assumptions and still find and eliminate the cause of signal loss.
Without an OTDR launch cable box, we could simply miss out the very fact that there is something wrong at the beginning of the FO line. Unless there were something seriously wrong at the beginning of the FO line, we wouldn’t notice it (the entire OTDR trace is very noisy, it is located low, close to the noise level, the dead zone is terrible, etc.). However, in practice, a launch cable is needed very rarely.
We discussed the issues related to measurements on optical cables. This topic is not easy.
If you want to become a good specialist in making optical fiber/cable measurements, you will need a lot of practice. There will always be cases that cannot be explained straight off or some non-typical situations and intricate arrangements for splice fusion.