With the operation of an OTDR, it requires a launch fiber/cable, and sometimes use a receive fiber/cable. The launch cable, is also called a “pulse suppressor” or “dummy fiber”, allows the OTDR to recover after the test pulse is sent into the fiber. This cable also provides a useful reference on the first connector of the fiber under test for measuring its loss and reflectance.
Normally, the first connector is the connector on the patch panel or optical distribution frame (ODF). A receive cable may be used on the far end to enable loss and reflectance measurements of the connector on the end of the cable under test as well, which will confirm the continuity of the cable under test.
Choosing the right length of launch cable depends on three factors, including:
- The optical fiber type that you will be measuring
- The maximum length of optical fiber that you intend to measure, which will determine the largest pulse width being used in the OTDR
- The connector types on the ODF
1) Optical Fiber Type
The optical fiber type and geometrical characteristics of the launch fiber should be similar to the fiber under test. For single-mode fiber, if the cable under test is G.652 and a G.655-style launch fiber is used, an additional loss may be obtained. However, if authenticated and accounted for, the loss is acceptable. This is especially important for multi-mode fiber, because the launch condition must be determined loss measurements accurately, and this is also true for the two different multi-mode fiber core dimension of 50 µm and 62.5 µm.
In addition, the launch fiber should ideally be finished at the polished end. There should not be any presence of internal splices, which would directly add to the total loss of the connector loss.
2) Pulse Width
When you have configured the typical maximum length, you must choose a pulse width to be used at this length. Note that an OTDR will measure the loss from the optical fiber, so it is important to consider loss rather than length. For example, a link with a 2.5 dB macro bend is equal to a fiber length of 10 km at 1550 nm.
A general rule of thumb is to take the length of the pulse width in meters (take the ns and divide it by 10, i.e. 2,000 ns = 200 m), and then add 20% for the “attenuation dead zone.” Although this varies across the range of pulse widths available on an OTDR (typically from 5 ns to 20 000 ns [20 µs]), this calculation serves to provide an upper-end/worst-case estimate.
Therefore, if you want to use a 6,000 ns pulse width, the length of the launch fiber should equate to 6,000 divided by 10, plus 20% = 720 m. Therefore, a 1 km launch cable would appropriate for all situations. However, if you were to then use a 10 µs pulse width, the length of the launch cable would be too short, falling under the required 1.2 km.
If you need a launch fiber that would suffice for all fiber lengths, the only solution would be to go with a length greater than the maximum pulse width of your OTDR. For example, if your OTDR has a maximum pulse width of 20 µs, a 2.4 km cable may be required.
For PON/FTTx MDU testing, a pulse width no greater than 500 ns would be used. Therefore, a launch fiber from 150 m to 300 m would be sufficient.
3) Connector Type
The first purpose of using the launch cable is to isolate the ODF connector in order to calculate its loss and reflectance. It is therefore VITAL for the selected launch fiber to have the correct ODF termination available. If the network is SC/UPC and your launch fiber is FC/UPC, using a “3 m jumper” to connect to the SC/UPC will result in two connector pairs being measured at the ODF, which is clearly not desirable.
As a result, there may be a requirement to keep more than one launch fiber in your accessories for testing fiber networks. Because most OTDRs have a fixed launch (for example, an SC/APC output), a wide range of SC/APC to xx/xPC cables can be used.
Another important use of the pulse suppressor/launch lead is to minimize the number of times the OTDR connector is used, ultimately eliminating the risk of damage to the OTDR connector and extending its service life.
When testing a fiber link without a launch cable, the results will only determine the reflectance of the first connector (pair). This causes an issue because the reflectance is associated with two pairs (OTDR and ODF), however the result DOES NOT account for the loss associated with the ODF connection.
In Figure 2, a 300 m launch cable and a 300 m offset/zeroing is used in order to present BOTH loss and reflectance of the ODF connection.
In Figure 3, the launch and receive cables are used in order to present BOTH loss and reflectance of both ODF connections.
In the example above, the same 300 m launch cable is used with a pulse width that is too large, and as a result, the pulse overtakes the launch cable. The benefit is therefore lost, and the network is 300 m longer than it should be.
Above 90 km, it is common to use 1550 nm/1625 nm for permitting macro bend detection. If macro bend detection is not required, the current practice is to just use 1550 nm.
In addition to allowing the OTDR to recover after sending a test pulse into an optical fiber, use of a launch fiber is invaluable to determining loss and reflectance on the cable under test. However, to achieve the desired results, care must be taken to select the suffice fiber type and length, in addition to the pulse width and cable termination required to isolate the ODF connector. When these factors are taken into account, along with the dynamic range when macro bend detection is needed, the result is more accurate insertion loss and reflectance measurements, and improved long-term OTDR performance.