OTDRs measure fiber loss, event loss, and event reflectance indirectly using small amounts of backscattered and reflected light. For this reason, they have certain limitations, particularly where the magnitude of the reflected light is much higher than the backscatter level. When this happens, the recovery time of the detector circuit to the lower (backscatter) level creates a blind spot called a “dead zone”.
Event dead zone (EDZ) and attenuation dead zone (ADZ) refer to the length of fiber after a reflective event in which reflective or non-reflective events cannot be measured. This length depends on the pulse width used and the recovery time of the OTDR’s detector after a reflection.
The initial dead zone (IDZ) is the length of fiber that cannot be measured after the OTDR connector. This is largely an historic specification. This length referred to the time the OTDR required to switch from transmitting the outgoing pulse to receiving the incoming backscatter and reflection. For older OTDRs, the IDZ was much larger than the ADZ and EDZ. Typical vintage OTDRs (pre-1990) with a 10-meter ADZ might have a 150-meter IDZ. Thanks to advances in electronics and optics, modern OTDRs have an IDZ that is essentially equal to the ADZ.
Launch cables are preterminated lengths of simplex cable — generally ranging from 150 meters to 2 kilometers — that are connected between the OTDR and the fiber under test (FUT).
Early Use of Launch Cables
The IDZ problem caused users to be frustrated with their inability to “see” (measure with an OTDR) the first few hundred meters of fiber. A typical trace of a 3-km span connected directly to the OTDR with a 150m IDZ and 10m ADZ (typical for older OTDRs) might look like the trace shown in Figure 1.
Figure 1. An OTDR trace showing 150 meters of IDZ at the start of a 3-km span.
Because of the IDZ, the user cannot measure the first 150m of fiber, or roughly 5% of the span.
Now the user adds a 1000-meter launch cable (Figure 2). There is only a 10m ADZ on the FUT, or 0.3% of the span that can’t be measured. This is a big improvement.
Figure 2. An OTDR trace showing 150 meters of IDZ and 10 meters of ADZ at the start of a 3-km span and a 1-km launch cable.
Older launch cables were very long and often referred to as “pulse suppressors” or “dead zone eliminators” even though they didn’t actually suppress any pulses or eliminate any dead zones. They simply exchanged one dead zone (the IDZ) for another, much shorter, one (the ADZ) — so were perhaps more aptly named “pulse avoiders” or “dead zone exchangers”.
Modern Use of Launch and Receive Cables
Loss is measured from the backscatter level (slope line) of the fiber in front of the event, to the backscatter level (slope line) of the fiber behind the event. It is the difference in the backscatter levels of the fiber preceding the event and the fiber following the event. Thus, all OTDR loss measurements are from backscatter to backscatter, or fiber to fiber.
Figure 3. OTDR losses for both reflective and non-reflective events are measured from the backscatter preceding the event to the backscatter following the event.
Even though modern OTDRs don’t have a large IDZ problem, they still have an ADZ at the beginning of the trace. We can see more of the trace, but still cannot measure the loss of the connector that is directly plugged into the OTDR. We need a section of fiber and a backscatter level preceding the first connector to measure its loss. Today, the primary purpose of a launch cable is to provide a fiber backscatter level preceding the first, or near end, connector, while the FUT provides the backscatter level following the event. Similarly, a receive cable is placed at the far end to provide a following fiber that enables measurement of the far end connector’s loss performance.
Practical Considerations in the Use of Launch Cables
Without an IDZ of hundreds of meters, does a launch cable really need to be 1000 meters long? Probably not. In order to measure the loss of the near end connector accurately, the OTDR requires a length of fiber just long enough to establish the preceding backscatter baseline (or following for the far end). Because the ADZ is strongly related to the pulse width used, a good rule of thumb is that the launch cable be 3-5 times the length of the pulse width to ensure a linear section upstream of the loss measurement.
In order for a launch cable to be optimally effective, it should meet the following criteria:
- It should (ideally) be the same type of fiber as the span to be tested.
- It needs the same type of connector as the span to be tested or must be compatible through a hybrid adapter.
- It should be 3-5 times the length of the pulse width.
Launch cable length and use can be highly dependent on several factors. Specific uses and techniques will be discussed in Launch Cables, Part 2: Using Launch and Receive Cables with an OTDR.
Fiber Technology Manager