What should be the minimum length for an OTDR launch cable?

Question: What should be the minimum length for an OTDR launch cable? Also, how does connecting a launch cable between the OTDR and the fiber being tested overcome the problem caused by connecting the fiber directly to the OTDRs output connector? Doesn't the "dead zone" created where the fiber is connected to the end of the launch cable cause the same problem?

Answer: The minimum length for a launch cable depends on the pulse width of the OTDR being used. A good rule of thumb is that a 10 nanosecond pulewidth equals a 1 meter dead zone. The launch cable must be slightly longer than the dead zone in order to view the first connector in your cable run. The connection between the fiber under test and the launch cable is commonly referred to as an "event dead zone" or an "attenuation dead zone". These secondary events create a much shorter blind spot (<1m typically) as compared to the initial dead zone of 5-10 meters or more.