Launch cable, sometimes is called launch box, fiber reel, or bare fibers. It is acturally a longer fiber jumper, but with much lower insertion loss of both cable and its connectors.
When users measure fibers with OTDR, launch box is often used to
1. Protect bulkhead on OTDR from contaminated
2. Change connector type from OTDR to fiber under test (from FC/UPC on OTDR to SC/APC on fiber under test)
3. Checking first connector condition of fibers under test
One more thing, receive cable is often used at end of fiber under test.
OTDR model 8000MAX, 980EXT, 980RTC supports deduct launch cable or receive cable length on OTDR curve.
See the reference,
Pulse width & Launch cable length
