Insertion and Substitution

Loss is a function of both wavelength (lower loss at higher wavelength) and source type (or more accurately, the mode fill typical or the source type, since LEDs launch light with a wider bean angle (higher mode fill) than a laser.) 

The source should match the system source in type (LED or laser) and wavelength (650 nm for LEDs and lasers) to accurately reproduce system loss. Systems using VCSELs at 850 nm may require testing with those sources.
– 650 nm LEDs
– 650 nm laser
– 850 nm VCSEL
– Connects to “launch” reference cable to connect to cable under test

The source requires a launch reference cable to mate the the cable under test. The launch cable is used as a the calibration point for source power (the “ 0 dB reference”) and reference connector to mate to the cable under test if the connectors are matable types.Otherwise a cable substitution test is used.
FOTM, Chapter 6,7,9,17, DVVC, Chapter 12,13, 14