Substitution method

In optical fiber technology, the substitution method is a method of measuring the transmission loss of a fiber. It consists of:

1. using a stable optical source, at the wavelength of interest, to drive a mode scrambler, the output of which overfills (drives) a 1 to 2 meter long reference fiber having physical and optical characteristics matching those of the fiber under test,
2. measuring the power level at the output of the reference fiber,
3. repeating the procedure, substituting the fiber under test for the reference fiber, and
4. subtracting the power level obtained at the output of the fiber under test from the power level obtained at the output of the reference fiber, to get the transmission loss of the fiber under test.