Editing Substitution method

{{for|the solution method to simple constrained optimization problems|Constrained optimization#Substitution method}}

In [[optical fiber]] technology, the '''substitution method''' is a method of measuring the [[transmission loss]] of a fiber. It consists of:
#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&nbsp;meter long reference fiber having physical and optical characteristics matching those of the fiber under test, 
#measuring the [[Power (physics)|power]] level at the output of the reference fiber, 
#repeating the procedure, substituting the fiber under test for the reference fiber, and
#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. 

The substitution method has certain shortcomings with regard to its [[accuracy]], but its simplicity makes it a popular field test method. It is conservative, in that if it were used to measure the individual losses of several long fibers, and the long fibers were concatenated, the total loss obtained (excluding [[Fusion splicing|splice]] losses) would be expected to be lower than the sum of the individual fiber losses. 

Some modern optical power meters have the capability to set to zero the reference level measured at the output of the reference fiber, so that the transmission loss of the fiber under test may be read out directly.

==See also==
*[[Cutback technique]]

==References==
*{{FS1037C}}

[[Category:Fiber optics]]


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