MODULATION
ook
DIRECT MODULATION
INDIRECT MODULATION
Common external modulators
MZI
Common external modulators
Subcarrier Modulation
Optical Coupler
Isolator
Circulator

Isolatorallows transmission in one direction but blocks all transmission in the other direction.

Used at the output of optical amplifiers and lasers primarily to prevent reflections from entering these devices, which would otherwise degrade their performance
Insertion loss: the loss in the forward direction, as small as possible
Isolation: the loss in the reverse direction, as large as possible
Typical insertion loss is around 1 dB, and the isolation is around 40–50 dB