With the increasing complexity of Photonic Integrated Circuits（PICs）

it is necessary to carry out their real-time monitoring in links by using the optical power monitor for the effective analysis of the device behavior without affecting the normal operation of the system.This paper summarizes the latest research advances in silicon-based optical power monitors with the emphasis on the optical power monitoring schemes in different absorption mechanisms

such as surface-state

two-photon

Ge and defect-state.and their respective advantages and disadvantages.Finally

it discusses in detail the great superiorities of using the defect-state absorption mechanism for photo-electric conversion.