After the nonlinear optical phenomena were discovered, people began to research the techniques
to detect the optical nonlinearities of materials. In this paper, a new optical nonlinear measurement
technique-4f coherent imaging system is recommended. The system has many advantages: single shot
real-time measurement, simple experimental apparatus, high sensitivity, being able to detect the magnitude
and sign of both nonlinear absorption and refraction at the same time, low requirement of beam spatial
distribution, and so on. This paper introduces the theory of the 4f system and makes a detailed review and expounds
development and application of the 4f coherent image system. The nerve of the experiment is improving the phase
diaphragm. The shape of the diaphragm from the double-slits to the small rectangular object, and transition
to a circular aperture, finally forming a circular phase diaphragm, which is a circular aperture in the center
add a phase object. Following these diaphragm changes, the sensitivity of the system is greatly improved.
The latest developments of the system are series-wound double 4f coherent imaging technique and the
time-resolved pump-probe system based on NIT-PO. The time-resolved pump-probe system based on NIT-PO can be
used to measure the dynamic characteristics of excited states nonlinear absorption and refraction.