4f phase imaging method for high sensitively measuring optical nonlinearity of material

The invention discloses a 4f phase imaging method for high sensitively measuring optical nonlinearity of a material, and belongs to the nonlinear photonics material and nonlinear optical information processing field. The 4f phase imaging method comprises the following steps that a laser beam is accurately and straightly emitted to a polyolefin (PO) baffle plate, a 4f system and a ring-shaped attenuation sheet after being expanded, and an emergent facular is imaged on a charge coupled device (CCD); partial split beam passing through the PO baffle plate permeates a reflection mirror, a convex lens, a reflection mirror and a beam splitting lens to be imaged onto the CCD after being reflected by the beam splitting lens; a beam passing through the convex lens is focused onto a sample to be tested, so that the sample produces optical nonlinearity; the PO baffle plate and a CCD image sensor are respectively arranged on an object plane and an image plane of the 4f system, and the ring-shaped attenuation sheet is arranged to be close to the CCD image sensor; when phase difference of one-half phi is generated in the center of the PO baffle plate, the light intensity is simultaneously and linearly attenuated in a central PO area, and the rest ring-shaped facular is attenuated at the same proportion by the ring-shaped attenuation sheet. Due to the adoption of the 4f phase imaging method, attenuation of the background light and the signal light in different proportions can be realized, and the measurement sensitivity is greatly improved.