Investigation of the third-order nonlinear refraction using 4f coherent imaging system with positive–negative bar phase objects

We report a new bar phase object (PO) with positive and negative phase shift in the 4f coherent imaging system to measure the third-order nonlinear refractive coefficient of nonlinear sample. The sensitivity of the measurement system can be enhanced by using the special positive–negative bar phase object (PNB-PO) compared to the traditional circular PO. It is shown that the increase of the sensitivity is more conspicuous in the negative nonlinear phase shift area for the positive nonlinear sample, and is almost twenty-fold near the nonlinear phase shift ΦNL=−π. Moreover, compared to the positive–negative circular phase object (PNC-PO), the fabrication of the PNB-PO is easier. The measurement system with PNB-PO is experimentally validated by measuring the nonlinear refraction of CS2.

► In the study, We use a unique positive–negative bar phase object (PNB-PO) to measure the optical nonlinear refraction with high sensitivity.
► Especially, the sensitivity reach almost twenty times near the nonlinear phase shift ΦNL=−π compared to the traditional circular PO.
► Furthermore, compared to the positive–negative circular phase object (PNC-PO), the fabrication of the PNB-PO is easier.
► Therefore, we can use it to measure the optical nonlinearity of certain films.