Studies Of The 4f Coherent Imaging Technique Used To Measure The Third-Order Nonlinear Refractive Index

Materials that possess nonlinear optical (NLO) properties have been investigated extensively for their potential applications in optical switches, optical limiting devices, and received much attention. Nonlinear optical measurement technology is one of the key technologies which are used to study the nonlinear optical materials. In recent forty years, a lot of methods were invented to measure the nonlinear refractive index of the nonlinear optical material, such as interferometric measurement,Degenerate Four-Wave Mixing,Kerr ellipsometry,Z-scan and so on. The 4f coherent imaging technique is a new method. It has many advantages such as one laser shot, simple optical path and lower sensitivity to statistical fluctuations of the laser beam, which raises people's interest and promotes the in-depth study.This paper introduces the theory of 4f coherent imaging technique, reviews the progress of this method in experimental improvements and application. According to the temporal distribution of Gaussian beam, an item of Gaussian time was introduced in the theoretical simulation .When the nonlinear absorption was omitted, how the nonlinear refractive index was affected by the laser's temporal distribution was studied . We improved the 4f coherent imaging system to measure the nonlinear refractive index when the pulse is unstable. The traditional reference system was designed as an another 4f system which has a longer focal length, and ensured that the input and the output of the reference optical system were coincident. This reference system can monitor the fluctuation of the laser energy and also can monitor the spatial distribution of the incident pulse at the same time. Inputting the reference figure, we can avoid the impact of the instability of the pulse. It becomes a reality that the improved 4f coherent imaging system can measure the nonlinear refractive index when the pulse is unstable. We used the pulse of the optical parametric generator (OPG) to measure the third-order nonlinear refractive index of C7H8 and verified the reliability of the improved 4f coherent imaging system.