Experimental Talbot effect with femtosecond laser illumination

Abstract

Talbot self-imaging effect and fractional Talbot effect of a grating under illumination of monochromatic continuous wave is an important basic diffraction phenomenon, and has been extensively studied. In this paper, we studied the Talbot self-imaging effect of gratings that was illuminated by femtosecond pulse. Femtosecond laser beam is not monochromatic but includes a broad range of spectrum distribution. In this case, observed optical field in the output plane can be regarded as a coherent superposition of the contribution from each frequency component. Based on this assumption, we study diffractive characteristic of femtosecond pulse, and we theoretical analyse the intensity distributions with Fresnel diffraction formula behind a grating under illumination of a femtosecond pulse laser. Intensity distribution on the Talbot imaging planes are obtained, which show that Talbot images of femtosecond pulse are not the same as that under continuous wave illumination. At the same time, experimental results are in good agreement with the theoretical analysis. Rectangular amplitude gratings with different periods (200 μm and 80 μm respectively) were illuminated by an expanded femtosecond pulse laser, and the Talbot images are obtained by using a CCD camera at different Talbot distances. Experimental results demonstrate that the femtosecond pulse laser can cause a large distortion of Talbot images (including the contrast of imaging and the decrease of diffraction efficiency), compared with that under continuous wave illumination. In addition, increasing of the Talbot distance can also result in distortion of a Talbot image. The theoretical and experimental results are helpful for further applications of the Talbot effects under femtosecond pulse illumination.