A top-hat intensity profile of a laser beam over its cross-section is often desired in micro-machining applications such as laser ablation, lithography etc. In laser drilling technology, the sharp edge is not obtainable for the normal Gaussian laser beam because of the heat penetration into the outer area of the irradiated region. While for homogeneous intensity distribution, the minimum edge roughness can be obtained. The above graph shows a standard laser beam shaping scheme using diffractive optics. The two key optics are the shaping element which gives a certain phase distribution to the laser beam, and the Fourier lens which focus the beam into the target plane.

We studied the spatial average intensity profile of an ultrashort laser pulse passing through a laser beam shaping system, which uses diffractive optical elements to reshape the Gaussian beam profile into a flat-top distribution. The Nonlinear Schrodinger Equation is solved numerically to simulate the nonlinear optical effects in this system. Our data and calculation show that this system works well for ultrashort pulses (> 100 fs). We also studied the effects of lateral misalignment, beam size deviation and defocusing on the beam intensity profile.

References:

1.  S. Y. Zhang, Q. Yang, and G. Lüpke, Spatial beam shaping of ultrashort laser pulses: theory and experiment, Appl. Opt. 44 (27), 2005, pp. 5818-5823.
2. S. Y. Zhang, Y. H. Ren, and G. Lüpke, Ultra-Short Laser Pulse Beam Shaping, Appl. Opt. 42 (4), 2003, pp. 715-718.

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Funding: DOE