I am working on femtosecond laser drilling .The material on which i am performing the drilling processes is just 4 micrometer thick pasted on glass . Using Bessel beam our task to drill micro hole without damaging the glass substrate with Perfect homogeneous side wall inside the drilled hole. During experiment we observe that the entrance aperture is bigger than the exit one and it has a slight taper shape with multi steps of different layers . Is there any suggestions to minimize this tapper issue and find the Homogeneous Hole. We know this effect is because of BESSEL concentric rings which have some energy value around the Central hole of beam.