how to eliminate the stripes when doing engraving

Engraving with RF co2 laser. 20KHz. dual-direction engraving

see the stripes on the engraving shape.  but when do a single-direction engraving, everything is ok.

see below:

According to the motion controller, the motion is not different with the motion and only the direction is changed. the acceleration stage , constant speed stage and deceleration stage is almost the same. the only difference is there is a longer rest time for the laser tube in the single direction application compared with dual direction. So it is mainly caused by the laser tube itself. So we change the PWM frequency of the laser tube , the stripes are disappeared when doing a dual-direction direction.