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Frequency modulation to amplitude modulation (FM-to-AM) conversion is an important issue that can prevent fusion ignition with high power lasers such as the Laser MegaJoule (LMJ). On LMJ, most of the FM-to-AM conversion is expected in the so-called frequency conversion and focusing system, which is a nonlinear system. However, we propose linear transfer functions to compensate the effect of frequency conversion on FM-to-AM conversion. We show that most of AM distortion can be reduced by practical systems: for beam intensity up to 3 GW/[cm.sup.2], the FM-to-AM conversion level can be divided by at least 2, and we almost cancel intensity modulation for intensities below 1 GW/[cm.sup.2]. OCIS codes: 060.5060, 070.2615, 140.3518, 190.2620.