as the voltage is always the same with PWM, if I filtering it, it'll be impossible to change dimming of the LED
but as it drive by the PIC on my serial LCD,
NO, see how PWM works:-http://www.thebox.myzen.co.uk/Tutorial/PWM.html
No the PWM frequency is controlled by the arduino, you just need to alter some values stored in registers.
I did say google it. Never mind I did it for you:-http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1152547089