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We experimentally demonstrate an all-optical approach for generating frequency-shift keying (FSK) radio-frequency (RF) signal based on nonlinear polarization rotation (NPR) in a highly nonlinear fiber (HNLF). A continuous wave probe beam is polarization-rotated by an optical amplitude-shift keying control beam via NPR in the HNLF. After removing the undesired control beam by a tunable optical filter, the polarization-rotated probe beam is converted to an intensity-modulated optical signal using a polarizer. By carefully adjusting the polarization controller before the polarizer and the optical power of the control beam, FSK RF signals are generated after photodetection. [ABSTRACT FROM AUTHOR]