The frequency conversion is one function of utmost importance in radio over fiber technologies. In this letter, we propose an optical implementation of a radio frequency down conversion based on a bandpass sampling technique realized by a semiconductor optical amplifier Mach-Zehnder interferometer used as an optical signal sampler. An optical pulse source generating 23-ps width pulses at a repetition rate of 7.8 GHz is employed to sample an intensity-modulated optical carrier carrying the signal to be frequency down converted. The frequency down conversion from a radio frequency signal at 37 GHz to an intermediate frequency at 2 GHz shows a positive conversion gain of 4 dB.