I report modulation format conversion technology that maps on-off keying to 4-level pulse amplitude modulation signals. The conversion technology is based on balanced detection and intensity modulation. Two input optical on-off keying signals into a balanced photo detector produce an electrical signal that drives an intensity modulator to generate an optical pulse amplitude modulation signal. Two 20Gbit/s on-off keying signals were successfully converted into a 40Gbit/s pulse amplitude modulation signal.