A wide band laser modulator is designed on the basis of electro-optic modulation principle. A trgvelling wave mode is adopted. A piece of 3x3x80 mm3 single crystal GaAs is used as the modulating crystal. The output power from the high frequency generator'is over 250W with 50ohm output impedance in the frequency range of 30 MHz to 400 MHz. The electrical matching between the modUlator and the generator is good. The bandwidth of the modulator is measured to be 400 MHz. Intensity modulation experiments are performed with a CO2 laser. High frequency HgCdTe infrared detectors in metal dewar are measured with this system to have a time constant below 1 ns.