We report a new type of all silicon surface-normal optical intensity modulator at 1.3-mu-m, which can be easily butt-coupled with a cleaved single mode fiber. The device utilizes free carrier effects in silicon to achieve phase modulation, and a built-in Fabry-Perot cavity to convert the phase modulation into intensity modulation. We have demonstrated 10% modulation depth with driving current density as low as 6 x 10(3) A/cm2, which is an order of magnitude smaller than the best result ever reported for silicon optical modulators. The measured 3 dB bandwidth is 40 MHz.