This paper discusses the possibility of applying the high extinction ratio optical modulator to a high stability and high frequency (over 100 GHz) optical reference signal generator. High-frequency reference signals are generated by a highly stable optical two-tone generator. ALMA (Atacama Large Millimeter/sub-millimeter Array) is a high-frequency radio interferometer array currently under development, and each of the ALMA antennas has a 10-band receiver, and its highest receiving frequency reaches 950 GHz. To receive such high frequencies, higher reference frequency is required for the 1st local (as much as over 100 GHz), and stability to maintain the signal coherence is also required. To address these issues, we have developed a new method to generate and transmit a reference signal in the form of frequency difference between two coherent light waves. One method to generate two optical signals is producing them from a pair of laser sources using optical phase lock loop for feed back control, however, optical phase lock loop has a stability problem in its operation. A good alternative method to the optical phase lock scheme is the LiNbO3 Mach-Zehnder optical intensity modulator which is capable of generating two highly stable optical signals (upper sideband and lower sideband components) by applying a sinusoidal microwave signal to an input laser signal. The two optical signals require phase stability better than 10-13 in the Allan standard deviation, vibration robustness and polarization maintaining capability. The signal coherence loss estimated from the phase stability of the two optical signals generated by the Mach-Zehnder modulator shows that the modulator has the ability to generate highly stable optical signals.