Phase stability measurement of an optical two-tone signal applied to a signal reference source for millimeter and sub-millimeter wave interferometer

Local signal generation with high phase stability is one of the key elements of an interferometer system. In the ALMA (Atacama Large Millimeter/submillimeter Array) project, we have plans to generate and transmit a micro/millimeter-wave LO (Local Oscillator) reference signal as an optical two-tone signal. To realize this, we introduce a method to generate an optical two-tone signal by double sideband suppressed carrier modulation with a lithium niobate (LiNbO₃) Mach-Zehnder optical modulator. The phase instability of the LO signal causes a loss of coherence in the interferometer. We measured the phase instability and estimated the coherence loss while assuming the highest frequency of ALMA LO. The observed Allan standard deviation for 22 GHz was 1.9 × 10^-14 at 1 s. Additional noise was found at a longer time scale than 4s; however, the reason is not well understood at this stage. If this additional noise was classified as white frequency noise, the estimated coherence loss was 5.2% when 938 GHz frequency and a 100-s integration time were assumed. On the other hand, if this additional noise was caused by some other effect, for example phase fluctuation of the amplifier, the coherence loss was estimated to be 0.33% at 938 GHz frequency (100s).