Photonic Millimeter-wave generation Developments for the ALMA Radio Telescope

This paper discusses the possibility of applying the high extinction ratio Mach-Zehnder LiNbOs optical modulator to a high stability and high frequency (over 100 GHz) optical reference signal generator for Astronomical Radio Interferometer. 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. The reference frequency is calculated using heterodyne conversion. Current optical two-tone generators of the ALMA baseline plan are based on an optical phase lock schemes. In the high-extinction ratio lithium niobate (LiNbO₃) Mach-Zehnder intensity modulator, the optical frequency difference between two optical signals is exactly twice (or four times) the modulation frequency. Compared to the optical phase lock scheme, the Mach-Zehnder modulator has significant advantages in terms of robustness to mechanical vibration and acoustic noise, stability (free from the influence of the input laser line-width), and capability of maintaining polarization state of the input laser.