Phase-locked oscillator operated in a fiber-coupled repeater station

Abstract

The ngVLA is a synthesis array of approximately 214 18-m antennas, and the antennas will be connected via a spoke-and-wheel connection arrangement and daisy-chain or some intermediate arrangement. These stations will be linked to the central timing system via long haul fiber optics. Repeater stations will be needed for reference signal regeneration for the most distant antenna stations. Otherwise, a large number of hydrogen masers will be required. An optical phase-locked oscillator (OPLO) can be realized at a remote repeater station that is synchronized with a transmitted reference signal from a reference station. We have been developing a wide-frequency range OPLO with an external modulation-type dual-wavelength generator that is capable of regenerating the dual-wavelength reference signal at a long-distance remote station. The remote station reference signal is thus coherent with the reference station and capable of tuning over a wide frequency range. At each antenna the reference is looped back to the central building where the measured round-trip phase is used for real-time /post-processing transmission phase compensation so that the LO signal to each antenna becomes coherent. This transmission phase stabilizer should detect the round-trip phase of each light-wave separately for chromatic dispersion compensation. We demonstrated the effectiveness of the OPLO with a post-processing-scheme transmission phase stabilizer. In this method, the OPLO’s phase-locked loop controls the microwave frequency of the voltage-controlled oscillator, not the optical frequency of the tunable laser. Thus, a wide-bandwidth PLL is not required.