Analog Phase Synchronization for Optical Aperture Synthesis using Digital Source Modulation

Radio and microwave frequencies are widely used for astronomy and satellite communications globally. Increasing the size of antennas can increase data throughput, signal strength, and resolution. Aperture synthesis antenna arrays are often used in astronomy instead of larger antennas due to their design feasibility, versatility, and smaller required size per telescope. Optical-infrared (terahertz frequency) telescopes are also used for astronomy research to observe different types of astronomical phenomena; however, aperture synthesis techniques in the terahertz range are difficult to implement due to the required precision for phase-delay matching and the need for sensitive interferometric equipment. Optical beamforming and interferometry using digital source modulation (OBI-DSM) is currently being experimented with by the National Research Council (NRC) Herzberg Astronomy & Astrophysics photonics aperture synthesis exoplanet resolver (PHASER) project with the intent of demonstrating how beams from widely separated optical-infrared telescopes can be combined. Conversion from the optical domain to the analog domain is accomplished using an avalanche photo diode (APD), which is then applied to a digital system that attempts to coherently phase adjust the optical signal phase using a thermo-optic phase shifter. This research project proposes to avoid a secondary domain shift by designing an analog channel-locking loop (CLL) control system which attempts to reduce the optical path length errors of the optical signal primarily caused by atmospheric turbulence with nano-meter phase control accuracy. A phase-locked loop (PLL) is used to generate a reference signal, which is used by the CLL to phase-lock all channels coherently. A CLL is designed and experimented with in simulation software to confirm the functionality of the selected components and to optimize the loop bandwidth to reduce noise entering the system. This leads to the optimization and design of a four-channel CLL PCB through which the theoretical analysis is verified. The purpose of this project was to develop an instrumented test circuit CLL which can be used for further noise analysis and testing.