Dual-Mode Laser for a Photonic Local Oscillator in the Submillimeter Band and The Molecular Composition of an Oxygen-Rich Asymptotic Giant Branch Star

Abstract

Generation of local oscillator power at Terahertz frequencies with conventional techniques is difficult and expensive. In this thesis, I demonstrate steps towards a THz source using the photonic local oscillator technique for submillimeter astronomy. An LT-GaAs photomixer illuminated by two laser signals generates a beat frequency through photoconductive mixing, equal to the difference of two laser frequencies which can be tuned from a few hundred GHz to around a few THz. To generate two frequencies in the same laser for a photonic LO, I have investigated the use of a Ti:Sapphire ring cavity laser. To generate dual-mode operation in the multi-mode laser, two intracavity solid Fabry- Perot etalons were installed. To characterize the spectral width of the photomixing product, the beat frequency was monitored with a commercial photodetector at 34 GHz. The spectral width of the beat frequency was less than 10 kHz. The output power from the LT-GaAs photomixer was found to increase linearly with the applied bias voltage. Unexpectedly large fluctuations in the output power were measured, due to dual-mode intensity variations from the Ti:Sapphire ring cavity laser. The reasons for these power fluctuations are thermal variations of the resonator cavity, mechanical variations, dust particles, air fluctuations, and mode competition. To reduce these power fluctuations, a power stabilization system using volume holographic gratings (VHGs) was developed, which greatly reduced the power fluctuations.<br /> I helped develop a 460/810 GHz dual-channel receiver, called the First Light APEX Submillimeter Heterodyne instrument (FLASH), for the Atacama Pathfinder Experiment (APEX) telescope located at Llano de Chajnantor in Chile0s Atacama desert. Using FLASH and the APEX-2A receivers, a large number of molecular transitions toward the Long Period Variable (LPV) star IK Tau were observed. Thirty four transitions of 12 molecular species, including maser lines, were detected. To determine the spatial distribution of the 12CO(3−2) emission, mapping observations were performed. Assuming local thermodynamic equilibrium (LTE), the rotational temperatures of molecules and the molecular abundances were obtained. By comparing the abundance of the individual molecules to those reported in the literature, we found an improvement over previously available observed abundances. To constrain the physical conditions in the circumstellar envelope, emission from the SO2 and CO molecules were modeled using a Monte Carlo method. From the model fits we could estimate the molecular column density and the kinetic temperature of the envelope.