The design and construction of a novel heterodyne spectrometer for airborne astronomy in the 50 µm - 200 µm wavelength range is described. along with laboratory measurements of its performance. A bulk, extrinsic Ge:Ga photoconductor is used as the mixer. Its low bandwidth, determined by the hole recombination rate, necessitates the use of a continuously tunable local oscillator. This is provided by a far-infrared laser sideband generator, which is based on a GaAs Schottky diode mounted at the feed of a comer-cube antenna, the latter combination acting as a reflective FIR modulator.
The first chapter of this thesis describes the astronomical and technical context of the project - in particular, the constraints which the astronomical goals set on the instrument, and the advantages and drawbacks of each of the various broad instrumental strategies that are available for spectrometer design. The chapter's last section provides a very brief overview of our most successful laboratory results, which are described at greater length in chapters 2- 4. In chapter two we describe the performance of Ge:Ga mixers as heterodyne mixers. We report on an extensive series of measurements of bandwidth, photoconductive gain, and direct detection responsivity for a series of highly compensated, NTD detectors grown specifically for this purpose. Chapter two also describes a nunber of experiments on FIR heterodyne performance, made using the direct, attenuated laser, rather than the output of the sideband generator, as the local oscillator. These confirm the expectation that germanium photoconductors are capable of quantum-limited noise performance with quantum efficiencies of ~10%, at much lower LO powers than required for Schottky diodes. Our best achieved noise temperature is TN(DSB) = 655K at PLO = 1.6µW, a factor of > 25 lower than the best reported corresponding figure for Schottky diodes.
Chapter 3 describes the operating principles and construction of our FIR laser, which formed a basic tool in nearly all our laboratory experiments. A brief discussion of the Lorenz instability in FIR lasers is also given, in connection with various observations we have made of spontaneous pulsations and excess low-frequency noise on the laser output, and which have recently been the subject of considerable study by other researchers. Chapter four describes FIR laser sideband generation using small-area Schottky diodes and comer-cube antennas. The construction and performance of our corner-cubes is outlined, including the first direct measurement of the main beam efficiency of a corner-cube antenna in the FIR, and a comparison with theory. The construction and measured performance of the rest of the sideband generator is also described. A detailed, quantitative model has been developed for the conversion efficiency obtainable from Schottky diodes in this application. We find that the low conversion efficiency (-39 db) measured in our experiments, and comparable to that found by other researchers, is inherent in the diode and well predicted by the model. For our particular experiment, the model predicts -28 db loss due to the diode, plus approximately -10 db loss due to the antenna coupling efficiency. The dependence of conversion efficiency on diode parameters is studied and guidelines for future optimization derived. Unfortunately, the severe conversion loss we measure, combined with low FIR laser power and (somewhat less significantly) poor optics transmission, leads to our presently available LO power being inadequate to obtain astronomically useful sensitivity, by a large factor.</p
