Design and Performance of Some Components for a Millimeter Wave Imaging Receiver

Doctoral thesis, 1996

This thesis deals with the design and performance of some novel devices for an imaging receiver, operating in the frequency band centered at 100 GHz, to be used in radio astronomy research. The emphasis is given on simple, small and reliable devices in order to reduce the complexity of an already complex receiver. An imaging receiver at millimeter wavelengths consist basically of several detectors (mixers) looking at different places of the sky, been the image a superposition of all the signals with certain spatial distribution. The coherent receivers used for millimeter wave radio astronomy are today of the heterodyne type, this means that they need an external local oscillator signal (LO) in order to obtain a intermediate frequency (IF) that is processed with conventional mic rowave techniques. This LO signal is, almost all the time, injected by quasi-optical means. Quasi-optical injection of the LO signal to an array of mixers poses the problem of individual power control to each mixer element. Some of the papers here presented introduce a novel approach where, by locally modulating the conductivity of a semicondu ctor, it is possible to achieve spatial modulation of a Gaussian beam. The underlying physics of this method is that the conductivity of a semiconductor can be changed by the generation of free carriers under photonic excitation, thus changing the refractive index of the material. In order to have an efficient splitting of the signal into an array of beams, from a single millimeter wave source, a phase interference device better known as kinoform was designed and tested at the operating frequency giving the expected results. The kinoform and semiconductor modulator were put together in a working device and the results are presented. In order to combine the observed signal, from the sky, and the LO signal we have to use a diplexer that also acts as a frequency selective device, used to reject the image sideband. The relatively long wavelength at 100 GHz makes the traditional design of diplexers quite complicated because of the large physical size needed and the fact that this diplexer must sit inside a dewar at cryogenic temperatures. We explored the design of a new type of diplexer, modeled its response and evaluated its performance. The practical results agree very well with theory. The design of optical elements in this imaging receiver was also something that was faced during this work and thanks to this, some novel results around the performance of lenses and its aberration properties were obtained. In particular we show that, using Teflon manufactured plano-convex lenses, there is no need for special shaping of the lenses to avoid coma effects due to the non-paraxiality of the mixers composing the array.