A 150 GHz interferometer for investigations of fluorescent lamp discharges

A 150 GHz interferometer system has been developed, and is being used, to gain new insights into many aspects of fluorescent lamp discharges. The diagnostic has shown to be particularly useful because of its superior temporal resolution (e.g., compared to Langmuir probes), its noninvasive character, and the straightforward manner in which its data are interpreted. Applications have included studies of the lamp over a broad range of parameters and modes of operation. This presentation describes the interferometer's setup and functionality, and discusses its performance and issues that potentially limit its performance. In a related presentation, results of applications are discussed. The interferometer has a Mach-Zehnder configuration, in which the discharge tube under test is situated in one leg, midway between two matched, Gaussian beam antennae. The antennae, which are 75 mm apart, serve as, respectively, input and output couplers between waveguide and free-space. Their antennae patterns each have a fundamental Gaussian mode shape, with ~4 mm diameter beam waist that is located midway between the antennae. The spatial resolution of the system can exceed the beam diameter if spatial scan measurements are made, since generally the discharges of interest have mild enough density gradients. The interferometer employs a heterodyne detection scheme, in which two oscillators produce radiation with nominally 75 GHz frequency and separated by frequency Deltaf, which is tunable, but typically set to about 50 MHz. One of the oscillators (the 'RF') is frequency-doubled to ~150 GHz, then split and directed 1) through the first leg containing the discharge, and 2) through the second leg, which bypasses the discharge. The other oscillator (the 'LO') is split, and both parts are mixed with the RF radiation from the two legs. This results in two ('IF') signals, both having the same frequency (2Deltaf), but with phase difference that is established, in part, by the presence of the discharge in one leg. The two signals are digitized and the phase difference, versus time, is determined through software. Maximum temporal resolution is equivalent approximately to three cycles of the IF signals, or about 30 nsec for the present setup. This can be improved upon in straightforward manner, if required by the application.