A two-color heterodyne interferometer for high-speed measurement of electron density in partially ionized gases and plasma*

The present work describes the design and bench testing of a common path two-color heterodyne interferometer for high-speed electron density measurements in partially ionized plasmas. Central to the approach are two stages of second harmonic generation in Mg:PPSLT and PPLN, first to produce the second probe wavelength from a single frequency 1.06 micron CW laser and then to generate an interference signal with a phase proportional to the electron density. Unlike conventional two-color interferometry, only a single phase measurement is needed to recover the electron density. We further describe an implementation of heterodyne detection in order to avoid the directional ambiguity of the fringe motion and separate the phase signal from low frequency noise. Phase measurements were obtained either by post-processing oscilloscope traces sampled at 10 GHz or by real-time 1 kHz monitoring via a phasemeter. Experimental evaluation of the two-color heterodyne interferometer was performed to quantify the noise floor, detection limit and background immunity. Under quiescent conditions, an oscilloscope acquisition noise floor of 1.5 mrad, corresponding to a density of 1.7  10¹² cm^-3 over a 200 mm path, was obtained at an effective sampling rate of 2.2 MHz. The instrument was also tested in a low pressure pin-to-sphere pulsed plasma which produced 1.4 A, 2 μs duration current pulses. Residual electromagnetic interference was found to increase measurement noise, resulting in a single-shot detection limit of 4 mrad (10¹⁴ cm⁻³ over the 5 mm path) at a 22 MHz acquisition rate.