Development of near-infrared laser heterodyne interferometry for diagnostics of electron and gas number densities in microplasmas

Laser heterodyne interferometry measures temporal variations of refractive index in tested materials, and in plasma diagnostics, we can calculate electron and gas number densities, which are both important plasma parameters, from the phase shift of the probing laser beam caused by the plasma. For interferometry diagnostics of microplasmas, which are typically under high-pressure conditions, the wavelength of the probing laser beam should be in the near-infrared (NIR) range considering the high electron collision frequencies in the microplasmas. We report on the development of a NIR diode laser heterodyne interferometry system and its application to the diagnostics of a pulsed DC microplasma source at atmospheric pressure.