Integrated frequency-modulated continuous-wave (FMCW) interferometer with in situ common-path referencing for plasma...

Millimeter-wave interferometry is a standard diagnostic for line-integrated electron density, but conventional implementations typically rely on discrete high-frequency sources, mixers, and separately aligned reference paths. Here, we report an integrated frequency-modulated continuous-wave interferometer for plasma diagnostics using a commercial 60-64 GHz radar system-on-chip and, critically, an in situ common-path reference architecture that enables phase-stable operation with industrial hardware. The key design feature is the intentional use of aperture-clipping reflection from the diagnostic-window flange as a stationary reference echo, while the transmitted beam core traverses the plasma and is reflected by a mirror. Because the reference and measurement echoes are generated by the same chirp and share the same transceiver, launch optics, and diagnostic port, differential phase extraction suppresses common-mode oscillator drift, thermal drift, and mechanical vibration without a separate reference arm. A high-duty-cycle chirp configuration and chirp-by-chirp FFT phase retrieval provide a temporal resolution of 120 μs. Experiments on the Keda Linear Magnetized Plasma device show that the common-path-referenced scheme reduces the phase standard deviation from 0.34° to 0.17° and suppresses the dominant 22 Hz vibration component by more than 12 dB, yielding a minimum detectable line-integrated density of 2 × 1014 m-2. These results demonstrate a compact and scalable interferometer architecture for plasma diagnostics based on commercially integrated millimeter-wave hardware.