A multichannel, two‐color, quadrature heterodyne interferometer is used to measure the line density in the DIII‐D tokamak. The unique feature of this real‐time vibration‐compensated interferometer is the combination of high speed (1 MHz), high resolution (2&pgr;/256), and wide range (±8193 fringes). Quadrature phase information from a CO2laser (10.6 &mgr;m) and a He–Ne laser (0.63 &mgr;m) are digitized with high‐speed (6 MHz) flash digitizers. Zero crossings of the signals are counted with digital circuitry yielding quarter fringe resolution with a 4‐MHz bandwidth. Further fringe resolution of 1/256 is provided at 350 kHz by a PROM which uses the digital signals as input to a look‐up table. Analog line density is presently available at 80 kHz with a system noise equivalent phase shift of ±2/256. Error monitoring is provided for low signal amplitude and exceeding the maximum fringe rate. In addition, a method to prevent coating of in‐vessel mirrors due to plasma and vessel wall cleaning discharges has been developed.