Density fluctuations in magnetically confined plasmas have been measured from the phase scintillations impressed onto a 10.6‐&mgr;m wavelength laser beam probe, using a Mach–Zender interferometer operating in the optical near field. By careful design, the effects of vibration and air‐borne sound waves, which limit the instrument’s sensitivity, have been reduced to a level such that the minimum detectable signal is 10−6rad in the frequency range 10 kHz–1 MHz, corresponding to a plasma density fluctuation level (&dgr;N/N) of 10−4.