A 10.6 μm infrared heterodyne receiver with an i.f. response centered at 2500 MHz has been employed to measure the spatial, spectral and amplitude distributions of the driven lower-hybrid wave density fluctuations in the Alcator Tokamak, using CO 2 laser scattering. The heterodyne receiver employs a cooled impedance matching network between the 1200 Ω Ge:Cu(Sb) photomixer and the 50 Ω i.f. preamplifier, and provides a quantum-noise factor as low as 8.6 dB at i.f. frequencies between 2000 and 2500 MHz.