A compact seven channel interferometer has been designed and built to measure electron density profiles in the COMPASS (compact assembly) tokamak. Two far-infrared (FIR) laser cavities are optically pumped with a single continuous-wave CO2 laser, generating two similar beams, with a small, tunable difference frequency (0.5-1.0 MHz). The COMPASS facility incorporates a complex set of poloidal field coils close to the vacuum vessel as well as a versatile set of close coupled “helical” resonant magnetic perturbation windings which severely restrict diagnostic access. As a result a novel approach to the optical circuit has been necessary. Wire grid polarizers are used to divide the laser power equally between channels and to overlay probing and local oscillator beams after the probe beams have made a double pass through the plasma. Gaussian beam-mode optics is used to minimize the size of the optical components.