Techniques for producing higher fidelity interferometer measurements by reducing sensitivity to spurious sources include reducing the coherence length of an electromagnetic beam. In addition, multiple surfaces within an optical system may be measured by electronically tuning the position of a coherence plane along the optical paths of an interferometer. A phase modulator is used in conjunction with a long coherence length electromagnetic source to generate beams for each leg of an interferometer. Providing a controlled broadband RF signal to the phase modulator increases the bandwidth of the beam and thereby reduces the coherence length of the beam. This reduces the spurious contributions to the output interference fringes from undesired surfaces along the beam path.