Optical inhomogeneity measurement of an 800  mm aperture parallel plate with a vibration-resistant and coherence-controllable interferometer

A vibration-resistant and coherence-controllable interferometer was proposed to measure the inhomogeneity of an 800 mm aperture parallel plate. The coherence of the interferometer’s laser was hybrid-modulated via external cavity feedback and noise current. Numerical and experimental results indicated that the spectral broadening of the laser was mainly attributed to external cavity feedback, while the coherence sidelobes were primarily suppressed by noise current modulation. The positioning efficiency of the zero optical path difference (OPD) of the interferometer was enhanced by matching the step length of the OPD scanner with the coherence length of the laser. Vibration-resistant interferometry of the parallel plate, insensitive to the polarization aberration of the interferometer, was achieved through a common optical path configuration and the tilt-phase iteration algorithm. The experimental results demonstrated that the root mean square (RMS) repeatability (±2 σ ) of the interferometer was better than 0.1 nm. The inhomogeneity of the 800 mm aperture parallel plate under test was 3.4 ppm. To the best of our knowledge, this is the first report on the inhomogeneity measurement of an 800 mm parallel plate using a coherence-controllable laser under vibration.