The importance of flatness, parallelism, and equality in thickness of the elements and spacers of a resonant reflector is discussed. Using the matrix method, the reflectivity vs wavelength is computed for several types of resonators, the thicknesses of whose elements were subject to variations. For a two-element device, the exact mechanical thickness of the spacer is not important, but the equality of thickness of the dielectric elements is critical. For a three- or four-element device, the exact thickness of the center element is least important to reflectivity, but the reflectivity decreases if the total error in orders on either side of the center approaches ½. The results indicate that it is extremely difficult to produce a practical device of this complexity. Pressure and temperature scanning is discussed. Experimental results were obtained for two-element devices showing the importance in equality of thickness and the sensitivity to nonparallelism of the spacer. The relative electric field strengths within the reflector were mapped to investigate susceptibility to radiation damage. The amplitude of the field and the position in the resonator where damage would first occur depends upon the reflectivity at the wavelength used.
T. A. Wiggins, C. E. Procik, and J. Pliva, "Optical Tolerances and Electric Fields in Resonant Reflectors," Appl. Opt. 10, 304-310 (1971)