The desirability of using confocal cavities in scanning interferometers is discussed and a method for fabricating such cavities is described. This method makes use of the fact that in a confocal cavity the minimum spot size, and hence the extent of any combination of cavity modes, is stationary with respect to small changes in the cavity length. A method for reducing wedge in the cavity to small values is also presented. Tolerances are discussed and performance figures for two interferometers produced this way are given.
John R. Johnson, "A Method for Producing Precisely Confocal Resonators for Scanning Interferometers," Appl. Opt. 6, 1930-1933 (1967)