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.
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