Double Michelson Interferometer for Contactless Length Change Measurements

The use of a two-channel, or double Michaelson, interferometer for measuring the absolute thermal strain epsilon of a small test sample (e.g., 4-in. long) is described. This method is contactless inasmuch as laser beams reflect off the ends of the sample. Real-time strain changes are recorded automatically to a resolution of lambda/8L sub s, where L sub s is the sample length. Analysis shows that the measurement errors are approximately proportional to the sample temperature excursion, with the strain error varying from .48 to 2 x 10 to the -7 power as Delta T sub s varies from 25 to 250 C. An average coefficient of thermal expansion for ultralow-expansion glass, measured with additional fringe interpolation by photo-detector signal analysis, would be known to + or - 1.2 times 10 to the -9th power/deg C.