Michelson Echelon

a spectral instrument consisting of a stack of glass or quartz plates of identical thickness, placed in optical contact so that they form a stairway of steps of equal height (Figure 1). It was first constructed by A. Michelson in 1898.

A parallel beam of light falling on the Michelson echelon is divided into several rays (depending on the number of plates), which traverse different paths in the material of the plates (in transmission Michelson echelons) or in air (on reflection from the steps coated with a reflecting layer in reflection Michelson echelons). Acquiring a path difference in this way, the beams interfere with one another as in a diffraction grating. Unlike the latter, the difference in the paths of two neighboring beams in the Michelson echelon amounts to several tens of thousands of wave-lengths of light, while the number of beams usually does not exceed 30 to 40.

Figure 1. Path of rays in a transmission echelon. Initial beam of light S is directed onto the echelon at nearly perpendicular incidence. Therefore, angle of diffraction 0 of rays passing through the echelon is also very small (broken line shows path for oblique incidence); d is difference between paths of rays from neighboring steps and is almost equal to height of step t.

The Michelson echelon is used as a spectroscope. Its resolving power is extremely high, and it is suitable for analyzing very narrow (of the order of 1 X 10^-11 to 2 X 10¹¹ m) spectral regions. Therefore, a preliminary monochromatization of the light is usually carried out, that is, a narrow spectral region is cut out of the radiation for analysis in the echelon. Reflection Michelson echelons, whose resolving power is approximately four times greater than that of transmission echelons, are used for studying regions of the spectrum that are opaque to the material of the echelon regions.