Construction and alignment of a high performance multipass vernier tandem Fabry-Perot interferometer

A multipass vernier tandem Fabry-Perot interferometer is easily constructed and aligned when the two scanning mirrors are mounted on a common scanning stage. Using such a system we obtain an ultimate resolution of 10 ^-3cm^-1 and a maximum scanning range, free of overlapping orders, of >50 cm^-1. Using a 5 plus 4 pass arrangement a contrast >10 is obtained which, together with a high overall luminosity, permits simultaneous resolution of both weak spectral features near zero energy shift and low-frequency Raman bands. In addition to constructional details of this system we discuss the alignment procedure for uniform transmission in tandem operation, and demonstrate how the alignment requirments ultimately limit the performance of any tandem system to an "effective finesse" of <1000. Spectra showing surface waves in silicon and shear waves in polystyrene illustrate the power of this technique.