The tandem Fabry-Perot interferometer developed by J.R. Sandercock is a highly sensitive spectrometer with a frequency resolution in the sub-GHz regime and a contrast of better than 1010. Therefore it is best suited for studying spin wave excitations in layered magnetic structures with monolayer sensitivity.
The frequency selecting element is an etalon consisting of two parallel optical mirror plates (flatness better than l/200), of rather high reflectivity (typically 92 - 96%). The etalon transmits light of wavelength l, if the plate distance is a multiple of l/2. In conventional interferometry using one etalon the analysis of inelastic excitations is hampered by the ambiguous assignment to the appropriate transmission order, since the transmission is periodic in l/2 in the mirror plate spacing. These ambiguities are avoided in a tandem arrangement.
The setup is schematically shown in the figure above. The light of a frequency stabilized laser (Dn = 20 MHz), which is typically an Argon-ion laser (l = 514.5 nm) is focused onto the sample with an objective lens. The light backscattered from the sample (elastic and inelastic contributions) is collected by the same objective lens and sent through a spatial filter for suppressing background noise before entering the tandem interferometer. The frequency selected light transmitted by the interferometer is detected by a photomultiplier or an avalanche photodiode after passing through a second spatial filter for additional background suppression. A prism or an interferene filter between the second spatial filter and the detector serve for suppression of inelastic light from common transmisssion orders outside the frequency region of interest. A computer collects the photon counts and displays the data.
The central part of the interferometer is displayed in the next figure The light passes in series through two Fabry-Perot etalons FP1 and FP2. Of both etalons one of the two mirrors is mounted on a common translation stage which is piezo-electrically driven. In order to illustrate the function of the tandem arrangement the figure displays schematically the transmission curves of the etalon (a) FP2, (b) FP1 and (c) of both etalons in series (tandem operation) as a function of the mirror separation of the first etalon, L1. Assuming that for a given value L1 both etalons are in transmission a change of l/2 in L1 puts FP1 into the next transmission order. Due to the common mounting of the movable mirrors the change in the spacing of the second etalon is smaller by a factor of cos q with q the angle between the optical axes of the two interferometers as displayed in the figure. Thus FP2 is now not in transmission; the transmission maxima of both etalons lie at different values of L1. The same arguments account for inelastic excitations, which are transmitted only if they belong to the common transmission order. The inelastic signal represents closely the scattering cross section of the sample.
For an experimental realization a planity of the mirror surfaces of better than l/200 and a parallelity of l/100 of the two mirrors of each etalon are necessary. To maintain the latter, a sophisticated active stabilization of the mirror alignment is mandatory, which is performed by analog feedback circuits or by computer control. In order to obtain the high contrast necessary to detect the weak inelastic signals the light is sent through both etalons several times using a system of retroreflectors and mirrors.
Modern interferometers are mostly set up in the (3+3)-pass arrangement. Special measures are taken to protect the detector from overload while scanning through the elastic peak. This is achieved by using an acoustooptic modulator or by a shutter system. Data collection is performed by a personal computer or by a multichannel analyzer.