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A PERIODIC LAMELLAR INTERFEROMETER FOR
THE FAR INFRA-RED
M. Tinkham, D. Martin
To cite this version:
JOURNAL DE PHYSIQUE Colloque C 2, suppliment au no 3-4, Tome 28, mars-avril1967, page C 2
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86A PERIODIC LAMELLAR INTERFEROMETER FOR THE FAR INFRA-RED
M. TINKHAM and D. H. MARTIN (*) (Read by P. E. Clegg)
Rbsum6. - On dkcrit un spectromktre de Fourier a faible rksolution pour l'infrarouge lointain constituk par un rkseau lamellaire
a
mouvement pkriodique en dents de scie et qui fournit en temps rkel, par amplification sklective sur 100 canaux intkgrateurs les 100 points du spectre ktudik.Abstract. - A Fourier transformation spectrometer for the far infra-red consists of a lamellar interferometer driven according a saw tooth law. It gives, by amplification on 100 integrating channels, the 100 points of the studied spectrum.
A periodic lamellar interferometer has recently been used at Berkeley to record spectra in the far infra-red, from about 5 cm-I t o 100 cm-'. This instrument differs from most others in use in this spectral region in that it is designed to produce spectra with relatively low resolution (up to 1 in lo2) relatively quickly, in 5 to 10 minutes.
The lamellar grating is driven at a constant velo- city and reverses with a periodic time of 4 seconds. Interferograms, recorded on both sides of the zero- path-difference position, are therefore produced every two seconds. Each Fourier harmonic of the interfero- gram, from the 10th to the 1 loth, is separately ampli- fied by a synchronous amplifier deriving its reference signal from a toothed disc rotating on the main shaft which drives the lamellar grating. The output for each harmonic is then smoothed by an integrating circuit which has a time-constant of 100 seconds. The final output-signals from the 100 channels can be conti- nu,ously displayed on an oscilloscope screen or can be recorded sequentially on a chart. The gain of each
(*) On leave from Queen Mary College, University of Lon- don.
channel is normally adjusted to give a smooth variation of signal intensity through the spectrum when no sample is in the beam ; the transmission spectrum of a sample is then displayed directly on the oscilloscope screen.
The detector is a gallium-doped germanium bolo- meter and responds well over the range 5 c/s to 55 c/s
which covers the frequencies of the 10th to 110th Fourier harmonics. With the maximum path-difference set at the value which gives a spectrum over the range from 5 cm-' to 55 cm-l, for example, a spectrum builds up during the time-constant of the smoothing circuits to give a noise-to-signal ratio of about 2
%
in most channels, rising to perhaps 5-10%
in the lower channels near 5 cm-'. The present noise level is predominantly due t o mechanical vibrations.The maximum path-difference can be set so that any infra-red frequency in the range 20 cm-I to 110 cm- l can be explored with the maximum resolu- tion of about 1 in 10' provided by the highest channels. The interferometer was developed in collaboration with a number of colleagues, particularly Dr. R. V. Alves.
