Ocean Optics ARCoptix ANIR - Manual (Page 32)

A: How the ARCoptix ANIR Works
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Figure A-1 shows incoming light is reflected back by a series of fixed and mobile mirrors acting as a
grating and generating a diffraction pattern. To obtain the interferogram one records the modulation of
the zero order in function of the motion of the mobile mirrors. The path difference (or depth of the
grating)
x is an important parameter and given by the distance between the fixed and the mobile
mirrors.
While in the Michelson interferometer one mirror changes its position, in the lamellar grating
interferometer a series of mirrors move. The configuration does not need a beam splitter. The LGI
splits the wavefront at the mirrors, contrary to the Michelson interferometer that splits the wave
amplitude at the beamsplitter. The whole arrangement of mirrors performs like a grating having a
variable depth. The interferogram is recorded by measuring the modulation of the zero order, reflected
back by the grating, as a function of the depth of the grating. Note that there is no dispersion in the
zero order. Exactly like in a conventional Michelson interferometer, the important notion, here, is the
phase delay generating at the grating, which is the double of the distance between the fixed series of
mirrors and the mobile ones. Figure A - 2 shows the resultant diffraction patterns for selected
positions of the movable mirror grating.
Figure A - 2.
The depth of the grating x determines the resulting efficiencies in the different diffraction orders. For
x = 0 the grating acts like a simple mirror while for x = /4 no light is found in the zero order. The
effect is wavelengths depended. For each wavelengths this conditions is realized for a different
x.
The zero order intensity A for a single wavelength 
0
can be written as a function of x and becomes:
   
0
2
0 0
cos      x A A Eq. A - 1