Laser fringe zero crossings

As mentioned earlier, it is also impractical to collect data at infinitesimally spaced values of 8. Data in Fourier transform spectroscopy is usually collected at discrete steps determined by a coincident interferometer with a mode-locked helium-neon (He-Ne) laser at X = l/o = 632.8 nm. The fringes of this laser are detected with a diode and sampling at every zero crossing of this line will provide data for retardation values spaced at 316.4 nm. In addition, one must insure that the mirror is moved slowly enough that the detector can relax between each sample so the data for each point is independent. 

In very high resolution spectrometers, such as the dynamic alignment spectrometer mentioned above, three laser beams are run in parallel to the infrared beam. These three laser beams are positioned at the apices of an equilateral triangle. Laser fringe measurements are taken on all three beams and any error, (that is, if all the zero crossings are not simultaneous) is corrected by tilting the fixed mirror. By this method, both mirror tilt and sampling frequency measurement are accomplished. 

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