Helium-neon laser interferogram

The data acquisition rate (sampling frequency) for mid-IR interferograms is usually equal to the frequency, Hz, of the interferogram that is generated by a laser (usually a helium-neon laser at 632.8nm) simultaneously with the infrared interferogram. f is equal to the product of the wavenumber of the laser and the... 

Another important advantage of FT-IR spectrometers is their outstanding frequency accuracy (Connes advantage), the basis for all achievements in difference spectroscopy. This accuracy of spectral frequencies is due to the precise and stable collection of the interferogram signal, triggered by the helium—neon laser. An accuracy in wavenumber of better than 0.01 cm can be achieved. 

The essential difference between conventional FT instruments and the step-scan devices is that, for successful operation, it is necessary to control the retardation (mirror) velocity in the case of the former and the retardation (mirror) position for the latter. In both cases, the method used to control the retardation involves a collinear or parallel helium/neon laser interferometer. In continuous scan operations the laser interference fringes are used to generate feedback signals to maintain constant mirror velocity, and in the step-scan mode the laser interferogram provides the means for the control of the mirror position via a feedback signal. 

The introduction of the Helium-Neon laser had a significant impact on the evolution of the optical part -the interferometer- of the Fourier transform spectrometer. It allowed for the direct calibration of the interferometer mirror displacement which resulted in a much more precise measurement of the interferogram signal and hence a much improved spectral recovery. 

The laser interferometer is used to measure the intervals between data point collection within each individual interferogram. The coherent monochromatic light yields an interferogram very unlike that produced by white light. The laser interferogram is virtually a cosine wave, with the period determined by the wavelength of the laser. The most commonly used laser is the helium-neon (He-Ne) laser, with a wavelength of 632.8 nm or 0.6328 ym. 

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