White-light interferogram

Figure 10 The use of a white-light interferogram to trigger data collection.

If the output of the photomultiplier is displayed on an oscilloscope, the sinusoidal envelope which contains a high frequency sinusoidal modulation is observed and as the path lengths of the two arms approach equality, the modulation depth of the higher frequency sinusoidal modulation in the envelope increases. If the interferometer is adjusted to produce a maximum modulation depth, zero path difference has been located. If a white light is then used to illuminate the interferometer, the photomultiplier should detect the expected eight to ten fringes and present the typical interferogram of a broadband source. Fig, 3. 

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. 

For a white light source (all frequencies), the detector sees the sum of the interferences of the recombined beams. At the zero path point, all the frequencies present in the source interfere constructively and a center burst is generated which is the largest signal in the interferogram (Fig. 3.4). 

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