Retardation zero, interferometer

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. 

The optical path difference (OPD) between the beams that travel to the fixed and movable mirror and back to the beamsplitter is called retardation, 8. When the path length on both arms of the interferometer are equal, the position of the moving mirrors is referred to as the position of zero retardation or zero path difference (ZPD). The two beams are perfectly in phase on recombination at the beamsplitter, where the beams interfere constructively and the intensity of the beam passing to the detector is the sum of the intensities of the beams passing to the fixed and movable mirrors. Therefore, all the light from the source reaches the detector at this point and none returns to the source. To understand why no radiation returns to the source at ZPD one has to consider the phases on the beam splitter. 

For a given solid angle through the interferometer, each optical frequency, then, has a specific value of the optical retaradation where the sine function is zero. If the interferometer is driven beyond this path difference, the phase of the modulation for that frequency is reversed, and energy at that frequency is removed from the spectrun rather than added to it as the optical retardation continues to increase. Therefore, in order to insure that this does not occur for any frequencies within the spectral bandwidth even at maximum path difference, the absolute maximum solid angle which can be used is... 

The instrument is capable of a 100 cm optical retardation. However, the method by which this path difference is achieved is different from the method of single-mirror motion employed by the previous two instruments. The Kitt Peak interferometer has two cat s-eye reflectors which move in a continuous-scan, reciprocating fashion. One reflector approaches the beam splitter while the other reflector retreats from it. Figure 10 shows the details of this interferometer. Both reflectors move on oil bearings. If the beam splitter is positioned so that the zero path difference occurs when one reflector is near the front of its track while the other... 

The detailed action at the beam splitter is complex, but as shown in Fig. 2.4(d), when the two mirrors are equidistant from the beam splitter, constructive interference occurs for the beam going to the detector, and destructive interference occurs for the beam going back to the source, for all wavelengths of radiation. The path length of the two beams in the interferometer are equal in this case, and the path length difference, called the retardation, is zero. 

Figure 2.21. Retardation versus time for a step-scanning interferometer. The velocity of the moving mirror is zero at each step and the mirtOT moves very quicMy between steps.

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