Cube corner interferometers

The Michelson interferometer is used in most FTIR spectrometers. However, some FTIRs use a cube-corner interferometer. 

Another solution is to use cube-corner mirrors, which consist of three mutually orthogonal plane mirrors. The mirror system reflects any incident ray back in the opposite direction. The first very high resolution cube-corner interferometer that really worked was the Oulu interferometer in Finland. The Oulu interferometer is basically a Michelson interferometer, where the moving and the fixed mirror are cube-corners. If the corners are perfect, the tilt problem completely disappears. The only disadvantage of this type of interferometer is a shearing problem, i.e. the lateral shift of the moving cube-corner. This is the... 

This interferometric dilatometer consists of a rather simple and small Michelson interferometer, in which the two arms are parallel, and of a 4He cryostat, in which the sample to be measured is hold. The sample is cooled to 4 K, and data are taken during the warm up of the cryostat. The optical path difference between the two arms depends on the sample length hence a variation of the sample length determines an interference signal. The Michelson interferometer consists of a He-Ne stabilized laser (A = 0.6328 xm), two cube corner prisms, a beam splitter, three mirrors and a silicon photodiode detector placed in the focal plane of a 25 mm focal length biconvex lens (see Fig. 13.1). 

The beams are backreflected by the cube corner prisms which are fixed, respectively, on the sample and on the sample holder. Since the cube corner prisms are able to make reflected beam exactly parallel to incident beam, this interferometer is tilt independent. The reflected beams get back to the beam splitter through the same path, but shifted by about 2 mm in the vertical direction. The beam splitter lets a part of the two beams go towards the photodiode sensor and lets the other part of beams reach the laser source (off axis, therefore giving no feedback effect). 

Figure 3.25 Cube-corner retroreflector interferometers with rotational scanning.

One potential disadvantage of refractively scanned interferometers is the effect of dispersion on the wavenumber accuracy. However, the variation of n with wave-number is known and this effect can be compensated accurately by the appropriate software. This disadvantage is more than offset by the fact that the stationary cube corners in each arm of the interferometer give it a remarkable immunity from the effects of tilt and shear. As a result, this interferometer is often used for process monitoring by FT-IR spectrometry. This interferometer has been installed as the key component of process monitors in well over 100 plants. 

In Figure 5.8, an example of an interferometer which realizes the OPD by rotating a pair of cube-corner retroreflectors is illustrated. As such an interferometer can lessen the effect of shocks and tilts to the spectrometer, restrictions on the place for siting the spectrometer can be relaxed. In this design of spectrometer, however, there is a limit to its spectral resolution, as it is difficult to attain a large OPD. Some spectrometers having such interferometers are, however, portable, and can be used for fieldwork also. 

Figure 5.8 Interferometer using rotational motion (schematically illustrated). A pair of cube-corner retroreflectors are placed on the both arms of an L-type rotator. The OPD is generated by rotating this rotator around the axis.

If a spectral measurement is disturbed, it is important to understand the origin of the disturbance first and take proper action to eliminate it. For example, if the reflectors on the two arms of the interferometer are plane mirrors (not cube-corner retroreflectors), the above case of the infrared radiation emitted by a high-temperature sample may be dealt with in the following way. It is effective to block half of the incident beam from the light source at a point close to the sample position where the incident beam is focused. Then, the infrared radiation from the sample will advance to the interferometer through the remaining half and will be reflected by the interferometer. When it comes back toward the sample. 

An alternative interferometer to the cube-corner is the cat s-eye interferometer, where the cube-corners are replaced by cat s-eye retroreflectors. These are components that consist of one parabolic and one spherical mirror, and reflect an incident ray back in the opposite direction. Cat s-eye interferometers are also used in high-resolution spectroscopy. 

Fig. 2 Typical Brillouin scattering setup using a tandem of triple-passed Fabry-Perot interferometers. M mirrors, SF spatial filters. FP Fabry Perot interferometers, c corner cubes. IF interferential filter, 1-12 half-wave plate. PM photomultipliers, P pinholes. A Gian analyzer. S fast shutter. G glass plate, and PC personal computer for data acquisition. (From Ref [13].) (View this art in color at WWW. dekker. com.)...

Figure 4.69 illustrates the principle of a traveling-wave Michelson-type interferometer as used in our laboratory. Such a wavemeter was first demonstrated in a slightly different version by Hall and Lee [184] and by Kowalski et al. [190]. The beams 5r of a reference laser and of a laser with unknown wavelength Xx traverse the interferometer on identical paths, but in opposite directions. Both incoming beams are split into two partial beams by the beam splitters BSl and BS2, respectively. One of the partial beams travels the constant path BS1-P-T3-P-BS2 for the reference beam, and in the opposite direction for the beam Bx. The second partial beam travels the variable path BS1-T1-M3-M4-T2-BS2 for 5r, and in the opposite direction for Bx. The moving corner-cube reflectors T1 and T2 are mounted on a carriage, which either travels with wheels on rods or slides on an airtrack. 

Fourier Transform or Interferometric Types. The biggest leap in MIR technology occurred approximately 20 years ago. The Michelson interferometer was incorporated into commercial infrared spectrometers. (Newer instruments have more rugged corner cubes and end-swivel mirrors, but these are just improvements on the basic idea.) The speed of this type of instrument is several orders of magnitude greater than that previously described for grating (dispersive)-based types. This improvement in measurement speed and precision has made MIR an extremely useful technique, even for quantitative analysis. 

---