The Interferometer Detector

The effective optical pathlength, d, when the light-filled sample cell is placed in the interferometer, depends on the change of refractive index of the liquid, Mi, and pathlength, Jt, of the cell according to the following equation 

Further, it is possible to relate a number of fringes (TV) (sensitivity), which move past a given point (or the number of cyclic changes of the central portion of the fringe pattern), to the refractive index change by the equation 

The larger N is for a given A/ , the more sensitive the detector becomes. Therefore / must be large and A as small as practical. The practical limit of the length of cell is 

It can be assumed that each of the beams focused on the Wollaston prism consists of two such perpendicular beams which, after the quarter-wave plate, result in two circularly polarized beams of opposite rotation. These beams will interfere with each other to yield the original linearly polarized beam. A second polarizer is placed at an angle (90-b) to the first one, allowing 35% of the signal to reach the photomulitiplier. A filter transmitting light at 546 nm precedes the photomultiplier. 

If the sample cell contains a higher concentration of solute than the reference cell, the refractive index in general will be higher and the interfering beams will be out of phase. The refractive index difference. An, and the phase difference, Ap, are related by 

---

The device is often operated with a refractive index detector in series in order to coincidentally measure the refractive index of the eluent. This is necessary to calculate (K) from the refractive index as given in equation (5). A common refractive index detector used for this purpose is that manufactured by the Wyatt Technology Corporation and it is described as the interferometer detector in chapter 11. As discussed above the molecular weight of a solute is determined from the intercept of the graph relating... 

The fringes contrasts are subject to degradation resulting from dissymmetry in the interferometer. The optical fields to be mixed are characterized by a broadband spectrum so that differential dispersion may induce a variation of the differential phase over the spectrum. Detectors are sensitive to the superposition of the different spectral contributions. If differential dispersion shifts the fringes patterns for the different frequency, the global interferogramme is blurred and the contrast decreases. Fig. 5 shows corresponding experimental results. 

At 10Hz in a typical Nd-YAG laser 1000Hz/- /Hz, and the typical finesse asymmetry is of the order of one percent. In order to detect a gw signal the laser frequency noise has to be lowered by six orders of magnitudes (compared to the noise of a free running laser), and the two arms made as identical as possible. In order to achieve this complex frequency stabilization methods are employed in all interferometric detectors, and in order to insure the perfect symmetry of the interferometer, all pairs of Virgo optical components are coated during the same run (both Fabry-Perot input mirrors then both end mirrors are coated simultaneously). 

Since cryogenics is not involved in interferometer detector (but the proposal of cooling mirrors has been done for VIRGO), we no longer deal with this type of detector. 

A sphere has a larger mass than a typical bar (L = 5D) resonating at the same frequency, and because it is equally sensitive for all directions and polarizations it has a cross-section (for the same material) that is about 75 times larger. A single sphere is also capable of determining the source direction and polarization. A spherical detector is the only detector for GWs with isotropic sky coverage and the capability of finding the location of the source. Both laser interferometers and bar detectors are unable to do this with just one detector six bar detectors would be needed to build an omni-directional observatory. 

FIGURE 8.15 An illustration of an FTIR instrument showing the light source, the interferometer, the sample compartment, and detector. 

Both the GC-MS and GC-IR instruments obviously require that the column effluent be fed into the spectrometer detection path. For the IR instrument, this means that the IR cell, often referred to as a light pipe, be situated just outside the interferometer (Chapter 8) in the path of the light, of course, but it must also have a connection to the GC column and an exit tube where the sample may possibly be collected. The infrared detector is nondestructive. With the mass spectrometer detector, we have the problem of the low pressure of the mass spectrometry unit coupled with the ambient pressure of the GC column outlet. A special method is used to eliminate carrier gas while retaining sufficient amounts of the mixture components so that they are measurable with the mass spectrometer. 

An FTIR instrument The three critical components (excluding the sample) are the source, the detector and the interferometer. In terms of enabling technology it is the interferometer that is critical to the measurement. 

Most detector systems require that the IR beam be modulated, where the source energy is adequately differentiated in the measured signal from the ambient background. One of the traditional approaches is to use some form of mechanical chopper , usually in the form of a rotating sector wheel, which modulates the beam by blocking the radiation in one or more sectors during a rotation. Note that this is not a requirement for FTIR systems where the interferometer naturally modulates the beam. 

Sources and detectors Specific discussions of sources and detectors have been covered elsewhere in this article. The issues here are more service and performance related. Most sources have a finite lifetime, and are service replaceable items. They also generate heat, which must be successfully dissipated to prevent localized heating problems. Detectors are of similar concern. For most applications, where the interferometer is operated at low speeds, without any undesirable vibrational/mechanical problems, the traditional lithium tantalate or DTGS detectors are used. These pyroelectric devices operate nominally at room temperature and do not require supplemental cooling to function, and are linear over three or four decades. 

For the usual interpretation, once the photon is detected at D2, nothing more from 2 remains in the interferometer because of the collapse of the wave-function. Therefore at detector Dj only the wave 2 from the usual source arrives. Since the path of this wave does not cross the phase shifting device and, even more, is only one wave, the coincidence count does not depend on the phase. 

---