Interferometric instruments

Circular dichroism employs standard dispersive or interferometric instrumentation, but uses a thermal source that is rapidly modulated between circular polarization states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polarized light, AA = — Aj, is recorded as a function of wavenumber. Relative differential absorptions... 

In this present chapter, we will describe the types of surface forces which can be expected in different types of systems (classified by conditions rather than as a description of different types of forces) and will concentrate primarily on three types of surface force measuring techniques. These are the SFA of Israelachvili (and as modified by others) (44-46), the AFM colloid probe technique (these two techniques are by far the most widely exploited), and the MASIF technique which provides high-resolution data from a purpose-built non-interferometric instrument. 

The detectors used in infrared spectroscopy are tailored to both the instrumental technique used and the range examined. In general, dispersive instruments are radiation-limited and require sensitive detectors, while interferometric instruments require detectors with a response rate fast enough to detect and transmit rapid energy changes to a recorder. The most common detectors are summarized in Table 4, along with some of the more salient features. Array detectors are still being developed for multiplex analysis in the NIR, but thus far the cost of these detectors has proven to be prohibitive to widespread use. 

Fig. 1 shows the block diagram of the vibrometer, in which the most sensible to small phase variations interferometric scheme is employed. It consists of the microwave and the display units. The display unit consists of the power supply 1, controller 2 of the phase modulator 3, microprocessor unit 9 and low-frequency amplifier 10. The microwave unit contains the electromechanical phase modulator 3, a solid-state microwave oscillator 4, an attenuator 5, a bidirectional coupler 6, a horn antenna 7 and a microwave detector 11. The horn antenna is used for transmitting the microwave and receiving the reflected signal, which is mixed with the reference signal in the bidirectional coupler. In the reference channel the electromechanical phase modulator is used to provide automatic calibration of the instrument. To adjust the antenna beam to the object under test, the microwave unit is placed on the platform which can be shifted in vertical and horizontal planes. 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

Figure 3.12. Experimental configuration and velocity profiles demonstrating the use of VISAR interferometric techniques in pressure-shear instrumentation to determine in-plane shear motion as well as longitudinal (P-wave) motion (Chhabildas and Swegle, 1980).

Independent arrays of telescopes have been discussed for decades but have generally not been successful, except for radio telescopes, where interferometry is a key virtue, aided by the fact that the individual telescope signals can be amplihed and combined while preserving phase information. This is not practical in the optical, thus there are significant inefficiencies in sensitivity by coherently combining the light from an array of optical telescopes. Instrumentation for an array of telescopes has also been a cause of difficulty. Perhaps the best known successful array has been the VLT with four 8-m telescopes, each with its own suite of science instruments, and the capacity to combine all telescopes together for Interferometric measurements. 

During the 1980s most mid-IR instruments moved away from dispersive methods of measurement (monochromator-based) to interferometric measurements based on the widespread introduction of FTIR instrumentation.18,19 These instruments provided the performance and flexibility required for modern-day mid-IR applications. At the heart of an FTIR instrument is a Michelson-style of interferometer. The critical elements in this style of instrument are the beamsplitter and two mirrors one fixed and the other moving (as illustrated in Figure 4.6). This type of measurement requires the production of an... 

Despite the fact that direct absorbance/transmittance measurements are well established in analytical chemistry owing to the simplicity of the instrumentation and their broad applicability and versatility towards a large number of analytes, most of the reported miniaturized optical devices are based on the measurement of variations of the real part of the refractive index, such as SPR sensors [84,109-111] or interferometric sensors [94,112]. 

The Degree Angular Scale Interferometer (DASI) is a very small interferometric array that operates at 26-36 GHz and the South Pole. After measuring the angular power spectrum of the anisotropy (Halverson et al., 2002) the instrument was converted into a polarization sensitive interferometer which detected the E mode polarization at 5.5a by looking at a small patch of sky for most of a year of integration time (Kovac et ah, 2002). The level agreed well with the solid predictions for adiabatic primordial perturbations. Since the measured quantity was the EE autocorrelation, the 5.5a corresponds to a 9% accuracy in the polarization amplitude. 

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