Rapid-scanning interferometer

Diffey W M and Beck W F 1997 Rapid-scanning interferometer for ultrafast pump-probe spectroscopy with phase-sensitive detection Rev. Sci. Instrum. 3296-300... 

Rapid scanning interferometers can be used to measure spectra of peaks eluting from a gas chromatograph without prior trapping of the sample (Ref 58)... 

Another major Hmitation arose from the need to employ a step-scan interferometer. This necessity arose from the relatively slow read-out rates of these first-generation FPAs, which were of the order of only a few hundred Hertz. The read-out rate (or frame rate) of a FPA detector determines the type of interferometer that must be used for FT-IR imaging, as the FPA cannot be triggered (for data transfer) any faster than its maximum read-out (frame rate) speed. As the first-generation FPAs were only capable of frame rates in the hundreds of Hz, and rapid-scanning interferometers required a faster frame rate, the use of step-scan... 

The main advantage of rapid scanning instruments is the ability to increase the signal-to-noise ratio (SNR) by signal averaging, which leads to an increase of signal-to-noise proportional to the square root of the time. It follows that in a rapid-scan interferometer the SNR is related to the number of scans by the following relationship ... 

Successful concepts for "reaction-modulated" IR difference spectroscopy use the multiplex advantage of FTIR spectroscopy or the availability of high-intensity laser IR sources. A kinetic photometer using tunable IR diode lasers as sources for the mid-infrared has been developed in our laboratory and will be described elsewhere [6]. It covers the time-domain from approx. 500 nsec to some seconds. A second approach is time-resolved FTIR spectroscopy using a rapid-scanning interferometer, several scans can be recorded per second and the time-domain of slow reactions thus be covered [7]. The following schemes illustrate both concepts ... 

Infrared radiation from an incandescent source, such as an SiC Globar, is collimated and passed through a rapid-scanning interferometer so that each wavelength in the spectrum is modulated at a different frequency. The beam of radiation is then focused onto the first window of the light-pipe and the infrared beam emerging from the second window is refocused onto a sensitive detector (typically a liquid-nitrogen-cooled mercury cadmium telluride (MCT) photoconductive detector). A typical system is... 

Steiner et al. [14] have reported the measurement of maps of octadecanephos-phonic acid (OPA) molecules deposited on a microstructured aluminum oxide/gold surface using an PEL source that was modulated by a rapid-scanning interferometer. The beam was then passed through a wire-grid polarizer and a photoelastic modulator so that the polarization was switched at a rate of 75 kHz. 

The movable mirror can either be moved at a constant velocity (a continuous-scan interferometer) or be held at equally spaced points for fixed short periods and stepped rapidly between these points (a step-scan interferometer). When the mirror of a continuous-scan interferometer is moved at a velocity greater than 0.1 cm s (the usual case for most commercial instmments), the interferometer is often called a rapid-scan interferometer. 

Figure 2.12. Typical interferogram of a blackbody-type source in the region of the centerburst measured by a rapid-scanning interferometer. The slight asymmetry indicates a very small amount of chirping.

Rapid-scan interferometers account for almost all the FT-IR spectrometers currently in use. In these instruments the moving mirror travels at a constant velocity (usually greater than 0.1 mm s ) during each scan. In a typical experiment the SNR of the spectmm is improved by repeated measurement and co-addition of the interferograms. Whereas the signal adds coherently, the measurement noise does not, as it is random and hence increases only with the square root of the number of scans, N thus, the SNR increases with y/N. This process is known as signal averaging. 

Figure 2.19. (a) Retardation versus time for a unidirectional rapid-scanning interferometer where the mirror speed is relatively slow during data collection, then resets rapidly for the next scan (b) velocity scan that corresponds to (a). The speed in the reset direction is greater than that for scanning. —A and +A indicate the limits of retardation at the beginning and end of travel, respectively. 

Figure 2.20. Retardation (a) and velocity (b) versus time fOT a bidirectional rapid-scanning interferometer. The speed of the moving mirror in both directions is tbe same.

There are experiments for which rapid-scan interferometers are not well suited. These experiments include depth profiling by photoacoustic spectrometry (Section 20.3), hyperspectral imaging (Section 14.5), fast time-resolved spectrometry... 

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