Filter Rayleigh

Figure 3.4-1 Optical diagram of a commercial Michelson interferometer for infrared and Raman spectroscopy (Bruker IFS 66 with Raman module FRA 106). CE control electronics, D1/D2 IR detectors, BS beamsplitter, MS mirror scanner, IP input port, S IR source, AC aperture changer, XI — X3 external beams, A aperture for Raman spectroscopy, D detector for Raman spectroscopy, FM Rayleigh filter module, SC sample compartment with illumination optics, L Nd.YAG laser, SP sample position.

Figure 3.5-1 Types of Raman spectrometers a scanning spectrometer with triple additive monochromators, S source, D detector b instrument with subtractive double monochromator combined with polychromator and array detector AD c Rayleigh filter RF with polychromator and array detector.

The transmission factor of the entire instrument (including interferometer and Rayleigh filter) is estimated to be t = 0.1. Finally, the radiant power of the Raman line equals ... 

Fig. 4.5 Basic diagram of a FT-Raman spectrometer. S, sample NF, notch filter for rejecting non-lasing radiation from laser RF, Rayleigh filter for rejecting radiation at laser frequency Ap, aperture wheel A, analyser I, interferometer.

Interference line filters reflect the whole spectrum with a high reflectivity except for the spectral line where the filter has the largest transmittance. The Rayleigh line intensity can thus be reduced by reflection on the filter. Combinations of such filters in a row make up a very efficient Rayleigh filter. 

For recording the weak Raman spectrum excited by the Nd YAG laser line at 1064 nm, interferometers are successfully used. They have to be combined with very powerful Rayleigh filters. The quantum noise of every strong line is distributed over the whole interferogram. By Fourier transformation it is distributed as white noise over the whole spectrum. To avoid this multiplex disadvantage all strong lines have to be removed from the spectrum to be analysed. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Similar to IR sensors, Raman sensors have profited from miniaturisation and improvement of light sources and optics. Essentially, a Raman sensor consists of (i) a monochromatic source, a (ii) sensor head, a (iii) filter separating the Raman lines from the excitation radiation and Rayleigh scattering and a (iv) spectral analyser. 

The implementation of notch filters makes the application of compact single-stage spectrographs, as used for chemical sensors, possible in the first place. Additionally, it permits to detect Raman lines much closer to the Rayleigh wavelength than previously possible. 

Similar work was performed by Shaw et al.3 in 1999 when they used FT-Raman, equipped with a charge coupled device (CCD) detector (for rapid measurements) as an on-line monitor for the yeast biotransformation of glucose to ethanol. An ATR (attenuated total reflectance) cell was used to interface the instrument to the fermentation tank. An Nd YAG laser (1064 nm) was used to lower fluorescence interference and a holographic notch filter was employed to reduce Rayleigh scatter interference. Various chemometric approaches were explored and are explained in detail in their paper. The solution was pumped continuously through a bypass, used as a window in which measurements were taken. 

Fig. 8.5. Steps in the analysis of V(z) for fused quartz (Kushibiki and Chubachi 1985). (a) V(z) on a linear scale (b) V(z) filtered to remove short period ripple due to lens reverberations (c) V(z) for Teflon VL (d) Best value of V" after subtracting long period error in Vf (e) Fourier transform of (d) (f) Final Fourier transform from which the Rayleigh wave velocity and attenuation are found using eqns (8.36) and (8.37). 225 MHz, Ao = 6.6 m.

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