Raman Scattering (contd...)

An experimental set-up in which Raman scattering is utilized for species concentration measurement is shown in Figure 7.12. It has a high power laser as a light source. For steady state measurements, a continuous wave laser may be adequate. Unsteady fields will require a pulsed (Nd:YAG) laser wherein the pulsing frequency can be used to track transients. In addition, laser power is concentrated within a pulse and will help in stronger signal generation. The temptation to use high power lasers must be balanced against the possibility of melting (in solids), evaporation (in liquids) and formation of plasma (in gases). Raman signals may also be corrupted by fluorescence effects, making the choice of the laser wavelength a crucial parameter. Dye lasers are quite suitable from this viewpoint, since they are tunable with respect to wavelength. The laser beam can be focused to a spot using short focal length optics, thus enabling measurements of high spatial resolution. The optical components that collect the Raman scattered signal are physically large so that as much of a solid angle is covered as possible, around the point of measurement. Scattering is usually the strongest in a direction normal to the incident beam. The measurement of the scattered light spectra needs to be elaborate when a variety of quantities such as temperature and multiple species concentrations are to be jointly determined. The polychromator is an optical filter that can be tuned to a band width around each frequency where intensity needs to be measured. Intensity can be measured by a variety of devices such as intensified diode array detector, photomultiplier tube, or CCD cameras that have an added advantage of mapping a planar region. The measurement system is greatly simplified if only one quantity, such as temperature, is of interest. Here, frequency shift for the material undergoing scattering and the incident wavelength are both known. An interference filter can be conveniently configured for the measurement of light intensity in the concerned frequency interval. When multiple species concentrations are to be determined, the laser wavelength may be tuned to one molecule at a time. This would call for adjustments in the polychromator for appropriate filter settings. The synchronization of the light source, filters, and the light detector can be accomplished using personal computers with software such as labView.

Figure 7.12: A laser-Raman experimental set-up for species concentration measurement, adapted from Mayinger (1994).