Filters notch

The basic layout of Raman sensors is similar to fluorescence probes. The common sensor form is that of a fibre optic probe, with excitation and collection fibres. As the excitation light comes from a monochromatic source no excitation filter is required, but a spectrally matched emission notch filter blocking the excitation wavelength is almost always part of the sensor head. 

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. 

Grover, R., Ibrahim, T.A., Kanakaraju, S., Lucas, L., Calhoun, L.C., and Ho, P.-T., 2004, A tunable GalnAsP-InP optical microring notch filter, IEEE Photon. Techno Lett. 16(2) 467-469. 

Fig. 6.3. Raman spectra of several minerals received using notch filter...

The charge-coupled device was first used in Raman spectroscopic applications in the late 1980s [45, 46], followed rapidly by the introduction of holographic notch filters [47]. This combination, coupled with the visibility of FT-Raman instruments introduced at around the same time, helped drive the growth of Raman outside the academic lab. Although the throughput advantage of... 

Because most Raman instruments rely on a notch filter to prevent CCD saturation from elastically scattered light, diffuse reflectance at the excitation wavelength is not... 

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