154
D. K. Pandey et al.
Raman lasers are optically pumped. However, this pumping does not produce a
population inversion as in conventional lasers. Rather, pump photons are absorbed
and “immediately” re-emitted as lower-frequency laser-light photons (“Stokes”
photons) by stimulated Raman scattering. Thus, these pump sources are employed
onto a sample by probing an intense beam or Stoke’s beam (copropagating or counterpropagating). Indeed, this energy exchange occurs only if the frequency difference
between the pump and the Stokes laser beam frequencies matched to a molecular
vibrational frequency of the sample. Moreover, silicon and optical fiber lasers are
also being used as a source of Raman spectroscopy. The optical fiber lasers are based
on Bragg’s fiber gratings which amplify easily and provide the continuous-wave
operation for the sample interaction. Furthermore, silicon laser sources came into
light in 2005 which was based upon the photonics of semiconductors due to its indirect bandgap. They emit the light as a waveguide and interact with a sample [16].
While using the lasers as a source, the following criteria must be satisfied to obtain
good quality and accurate Raman spectra. The frequency of the lasers should be
highly stable within the two or more frequent measurements. The lasers with narrow
bandwidth are highly recommended as they directly influence the resolution of the
obtained spectra.
3.2 Filters
The spectrophotometer consists of the various types of filters, which have various
application at different stages of the Raman scattering process. For the isolation of
a single beam of laser light having a certain wavelength, the bandpass filters are
used. Further, the low pass filter allows the shorter wavelength radiation than certain
limiting value, while some of the filters are used to block the unwanted band of wavelengths, known as band block filters. As the spectrophotometer can be dispersive, it is
needed a combined setup of notch filters and a high-quality grating monochromator.
Moreover, the monochromator can be used having double or even triple gratings
with super notch filters (long-wave pass-LWP) and rejection filters (short wave passSWP). These filters are optical filters, attached with a spectrophotometer and help
to differentiate the scattering radiations. For example, to separate relatively weak
Raman lines from intense Rayleigh scattered radiations the edge filters and holographic filters are used [3, 9, 12, 14, 17]. The filter cut off of all the filters is based
upon the optical density of filters. Further, the greater optical density of filters blocks
the larger amount of light [9].
3.3 Raman Spectrophotometer
A conventional spectrophotometer is designed to receive light with several wavelengths and separate these wavelengths then ‘detect’ each wavelength by the detector,
D. K. Pandey et al.
Raman lasers are optically pumped. However, this pumping does not produce a
population inversion as in conventional lasers. Rather, pump photons are absorbed
and “immediately” re-emitted as lower-frequency laser-light photons (“Stokes”
photons) by stimulated Raman scattering. Thus, these pump sources are employed
onto a sample by probing an intense beam or Stoke’s beam (copropagating or counterpropagating). Indeed, this energy exchange occurs only if the frequency difference
between the pump and the Stokes laser beam frequencies matched to a molecular
vibrational frequency of the sample. Moreover, silicon and optical fiber lasers are
also being used as a source of Raman spectroscopy. The optical fiber lasers are based
on Bragg’s fiber gratings which amplify easily and provide the continuous-wave
operation for the sample interaction. Furthermore, silicon laser sources came into
light in 2005 which was based upon the photonics of semiconductors due to its indirect bandgap. They emit the light as a waveguide and interact with a sample [16].
While using the lasers as a source, the following criteria must be satisfied to obtain
good quality and accurate Raman spectra. The frequency of the lasers should be
highly stable within the two or more frequent measurements. The lasers with narrow
bandwidth are highly recommended as they directly influence the resolution of the
obtained spectra.
3.2 Filters
The spectrophotometer consists of the various types of filters, which have various
application at different stages of the Raman scattering process. For the isolation of
a single beam of laser light having a certain wavelength, the bandpass filters are
used. Further, the low pass filter allows the shorter wavelength radiation than certain
limiting value, while some of the filters are used to block the unwanted band of wavelengths, known as band block filters. As the spectrophotometer can be dispersive, it is
needed a combined setup of notch filters and a high-quality grating monochromator.
Moreover, the monochromator can be used having double or even triple gratings
with super notch filters (long-wave pass-LWP) and rejection filters (short wave passSWP). These filters are optical filters, attached with a spectrophotometer and help
to differentiate the scattering radiations. For example, to separate relatively weak
Raman lines from intense Rayleigh scattered radiations the edge filters and holographic filters are used [3, 9, 12, 14, 17]. The filter cut off of all the filters is based
upon the optical density of filters. Further, the greater optical density of filters blocks
the larger amount of light [9].
3.3 Raman Spectrophotometer
A conventional spectrophotometer is designed to receive light with several wavelengths and separate these wavelengths then ‘detect’ each wavelength by the detector,