Noncollimated beam

FIGURE 14 Schematic diagram of a noncollimating beam displacement-type, compact, polarization-independent optical circulator. 

In the spherical lamellar grating the surface at zero path difference has not a plane but a concave shape. This interferometer operates with a noncollimated beam and is, therefore, limited to lower spectral resolution than the plane version otherwise the spherical configuration is the most simple form of a lamellar grating instrument. Only a source and a detector need to be added adjacent to the center of curvature to form a working far infrared interferometer. 

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

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