The helium-neon laser is consistently pumped electrically making use of a high dc voltage of up to 1 kV. The obtain medium consists of a gas mixture of about 10 components helium come each component of neon in ~ a push of about 3 Torr. The acquire of this medium is incredibly low, hence the necessity for Brewster-angle home windows to get rid of reflection lose of the light polarized in the aircraft that has the axis of the laser and also the typical to the Brewster window. Because of this requirement, the light calculation is necessarily plane polarized.
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The pump excites helium atom by electron impact, and also resonant energy transfer to neon atoms then occurs via collisions that the gaseous atoms (Figure 4). This create a populace inversion in the neon atoms, enabling the laser change to take place at 632.8 nm. Following emission, the neon atoms decay nonradiatively under to their metastable 2p53s1 level native which castle decay earlier down to the ground state via collisional de-excitation through the wall surfaces of the tube. This device restricts the maximum achievable power output to ca. 50 mW, due to the fact that of the should depopulate the metastable neon atoms by wall surface collisions.
An advantage of utilizing helium-neon lasers because that Raman spectroscopy is the they typically operate in the fundamental TEM00 mode, i beg your pardon is critically dependency on the proportion of the size of the pipe to that is diameter. Thus they space designed to have tiny tube diameters of about a couple of millimetres and lengths that 0.15–0.50 m, the tiny diameter additionally aiding collisional de-activation v the walls as result of the relatively large surface to volume ratio of the tube. Another advantage of this laser because that Raman spectroscopy is the the linewidth that the 632.8 nm emission line is ca. 1.5 GHz.
A disadvantage of the helium-neon laser is that it emits a huge number the spontaneous emission lines originating native the excited neon atoms. These plasma emissions room observed in the Raman spectrum together sharp lines uneven they room filtered out, for example, through a pre-monochromator or one interference filter. It must be mentioned, in passing, the these plasma lines space not always unwanted, since they deserve to be an extremely useful for wavenumber calibration the the Raman spectrometer.
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The short output strength of −1 region lie well within the quantum effectiveness curve the this detector. The is likewise possible, though less common, to operate the helium-neon laser ~ above weaker transitions that encompass the environment-friendly 543.5 nm line.