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As we know that, the He-Ne laser emits many wavelengths including the red(632.8nm), green etc.

How to select a particular wavelength? (For eg: Red) Please explain briefly.

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You have to introduce some sort of frequency selection inside resonator.

For HeNe lasers the most straightforward way is to change dielectric coating on resonator mirrors so that they only reflect 99.9% for the wavelength you need (i.e. green).

You can also introduce tunable element like Lyot filter but this could be challenging as HeNe laser has somewhat low gain and therefore very sensitive to looses.

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  • $\begingroup$ For red lasers, which material generally we use for coating. Why is so? $\endgroup$
    – user142930
    Commented Mar 10, 2017 at 20:44
  • $\begingroup$ Typically multilayer dielectric coatings are used, which could have like 50 layers of different dielectrics (for example SiO2 and TiO2). Metallic mirrors are too bad for HeNe resonator. $\endgroup$ Commented Mar 10, 2017 at 20:58

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