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Mirror coatings are used to select wavelength. Tube length is a factor in the sense that the non-red wavelengths have very low gain and since mirrors are not perfect there is always some loss, and the longer tube is needed to increase the gain to compensate for those losses.
The HR mirror is usually broadband and will reflect all wavelengths equally while the OC mirror transmits all but the one needed. A cool experiment is to take a non-red HeNe and align a red-coated mirror with the laser cavity. You'll need a very very stable alignment jig but you can make a the HeNe hop between lasing colors!
The HR mirror is usually broadband and will reflect all wavelengths equally while the OC mirror transmits all but the one needed. A cool experiment is to take a non-red HeNe and align a red-coated mirror with the laser cavity. You'll need a very very stable alignment jig but you can make a the HeNe hop between lasing colors!