Temperature effects in Hе-Nе laser
elium-neon laser is the most commonly used laser for the visible spectral region (0.63 µm). Active medium of Hе-Nе laser is represented by the gaseous mixture, to which the necessary energy is supplied in electric discharge. Due to high quality of beam, Hе-Nе laser is commonly used for metrological purposes. It seems that its characteristics are well known. The measurements of temperature along the active element of He-Ne laser did not occupy the minds of researchers. Their attention compelled to the matters of temperature variation in media of high-power solid-state lasers where the thermal lens occurs for obvious reasons. However, the interest in the problem of temperature distribution along the active element of Hе-Nе laser is the result of the research of capability of development of the lasers with non-traditional geometry of active elements [1, 2]. It is well known that in order to ensure the mechanism of He-Ne laser radiation generation it is required to create the conditions for the collision of electrons with cold tube walls. In case of increase of the diameter of gas-discharge tube, the generation conditions deteriorate. However, in order to develop the cone-shaped He-Ne laser the dependences detected by us can help in the increase of radiation power.
Using the pyrometer IR 260–85, the temperature along the active element of He-Ne – laser GL-110 with the length of 30 cm was measured. Active element (Fig. 1) is mounted in cylindrical container, and it has cold cathode located in parallel with the main discharge gap. Molybdenum cylinder installed in alignment with the main discharge gap serves as anode. Under normal operating conditions of laser, the values of temperature of both edges of element tube, which do not touch the discharge, are approximately identical. It is stipulated by the natural air cooling. But the edge located closer to cathode cylinder is heated by several degrees. Maximum of temperature distribution is located approximately in geometrical center of the tube (Fig. 2). Forced cooling of one tube edge resulted in the respective temperature decrease at other points as well for all values of discharge current.
Of course, the hypothesis of the influence of glass heat conductivity on temperature distribution suggests itself. For the purpose of check of this hypothesis, the equation of heat conductivity was solved. Active element of gas laser was considered as unrestricted cylinder. Practically, the cylinder can be deemed unrestricted if its length is significantly greater than the radius of its base, which is applicable to gas laser. As a result of numerical experiments, it was established that, as expected, in He-Ne lasers with the approximate length of one meter the occurrence of temperature maximum in the element center is not observed. In the paper  it is shown that cone-shaped tube in He-Ne laser ensures the radiation power by 1.8 times higher than cylindrical construction. Use of temperature effects additionally enhances the power increase.