B.S.T.J. Briefs: Thermal Gas Lens Measurements

T h e r m a l G a s Lens M e a s u r e m e n t s Ity A. C. B E C K ( M a n u s c r i p t received J u n e 12, 1964) The refractive index of a gas is an inverse function of its temperature under isobai ic conditions. Therefore a cool gas flowing into a heated tube will have a lower refractive index near the heated walls than in the center, and the combination becomes a convex lens which will focus a light beam transmitted through the tube. 1,2 A simple arrangement was built to get some information about the behavior of such a device. It is sketched in Fig. 1. The gas flowed through a 5-inch long electrically heated brass tube with a j-inch inside diameter. The tube was mounted in a large polyfoam cylinder to reduce external heat, losses. A single-mode light beam from a helium-neon gas laser oscillating at X = 0.63 micron was collimated at a diameter of about £ inch with glass lenses and then sent along the axis of the heated tube, through which a gas was flowing. The light was intercepted on a screen about 10 to 20 feet away. When this system, acting like a convex lens, is placed in such a collimated beam, the light goes through a focus at the lens focal length, and then expands to form a much larger area of light on the screen. A Foucault knife edge cutting the beam at the focus was used for measuring the focal length of the lens. The reciprocal of the lens focal length in meters (often called the focusing power or convergence of the lens, and usually expressed in diopters) is plotted as a function of wall temperature rise on Fig.