An Improved Thermal Gas Lens for Optical Beam Waveguides

The quality of thermal gas lenses can be significantly improved if the heated gas is exhausted radially. This type of exhaust is typically created in a counter-flow lens where opposing gas flows pass through two closely spaced lens sections and exhaust radially through a small gap between these sections. We found that the stability of the focusing behavior depends on a laminar flow transition region of sufficient length, a properly chosen width and shape of the exhaust gap, and on mechanical symmetry and smoothness. We describe the guiding properties of a single lens as well as a beam waveguide consisting of up to eleven counter-flow lenses. Off-axis injections of the light beam with displacements amounting up to 45 percent of the lens radius resulted only in changes of the beam width (less than 23 percent for 11 lenses), whereas the gaussian profile was essentially maintained.