Shift and broadening of saturated absorption resonances due to curvature of the laser wave fronts

Abstract

Using frequency offset-locked spectroscopy of methane at 3.39 μm we show that the saturated absorption peaks can be significantly blue or red shifted if the wave fronts of the laser beam are spherical in the gas cell. Good agreement with theoretical prediction is found. We discuss suitable conditions to avoid this geometry-induced shift in optical frequency standards.