Determination of saturation parameter in iodine and precise molecular linewidth measurements in He-Ne/I-2 standard at 633 nm

A new method for linewidth studies in a primary wavelength standard is proposed, resulting in the determination of a saturation parameter in iodine, the physical quantity that defines the amplitude and the power broadening of the inverted Lamb dip in the absorption medium. Precise linewidth information is obtained by comparison of the experimental beat-frequency data of the laser locked to the zero crossings of the discriminant curve, with the results of calculations that use the high-power gas laser theory with Gaussian intensity distribution beam profile. The method is based on the variation principle, the saturation parameter value being found by the computer fit, which gives a constant value of the homogeneous linewidth of iodine for several laser modulation width values. The saturation parameter in iodine has been determined for the first time. Its dependence on iodine pressure is found to be parabolic. The relation between the saturation parameter and the laser output power is presented, giving the possibility to calculate the saturation parameter of an auxiliary He-Ne/I-2 laser at 633 nm.