Properties of low order transverse modes in argon ion lasers.

A study has been conducted to determine the trade-offs among beam divergence, output power, and mirror alignment tolerances of high power argon ion lasers operating in various low order transverse modes. The radial intensity distribution and angular divergence of the actual laser modes from the active cavity have been measured and are compared with the theoretical passive cavity modes. The intensity distributions of higher order laser modes are found to differ markedly from those predicted by theory. The differences consist primarily of enhancement of the intensity of the off-axis peaks relative to that of the center peaks. These differences are caused by the relative high small-signal gain and the saturable gain characteristics of the medium. The beam divergence in free space of different transverse modes agrees very closely with that predicted for the passive cavity modes. The three lowest order transverse modes (i.e., TEM(00), TEM(01*), and TEM(10)) diverge at 1.1, 2.4, and 3.6 times the rate of a diffraction limited gaussian beam having the same diameter at the 1/e(2) intensity points. The mirror alignment tolerances associated with different transverse modes have been measured for both double concave and plano concave cavity configurations using a wide variety of spherical mirrors. For a given cavity configuration, as the order of the transverse mode increases, the alignment requirements become progressively less stringent. In accordance with theory, the alignment tolerances are found to be less severe for double concave cavities than for plano concave cavities. For a given cavity configuration, higher order transverse modes have a larger beam diameter and consequently couple to a larger volume of the laser medium. Therefore, a greater power output may be obtained in a given length by using a higher order transverse mode. Data on the relative power available in different transverse modes as a function of cavity configuration are given. A knowledge of the trade-offs among beam divergence, cavity stability, and power output for different transverse modes allows the optimum transverse mode and cavity configuration for a given laser application to be selected.