Владимир Григорьевич Низьев

Formation TE modes.pdf

V. G. Niziev, and D. Toebaert

Institute on Laser and Information Technologies, Russian Academy of Sciences, Shatura, Moscow Region, 140700, Russia

ELAS NV, Slachthuisstraat 53-59, B-7700 Moeskroen, Belgium

Applied Optics 51, 954-962 (2012)

Abstract The developed iteration algorithm for simulation of lasers with an open resonator was employed in the study of transverse mode formation. The simulations of an axially symmetrical resonator rely on an analytical description of radiation diffraction from a narrow ring. Reflection of an incident wave with a specified amplitude-phase distribution from the mirror is calculated by the Green-function method.The model also includes an active medium homogeneous along the resonator axis that is represented by the formula for saturating gain. The calculations were performed for two types of lasers: with on-axis and off-axis gain maximum. In the first type of laser one can obtain either a principal mode or“multimode” generation. The latter means quasi-stationary generation with regular or chaotic oscillations.In the second type of laser high order single-mode generation is possible. Experimental results obtained on a fast axial flow 4 kW CO2 laser are also presented. They are in good agreement with the calculations.

Conclusion A numerical model for simulation of an axially symmetrical resonator with an active medium is developed.The model is based on an analytical formula of radiation diffraction from a narrow ring. Reflection of an incident wave with a specified amplitude-phase distribution from the mirror is regarded as a Green function problem. It also includes an active medium homogeneous along the resonator axis and is simulated by the formula for saturated gain.The calculated results are presented for two types of lasers: with on-axis gain maximum and with off axis gain maximum. In spite of the fact that the gain dip is very small in the second case, the transverse mode formation is qualitatively different in these two cases.In the first case (typical for low power diffusion cooled tube lasers) we can obtain either principal mode generation or “multimode” beam. The last one is shown to be quasi-stationary generation. It is realized in the form of multi pass ray mode ifthe resonator parameters correspond to the conditions of paraxial resonance. If the resonator parameters are out of paraxial resonance irregular field oscillations are observed.In the second case (typical, for example, for a FAF CO2 laser), the single modes of different order starting from the principal one can be obtained consequently at the increasing current. Oscillations leading to mode formation can be regular for paraxial resonance conditions or irregular when the resonator parameters are out of paraxial resonance.The calculated results are in good agreement with the experimental investigations performed on the FAF 4 kW CO2 laser. The calculated phase radial distribution is close to the idealized LG mode TEM10. The calculated phase distribution of the real mode having a peak and two rings is rather smoothed in comparison with the idealized LG mode TEM20.