Suppression Of Higher Order Modes In High Power Waveguide Laser

As a new type of waveguide laser,the gain-guided index-antiguided(GG-IAG)waveguide is widely used in high power lasers.When the pump power is not too high,the GG-IAG waveguide laser can maintain the single mode output laser in superlarge mode field.However,with the continuous improvement of the pump power,the higher order modes of lasing will still be caused due to the problem of its own waveguide structure.In order to suppress the higher order modes and improve beam quality in large area high power waveguide laser,based on gain-guided index-antiguided theory,a new symmetric layered waveguide structure is designed in this paper,and an interval layer is proposed to add between waveguide layer and cladding layer in traditional symmetric GG-IAG waveguide structure.As a result,using the selectivity of interval layer to the modes,while reducing the leakage loss of fundamental mode,the threshold gain coefficient difference between fundamental mode and higher order mode will be further increased.When the gain in the waveguide layer is between the stable oscillation threshold gain coefficient of fundamental mode and higher order mode,the fundamental mode can occupy a great advantage in the mode competition.Thus,higher order modes are suppressed and the single mode output is guaranteed.From the theoretical analysis and numerical calculation,the guided mode principle of this waveguide structure and the method of restraining higher order modes are studied in detail.The research contents of this paper include the following parts:A theoretical analysis modal is given from the perpective of space ray optics.From the waveguide condition,the incident angle corresponding to the mode of transmission waveguide is obtained.Then,we can get the reflection coefficient of each mode according to the optical path transmission modal and the multi beam interference principle.Further,the equivalent loss coefficient and threshold gain coefficient of each mode can be obtained.A theoretical analysis modal is given from the perpective of wave optics.Starting from the Helmholtz equation,the field distribution of each mode is obtained by combining the dual function characteristics of fundamental mode field distribution and the odd function characteristics of higher order mode field distribution.According to the continuous boundary condition of the tangent components of the electromagnetic fields at the boundary in each order mode,we can obtain the corresponding eigenequations.Further,the leakage loss of each mode can be obtained.Based on the above two theories,the mode leakage loss and the field distribution offundamental mode and higher order mode in the new symmetrical layered GG-IAG waveguide structure are numerically calculated.This paper gives the parameters optimization process which can suppress the high order mode.The calculation results show that,the numerical results of two theories are consistent,the mode loss of fundamental mode can be greatly reduced on the premise of the maximum leakage loss in the higher order mode by reasonably selecting the interlayer parameters.In this way,compared to the GG-IAG waveguide with no interlayer,we can not only reduce the threshold gain coefficient of fundamental mode,improve the efficiency of the laser,but alse make the threshold gain coefficent of higher order mode far larger than fundamental mode,so that higher order modes can be suppressed more effectively and the beam quality is improved.