Study On The Pumping Effect Of Solid-State Laser

The pumping effect of laser-diode-pumped solid-state laser (DPL) is analyzed and discussed in this paper and evaluation parameters are proposed. Thermal effects are caused by the heat consumption created by partial absorption of the pumping energy by the laser crystal. These thermal effects are important factors in DPL performance. In side-pumped solid-state lasers, effects of pump light non-uniform distribution along the axial, radial and tangential directions on thermal lens effect are investigated. First, research has been conducted on thermal effects caused by different LD dimensional duty factor, those in thin-walled tubes with different radii and in different tangential position of a thin-walled tube. Second, the effect of pump light non-uniform distribution on oscillating laser beam quality is examined using a laser rate equation. In end-pumped lasers, dimensional property of output beams from LD array propagating through optical fiber is investigated and the effect of LD beam direction angles on the optical field of the optical fiber exit surface is analyzed. The influence of output optical field distribution on oscillating laser is discussed and validated by experiments.There are four key issues discussed in this dissertation:Firstly, the effects of pump light non-uniform distribution along the axial directions on thermal lens effect are investigated in a LD side-pumped laser. Radial non-uniform distribution of thermal lens caused by non-quadratic distribution of heat consumption is probed. This discussion considers two analytical areas:1. A method of analyzing piecewise continuous distribution is proposed to study the effect of LD axial non-uniform distribution on thermal lens and stability of the cavity.This analysis demonstrates that depending on the independent pump light beams, the laser crystal is expected to be separated into several segments and layers. Each layer is thin enough to be supposed with a constant heat consumption density and can be equivalent to a thin lens. Its equivalent thermal lens focal length can be calculated, as can each segment and the entire crystal bar. Based on this theory, the effect of LD axial non-uniform distribution on thermal lens and stability of the resonator are studied. The results indicate that the higher dimension duty factor the LDs occupies, the more even the axial heat consumption in the crystal will be, which leads to a weaker thermal lens effect, and a longer equivalent thermal focal length. The higher dimension duty factor the LDs occupies, the longer cavity length of stable cavity will be. That means that, a cavity with the same cavity length but different LDs dimension duty factor could be a stable cavity, a metastable cavity, or an unstable cavity. The effect of LDs dimension duty factor can not be ignored in side-pumped DPL design.2. A ring-area analysis method is proposed to study the radial thermal lens distribution caused by heat consumption in non-quadratic distribution:The hypothetical thin crystal layer can be separated into a number of narrow rings according to different radial radii, the heat consumption density of which is supposed to be constant. In that case, the equivalent focal length of a ring with a specific radial radius can be calculated by temperature gradients between two adjacent rings. This method can be generalized to the entire crystal.The results show that, due to the actual pumping conditions, the radial heat consumption in crystal often has a non-quadratic distribution, which creates different equivalent focal lengths in crystal pipelines with different radial radii, and aberrations of rings between center and edge of the pipeline.Secondly, the effects of pump light non-uniform distribution along the tangential direction of the thermal lens effect and influences on circular symmetry of the oscillation light are investigated in a LD side-pumped laser. Three new parameters for describing the thermal tangential non-uniformity are proposed:the ring-roundness, the disk-roundness and the weighted disk-roundness. Ring-roundness is defined as the tangential phase delay variance over two ends of thin-walled crystal pipe with a certain radial radius. With other parameters constant, a higher pump power, a smaller number of pumping sources, and the LD angular deviation and pumping distance deviation during the assembly process can create decreased tangential uniformities of thermal effects. The weighted disk-roundness can be used to determine the oscillation light circular symmetry caused by thermal lens effect in a specific cavity.Thirdly, the overlap value of pump and oscillating light:C is defined as the amount of pump power absorbed within the radius of TEMoo oscillating light as a percentage of the entire absorbed pump power. The C-value method is used to describe pump light distribution in a LD side-pumped laser, and the impact of pump light on oscillating light beam quality is investigated. Found that there is a fixed correspondence between C value and M2of oscillating light; the greater the C value is, the better the beam quality will be. And this trend is applicable in different resonator structures. Compared with the traditional iterative method, the C-value method has the advantages of easy computing, short time, and no need to assume the mode types of oscillating light in cavity. Finally, output beam properties of fiber-coupled LD array and their impacts on oscillating light are examined.Results show that, deviating from Gaussian distribution, output light intensity on the exit surface of the optical fiber is in multi-peak distribution. Positions of peaks are sensitive to LD beam directions, and will change with the variation of LD beam direction angle, spatial location, and fiber length. This phenomenon is a result of various factors. Pump light with multi-peak distribution has a direct impact on the mode structure and output efficiency of the oscillation light. The more pump light intensity peaks concentrate in center, and the more convex of the light intensity is, the better oscillation light beam quality will be, which should be specially considered in DPL design.