Experimental study of thermal lensing of Nd: YAG laser

Abstract

A wavefront method of measuring the thermal lensing of solid-state lasers is proposed. This method is easy to implement and has a high spatial resolution for diagnosing thermal lensing. By this method, the thermal lensing of Nd.YAG laser is studied in detail. And this work provides a means for studying the thermal effects of laser medium and many instructional parameters for optimizing the design of the laser cavity. Beam focusing and higher-order wavefront distortion due to heat dissipation in a laser gain medium are important considerations in the design of solid-state lasers because wavefront distortion can profoundly influence the performance of a given laser design. Although the effects of heat dissipation in laser materials can be calculated for a particular geometry with appropriate boundary conditions, the material inhomogeneity as well as the nonuniform optical pumping are difficult to be evaluated by numerical modeling alone. Therefore, an accurate characterization of thermal lensing for each rod of laser material installed in a particular laser cavity can be achieved only by careful measurement of the transmitted wavefront.When a beam of light acting as a probe is transmitted through a heated laser rod, the thermal lensing can be measured with an interferometer or with a wavefront sensor such as the Shack-Hartmann wavefront sensor. For measurements of the thermal lensing, where many waves of curvature may be imposed on the probe beam, wavefront sensors offer a practical easy-to-use alternation to interferometers.Wavefront sensors are insensitive to vibration and able to measure larger wavefront distortion. They can make differential measurements that yield only thermal lensing information and ignore aberrations in both optical elements used to implement the measurement and the cold laser rod itself. Wavefront sensors operate on the principle that light travels in a straight line. If we adopt a definition for the wavefront as the surface normal to the direction of propagation of light, then waveffont sensors actually measure the slope of this surface. When the wavefront is reconstructed from the measured slope data,a lot of information on focusing and high-order optical aberrations can be obtained by using expansion of Zernike polynomials. This information can be used to optimize the cavity designs by abnegating the laser rods or cavity optics that do not meet the specifications,and * E-mail: kloudey@ sina. tom selecting appropriate materials or optics according to the requirements.The heat dissipation in a laser medium will produce changes in the refractive index and the shape of the laser rod as well as the bireffingence phenomenon at the same. These factors constitute the so-called thermal lensing, that is f;1 = f~o + fon~d q_f~ (1) where fth is the thermally induced focal length, ftho is the thermally induced focal length due to the change of refractive index only, fena is the focal length caused by the end face effect of the rod only, and fb~ due to the birefringence phenomenon. Also the three fact...

Technique for thermal lens measuring using the Gerchberg-Saxton algorithm in high power laser systems

“…For the first approach, Shack-Hartmann Wavefront Sensors (SHWS) is most prevalent [4,7,8]. For second approach, phase-shift interferometry is more common [9][10][11].…”

Technique for Thermal Lenses Measuring Using the Gerchberg-Saxton Algorithm in High Power Laser Systems

“…Currently, determination of the phase profile is achieved by using such methods as phase shift interferometry [7,8] and Shack-Hartmann Wavefront Sensors (SHWS) [4,9]. However, these techniques are resource-intensive and their accuracy is not always sufficient for determining the contribution of aberrations.…”