A simple method for measurement of high power, inline beam quality of high-power lasers

Abstract

Over the past few decades, applications of high-power fiber laser in the area of material processing, medicine, defence, high precision micro-machining etc., is rapidly increasing. The primary reason behind thriving applications is the exceptional power scaling which is due to unique advantages like better thermal management, compact size, cost effectiveness, high quantum efficiency and excellent beam quality. High power fiber laser has a gain medium which is a long cylindrical geometry of optical fiber and thus, provides a high surface to volume ratio. The heat generated during lasing is distributed over longer fiber length and provides better thermal management as compared to other solid-state and gas-based lasers. However, power scaling of high-power fiber lasers has required the use of Large Mode Area(LMA) fibers to increase the dimension of gain medium(fiber core) to accommodate more power. The excitation mechanism of those lasers is also becoming more complex, which is leading to the presence of higher order waveguide modes supported by these larger fibers in the output beam profile. Therefore, the measurement of the beam quality of the laser (represented by the parameter M2 ) is imperative. Conventional M2 measurement methods such as moving knife-edge, variable aperture, moving slit method etc. uses CCD, CMOS or InGaAs detectors and these detector-based methods are not reliable at high power, because of their very low saturation powers. Therefore, with these measurement techniques, high-power beams need to be attenuated and this attenuation has the potential for substantial distortion of the inherent beam profile thereby degrading the beam quality. Other methods using modal decomposition of different constituent eigenmodes are also suggested to measure M2. However, they are complex and not very accurate. In this work, a high power 100Watt Ytterbium doped fiber laser is built, and its beam profile is characterized with a newly proposed method. A simple, cost-effective and alternative thermal imaging-based measurement technique is implemented to measure both beam profile and beam quality of high-power laser beam at full power without additional attenuation or beam diversion. This technique is wavelength agile, robust and can be easily implemented in any high-power laser laboratory where the thermal camera is readily available.