Beam quality is an extremely critical parameter in laser applications. It is generally considered to be a qualitative evaluation of the transmission characteristics of laser beams, which is of great significance to theoretical analysis and laser design, manufacturing, testing, and practical applications. For different application purposes, people have many definitions of laser beam quality, and different evaluation parameters have been proposed.
However, with the passage of time, there has been no defined definition of beam quality, and no standard measurement method has been established, which has brought inconvenience to scientific research and application. The universality of the beam quality factor, analyzing and controlling the factors that affect the beam quality, is of great significance.
The M2 factor, also called beam quality factor or beam propagation factor, is a common measure of the beam quality of a laser beam. The laser beam quality M2 factor is recognized by the international optical community and recommended by the International Organization for Standardization (ISO). The M2 factor overcomes the limitations of commonly used beam quality evaluation methods. It is of great significance to use the M2 factor as an evaluation standard to monitor the quality of the laser system and assist in the design. The M2 factor evaluation method is often used for low-power lasers to produce laser beams with continuous light intensity distribution on the beam section. Because the second moment of the beam is used to define the beam width, the requirements for measuring instruments are relatively high. The characteristics of the laser beam can be expressed by several parameters such as power, wavelength, and beam quality. The beam quality M2 factor is an essential parameter that characterizes the high brightness of the laser beam and good spatial coherence. Compared with other evaluation methods, the M2 factor can better reflect the essence of the beam quality, has a strong universality, and integrally reflects the spatial distribution of light intensity. When the beam quality factor is 1, it has the best beam quality.
The M2 factor is not suitable for evaluating the beam quality of high-energy lasers. The resonant cavity of high-energy lasers is generally unstable. The output laser beam is irregular, and there will be no "light waist". Moreover, for high-energy laser beams with discrete energy distribution, the second-order moment defines that the calculated spot radius is far from the actual.
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