对激光光束质量一些问题的认识

Some aspects of laser beam quality

从对激光光束质量研究进展开始,讨论了在傍轴和非傍轴范畴内激光光束质量的一些问题,主要是非傍轴光束的光束质量评价。光束传输的非傍轴矢量二阶矩理论可用于计算多种非傍轴光束的M2因子,但对M2因子的物理解释却有困难。例如,在非傍轴范畴,M2因子依赖于束腰宽度与波长之比,并可小于1。但远场发散角之值可趋于90,°这将导致很大的M2因子值。另一方面,若将公式中的光强用时间平均坡印廷矢量z分量代替,桶中功率(PIB),或等价地,!参数可推广用于评价非傍轴光束的远场光束质量。PIB表征了光束功率的可聚焦能力,是一种简明、直观评价不同激光光束的方法。然而,在!参数的公式中的束宽是按功率含量百分比定义的,因此双曲线传输规律不再严格成立。

Starting from a short review on the advances in laser beam quality, some aspects of laser beam quality in both paraxial and nonparaxial regimes are discussed, where main attention is focused on the characterization of beam quality in the nonparaxial regime. The nonparaxial vectorial second-order moments theory of beam propagation can be used to calculate the M^2 factor of a variety of nonparaxial beams, but the interpretation of its physical meaning is still open. For example, in the nonparaxial regime, the M^2 factor depends on the ratio of the waist width to the wavelength and may be less than one, but the maximum far-field divergence angle may approach 90°, which results in a very large M^2 factor. On the other hand, the power in the bucket （PIB）, or equally, β parameter can be extended to characterize beam quality of nonparaxial beams in the far field, if the intensity in the formulae is replaced by the z component of the time-averaged Poynting vector. The PIB provides a measure of the power focusability of beams, and is a simple and straightforward method for characterizing different laser beams. However, the beam width in the formulae of the PIB and β parameter is defined as the percentage of the power content, thus the hyperbolic propagation law does not hold strictly.