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Neglecting divergence due to poor beam quality, the divergence of a laser beam is proportional to its wavelength and inversely proportional to the diameter of the beam at its narrowest point. For example, an ultraviolet laser that emits at a wavelength of 308 nm will have a lower divergence than an infrared laser at 808 nm, if both have the ...
In laser science, the beam parameter product (BPP) is the product of a laser beam's divergence angle (half-angle) and the radius of the beam at its narrowest point (the beam waist). [1] The BPP quantifies the quality of a laser beam, and how well it can be focused to a small spot.
From the above expression for divergence, this means the Gaussian beam model is only accurate for beams with waists larger than about 2λ/π. Laser beam quality is quantified by the beam parameter product (BPP). For a Gaussian beam, the BPP is the product of the beam's divergence and waist size w 0. The BPP of a real beam is obtained by ...
The beam divergence of a laser beam is a measure for how fast the beam expands far from the beam waist. It is usually defined as the derivative of the beam radius with respect to the axial position in the far field, i.e., in a distance from the beam waist which is much larger than the Rayleigh length. This definition yields a divergence half-angle.
Laser physicists typically choose to make θ the divergence of the beam: the far-field angle between the beam axis and the distance from the axis at which the irradiance drops to e −2 times the on-axis irradiance. The NA of a Gaussian laser beam is then related to its minimum spot size ("beam waist") by
International standard ISO 11146-1:2005 specifies methods for measuring beam widths (diameters), divergence angles and beam propagation ratios of laser beams (if the beam is stigmatic) and for general astigmatic beams ISO 11146-2 is applicable.
The equation for the divergence of a pure Gaussian TEM 00 unfocused beam propagating through space is given by =, (1) where D 00 is the diameter of the beam waist, and λ is the wavelength. Higher mode beams often start with a larger beam waist, D 0, and/or have a faster divergence Θ 0. In this case Equation (1) becomes
The beam quality of a laser beam is characterized by how well its propagation matches an ideal Gaussian beam at the same wavelength. The beam quality factor M squared (M 2) is found by measuring the size of the beam at its waist, and its divergence far from the waist, and taking the product of the two, known as the beam parameter product.