Beam parameter product
2024-4-6
Definition:
Abbreviation: BPP Product of beam radius at focus and far-field divergence angle.
The beam parameter product (BPP) of a laser beam is the product of the beam radius (at the beam waist) and the half divergence angle (far field). Commonly used units are mm mrad (millimeters times milliradians). BPP is usually used to characterize the beam quality of laser beams: the greater the beam parameter product, the worse the beam quality.
If you define the BPP of a non-Gaussian beam, you need to redefine the beam radius and divergence angle. The minimum beam parameter product that a diffraction-limited Gaussian beam can achieve is λ/π. For example, the minimum beam parameter product for a 1064 nm beam is approximately 0.339 mm mrad.
Figure 1: Beam parameter products and M2 values for different types of lasers. Because the laser has a longer wavelength, its beam parameter product is larger than that of a diffraction-limited solid-state laser, but smaller than that of a lamp-pumped system.
The BPP values of non-circular cross-section beams in the vertical and horizontal directions are different.
When the beam is transmitted in an aberration-free optical system, such as a thin lens, the BPP value does not change. If the lens converges the beam to a radius smaller than the beam waist radius, the beam divergence angle will increase accordingly. To measure BPP, the beam needs to be converged to a suitable size, which depends on the instrumentation used (e.g., beam profiler) and the size of the space (several Rayleigh lengths are required).
Non-ideal optical devices will damage the beam quality and increase the BPP value. In some special cases, small aberrations of optical elements (such as spherical prisms) will also reduce the BPP value of the beam, and then the beam distortion will be compensated by the element.
A commonly used related quantity is the diameter-divergence angle product.
Abbreviation: BPP Product of beam radius at focus and far-field divergence angle.
The beam parameter product (BPP) of a laser beam is the product of the beam radius (at the beam waist) and the half divergence angle (far field). Commonly used units are mm mrad (millimeters times milliradians). BPP is usually used to characterize the beam quality of laser beams: the greater the beam parameter product, the worse the beam quality.
If you define the BPP of a non-Gaussian beam, you need to redefine the beam radius and divergence angle. The minimum beam parameter product that a diffraction-limited Gaussian beam can achieve is λ/π. For example, the minimum beam parameter product for a 1064 nm beam is approximately 0.339 mm mrad.
Figure 1: Beam parameter products and M2 values for different types of lasers. Because the laser has a longer wavelength, its beam parameter product is larger than that of a diffraction-limited solid-state laser, but smaller than that of a lamp-pumped system.
The BPP values of non-circular cross-section beams in the vertical and horizontal directions are different.
When the beam is transmitted in an aberration-free optical system, such as a thin lens, the BPP value does not change. If the lens converges the beam to a radius smaller than the beam waist radius, the beam divergence angle will increase accordingly. To measure BPP, the beam needs to be converged to a suitable size, which depends on the instrumentation used (e.g., beam profiler) and the size of the space (several Rayleigh lengths are required).
Non-ideal optical devices will damage the beam quality and increase the BPP value. In some special cases, small aberrations of optical elements (such as spherical prisms) will also reduce the BPP value of the beam, and then the beam distortion will be compensated by the element.
A commonly used related quantity is the diameter-divergence angle product.
