Quantifying the quality of a laser beam with the Beam Parameter Product and the M2 factor
Friday, September 23, 2022
All laser beams diverge. Their rays slowly spread out as they travel, diluting the laser’s power. This happens because of diffraction, a fundamental – and unavoidable – property of light.
We cannot completely eliminate divergence. The best we can hope for is a beam with the smallest divergence allowed by the laws of diffraction. Such a beam is said to be diffraction-limited. Beams with a Gaussian beam profile are diffraction-limited.
Beam quality quantifies how close a laser is to this ideal diffraction-limited Gaussian beam.
The advantages of high beam quality
High beam quality comes with considerable advantages:
Smallest spot size
Lasers of high beam quality can be focused down to the narrowest spots. This allows for smaller feature sizes and high-resolution marking and engraving.
Efficient at different depths
Because the divergence is small, the power density remains high for long distances before and after the focal spot. This makes longitudinal alignment more forgiving and means the laser is still effective deep within a cut.
Increased working distance & durability
Lasers with high beam quality allow an increased distance between the focusing objective and the workpiece, meaning the optics are less at risk of damage from fumes and material spatter.
Cheaper optics
Smaller beam diameters throughout the optical path mean that smaller, cheaper mirrors and lenses can be used.
Consistent beam profile
Gaussian beam profiles remain Gaussian throughout propagation. Only the beam radius changes. This is not the case with other profiles, like flat-tops or irregular profiles.
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Quantifying beam quality: Beam Parameter Product (BPP) and M2
The 2 most common metrics to assess laser beam quality are the Beam Parameter Product (BPP) and the M2 factor.
Beam Parameter Product (BPP)
The Beam Parameter Product is defined as the product of the beam radius at the beam waist and the beam divergence half-angle measured in the far field. The units of BPP are often mm mrad.
Lower values of BPP indicate higher beam quality, but BPP varies with wavelength. A high-quality CO2 laser will have a higher BPP than a high-quality YAG laser, just because a CO2 laser’s wavelength is longer.
M2 factor
M2 provides a good way to compare the beam quality of lasers, even if they operate at different wavelengths.
The M2 factor is defined as the BPP divided by λ/π, where λ is the wavelength. Thanks to this division, the best possible value of M2 is always 1, regardless of the laser.
In real life, high quality beams have an M2 just above 1. Because it’s hard to keep good beam quality as power increases, certain high-power laser applications have an M2 in the hundreds.
Measuring M2 requires multiple beam profiles at different points along optical path. Each profile is then used to determine the beam radius at that point. To be ISO-compliant, the D4σ beam definition must be used. Other technical considerations must also be taken into account for a reliable measurement (e.g. background noise subtraction, appropriate camera calibration and resolution, etc.).
Gentec-EO’s Beamage-M2 simplifies M2 measurement and improves repeatability thanks to an automated measurement system and intuitive software. It is equipped with 50mm optics and its many available accessories make it compatible with just about any wavelength, beam size, and power.
