Laser Beam Quality and M2 Measurement: Beam Amplification and Quality Variables
Learn everything you need to know about laser beam quality.
The laser beam quality has a transforming effect on the throughput of a machine if maintained and improved. In principle, the ideal energy profile of laser output is defined by a “normal” or Gaussian distribution—smoothly bell-shaped and symmetrical, reaching a peak at the central optical axis. This peak is defined as the fundamental mode, (TEM00). While this is the ideal, it is only approximated in reality. This energy profile results in the lowest divergence, reducing the widening of the beam with distance from the source. Idealized beam quality limits beam diffraction. It minimizes the focused dot size to the smallest possible, resulting in the highest energy concentration. Optics, laser source, and path imperfections all affect the distribution adversely, and managing these issues is the result of good design. This article will discuss laser beam quality, M2 beam quality, the importance of laser beam quality, its characterization, and how it is measured.
Laser beam quality defines the aspects of the beam illumination pattern and the merits of its transformation and propagation. It is measured from the beam pattern as it illuminates, and the beam space-bandwidth product (SBP) criterion. This latter is a data measurement, representing the number of receiver pixels required for achieving maximum information capacity. Observation of beam pattern symmetry allows the spatial mode properties to be assessed, which shows any obstruction or distortion in the beam path. Measurement of the focused dot size allows a measurement of the multiplier of the diffraction limit to be assessed. It also helps to understand the source and optic distortion of the beam's energy distribution.
M2 is the parameter that defines laser quality in the real world, where M2=1 is the value for perfect Gaussian energy distribution in the beam. M2 is measured either manually or with an automated instrument, and it’s calculated from the relationship between waist diameter at the focal point (D0), divergence angle (θ0), and wavelength (λ).
θ0 = M2 4λ/πD0
Laser beam quality defines several properties in the resulting processing of materials as listed below:
- A small dot at the focal point results in a high energy concentration. Increasing the effectiveness of the laser (as the energy is applied to a smaller area of material) increases the effective power.
- Greater point energy concentration results in more rapid vaporization, reducing the heat-affected zone (HAZ).
- A smaller beam diameter at the top of the cut results in a narrower cut. This allows the creator precision in finished part dimensions.
Certain beam properties are more important in some applications than others. The most common ways to characterize laser beam quality are listed below:
The Strehl ratio is the ratio of the peak intensity of the laser spot compared with the theoretical, diffraction-limited ratio of an optically perfect system. Values range from 0 (no spot of energy) to 1 (optically perfect, the intensity at the theoretical maximum). The Strehl ratio was originally developed for assessing the quality of astronomical instruments. It is finding commercial use in laser cutting equipment as a way to quantify the optical path quality of a device.
This quantification is mathematically simple. It is more closely related to the functional rather than the theoretical, potentially making it a more understandable reference. The laser beam parameter product is the product of the beam waist radius (i.e. the radius of the focused dot) and the divergence half-angle (in radians or more usefully milli-radians)—i.e., an easily measurable physical property.
M2 is a calculated value that defines the divergence of the actual distribution from the Gaussian ideal. It results in a value between 1 and sometimes 10 or higher, where 1 is a perfect, theoretical beam and higher is degrading quality, as defined in ISO Standard 11146. In reality, laser cutters with M2 as high as 10 can be effective in heavy work, but lower is desirable.
Power in Bucket (PiB) is not yet measured by a universally accepted measurement. However, the measurement of total energy at the lens aperture (diameter D) and at a focal point aperture of (𝜆(beam principle wavelength)/D) are both required.
Vertical axis beam quality is calculated from the:
Vertical axis beam quality formula.
Horizontal power distribution for a given power is calculated as the ratio:
Horizontal power distribution formula.
A higher-quality beam allows greater separation between the optics and the cut surface. This reduces deposition on the lens and obstruction of the beam. It also allows smaller diameter beams to be produced. This reduces the overall cost of the optics. A higher-quality beam can be better focused to a point. It increases the energy density per unit area at the cut. This results in a smaller heat-affected zone (HAZ) and lower kerf in the cut. A greater distance to the cut also allows the use of diode-pumped lasers, as the pump beam requires optics before entering the laser crystal.
Low beam quality has serious impacts on laser cutter performance. Greater beam divergence results in more HAZ and greater kerfing. Low beam quality lessens the ability to tightly focus the beam to a point. This results in lower specific energy, requiring higher overall power to compensate for the loss of efficacy in the beam. The poorer focus must be compensated for by lower separation between optics and cut surface. This increases the risk of debris and deposition obstruction.
The ideal laser beam quality ratio in terms of the Strehl ratio is 0.8. It is considered a practical optimum because this value has a theoretical maximum of 1. For CO2 lasers, the M2 value typically lies between 1.1 and 1.3. Diode lasers typically range from 1.1 to 1.7. High-energy devices can be around 3 or 4, and values as high as 10 are not uncommon. PiB values commonly lie between 0.2 (quite poor) and 0.6 (good).
The Strehl ratio is not easy to measure. A pixelated digital light meter capable of tolerating intense energy levels is required. It measures at the optic exit point and at the focal point, which will map the energy levels. This is used in conjunction with the diffraction-limited theoretical value for the laser.
The most easily assessed measurement is the BPP (beam parameter product). This can easily be measured with simple instruments. By increasing the laser optics to target distance to a large, known value and measuring the large disc of the incident light, the divergence half-angle can be calculated. By bringing the laser optics to the focal length and measuring the diameter of the dot of light that results, the second measurement gives the focal radius. Multiplication of these two values gives the BPP.
An essentially identical method can be used for calculating M2, and with a known laser wavelength, the value is simple to calculate. The PiB (power in a bucket) parameter is also difficult to measure. It requires an energy measurement at multiple points across the axis of the beam, both at the exit of the optics and at the focal point. These measurements are not easy to achieve, requiring a fairly scientific approach.
For more information, see our guide on How to Measure Laser Beam Quality.
Manipulating a particular laser's quality to improve (lower) the M2 value is advanced optical physics. It is tackled using phase reshaping, continuous phase elements, and a variety of other approaches. In a commercial application, the best approach is to upgrade either the laser module, the optics, the beam path guides, or all three. Imperfect optics, beam occlusion, and poor-quality lasers are relatively easily changed to improve beam quality.
Laser beam quality is affected by:
- The theoretical Gaussian distribution that the laser could produce, and the difference between that energy profile and the actual output. This is directly attributable to injection laser quality, or the clarity and uniformity of the lasing elements.
- The optical quality and alignment of the collimation lenses.
- The optical quality of the focal lens arrangement.
- The alignment of the beam to the optical path.
- Occlusions and contamination in the optical path, both before and after the focal arrangement.
Nonlinear optics (NLO) is the science that describes how high-intensity energy beams interact with matter, undergoing quantum effects that change the nature and wavelength of some or all of the light. In smaller, low-power devices these effects are negligible, but in extremely powerful industrial devices, the effect can be marked.
These effects can be harnessed using phase manipulation to improve beam quality through the intelligent and adaptive design of the optical pathway components. Second harmonic generation is an example of an NLO effect, whereby a ruby laser triggers the emission of coherent UV light from a quartz crystal.
Beam quality is either a direct or indirect measure of the distribution of energy across the beam and the divergence of the beam. Both of these are very influential in the eventual application of energy to vaporize a target to cut material. These factors influence the ability of the beam to be focused, reducing (as they degrade) the specific energy at the cut point and the divergence of the beam after the focal point, resulting in kerfing.
No, having a low-quality beam is not good. As beam quality degrades, the specific power (energy per mm2) drops, making the effective laser power lower and slowing or preventing effective cutting.
Yes, a low value of M2 is the ideal laser beam quality. Although, M2 is not the only measurement of value in laser cutting. Other approaches to analysis also provide good approximations for laser cutter effectiveness. However, no laser cutter with a high PiB or BPP is likely to have a disadvantageous M2 value, so each of the beam quality measures has a similar utility.
No, high beam quality is not safe, but desirable. Higher beam quality usually indicates higher specific energy (W/mm2) at the focal point. This can result in higher energy concentration in the scattered light when cutting reflective materials. Additionally, the higher energy intensity will cut better. This is a user hazard, but when proper safety precautions are practiced, the risk is mitigated by appropriate filter barriers at the machine, machine cover/guard interlocks, and user goggles.
This article presented laser beam quality, explained what it is, and discussed it in detail. To learn more about laser beam quality, contact a Xometry representative.
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