What M² value indicates a good laser for cutting?

For single-mode fiber lasers (up to ~2kW): M² < 1.1 is excellent. For multi-mode fiber lasers (3-30kW): M² = 1.5-5.0 is normal and acceptable for cutting. For CO₂ lasers: M² = 1.1-1.3 (low power) to 3-5 (high power industrial). Lower M² means smaller focused spot, but even M² = 3-5 is adequate for thick plate cutting where spot size is less critical.

What equipment do I need to measure M²?

Minimum equipment: (1) focusing lens with known focal length, (2) beam profiler camera (CMOS or CCD) or scanning slit profiler, (3) translation stage or rail for moving the profiler along the beam axis, (4) attenuator for high-power beams. For convenience, commercial M² measurement systems (Ophir, Thorlabs, Spiricon) integrate all components. Budget: $5,000-15,000 for manual setup, $15,000-40,000 for automated systems.

M² Measurement Tutorial: How to Measure Laser Beam Quality Factor

Q: How do you measure M² of a laser beam?

M² is measured by performing a "caustic scan": focus the beam through a lens, measure beam diameter at 10+ positions along the propagation axis (5+ on each side of the waist), fit the data to a Gaussian beam propagation equation, and extract M². ISO 11146 defines the exact procedure. An automated M² measurement system (Ophir BeamSquared, Thorlabs M2MS) performs this in 5-15 minutes.

Q: What equipment do I need to measure M²?

Minimum equipment: (1) focusing lens with known focal length, (2) beam profiler camera (CMOS or CCD) or scanning slit profiler, (3) translation stage or rail for moving the profiler along the beam axis, (4) attenuator for high-power beams. For convenience, commercial M² measurement systems (Ophir, Thorlabs, Spiricon) integrate all components. Budget: $5,000-15,000 for manual setup, $15,000-40,000 for automated systems.

⚡ Key Takeaway

M² is measured via ISO 11146 caustic scan: focus the beam, measure beam width at 10+ positions through focus, fit to Gaussian propagation model. An automated system does this in 5-15 minutes. Target M² < 1.1 for single-mode fiber, 1.5-5.0 for multi-mode cutting lasers.

M² (beam quality factor) determines your focused spot size and thus your cutting capability. This tutorial walks through the complete measurement procedure from equipment setup to data analysis. For a broader overview of beam quality metrics including BPP and divergence, see our Beam Quality Guide.

Published: February 11, 2026

Last Updated: February 11, 2026

Skill Level: Laser Engineer / Optics Specialist

1. What M² Tells You

Physical Definition

M² is the ratio of your beam's divergence-waist product to that of an ideal Gaussian beam. M² = 1.0 is the theoretical limit (perfect TEM₀₀ mode). Real beams always have M² ≥ 1.0.

w(z) = w₀ · √(1 + (M² · λ · z / (π · w₀²))²)

w₀ = beam waist radius, λ = wavelength, z = distance from waist

Practical Impact

A beam with M² = 2 produces a focused spot with 2× the area of a M² = 1 beam using the same optics. This means 50% lower power density at the workpiece. For cutting:

• M² = 1.05: Single-mode fiber (up to ~2kW) — precision cutting

• M² = 1.5-3: Multi-mode fiber (3-12kW) — general metal cutting

• M² = 3-5: High-power industrial (12-30kW) — thick plate

2. Required Equipment

Component	Purpose	Recommended	Budget
Beam profiler	Measure beam width at each z-position	Ophir-Spiricon SP928, Thorlabs BC106N	$3,000-8,000
Focusing lens	Create artificial beam waist for measurement	f = 100-300mm, AR coated for laser wavelength	$100-500
Translation stage	Move profiler along beam axis	100mm+ travel, motorized preferred	$500-3,000
Attenuator	Reduce beam power to camera-safe levels	ND filters or wedge beam sampler	$200-1,000
M² analysis software	Fit beam width data to propagation model	Ophir BeamGage, Thorlabs BeamAlyzer	Included with profiler

Integrated systems: For routine measurements, consider an all-in-one M² system (Ophir BeamSquared: ~$25,000, Thorlabs M2MS: ~$18,000) that automates the entire scannable, data acquisition, and fitting process.

3. ISO 11146 Measurement Procedure

Step 1

Set up the beam path: Collimate the beam, insert focusing lens (f = 100-300mm), and mount the beam profiler on a translation stage behind the lens. Ensure the beam propagation axis is aligned with the stage travel direction.

Step 2

Locate the beam waist: Move the profiler to the approximate focus position and find the smallest beam width. Record this as the waist position z₀. The waist should be approximately at the back focal point of the lens.

Step 3

Measure beam width at 10+ positions: ISO 11146 requires minimum 10 measurements: ≥ 5 within one Rayleigh range of the waist (near-field) and ≥ 5 beyond two Rayleigh ranges (far-field). Use the D4σ (second moment) beam width method for all measurements.

Step 4

Fit the data: Plot beam width² vs z-position. This should form a hyperbola. Fit to w²(z) = A + B·z + C·z² where M² = (π / λ) · √(AC - B²/4). Most software does this automatically and reports M² for both X and Y axes.

Step 5

Validate the result: Check the fit quality (R² should be > 0.99). Repeat the measurement 3× and verify reproducibility within ±5%. Report M²x and M²y separately if the beam is non-circular. Also compute effective M² = √(M²x × M²y).

4. Common Measurement Pitfalls

❌ Too few far-field points

Without adequate far-field data, the fit cannot accurately determine beam divergence. ISO 11146 explicitly requires ≥ 5 points beyond 2× Rayleigh range. Missing these gives artificially low M².

❌ Using 1/e² width instead of D4σ

ISO 11146 mandates D4σ (second moment) beam width. The 1/e² clip-level method underestimates the width of non-Gaussian beams and gives artificially good M² values.

❌ Camera saturation

A saturated camera clips the peak of the beam profile. This artificially widens the measured beam width and inflates M². Always check the peak pixel value is below 80% of camera dynamic range.

❌ Background noise

Stray light or camera noise adds to the second-moment width calculation, inflating M². Apply proper background subtraction. ISO 11146 recommends the iterative baseline subtraction method.

Frequently Asked Questions

How do you measure M² of a laser beam?

Focus the beam through a lens, measure the beam width (D4σ method) at 10+ positions through the focus, fit to the Gaussian propagation model, extract M². ISO 11146 defines the full procedure. Automated systems complete this in 5-15 minutes.

What M² is good for laser cutting?

Single-mode fiber: M² < 1.1 (excellent). Multi-mode cutting lasers (3-12kW): M² = 1.5-3.0 (standard). High-power (12-30kW): M² = 3-5 (acceptable for thick plate). See the BPP guide for how M² relates to focused spot size and processing parameters.

What equipment do I need?

Minimum: beam profiler ($3-8K), focusing lens ($100-500), translation stage ($500-3K), attenuator ($200-1K). Or purchase an integrated M² system ($15-40K) for automated measurements. The profiler software typically includes M² fitting algorithms.

Related Guides

Beam Quality Guide

M², BPP, and divergence overview

BPP: Beam Parameter Product

Deep-dive into BPP metrics

Focus Position Guide

Applying beam quality to cutting

Measurement procedure follows ISO 11146-1:2021 (laser beam widths, divergence angles, and beam propagation ratios). Equipment pricing reflects 2025-2026 North American/European market. Always follow laser safety protocols (IEC 60825) when performing beam measurements.