What is a good BPP for laser cutting?

For fiber laser cutting: BPP < 0.4 mm·mrad (single-mode, excellent for thin sheet precision), BPP = 0.5-2.0 mm·mrad (multi-mode, standard for general cutting), BPP = 2-8 mm·mrad (high-power industrial, thick plate cutting). For CO₂ lasers: BPP = 3-7 mm·mrad is typical. Lower BPP enables smaller spots and higher power density, but even BPP = 4-8 is adequate for thick plate cutting.

BPP (Beam Parameter Product): The Complete Technical Guide

Q: What is BPP in laser technology?

BPP (Beam Parameter Product) is the product of beam waist radius (w₀) and far-field half-divergence angle (θ): BPP = w₀ × θ, measured in mm·mrad. It is a wavelength-independent measure of beam quality. Lower BPP means the beam can be focused to a smaller spot. BPP is related to M² by: BPP = M² × λ / π. An ideal Gaussian beam has BPP = λ/π (0.34 mm·mrad at 1.06μm).

Q: What is the difference between BPP and M²?

M² is wavelength-normalized and dimensionless (M² = BPP × π / λ). BPP is wavelength-dependent and has units of mm·mrad. BPP allows direct comparison of beams at different wavelengths: a 1.06μm fiber laser with BPP = 0.4 mm·mrad is not "better" than a 10.6μm CO₂ laser with BPP = 3.5 mm·mrad — when normalized by wavelength (M²), they may have similar beam quality. Use M² for same-wavelength comparison, BPP for cross-wavelength comparison.

⚡ Key Takeaway

BPP = w₀ × θ (beam waist radius × half-divergence angle, in mm·mrad). It determines focused spot size and is wavelength-independent, unlike M². Ideal Gaussian: BPP = λ/π = 0.34 mm·mrad at 1.06μm. Lower BPP → smaller focus → higher power density → faster, thinner cutting.

BPP is the fundamental metric that connects laser source properties to cutting performance. While M² is the most commonly cited beam quality factor, BPP provides additional insight by retaining wavelength information and directly predicting spot size. This guide covers the physics, practical applications, and manufacturer data for all major laser types.

Published: February 11, 2026

Last Updated: February 11, 2026

Skill Level: Laser Physicist / Applications Engineer

1. BPP Physics and Calculation

Definition

BPP = w₀ × θ [mm·mrad]

w₀ = beam waist radius (1/e² half-width) in mm

θ = far-field half-divergence angle in mrad

BPP is conserved through ideal optical systems — a lens cannot improve BPP, only trade waist size for divergence and vice versa. This is the optical analog of the Heisenberg uncertainty principle.

Relationship to M²

M² = BPP × π / λ

λ = wavelength in mm (1.06μm = 0.00106mm)

Ideal Gaussian: M² = 1.0, BPP = λ/π

At 1.06μm (fiber)

BPP_ideal = 0.34 mm·mrad

At 10.6μm (CO₂)

BPP_ideal = 3.37 mm·mrad

2. BPP Values by Laser Type

Laser Type	Wavelength	Typical BPP	Equivalent M²	Typical Application
Single-mode fiber	1.06-1.08μm	0.35-0.40	1.03-1.18	Precision thin sheet cutting
Multi-mode fiber (3-6kW)	1.06-1.08μm	0.5-2.0	1.5-5.9	General metal cutting
High-power fiber (12-30kW)	1.06-1.08μm	2.0-8.0	5.9-23.6	Thick plate, welding
Disk laser (Trumpf TruDisk)	1.03μm	2.0-8.0	6.1-24.4	Cutting & welding
CO₂ (RF-excited)	10.6μm	3.5-7.0	1.04-2.07	Metal & non-metal cutting
Direct diode	0.9-1.0μm	15-60	47-200	Welding, cladding, heat treating

Important nuance: A CO₂ laser with BPP = 3.5 and a fiber laser with BPP = 0.35 have similar M² (~1.04 vs ~1.03) because BPP scales with wavelength. The fiber laser's smaller absolute BPP means it still produces a physically smaller focus spot — critical for cutting performance.

3. BPP Impact on Cutting Performance

Focused Spot Size

d_focus ≈ 4 × BPP × f / D

f = focal length, D = beam diameter at lens. Lower BPP directly gives smaller focus.

Example: BPP = 0.4 mm·mrad, f = 150mm, D = 25mm → d_focus ≈ 0.096mm = 96μm. With BPP = 4 mm·mrad → d_focus ≈ 0.96mm = 960μm.

10× BPP = 10× spot diameter = 100× spot area = 100× lower power density.

Processing Implications

BPP < 1 mm·mrad: Thin sheet precision — smallest kerf, highest speed, cleanest edges. Optimal for 0.5-3mm materials.

BPP 1-4 mm·mrad: General cutting — good balance of speed and edge quality across 1-25mm range. Most productive for mixed production.

BPP > 4 mm·mrad: Thick plate and welding — larger spot distributes energy over wider area, better for deep penetration and thermal processes.

4. BPP and Fiber Delivery

When a laser beam is coupled into a delivery fiber, the fiber core diameter and numerical aperture (NA) set a minimum BPP. The delivered BPP is always equal to or worse than the source BPP:

Fiber Core (μm)	NA	Fiber BPP (mm·mrad)	Typical Use
14	0.08	0.56	Single-mode precision cutting
50	0.08	2.0	Multi-mode general cutting
100	0.12	6.0	High-power cutting
200	0.15	15.0	Welding, cladding

BPP_fiber = (core_radius) × NA × 1000 [mm·mrad]

Frequently Asked Questions

What is BPP?

BPP (Beam Parameter Product) = beam waist radius × half-divergence angle, in mm·mrad. It is a wavelength-independent measure of beam quality that directly determines the minimum achievable focused spot size. Lower BPP = smaller focus = higher power density.

What is the difference between BPP and M²?

M² is dimensionless and normalizes BPP by wavelength (M² = BPP × π / λ). Use BPP to compare beams at different wavelengths (e.g., fiber at 1.06μm vs CO₂ at 10.6μm). Use M² to compare beams at the same wavelength. See our M² Measurement Tutorial for practical measurement procedures.

What BPP do I need for laser cutting?

Thin sheet precision (<3mm): BPP < 1 mm·mrad. General cutting (3-12mm): BPP = 1-4 mm·mrad. Thick plate (> 12mm): BPP = 2-8 mm·mrad. Higher BPP is acceptable for thicker materials where absolute spot size is less critical than total power delivered.

Related Guides

Beam Quality Guide

Full M², BPP, and divergence overview

M² Measurement Tutorial

Step-by-step ISO 11146 procedure

Focus Position Guide

Applying beam knowledge to cutting

BPP values sourced from published specifications of major laser manufacturers (IPG, Trumpf, nLIGHT, Coherent, Raycus) and peer-reviewed literature. Fiber delivery BPP calculations assume equilibrium mode distribution. Actual delivered BPP may vary with fiber bending, length, and launching conditions.