In fibre laser engineering, few parameters influence cutting, welding, and marking performance as much as beam propagation. And at the centre of that behaviour lies a key concept: Rayleigh length.

Rayleigh length tells us how far a beam stays tightly focused before diffraction causes significant expansion. In practical terms, it defines how stable and forgiving your laser process will be.

“A laser beam with a long attention span gives you fewer headaches.”

Why Rayleigh Length Matters in Fibre Lasers

Fibre lasers naturally deliver high brightness and low divergence — ideal conditions for a longer Rayleigh length. This brings several advantages:

• Stable cutting across varying material thicknesses
• Consistent welding penetration, even with small standoff variations
• Better tolerance in robotic and multi-axis systems
• Improved process reliability in real-world environments

“A long Rayleigh length is your built-in safety margin.”

How Divergence Angle Predicts Rayleigh Length

Most fibre laser datasheets specify far-field divergence rather than beam waist. Fortunately, we can calculate Rayleigh length directly from divergence.

For a Gaussian beam:

If you have half-angle divergence θ\thetaθ:

$$z_R = \frac{\lambda}{\pi \theta^2}$$

If you have full-angle divergence Θ\ThetaΘ:

$$z_R = \frac{4\lambda}{\pi \Theta^2}$$

This is extremely practical when evaluating cutting heads, fibre delivery systems, and beam quality across different manufacturers.

“Never ignore the divergence figure — it’s the small number that decides the big picture.”

Quick Example: Single-Mode Fibre Laser

• Wavelength: 1070 nm
• Full-angle divergence: 1.5 mrad

Rayleigh length becomes:$$z_R \approx 0.61\, \text{m}$$

This means the beam remains near its minimum size for more than 60 cm, helping explain why single-mode fibre lasers excel in fine cutting, micro-welding and precision processing.

What This Means for System Designers

Understanding Rayleigh length is essential when:

• Selecting focusing optics
• Designing cutting/welding heads
• Integrating lasers with robots
• Troubleshooting inconsistent cut quality
• Comparing beam parameters between suppliers

“If your Rayleigh length is short, your process window becomes short-tempered.”

✅ Key Takeaway

Using divergence angle to calculate Rayleigh length gives engineers a fast, reliable way to evaluate beam behaviour without directly measuring the waist. It’s a simple formula with significant impact on real-world fibre laser performance.

In short:
Diffraction is unavoidable — but poor understanding of it is not.