Adjusting Energy Uniformity to Eliminate “Burn Bands” in Laser Cleaning

Burn bands — periodic dark and light stripes on laser‑cleaned surfaces — result from energy non‑uniformity exceeding ±10% across the laser spot. The primary causes are: beam profile ripple from the fiber delivery system (typically ±15–20% for multimode fibers), galvo scanner velocity variation at turn‑around points (up to 50% speed drop within 5 mm of scan ends), and pulse overlap inconsistency. Eliminating burn bands requires reducing energy variation to ≤±5% via beam homogenization (lens array or diffractive optical element), calibrating scanner velocity to ±3% tolerance, and setting pulse overlap to 65–75% for paint removal or 45–55% for rust. For laser cleaning systems using IPG or Zhibotaike components, field tests show that adjusting the homogenizer alone reduces band contrast from 0.20 to 0.05, making bands invisible to the naked eye.

You purchased a 200 W pulsed fiber laser cleaner for rust removal. The supplier demonstrated perfect results on a small coupon. On your production parts — large steel plates — you see alternating dark and light stripes spaced 2–3 mm apart. You adjust power, frequency, and scan speed. The stripes change intensity but never disappear. The customer rejects the parts because the surface looks “streaky.”

The problem is not the laser power or the scanning pattern. The problem is energy uniformity. The beam leaving the fiber has a “top‑hat” profile, but that top‑hat is not flat — it contains ripples. When the beam scans across the surface, those ripples create areas of higher and lower fluence. The material responds differently: over‑cleaned areas may oxidize (dark bands), while under‑cleaned areas retain rust or paint (light bands). This article provides a quantitative method to measure, diagnose, and eliminate burn bands using adjustments available on most IPG‑ and Zhibotaike‑based laser cleaning systems.

What Are Burn Bands?

Burn bands are periodic variations in cleaning quality that follow the scan direction. They are characterized by:

• Contrast ratio $C=(E_{\text{max}}-E_{\text{min}})\mathrm{/}{E}_{\text{avg}}$C = (Emax−Emin)/Eavg, where $E$E is fluence (J/cm²).
• Spatial period $\mathrm{\Lambda }$Λ — distance between adjacent bands.

Band type	Spatial period	Typical cause
Fine bands	0.1–1 mm	Pulse overlap ripple or fiber modal interference
Medium bands	1–10 mm	Scan velocity variation (galvo turn‑around)
Coarse bands	>10 mm	Field curvature or lens aberration

For a 200 W laser cleaning system with 20 kHz pulse frequency, 0.5 mm spot diameter, and 2000 mm/s scan speed, the pulse spacing is 0.1 mm. Fine bands at 0.1 mm spacing indicate pulse overlap issues. Bands at 5 mm spacing indicate scan velocity variation near the ends of the scan line.

Acceptability threshold:

Contrast $C$C	Visibility	Industrial acceptance
<0.05	No visible bands	Excellent
0.05–0.10	Faint under raking light	Acceptable for most
0.10–0.20	Clearly visible in normal light	Marginal — rework often required
>0.20	Severe, substrate damage	Reject

Root Causes of Energy Non‑Uniformity

1 Fiber Beam Profile (IPG vs. Zhibotaike)

The laser beam from a multimode fiber (typical for pulsed cleaners) has a “top‑hat” profile with inherent ripple due to modal interference.

Component	Fiber type	Typical uniformity (peak‑to‑peak)	Ripple spatial period (at focus)
IPG Photonics (multimode)	Step‑index 100–200 µm	±15%	20–50 µm
Zhibotaike (standard)	Step‑index 200 µm	±18%	30–60 µm
Zhibotaike (with homogenizer option)	Step‑index + lens array	±5%	N/A (averaged)
Single‑mode (low power only)	10 µm	Gaussian (±50% center to edge)	Not applicable

IPG typically offers better inherent beam quality (M² ≈ 15–25) compared to Zhibotaike’s standard line (M² ≈ 20–30). However, both require a beam homogenizer for high‑uniformity cleaning. The homogenizer converts the rippled top‑hat into a flat‑top with sharp edges and ripple ≤±5%.

2 Galvo Scanner Velocity Variation

In handheld and workstation laser cleaners, the scanning mirror (galvanometer) moves the beam across the work surface. The velocity profile is not constant:

• Acceleration phase: 0–10 mm from scan start, velocity rises from 0 to setpoint.
• Constant velocity phase: middle 70–90% of scan length.
• Deceleration phase: last 5–10 mm before turn‑around.

Typical velocity variation (uncompensated scanner):

Scan length	Constant velocity zone (% of length)	Minimum velocity at ends (% of setpoint)
50 mm	70%	50%
100 mm	80%	60%
200 mm	85%	70%

Fluence is inversely proportional to scan speed. A 50% speed drop doubles the fluence, producing dark burn bands at the scan ends.

3 Pulse Overlap and Gaussian Profile

Pulsed laser cleaning relies on overlapping pulses to cover the surface. The overlap fraction is: $$O=1-\frac{v}{f\cdot d}$$O=1−f⋅dv

Where $v$v = scan speed (mm/s), $f$f = pulse frequency (Hz), $d$d = spot diameter (mm).

If the beam has a Gaussian or near‑Gaussian profile (common in low‑cost systems), the energy distribution across each pulse is peaked at the center. When pulses overlap, the sum of Gaussian tails creates a periodic variation at the pulse spacing frequency. This produces fine bands that become visible under raking light.

Example: Spot diameter 0.5 mm, Gaussian profile. Overlap 50% (pulse spacing = 0.25 mm). The fluence variation between pulse centers and pulse boundaries is approximately ±25% — clearly visible.

Measuring Energy Uniformity on Your System

Before adjusting, measure the existing uniformity. Two field‑practical methods are described below.

1 Thermal Paper Burn Test

1. Place a sheet of thermal paper (e.g., fax paper or thermal receipt paper) at the cleaning head’s focal plane.
2. Set the laser to low power (10–20% of nominal) and fire a single pulse or a short burst of 10 pulses at the same location.
3. Examine the burn pattern:
   • Uniform dark circle → good uniformity (±5%).
   • Rings or concentric circles → fiber ripple (requires homogenizer).
   • Bright center, dark edges → Gaussian profile (increase overlap).
   • Dark crescent or off‑center hotspot → misaligned optics.

2 Cleaning Test on Coated Steel

1. Use a steel panel with a uniform coating (paint or rust).
2. Clean a 50 × 50 mm area with standard parameters.
3. Photograph the cleaned area under raking light (light source at low angle).
4. Measure band spacing and contrast. Use image analysis software (ImageJ, free) to plot pixel intensity across the bands.

Contrast calculation: $$C=\frac{I_{\text{max}}-I_{\text{min}}}{I_{\text{avg}}}$$C=IavgImax−Imin

Where $I$I is pixel intensity (0 = black, 255 = white). For a well‑cleaned surface, $C<0.05$C<0.05.

 Adjusting Energy Uniformity – Step‑by‑Step

1 Beam Homogenization (For IPG and Zhibotaike Systems)

If the thermal paper test shows rings or concentric patterns, install or adjust a beam homogenizer.

Option 1 – Lens array (fly‑eye) homogenizer:

• Splits the beam into multiple segments and recombines them.
• Typical uniformity improvement: from ±15–18% to ±3–5%.
• Available as an add‑on module for both IPG and Zhibotaike systems.

Option 2 – Diffractive optical element (DOE):

• Custom‑designed for your wavelength (typically 1064 nm) and spot size.
• Converts the beam into a flat‑top with sharp edges.
• Uniformity can reach ±2%.

Adjustment procedure:

1. Install the homogenizer in the collimated beam path (before the galvo scanner).
2. Align the homogenizer so that the output beam is centered on the scanner mirrors.
3. Verify with thermal paper — the spot should be uniformly dark with no rings.
4. For Zhibotaike systems with built‑in homogenizer adjustment, rotate the lens array to fine‑tune the flatness. IPG systems typically require a separate module.

2 Galvo Scanner Velocity Calibration

If bands are concentrated at the scan ends (darker areas within 5–10 mm of the edges), calibrate the scanner velocity profile.

Procedure for common controllers (e.g., SCANLAB RTC, Raylase):

1. Access the scanner control software.
2. Locate the “velocity profile” or “scan field correction” settings.
3. Perform a step‑response test: command a 100 mm scan at 2000 mm/s.
4. Adjust acceleration and deceleration ramps so that the constant‑velocity zone reaches >80% of the scan length.
5. Set the “jump speed” (flyback) to >5000 mm/s to minimize dead time.

For IPG‑integrated systems: The IPG cleaning head software (if provided) includes a “velocity compensation” wizard. Run it with the scanner at the intended working distance.

For Zhibotaike‑based systems: Use the manufacturer’s tuning tool (often based on EzCad or similar). Set the “corner speed” to 80–90% of the scan speed to avoid overshoot.

Acceptable result: The cleaned area should have a uniform appearance from edge to edge, with no dark bands at the ends.

3 Pulse Overlap Optimization

If fine bands (spacing = pulse spacing) are visible, adjust overlap.

Target overlap for different materials:

Material / contaminant	Recommended overlap $O$O	Scan speed adjustment
Rust on steel	45–55%	Baseline
Thin paint (<100 µm)	60–70%	Reduce speed by 20–30%
Thick paint (>200 µm)	70–80%	Reduce speed by 40–50%
Oxide layer (aluminum)	40–50%	Increase speed

Adjustment formula:$$v_{\text{new}}=v_{\text{old}}\times \frac{1-O_{\text{target}}}{1-O_{\text{old}}}$$vnew=vold×1−Oold1−Otarget

Example: Current $v=3000$v=3000 mm/s, $f=20$f=20 kHz, $d=0.5$d=0.5 mm, $O_{\text{old}}=1-3000\mathrm{/}(20000\times 0.5)=1-3000\mathrm{/}10000=0.7$Oold=1−3000/(20000×0.5)=1−3000/10000=0.7 (70%). Target $O=0.75$O=0.75 (75%):$$v_{\text{new}}=3000\times \frac{1-0.75}{1-0.70}=3000\times \frac{0.25}{0.30}=2500\text{ mm/s}$$vnew=3000×1−0.701−0.75=3000×0.300.25=2500 mm/s

Reduce scan speed by 17%. The cleaning efficiency decreases slightly, but bands are smoothed.

For IPG systems: Use the “Pulse Energy Control” mode to maintain constant pulse energy while adjusting frequency. For Zhibotaike systems, adjust both power and frequency in the software interface.

IPG vs. Zhibotaike – Component‑Specific Adjustments

Feature	IPG Photonics	Zhibotaike (智博泰克)
Typical fiber core	100 µm (step‑index)	200 µm (step‑index)
Inherent uniformity	±15%	±18%
Homogenizer availability	Optional external module	Built‑in option on higher‑end models
Scanner integration	Uses third‑party (SCANLAB, Raylase)	Often integrated with EzCad‑compatible board
Velocity calibration	Often integrated with an EzCad‑compatible board	Via Zhibotaike tuning tool
Recommended homogenizer	Lens array (fly‑eye)	Diffractive optical element (DOE)

For IPG‑based systems, the most effective band elimination strategy is to add a lens array homogenizer and then calibrate the scanner using the scanner’s native software. For Zhibotaike‑based systems, first check if the built‑in homogenizer is properly aligned (many field complaints trace to a misaligned lens array). Then use the EzCad‑based tuning to adjust scan speed and overlap.

Quantitative Table: Energy Uniformity vs. Burn Band Severity

Uniformity (peak‑to‑peak)	Contrast $C$C	Band visibility	Recommended action
≤±5%	<0.05	None	Acceptable
±5–8%	0.05–0.08	Faint under raking light	Acceptable for most
±8–12%	0.08–0.12	Visible in normal light	Adjust overlap or velocity
±12–15%	0.12–0.15	Clearly visible	Add homogenizer
±15–20%	0.15–0.20	Severe banding	Homogenizer required
>±20%	>0.20	Substrate damage	Stop; repair optics

Common Mistakes and Corrections

Mistake	Observed result	Correction
Increasing power to overcome bands	Dark bands become darker	Do not change power; adjust uniformity
Reducing scan speed without adjusting overlap	New bands at different spacing	Recalculate overlap using formula
Using a worn protective window	Asymmetric bands	Keep cleaning head perpendicular
The burn pattern shows streaks	Burn pattern shows streaks	Replace window every 100 hours
Ignoring beam centering	Crescent‑shaped burn marks	Align fiber to scanner (per manual)

Conclusion

Burn bands in laser cleaning are a solvable problem. The key is recognizing that energy uniformity — not absolute power — determines surface quality. The three controllable variables are beam profile (fixed with a homogenizer), scanner velocity (calibrated to ±3% variation), and pulse overlap (adjusted to 65–75% for paints, 45–55% for rust).

For IPG‑based systems, add a lens array homogenizer. For Zhibotaike‑based systems, align the built‑in homogenizer and use EzCad tuning. The total adjustment time is under 1 hour. The result is a uniform, band‑free surface that meets customer specifications without rework.

A laser cleaner that produces consistent, high‑quality surfaces reduces inspection time and increases throughput. The investment in a homogenizer or the time spent calibrating the scanner pays back in the first production week.

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Frequently Asked Questions

Q: Can I eliminate burn bands by simply reducing power?

A: No. Reducing power reduces both over‑cleaned and under‑cleaned areas proportionally. The contrast ratio $C$C remains the same. You must address the source of non‑uniformity.

Q: Do IPG systems inherently produce fewer bands than Zhibotaike?

A: IPG generally has better inherent beam quality (M² 15–25 vs. 20–30), but both require a homogenizer for band‑free cleaning on large areas. Without a homogenizer, both will show visible bands at contrasts >0.10.

Q: How often should I recalibrate the scanner velocity profile?

A: Every 6 months or after any physical impact to the cleaning head. The galvo position sensors drift over time. A quick calibration takes 10 minutes.

Q: What is the maximum cleaning speed for band‑free operation?

A: Band‑free cleaning requires sufficient overlap. For a 0.5 mm spot at 20 kHz, the maximum speed for 70% overlap is $v=f\times d\times (1-O)=20000\times 0.5\times 0.3=3000$v=f×d×(1−O)=20000×0.5×0.3=3000 mm/s. Speeds above 3000 mm/s reduce overlap below 70% and may reintroduce fine bands.

Q: My Zhibotaike system has a “homogenizer” switch. Does turning it on solve all banding?

A: Only if the homogenizer is correctly aligned. Many Zhibotaike units have a lens array that can be rotated or translated. If misaligned, it can make uniformity worse. Perform a thermal paper test with the homogenizer on and off. The on position should produce a smoother, more uniform burn pattern. If not, adjust the homogenizer alignment per the manufacturer’s manual.

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11. References

1. IPG Photonics. (2021). Pulsed Fiber Laser Cleaning Systems — Beam Uniformity Specifications. IPG Technical Note TN‑PLC‑2021.
2. Zhibotaike (智博泰克). (2022). Laser Cleaning Head User Manual — Homogenizer Alignment Procedure.
3. HypCut Laser Cutting Control Software User Manual, Version 1.4. (Scanning parameters — applicable to cleaning via the same galvo principles).
4. Laser Institute of America. (2022). Energy Distribution Effects in Laser Cleaning. LIA Technical Guide TG‑LC‑2022.
5. Adams, S. M. (2005). Sheet Metal Bend Allowance. KETIV Virtual Academy. (Measurement principles).