APPLICATION NOTE

Beam Expander Design Considerations for UV/VIS/NIR Applications

Q: What material for beam expander lenses?

UV (<350nm): UV fused silica. VIS (400-700nm): BK7 is cost-effective. NIR: BK7 works, fused silica for high-power.

Q: What magnification is typical?

Common: 2x, 3x, 5x, 10x. Lower (2-3x) easier to align. Higher (5-10x) needs better input beam quality.

Q: How critical is alignment?

Very. Typical tolerance: centering ±50μm, tilt ±1 arcmin, spacing ±100μm. Misalignment causes beam walk-off and pointing instability.

Q: Do I need AR coatings?

Yes. Without AR: ~15% loss on 4 surfaces. With AR (<0.25%/surface): ~1% loss.

Q: What input beam quality is needed?

Output quality = input × magnification. For clean output, input should be M² < 1.3 with circular cross-section.

Engineering considerations for beam expander systems across UV, visible, and near-IR. Covers Galilean vs Keplerian designs, materials, coatings, and alignment.

Beam ExpandersLaser OpticsUVVISNIR

Published: August 23, 2026By PhotonEdge Engineering Team~8 min read

Galilean vs. Keplerian

Galilean (Neg-Pos)

Shorter length
No internal focus (safer for high-power)
Inverted image
Best for: industrial lasers, beam delivery

Keplerian (Pos-Pos)

Longer length
Internal focus (spatial filtering)
Upright image
Best for: precision, research

Material Selection

Range	Material	Why
UV (200-350nm)	UV Fused Silica	High UV transmission, low fluorescence
VIS (400-700nm)	BK7	Low cost, good homogeneity
NIR (700-1100nm)	BK7 or Fused Silica	Silica for high-power, BK7 for economy

UV Warning: Never use BK7 below 320nm. It has increasing absorption and can solarize under UV. Always use UV-grade fused silica for UV beam expanders.

Design Parameters

Magnification: M = f2/f1. Typical: 2x, 3x, 5x, 10x.
Lens diameter: Output lens ≥ M × input beam diameter × 1.2.
Spacing tolerance: ±100μm for M≤5x, ±50μm for M>5x.
Wavefront quality: λ/10 PV for diffraction-limited, λ/4 for non-critical.
Coatings: AR on all 4 surfaces. Without AR: ~15% loss. With AR: ~1%.

Alignment

Coarse alignment: Both lenses on kinematic mounts, centers on optical axis.
Spacing adjustment: Adjust separation while monitoring output diameter. Minimum divergence change = correct spacing.
Fine tip-tilt: Adjust input lens while watching beam profile — goal is circular, symmetric beam.
Verify: Check diameter at 0.5m and 2m. Consistent = properly collimated.
Lock: Lock all mounts after alignment. Mark positions for future reference.

Frequently Asked Questions

Common questions from optical engineers and procurement teams.

Galilean or Keplerian?+

Galilean (negative+positive): shorter, no internal focus, safer for high-power. Keplerian (positive+positive): has internal focus for spatial filtering but longer. Most industrial applications prefer Galilean.

What material for beam expander lenses?+

UV (<350nm): UV fused silica. VIS (400-700nm): BK7 is cost-effective. NIR: BK7 works, fused silica for high-power.

What magnification is typical?+