How to Choose a Beam Expander for Large Aperture Testing
By Zygo | Published: 2026-10-19
Category: Buying Guides
Selecting the right beam expander for large aperture interferometry requires matching aperture size, wavefront quality, and phase shifting method. This guide provides practical decision rules and trade-offs.
Quick answer
For large aperture testing, choose a beam expander whose output aperture matches your optic's clear aperture, with wavefront quality that meets your measurement tolerance, and a phase shifting method compatible with your interferometer.
Aperture size drives the first decision
The beam expander's output aperture must be at least as large as the optic under test. If the expander is undersized, you clip the beam and introduce diffraction artifacts. Oversizing adds cost and bulk without benefit. Measure your largest test part's clear aperture and add a small margin—typically 10-20%—to account for alignment tolerance.
Wavefront quality vs. budget
High-precision interferometry demands a beam expander with wavefront error below λ/10 or better. Lower-spec expanders (λ/4) are acceptable for shop-floor pass/fail testing but will limit absolute accuracy. Trade off wavefront quality against cost: a precision expander can cost several times more than a standard version.
If you are uncertain about your required wavefront, test a known flat reference first with your candidate expander. The measured error will reveal the expander's contribution.
Mechanical phase shifting considerations
Mechanical phase shifting (MPS) uses a motorized stage to move the reference mirror, producing the phase steps needed for analysis. It is robust and works with most interferometers, but introduces a potential source of vibration and requires careful calibration.
For large aperture expanders, the moving mass is higher, so the MPS stage must be sized accordingly. The ZYGO 18" Beam Expander- Mechanical Phase Shifting (SKU 6068-0471-12) is designed with this in mind, but verify that your interferometer's control electronics support the MPS drive voltage and connector type.
Decision rules for beam expander selection
- Rule 1: Match aperture to your largest test part: Measure the clear aperture of your largest optic. Choose a beam expander with an output aperture at least 10% larger. For example, if your largest part is 16 inches, an 18-inch expander (like the ZYGO 18" model) provides sufficient margin. If you test parts smaller than the expander's aperture, you waste money on unused capacity.
- Rule 2: Choose wavefront quality based on measurement type: For absolute metrology (e.g., reference sphere calibration), select λ/10 or better. For comparative testing (e.g., production pass/fail), λ/4 is often sufficient. When in doubt, use the expander's wavefront specification as a systematic error budget item.
- Rule 3: Verify phase shifting compatibility: Confirm that your interferometer supports the phase shifting method of the expander. Mechanical phase shifting requires a compatible drive interface and software control. The ZYGO 18" beam expander uses mechanical phase shifting; ensure your controller can drive it. Contact the manufacturer with your interferometer model to confirm compatibility before purchasing.
Product highlights
- 18" Beam Expander- Mechanical Phase Shifting: The ZYGO 18" Beam Expander- Mechanical Phase Shifting (SKU 6068-0471-12) offers a standard mechanical phase shifting design for large aperture interferometry. This expander is purpose-built for testing optics up to 18 inches, with a mechanical phase shifter that integrates directly into ZYGO interferometer setups.
18" Beam Expander- Mechanical Phase Shifting
Who should use this approach
Best for
- Optical engineers setting up an interferometer for large aperture testing (≥12 inches)
- Quality labs that need to match beam expander specifications to measurement uncertainty budgets
Not for
- Users who only test small optics (under 6 inches) and can use a standard beam expander without mechanical phase shifting
