Differential Plane Mirror Interferometer: How DPMI Works
By Zygo | Published: 2026-11-01
Category: How-to Guides
Learn how a differential plane mirror interferometer (DPMI) measures displacement and angle with two reference mirrors, and follow a setup sequence that avoids common alignment mistakes.
Quick answer
A differential plane mirror interferometer (DPMI) splits one laser beam into two paths that reflect from two separate plane mirrors, then recombines them so the interference signal reports the difference between the two paths. Set it up by defining both reference mirrors first, aligning each return beam independently, and only then zeroing the differential reading.
What makes the DPMI different from a single-pass plane mirror setup
A conventional plane mirror interferometer sends one measurement beam to a target mirror and compares it with a fixed internal reference. A DPMI instead uses two external plane mirrors: one as the displacement reference and one as the angular reference. The interference you observe is the difference between those two external paths, so common-mode motion such as stage drift or thermal expansion in the mounting structure largely cancels.
That difference matters when you care about relative motion, such as the straightness or angular deviation between two surfaces, rather than absolute distance from the interferometer head. The trade-off is that both reference mirrors become part of the metrology loop. If either reference mirror moves or is poorly mounted after zeroing, the reading drifts even though the target has not moved.
Choosing the reference mirror arrangement
The supplied DPMI description specifies use with a plane mirror target and either an angular or displacement reference mirror. That wording is a decision point, not a suggestion: the reference mirror you choose sets what the differential measurement means. An angular reference configuration is the right choice when you want to track tilt or straightness relative to a stable surface. A displacement reference configuration is the right choice when you want the difference between two linear positions.
Because the catalog does not list available option types for this product, confirm your intended reference-mirror configuration and mounting interface with Zygo before ordering. Do not assume that a mirror you already own will serve as the reference; its flatness, mounting, and coating need to match the measurement you are claiming.
Where DPMI setups go wrong
Most DPMI problems are not optical failures; they are mechanical ones. A reference mirror clamped to a flexing bracket, a target mirror that tilts as the stage moves, or a beam that clips an aperture after warm-up will all produce plausible-looking data that is wrong. The second common failure is zeroing too early: if you zero before the laser and stage have reached thermal equilibrium, the differential reading will drift for the rest of the session.
A third issue is confusing the two return beams. In a differential layout, both paths return to the interferometer, and swapping which return you align first can make a misaligned reference look acceptable. Label the beams physically at the head before you touch any adjustment.
DPMI setup sequence
- Define the measurement before mounting anything: Decide whether you need an angular reference mirror or a displacement reference mirror, and write down which surface is the target and which is the reference. Common mistake: choosing the reference mirror based on what is already on the bench rather than on what the measurement requires. If the answer is ambiguous, stop and confirm the configuration with Zygo before proceeding.
- Mount both reference mirrors rigidly and independently: Attach the reference mirror and the plane mirror target to mounts that do not share a flexing member. Check that each mount can be adjusted without disturbing the other. Common mistake: clamping both mirrors to the same thin plate, which couples their motion and defeats the differential measurement. Push gently on each mount after tightening; any visible movement means the mount is not ready for metrology.
- Level and square the interferometer head to the target path: Set the head so the measurement beam hits the plane mirror target near its center of travel and returns through the aperture across the full stage range. Common mistake: aligning only at the start position, then losing the return beam at the end of travel. Move the stage through its full range before locking anything down.
- Align each return beam separately and label it: Block one path at a time and steer the other return to its detector or camera position. Mark which return is the target path and which is the reference path at the head. Common mistake: adjusting both returns at once and mistaking a misaligned reference for a well-centered target. Both returns should be centered and stable before you attempt to observe fringes.
- Observe fringes and confirm the differential response: With both paths open, look for clean fringes and then deliberately move only the target mirror a small known amount. The fringe count should change; moving only the reference mirror should also change it, confirming that both paths are live. Common mistake: accepting fringes that do not respond to either mirror, which usually means one path is blocked. If only one mirror changes the reading, one return beam is not reaching the interferometer.
- Let the system reach thermal equilibrium, then zero: Run the laser and stage for the warm-up period your system requires, watch the differential reading for drift, and only then set zero. Common mistake: zeroing immediately after power-on, which bakes the warm-up drift into every subsequent reading. Record the drift rate before zeroing; a stable reading is the precondition for a trustworthy zero.
- Verify with a repeat measurement before collecting data: Move the target away and back to the start position and confirm the differential reading returns to zero within your tolerance. Common mistake: skipping the return-to-zero check and discovering backlash or mount creep only after a long measurement run. If the reading does not return to zero, investigate the mounts before adjusting the optics.
Product facts
- Differential Plane Mirror Interferometer: Differential plane mirror interferometer designed for use with a plane mirror target and either an angular or displacement reference mirror. The reference-mirror choice defines whether your differential reading represents angle or linear displacement, so settle that before mounting optics.
Differential Plane Mirror Interferometer
