DPMI Reference Mirrors: Setup and Alignment Guide
By Zygo | Published: 2026-11-04
Category: How-to Guides
Learn how to choose and align DPMI reference mirrors for stable differential plane mirror interferometer measurements. Step-by-step setup with practical tips.
Quick answer
Use the reference mirror style that matches your DPMI configuration—linear, linear reversed, or angular—and align it so the return beam overlaps the measurement beam before tightening any mounts. Verify stability with a short scan, then proceed with your measurement.
Why the reference mirror style matters for DPMI measurements
The DPMI splits a laser into two beams that travel different paths before recombining. The reference mirror defines the fixed optical path; the measurement mirror moves with the test surface. If the reference mirror is the wrong style for your setup, the beams may not overlap correctly or the interferometer may not achieve the intended differential measurement.
ZYGO offers DPMI reference mirrors in three styles: linear, linear reversed, and angular. The linear style suits standard configurations where the reference path runs parallel to the measurement path. The linear reversed style accommodates layouts where the reference path must be reversed relative to the measurement path. The angular style is for configurations that require an angled reference return. Choosing the right one depends on your DPMI mounting and beam routing—check your interferometer manual or the DPMI configuration before ordering.
When alignment is the real challenge
Even with the correct mirror, alignment can be tricky because the DPMI is sensitive to tilt and translation of both the reference and measurement mirrors. A common mistake is to align the reference mirror by maximizing fringe contrast alone; that can hide a lateral offset that later causes measurement drift. Instead, align for beam overlap first, then fine-tune for contrast.
If your setup uses an angular reference mirror, remember that small angular adjustments produce large beam displacements. Make coarse adjustments with the mount locked, then use fine controls. And always check that the reference mirror is securely fastened—vibration or thermal expansion can shift it during a long scan.
Step-by-step DPMI reference mirror setup
- Identify your DPMI configuration: Check your interferometer manual or the beam path to determine whether you need a linear, linear reversed, or angular reference mirror. If unsure, contact ZYGO support with your DPMI model and setup details. Ordering the wrong style is the most common and costly mistake—verify before purchase.
- Mount the reference mirror securely: Attach the mirror to its mount using the provided hardware, but leave the adjustment screws slightly loose for alignment. Ensure the mount is rigid and free from play. A loose mount will cause drift during measurement; tighten only after alignment.
- Rough-align for beam overlap: With the laser on and the measurement mirror temporarily blocked, adjust the reference mirror mount until the return beam roughly overlaps the outgoing beam. Use a target or IR card to visualize the spot. Do not rely on fringe contrast yet—overlap is the first checkpoint.
- Fine-tune for fringe contrast: Unblock the measurement mirror and adjust the reference mirror tilt to maximize fringe contrast. Make small adjustments and wait for the fringes to stabilize before evaluating. If contrast is low, recheck beam overlap before making large tilt changes.
- Lock the mount and verify stability: Gently tighten the adjustment screws while watching the fringes. Then perform a short scan (e.g., 30 seconds) to check for drift. If drift exceeds your tolerance, re-adjust and lock again. A stable fringe pattern over time is the final confirmation of proper alignment.
Product facts
- Differential Plane Mirror Interferometer (DPMI) Reference Mirrors: DPMI Reference Mirrors are offered in linear, linear reversed, and angular styles to suit different configurations. Selecting the correct style ensures the reference path matches your DPMI layout and avoids alignment headaches.
Differential Plane Mirror Interferometer (DPMI) Reference Mirrors
