Precision Displacement Measurement via a Distance Measuring Interferometer (DMI)

Why DMI Is Needed Important characteristics of ``International Technology Roadmap for Semiconductors: 2001'' published by the SIA.

General System LaserIF Electronics Receiver DMI system is comprised of three parts: 1.IF 2.Laser 3.Electronics

DMI History YearDescriptionDMI Resolution 1887Michelson-Morley Experiment. Leads to Michelson interferometer. NA 1902Pieter Zeeman wins Nobel Prize for effects of magnetic fields on atomic spectra. Leads to Zeeman split laser. 1960Bell Laboratories develop first HeNe laser 1964Airborne Instrument Labs, Division Cutler Hammer, first commercial displacement IF 1965Zeeman HeNe Laser (HP) 1968Perkin-Elmer “Lasergage” homodyne IF 1970Zeeman laser IF (HP) / MHz Heterodyne, with 2-pass (Zygo) /512 ~1996Current electronics (HP/Zygo). /2048

Michelson Interferometer  Michelson-Morley experiment (c.a. 1887).  Typical use of the Michelson interferometer is to compare a test optical surface against a known high quality reference flat. The output of the measurement is a light fringe pattern viewed from a diffuse surface. These fringes are spatial fringes.

Michelson Interferometer, Using Polarized Light Laser I Photodetector Heterodyne

Single Axis Interferometer 10706B Plane Mirror IF

Multiaxis IF 10735A

Design Considerations CNC Protective Covers

Design Considerations Split Frequency Limit on Velocity |||||||| Frequency (MHz) For a four pass plane mirror IF.

Design Considerations Miscellaneous Vacuum compatibility. Low adjustability. Beam size. Metric vs. U.S. Customary.  CTE between parent structure and IF parts. Peak-to-Valley (PV) wavefront per optic. Remote Receiver fibers (bend radius).

DMI System Errors Deadpath & Environment Deadpath: Difference in physical optical path between Reference and Measure. L0L0 Therefore, in this example the deadpath is L=4L 0. This is assuming that the air space between the PBS and the two quarter wave plates are equal.

DMI System Errors Deadpath & Environment  Edlen’s Equation Metrologia, Vol. 2, No. 2, Pg. 71, 1966 Air Temperature, T (°C) Barometric pressure, P (mmHg) Relative humidity, H (%) Gas composition. Typically not measured. or, therefore

DMI System Errors Abbe Error Abbe error can be eliminated through with a  measurement, and by knowing L.

DMI System Errors Cosine Error Reduced through proper alignment. Part of the accuracy budget, and not the repeatability budget. As an example, you can expect a 10706B to have a cosine error of 0.05 ppm (50 nm for a 1 m travel).

DMI System Errors Errors Summary 1.Make the ambient environment tightly- controlled and stable, and apply atmospheric compensation tools. 2.Minimize deadpath distances and Abbe offsets, and subtract in the processing. 3.Properly align the optics.

Summary DMI is currently the most accurate and sensitive linear translation measurement scheme. Additionally, it has a near limitless translation measurement bandwidth. Relative, not absolute. Noncontact. Near coaxial measurement of translation axis. Resolution (Agilent 10897B electronics and two-pass IF): 1.2 nm. Accuracy: ~2-3 nm. Max. range: > 10 meters. All 6 DOFs of a rigid body, are indirectly measurable. Max. velocity (two-pass IF): 2 m/s. Typical beam diameters: 3, 6 and 9 mm. 9mm is preferred.