What is a Fizeau Interferometer?

A Fizeau interferometer is a laser interferometer configuration commonly used for measuring the surface shape and transmission quality of optics and optical-grade surfaces.

In a Fizeau laser interferometer,  a beam splitter divides the beam from a laser source. The “reference beam” is directed to a high-quality surface. The “test beam” is directed to the test surface, or through a test optic to a high-quality return mirror.

Fizeau Interferometers Measure Surface Heights or Transmitted Wavefront Error

When the two reflected beams pass back through the beam splitter, they recombine at the sensor (camera). When recombined, the beams form an interference pattern. This pattern is due to changes in phase caused by the differences between the test and “perfect” reference surfaces.  The sensor records this interference pattern as a single measurement frame.

By combining several frames of data, one can measure the heights of all points on the test surface, or quantify the change in wavefront caused by the beam passing through the optic. There are several ways to acquire multiple frames.

Temporal Phase Shifting Fizeau Interferometer

In a “phase-shifting” Fizeau interferometer, the reference is moved in precise increments relative to the test surface. These increments are fractions of the source wavelength, or tens to hundreds of nanometers.  The instrument acquires a frame of data after each incremental move. The complete sequence of measurement frames is then used to calculate surface height or wavefront error.

Phase-Shifting Interferometers Are Limited by Vibration and Turbulence

The challenge with phase-shifting interferometers is that time elapses during the acquisition of the measurement data. While the time is brief (hundreds or thousands of milliseconds) it is long enough that vibration and air turbulence can degrade or destroy the measurement.

To overcome the environment, temporal phase-shifting systems must be isolated from vibration and turbulence. Typically, this is accomplished by moving the systems from the production floor into a dedicated metrology lab, and by using heavy vibration isolation tables. Such measures can be expensive and require valuable production space.

A Dynamic Fizeau Interferometer Overcomes Vibration and Turbulence

In a dynamic Fizeau interferometer all phase shifting is accomplished spatially, rather than over time. Extremely short acquisition time means that vibration and turbulence are effectively frozen, producing excellent measurements  despite vibration or turbulence.

Dynamic Fizeau interferometers measure in noisy environments without vibration isolation. In fact, a dynamic Fizeau interferometer will measure more accurately if vibration is present and several data frames are averaged.

Because they can operate without isolation, vibration-immune Fizeau instruments measure effectively in production floor environments. They serve in cleanrooms where noisy pumps and air handlers make temporal phase-shifting measurements challenging. Dynamic systems can even be used inside pressure chambers to measure space-bound optics during cryo-vacuum testing that simulates conditions in space.

4D Technology’s Range of Fizeau Laser Interferometers

Fizeau interferometers from 4D Technology have continually led the industry for high performance and innovation.  We pioneered the development of dynamic interferometry; led the field with high resolution metrology for measuring steep slopes and aspheres; developed short coherence source models for measuring plane-parallel optics; and produced a vast array of Fizeau mounts and accessories. 

With available wavelengths from 193 nm through 10.6 microns, apertures from 33 mm to 600 mm, and industry-leading analysis software, 4D Fizeau interferometers beat all other systems for value, flexibility and long-term investment value.