How Interferometers Work & How We Save You Time & Money

A simple way to think interference is to consider two waves on the surface of a pond colliding with one another. If you throw two rocks into a pond, they both create concentric waves that move away from where the rocks entered. If the waves intersect, traveling in opposite directions, you may see them interfere with one another, creating a certain result, like a bigger or smaller wave, or no wave at all.

That result is similar to the interference patterns we observe using interferometry, as light waves can behave similarly to how the waves on a pond do.

In terms of light waves creating a pattern, as two or more interact, the heights of the separate waves can be added together, resulting in an interference pattern. From there, we can determine specific, meaningful properties of the waves, such as wavelength type or refractive index changes.

But, how is the principle of interference applied to interferometry technology? Well, an interferometer will split a beam of light—rather than test two separate light sources against one another—into two beams that then travel different routes, or optical paths, and are then recombined to produce an interference pattern, or interferogram, that is observed and collected using a detector.

Now, there are different types of interference. Two of those include constructive interference and destructive interference. Total constructive interference happens when the peaks and troughs of two or more waves perfectly meet up. When added together, you can construct a larger wave, the size of which is equal to the sum of the heights and depths of the two waves at each point where they are physically interacting.

On the other hand, total destructive interference occurs when the peaks of one or more waves meet and match the troughs of an identical wave. Adding these together results in them cancelling each other out, hence the term “destructive.”

In phase shifting interferometers (PSI), the reference is moved in precise increments proportionately to the test surface using mechanical oscillation. This method is used for sectional surface characterization that requires the interference data acquired during each phase shift to be digitized.

Each increment is a tiny fraction of movement, adjusting the reference surface by mere nanometers. The interferometer captures a new frame of data each time the reference is changed. Once completed, a full 3D image is created, showing the determined surface height or wavefront error.

Fizeau interferometers are commonly used for testing surface figure, flatness, and parallelism of components such as optical flats. Generally, a fabricated lens or mirror is compared to a reference piece that has the desired shape. This is accomplished by using a beam splitter to divide the beam from a laser source.

The reference beam is directed at a high-quality surface, while the test beam is directed to a test surface, or through a test optic and then onto a reflective mirror. As the reflected beams return through the beam splitter, they recombine at the sensor, or camera. This creates an interference pattern, which occurs because of the differences in phase of the two different laser beams as they reflect off of each distinct surface.

The camera records this information as a single measurement frame. It is possible to combine several measurement frames in order to measure the heights of every point on the test surface, or measure the change in wavefront caused by the beam passing through the optic. Wavefront measurement is used to determine the optical quality of an optical component or system. Multiple frames of data are acquired using methods such as phase-shifting.

A Twyman-Green interferometer, like the Michelson interferometer, is a type of amplitude-splitting laser interferometer used to test optical components, assemblies, systems, and optical-grade mechanical surfaces. In fact, it is a variant of the Michelson interferometer. Testing involves measuring the surface shape and transmitted wavefront quality.

These devices work similarly to a Fizeau interferometer, where a light source produces a beam that’s collimated and split in two by the beam splitter. Each beam travels to their respective surfaces, recombining to form an interference pattern.

One particular advantage of the Twyman-Green interferometer is its ability to measure in small locations based on its compact design, resulting in easy positioning despite difficult-to-reach locations. Twyman-Green interferometers are well-suited for measuring larger, concave optics, focal optical systems, aspherical optics or lenses, and moving systems such as adaptive optics and flexible membranes.

PSI are limited by their sensitivity to vibration, and are often moved off the shop floor into metrology labs for controlled measurements. Phase shifting requires sequential adjustments, which has a long enough exposure time that the interferometer inadvertently picks up on any vibrations occurring.

This reduces the accuracy of the measurements taken. To combat the shop floor’s mechanical disturbances, dynamic interferometers are used, as they are impervious to most noise and vibration issues.

These instruments take all measurement data simultaneously, making accurate readings possible in a hectic environment. Dynamic interferometers make it possible to capture images in four quick phase shifts, with incredibly short exposure time. This permits single frame, measurable recordings.

A white light interferometer relies on white light, rather than a laser source. It is commonly used to measure surface roughness of an object using the same principles as the Michelson interferometer.

Using a beam splitter, the light is divided into two separate beams that interact with the test object and reference object, similarly to a Fizeau interferometer. The beams recombine, forming an image through the sensor camera after being reflected off the object’s surfaces, ultimately creating an interference pattern. This phenomenon is used to measure the roughness of the test surface, as well as other sectional topography, including waviness.

White light interferometers are capable of measuring objects with sub-nanometer precision over a wide FoV. White light interferometry is old in comparison to other interferometric techniques, however, the use of white light can eliminate coherence noise such as spurious fringes and speckles that are present when using a laser source, as well as ambiguities in height that are present while using monochromatic interferometry.

Visual scanning interferometry (VSI), or coherence scanning interferometry (CSI) is an example of a technique applied when using white light as the light source during interferometric measurement. VSI can be used for high resolution measurements of samples with large step heights.