What Is an Interferometer?
An interferometer is a device that measures the surface shape, refractive index, size, etc. of an object by measuring the interference phenomenon that occurs when light is shone on the object.
The interference phenomenon is the superposition of multiple waves that strengthen or cancel each other to form a new waveform. It is most noticeable when the waves originate from the same wave source or have the same or close frequencies.
There are several types of interferometers, including Michelson interferometers, Mach-Zehnder interferometers, and Fizeau interferometers.
Uses of Interferometers
The main application of interferometers is the non-contact observation of object surfaces, including flat plates such as glass, metal, and ceramics. For example, glass-based applications include cover glass for smartphones and other devices, glass for LCDs, prisms used in color-resolved optics, and semiconductor mask substrates, while metal-based applications include molds and aluminum disks.
Other special items such as silicon wafers used in semiconductor devices for electronic equipment and hard disk substrates can also be measured. Radio interferometry is also a mechanism used in astronomical observations with radio telescopes.
Principle of Interferometers
Major interferometers split the light emitted from the light source into two beams, and one beam is transmitted through the sample and interferes with the other beam. The optical distance varies depending on the refractive index and distance of the transmitted sample, resulting in the appearance of a pattern of interference fringes.
By analyzing this pattern of interference fringes, the interferometer can measure the surface profile of the sample and the shape of the transmitted wavefront. The size of a sample that can be measured with an interferometer is a few to several dozen centimeters at the most. If the sample to be measured is large, it must be cut into pieces, etc.
Types of Interferometers
Interferometers are available that can measure polished surface planes such as glass planes, wafers, and mirrors, as well as spherical surfaces such as optical lenses, steel balls, and plastic lenses. There are also multi-axis interferometers that measure in two or three axes, and these multi-axis interferometers provide more sensitive measurements in a smaller space.
Laser scanning interferometers can also measure non-planar surfaces such as cylindrical surfaces. They are used to measure glass, fiber end faces, ceramics, ground metal surfaces, and plastics in injection-molded products. Various types of interferometers are available, so the choice should be made according to the application.
Other Information on Interferometers
1. Interference Phenomena
When multiple waves are superimposed, the amplitude of the new wave at a point coincides with the sum of the amplitudes of all the waves affecting that point. In this case, the waves strengthen each other where their phases match and weaken each other where their phases are reversed, which is called interference. An interferometer is a device to observe the state of an object by observing the interference fringes produced in this case.
2. Fizeau Interferometer
A Fizeau interferometer is an interferometer that uses a laser as its light source. The measurement mechanism is as follows.
- The irradiated laser beam is transmitted through a diverging lens, a beam splitter, and a collimator lens, in that order, and becomes a collimated beam.
- The light reaches a flat glass plate, which is the reference plate, and some of the light is reflected by the reference plane on the underside of the reference plate. The remaining light, on the other hand, is transmitted through the reference plate before reaching the sample to be measured and reflected.
- The light reflected from the reference plane and the light reflected from the sample return to their original optical paths, and interference fringes are generated due to the difference in optical path length. The interference fringes are observed with a detector.
Fizeau interferometers are characterized by their simple configuration and high accuracy for plane and spherical measurements.
3. Analysis
Numerical analysis is performed using the obtained interference fringes. One of the typical methods is the Fourier transform.
By performing the Fourier transform, the frequency spectrum is extracted from the interference fringes. By inverse Fourier transforming the extracted spectrum, phase information of the measured material is obtained.