One notable application of interferometry is the Fabry-Pérot interferometer, which is widely used in various fields like optics and spectroscopy. This device consists of two parallel, partially reflecting mirrors with a known separation distance between them. When a monochromatic light source is directed between them, multiple reflections occur, leading to constructive and destructive interference patterns. Thus, by directing light between these mirrors and analyzing the resulting interference patterns, researchers can precisely determine wavelengths, study the fine structure of spectral lines, and investigate the properties of optical components. Fabry-Pérot interferometers have applications in areas such as telecommunications, astronomy, and even material science.

An application of interferometry is the Fourier Transform Infrared Spectroscopy. This apparatus is used to measure intrinsic physical properties of materials such as thermal conductivity through reflectance and transmittance. This apparatus comprises a black body source which emits IR radiation which passes through a collimator and renders the radiation parallel to the sample, until it illuminates the beam splitter (BS) in the Michelson interferometer. The beam is partially transmitted through the BS and reflected from a fixed mirror M1, and it is also partially reflected from the BS then from the movable mirror M2. The moving mirror changes the path difference ∆ that the two light beams traverse. At the BS, the two reflected beams recombine and produce an intensity I(∆) dependent on the path difference called the interferogram. This interferogram undergoes rigorous computation and is converted into a complex dielectric function in order to be utilized by material physicists to find a fingerprint of the sample understudy. The reflectivity spectrum obtained can be used to find the resonance of the optical phonon and calculate the thermal conductivity of the sample.

a specific type of interferometer called the point diffraction interferometer (PDI). The PDI is commonly used in the field of optics for testing and measuring the surface shape and wavefront quality of optical components, such as lenses and mirrors. It operates by illuminating the optical component with a collimated beam of light and using a pinhole or small aperture to create a point source of light. The light passing through the pinhole diffracts and interferes with the light reflected or transmitted by the optical component. By analyzing the resulting interference pattern, researchers can determine the surface shape deviations, aberrations, and other optical properties of the component under test. The PDI offers high precision and accuracy, making it valuable in industries like optics manufacturing, microscopy, and laser technology.

The Point Diffraction Interferometer is an optical device used to measure phase differences in optical systems as well as in microscopy. The PDI is made up of a point source, such as a laser which provides constant light, a pinhole to ensure that the incoming light waves are in a spherical pattern, a lens to ensure the light waves are parallel, and an imaging system to capture the interference pattern formed by the light waves. The PDI has a variety of applications, but I find its capacity of high resolution microscopy the most fascinating. PDI’s can, in real time, enhance the low resolution images from a microscope by measuring the amount of optical aberration, and correcting for it.

A Rayleigh interferometer uses slits to split a beam of light into two from a single source. The two beams are recombined after traversing two optical paths with compensating plates, and the interference pattern after recombination allows the determination of the difference in optical path lengths. While it is simple to construct, it requires a point or line source of light for good fringe visibility, and the fringes must be viewed with high magnification.

One interferometer that I have found to be interesting is the Mach-Zehnder interferometer. It consists of a source of light, two screens at the end, two full mirrors (a term I use to describe regular mirrors), and two half-slivered mirrors. A half-slivered mirror only allows half of the light that hits it to reflect off of it; the other half gets refracted through. The source hits the first half-slivered mirror, and the light can take two different paths from there. It can reflect off the mirror, head towards one full mirror, reflect off of that, head towards the other half mirror, and refract through the final mirror to one screen. It can also refract through the mirror, head towards another full mirror, reflect off of that, head towards the other half mirror, and reflect off of the final mirror to the same screen. I have made a diagram below, where light can only follow the arrows:
___________________________________________________________________
[SOURCE] >>> [HALF MIRROR] >>>>>>>>>>>>[FULL MIRROR]
V V
V V
V V
V V
V V
[FULL MIRROR] >>>>>>>>>>> [HALF MIRROR] >>>>>> [LIT SCREEN 1]
X
X
X
[UNLIT SCREEN 2]
___________________________________________________________________

The reason why Screen 2 did not receive any light has to do with the wavelengths. There are technically two paths to get to Screen 2, but at the end, the paths converge at opposite points on the wavelength cycle (based on the time it took them to get there) and destroy each other by cancelling out. On the other hand, the paths going into Screen 1 converge at the same wavelength point and thus do not cancel out. The ability to split a path of light into two paths that we control by the mirror placement and join them back together is revolutionary when we think about anything we might want to monitor. Heat transfer, pressure, density, flow visualization, and fiber-optic communications are all examples of topics where the Mach-Zehnder interferometer can be of great use.

Interferometry has a lot of application, one of them is its use in astronomy. In the domain of celestial observation, an astronomical interferometer comprises a collection of individual telescopes, mirror segments, or radio telescope antennas functioning collectively as a unified instrument, to provide higher resolution images of astronomical objects. These interferometers work by combining the signals from the multiple telescopes, using a complex system of mirrors, and delivers them to the astronomical instruments where it is combined and processed. All of this is principally conducted using Michelson interferometers. Some well-known observatories that utilize interferometric instrumentation include the Very Large Telescope (VLT), the Navy Precision Optical Interferometer (NPOI), and the Center for High Angular Resolution Astronomy (CHARA).

The Angle-resolved low-coherence interferometry is a biomedical imaging technology which uses the properties of scattered light to measure the size of cell structures, including their cell nuclei. This technology could be used as a clinical tool for early detection of dysplastic, or precancerous tissue.

It combines low-coherence interferometry with angle-resolved scattering to solve the inverse problem of determining scatterer geometry based on far field diffraction patterns. Like other types of interferometers, it uses a broadband light source in an interferometry scheme to achieve optical sectioning with a depth resolution set by the coherence length of the source. Angle-resolved scattering measurements capture light as a function of the scattering angle and invert the angles to deduce the average size of the scattering objects via a computational light scattering model.

This system makes it possible to measure average scatter size at various depths within a tissue sample.

Michelson Stellar interferometers are mostly used in astronomy, so this type of interferometers is often known as astronomical interferometers. They are explicitly used in cutting-edge research of exoplanet identification and incredibly high-resolution star images. The interferometer consists of four mirrors, a set of two pinholes, a positive lens, and a detector (the illustration of its structure is attached). First-ever measurement that was made by this interference was a measurement of the stellar diameter of the Betelgeuse star in December 1920. The diameter was found to be 240 million miles (~380 million kilometers), about 300 times larger than the Sun.
Overall, Michelson Stellar interferometers are capable of taking high angular resolution measurements of stars and galaxies, making them very useful for astronomical investigations.

One interesting interferometer is the Twyman-Green interferometer. The Twyman-Green interferometer is a variation of the Michelson Interferometer. It is used industrially to test optical components such as lenses. In a Twyman-Green interferometer, a source beam is collimated and then split up by a beam splitter. There is a high quality reference mirror in the setup to which a reference beam is directed. Similarly, the test beam is directed to the test surface. The shape of the test wavefront must resemble the curvature of the test surface, which can be achieved by using a diverging lens or bean expander. When two reflected beams go through the beam splitter, they combine again, and form an interference pattern, caused by differences between the surface being tested, and the ideal or reference surface. Phase shifting interferometry combines data to measure the height of different points on the test surface in order to quantify its shape precisely. This interferometer can be used to measure in unusual and small locations. Furthermore, it does not require expensive vibration isolation.

Young’s double slit inferometer is the first version of the current double slit inferometer, which is used to prove the wave theory of light. Because of this inferometer, the first evidence of light as a wave has been made. For this inferometer, monochromatic light, which only has one wavelength or frequency, is used as a light source. A screen is placed on the other side and two slits are placed in front of this light source, creating two more light sources behind these two slits before reaching the screen. The wave crest and trough from the light source create interference fringes. The brighter and duller fringes are seen on the screen. The brighter fringes are created because the two waves (crests) from two new light sources combined, making a brighter fringe. The darker fringes are made because the wave crest and wave trough from two waves of two new light sources are cancelled with each other, creating duller fringes on the screen. This shows the interference of light, which proves the theory that light is a wave.

I’m an avid user of the Wolfram Language, and found a very interesting demonstration of the aforementioned Fabry-Pérot interferometer. Link Here

Sagnac interferometers are used in inertial navigation systems (INS) to measure the rotation of a vehicle. This information can be used to determine the vehicle’s heading and orientation. INS are used in a wide variety of vehicles, including aircraft, ships, and submarines.
For example, a Sagnac interferometer in an aircraft could be used to measure the aircraft’s roll, pitch, and yaw. This information could then be used to control the aircraft’s autopilot system.
They are very accurate and reliable, making them ideal for use in navigation systems.

One fascinating application of interferometry is the use of a Fabry-Pérot interferometer in astronomy, particularly in the study of spectral lines and the determination of the properties of distant celestial objects.

The Fabry-Pérot interferometer consists of two parallel, highly reflective mirrors with a small gap in between. Light from a distant source (e.g., a star) passes through this gap. The light is partially transmitted and partially reflected back and forth between the mirrors. This creates a series of interference patterns, with some wavelengths of light constructively interfering and others destructively interfering.

Astronomers use the Fabry-Pérot interferometer to study the spectral lines of stars and galaxies. By measuring the interference patterns produced by different wavelengths of light, they can precisely determine the wavelengths and frequencies of spectral lines. This information is crucial for understanding the chemical composition, temperature, velocity, and other properties of celestial objects, helping astronomers gain insights into the nature of distant cosmic phenomena.

I was among the many many astronomy and physics enthusiasts around the world to be elated to see the first ever captured image of a black hole, and if I remember correctly it was made possible by Very-Long-Baseline Interferometry, which combines the power of several telescopes to simulate a telescope with a much bigger primary mirror diameter.

I’ve also seen application in Interferometric synthetic aperture radar or InSAR, which could help detect earthquakes by perceiving minuscule movements in the earth’s surface.

One that I found to be incredibly cool is The “Sagnac Interferometer,” also known as a Sagnac interferometer gyro. It is a specialized interferometric device used for highly accurate measurements of angular velocity. Imagine a closed-loop path where beams of light travel in opposite directions, and when you rotate this system, you can actually measure the rotation itself! It operates on the principles of interference and the Sagnac effect, which is based on the theory of relativity. In a Sagnac interferometer, a beam of light is split into two counter-propagating beams that travel in opposite directions along a closed loop, typically in the form of a ring or fiber optic coil. When the system rotates, one of the beams travels a slightly longer path than the other, leading to a phase shift when they recombine. This phase shift is proportional to the angular velocity of the interferometer’s rotation.

Sagnac interferometers are widely used in inertial navigation systems, where precise measurements of angular velocity are essential for determining changes in orientation and position. They have applications in aircraft navigation, spacecraft guidance, and other systems that require accurate motion sensing.

One specific type of interferometer is the Fabry-Pérot interferometer, which is widely used in various scientific fields, including optics and spectroscopy. It consists of two partially reflecting parallel mirrors placed very close to each other, allowing light to bounce back and forth between them multiple times. The Fabry-Pérot interferometer is often used in spectroscopy to measure the spectral characteristics of light or other electromagnetic waves. When light enters the interferometer, it undergoes multiple reflections between the mirrors. Depending on the wavelength of the incoming light and the spacing between the mirrors, certain wavelengths of light will interfere constructively, leading to a transmission peak in the output. Other wavelengths will interfere destructively, resulting in transmission minima. By scanning the mirror spacing or the incoming wavelength, scientists can precisely measure the wavelengths of light in a spectrum. This interferometer allows for the analysis of atomic and molecular spectra, the determination of the spectral lines of gases, and the characterization of lasers and optical filters. Its ability to provide high-resolution spectral information makes it an essential tool in research and various branches of science & technology.

I know something about OCT. It utilizes low-coherence interferometry to capture micrometer-resolution, cross-sectional images of biological tissues. This technique is non-invasive and enables high-resolution imaging of tissue microstructure in vivo. OCT operatas by splitting a light beam into a sample and reference arm. The light reflected from the sample and reference arm recombines, producing an interference pattern. By anelyzing the interference pattern, OCT can generate high-resolution images of biological tissues, making it an invaluable tool in ophthalmology for imaging the retina and anterior segment of the eye.

Wow, this is a lot of information. After a long research (and reading through all these investigations from you guys), I found many interferometers, and it becomes more complicated the deeper I delve.

In conclusion, I would like to focus on astronomy-related ones. The Michelson Stellar Interferometer, as Gaukhar previously mentioned, initially impressed me the most. It is primarily associated with the field of optics and experimental physics, for precise measurements of the speed of light and spectral line wavelengths, rather than astronomy. However, it has played a significant role in measuring the angular diameters of stars, such as Betelgeuse, which is enormous, at 240 million miles.

Now, moving on to some interferometers with a closer connection to astronomy.

In general, these large instruments work by combining light from multiple telescopes to achieve high-resolution observations.

Firstly, these interferometers consist of multiple telescopes or antennas. Light collected by these telescopes is carefully directed to a central location where it is combined. To create interference patterns (basically the piececs of information we want to finnally build our nicely done image), interferometers ensure that the light from each telescope arrives at the combining point with precisely matched path lengths. This is crucial for preserving the phase information of the incoming light. When the light from the telescopes is combined, it generates the interference patterns we want, often referred to as fringes. These fringes contain valuable information about the target object.

After this whole process, these machines bassically record these interference patterns using specialized detectors or cameras and then use that data for detailed analysis to create the images.

Also, many interferometers use adaptive optics systems to correct for distortions caused by Earth’s atmosphere (I left an investigation in case someone would like to see how it works). This correction ensures a clearer and more stable input beam, and instruments like GPI (which I will discuss in a moment) use coronagraphs to block out the light from the central star, revealing faint objects like exoplanets or circumstellar disks.

Moving on to the ones I liked the most:

VLT Interferometer (VLTI) & Keck Interferometer:
The Very Large Telescope Interferometer, operated by the European Southern Observatory (ESO) and located at the Paranal Observatory in Chile, is truly remarkable. This massive piece of technology combines the light from the four 8-meter VLT telescopes, along with auxiliary telescopes, to create an interferometric array. VLTI has been instrumental in observing and characterizing binary star systems, young stellar objects, and the environments around supermassive black holes.

The Keck Interferometer on Mauna Kea in Hawaii operates similarly but links the two 10-meter Keck telescopes to function as an optical and infrared interferometer. It has been used to study the structure of active galactic nuclei, exoplanets, and the disks around young stars. This one is a powerful tool for observing objects outside our solar system.

The CHARA Array: The Center for High Angular Resolution Astronomy (CHARA) is another optical interferometer located on Mount Wilson in California. It’s specifically designed for high-resolution imaging of stars.

There is also the Event Horizon Telescope (EHT), ALMA (Atacama Large Millimeter/submillimeter Array), and Gemini Planet Imager (GPI), but if I continue, this will never end. Anyway, I am including the links in case someone would like to delve even deeper. 🙂

One of the specific one that I found is Michelson interferometry,which is commonly used in various application.including measuring the properties of light. And detecting gravitational waves .
It can be used to measure the speed of light and detecting gravitational waves, even spectroscopy .
This are just a few example of Michelson interferometry .is applied in various field for precise measurements and observation.
In general, for measuring different physical phenomena.