In the modern world of science and technology, there is an extremely unique measurement method that can observe changes smaller than laser wavelengths, capture micro-nanoscale deformations without touching objects, and obtain critical information from the ocean to deep space. This method is called interferometry.
Interferometry is considered the “crown of precision measurement” because it can detect subtle changes that ordinary instruments cannot discern. For example, the first direct observation of gravitational waves by humans was achieved by relying on the Michelson interferometer with a long arm of 4,000 meters. In precision manufacturing, nano-defects on the lens surface can also be magnified by interference fringes. In communication, metrology, and materials science, interferometers are the basic equipment.
The importance of interferometry is reflected in:
- Extremely high accuracy (sub-wavelength level)
- It can be used to detect small displacements, vibrations, refractive index, and even temperature changes
- It is widely used in scientific research, industrial manufacturing, optical communication, astronomy, and other fields
- Non-contact, high-sensitivity measurements are possible
What Is Interferometry
At its core, interferometry uses interference fringes formed by the superposition of light waves to extract information. When two beams of light meet again in space, if there is a light path difference, a light and dark texture will appear, which is an interference fringe.
In practice, the laser beam is usually divided into two beams, one illuminating the object being measured and the other as a reference light. When the two beams of light are remerged, interference fringes will be produced due to the difference in optical path. By analyzing the changes in the stripes, information such as length, microdisplacement, surface topography, material refractive index, and micro-vibration can be measured.
Interferometry is unique in that it enables subwavelength-level accuracy. This is because the wavelength of light is extremely short (about 500–1500 nm), and any small change in the optical path can cause detectable fringe movement, allowing for extremely sensitive measurements.
Fundamentals of Light Interference
The basis of the phenomenon of interference lies in the fluctuation of light. Light is an electromagnetic wave with characteristics such as wavelength, frequency, and phase, so it can be superimposed. When two beams of coherent light meet, interference fringes are formed based on the phase relationship.
Mutual-long interference and destructive interference
- Constructive Interference: Two beams of light are aligned with the crests to form bright stripes.
- Destructive Interference: Light peaks meet troughs, forming dark stripes.
Optical path difference and phase difference
Optical Path Difference (OPD) refers to the total distance difference between two beams of light that pass through different paths and reach the point of convergence. The optical path difference is directly proportional to the phase difference and can cause observable fringe changes even if the OPD is small.
Measurement principle of interference stripes
The movement of interference fringes corresponds to changes in optical path difference, so they can be used to measure:
- Small displacement
- Surface topography of objects
- Change in refractive index
- Vibration and dynamic changes
Implementation of sub-wavelength accuracy
Interferometers measure not absolute distance, but phase changes. The phase varies continuously and can be divided into very small parts, so the measurement accuracy can be 1/100–1/1000 wavelength.
Operation Principles
The interferometer workflow can be represented by arrows as follows:
Light source → spectroscopy → reflection → conjunction → streaks forming → signal interpretation → calculation of physical quantities
Each step is detailed:
Light source
- Provides a stable laser beam.
- Commonly used He-Ne lasers or semiconductor lasers.
Spectroscopy
- The laser is divided into two beams: the reference light and the measurement light.
- Ensure that the two beams of light are coherent.
Reflection / Transmission
- The measuring light illuminates the object under measurement, and the reference light remains unchanged.
- The optical path difference is caused by the object being measured.
Combined bundles
- The two beams of light are re-superimposed to form interference fringes.
Stripe formation
- The light and dark stripes reflect the light path difference.
- The movement of the stripe corresponds to the change in the physical quantity.
Signal interpretation
- The streaks are collected by a photodetector or camera.
- Software or an electronic system reads stripe changes.
Calculate physical quantities
- The fringe changes are converted into measurements such as displacement, angle, and refractive index.
Why Interferometry Is Useful
The outstanding advantages of interferometers are their non-contact, high accuracy, versatility, and extreme sensitivity. It can not only measure small displacements and vibrations, but also detect refractive index, surface topography, and dynamic changes. Interferometry is an irreplaceable technology in the fields of exact science, industrial manufacturing, optical communications, and astronomy.
Interferometer Types
Michelson Interferometer
The Michelson interferometer splits the light into two beams, which are reflected on two mirrors and then merged to form interference fringes. The relationship between optical path difference and phase difference directly reflects the change in the displacement or refractive index of the measured object.
Applications include length measurement, wavelength calibration, and gravitational wave detection (e.g., LIGO). Even a few hundred nanometers of displacement of a single mirror is clearly represented in the stripes.
Twyman–Green Interferometer
Twyman–Green interferometers are primarily used to test the surface shape of optical components. It shows the surface shape error by the interference of the reference light and the light to be measured. This type of interferometer adapts to different reflectance samples, improves measurement accuracy through polarization technology, and avoids instrument reflection interference.
Dual-Frequency Laser Interferometer
The dual-frequency interferometer uses two lasers of different frequencies, f1 and f2, to accurately measure displacement, velocity, and refractive index. This type of interferometer has strong anti-interference ability and high stability, and is widely used in precision lithography, automation equipment control, and high-precision metrology systems.
Applications of Interferometry
1. Linear displacement measurement
The multi-frequency beam is separated by a polarization splitter to measure the frequency shift of light on a moving object to obtain displacement information. It is suitable for precision machining platforms, motion control systems, etc.
2. Angle Measurement
Using a rotating mirror, the frequency difference of the echo light can be converted into angular information. Measurable range ±10° with resolution up to 0.1 arcsec.
3. Refractive index and material detection
Optical path difference is sensitive to changes in refractive index and can be used for gas or liquid refractive index measurements, as well as for internal stress detection of materials.
4. Surface topography and optical component inspection
Interference fringes can reveal small wavefront errors in lenses, mirrors, and optical components, making them an essential tool for precision optical fabrication.
5. High-precision metering system
National Metrology Institutes, laboratories, and industrial high-precision platforms use interferometers for standard length, angle, and deformation measurements.
6. Earthquake, ocean, and atmosphere research
Interferometric radar (InSAR) can be used for surface deformation, wave monitoring, and earthquake precursor analysis.
7. Astronomical Observations and Gravitational Wave Detection
Astronomical telescope resolution improvements, gravitational wave detection, etc., require long baseline interferometry technology to capture weak signals.
Interferometry vs. traditional measurement methods
Indicators | Interferometry | Traditional measurement methods |
Accuracy | at the nanoscale, subwavelength level | from micron to millimeter |
Measurement method | non-contact | mostly contact |
Sensitivity | extremely high | average |
Stability | depends on the stability of the light source | Affected by wear and temperature |
Speed | High-speed real-time measurement | Medium and low speed |
Measurable content | displacement, angle, refractive index, topography, vibration | displacement or angle |
Applications | Optics, metrology, astronomy, communications, industrial measurement, and machining | limited |
Cost | higher | lower |
Conclusion
Interferometry uses the fringes formed by the superposition of light waves to “amplify” small changes that are invisible to the naked eye into measurable information. Compared with traditional measurement methods, it has higher accuracy, higher sensitivity and a wider range of applications. From optical fabrication and precision metrology to astronomy and gravitational wave detection, interferometers are indispensable tools in modern science. With the development of laser and optical technology, interferometry will play a central role in more fields.
