What Is an Interferometer?

Interferometer is a general term for a wide range of experimental techniques. The idea is to use the superposition of waves to obtain the phase information of the waves, thereby obtaining the physical quantities of interest in the experiment. Interferometers are not limited to optical interferometers. Interferometers are widely used in astronomy (Thompson et al, 2001) ^[1] , optics, engineering measurement, oceanography, seismology, spectral analysis, quantum physics experiments, remote sensing, radar and other precision measurement fields (Hariharan, 2007) ^[2] .

Chinese name: Interferometer

Interferometer basic principles

The superposition of two columns of quasi-monochromatic waves with a fixed phase difference will cause the amplitude to change, so that the phase information of the wave can be obtained by measuring the amplitude that is easier to measure.

Two columns of vibrations with the same frequency waves can be described at one point with the following formula

Then the vibration of the two waves after superposition is

Triangulation gives the amplitude where the superposition is

It can be seen that the amplitude after superposition is related to the initial phase difference between the two columns of waves. Because the amplitude change depends on the cosine function of the phase difference, this amplitude change sometimes appears as periodic stripes in space. Such fringes are sometimes called interference fringes, and amplitude changes due to changes in phase difference are sometimes called fringes.

Interferometer classification

Interferometer classification has different division methods

Differentiate by structure

Interferometers can be divided into two types: single-path interferometers and multi-path interferometers. The difference lies in whether the interfering waves propagate through the same path. For example, the Michelson interferometer is a common multipath interferometer, and the Sagnac interferometer, isosmotic interference and equal thickness interference are single-path interferometers (Zhong Xihua, Chen Ximou, 2002) ^[3] .

Distinguish by interference light source

Interferometers can be divided into two types: wavefront decomposition and amplitude decomposition. The difference is whether to use the wavelet sources at different positions on the wavefront to form interference. For example, the Young's double-slit interference belongs to the wavefront decomposition interferometer (Zhong Xihua, Chen Ximou, 2002) ^[3] ; and the isotropic interference and equal thickness interference are the amplitude decomposition interferometers.

Interferometer application

The applications of interferometers are extremely wide, mainly in the following aspects:

Interferometer length measurement

In a two-beam interferometer, if the refractive index of the medium is uniform and constant, the movement of the interference fringes is caused by the difference in the geometric distance of the two coherent light. According to the number of movements of the fringes, accurate length comparison or absolute measurement can be performed. Michelson interferometers and Fabry-Perot interferometers have been used to express international meters in the wavelength of the red line of cadmium.

Interferometer refractive index measurement

The geometric distance of the two beams remains unchanged, and the change of the refractive index of the medium can also cause the change of the optical path difference, which causes the fringe movement. A Rayleigh interferometer is a typical interferometer that performs relative measurement of refractive index through fringe movement. Mach-Zinter interferometers used in wind tunnels are used to observe changes in the refractive index of airflow in real time.

Interferometer wavelength measurement

Any method of measuring the standard meter ruler in terms of wavelength is also a method of measuring the wavelength in standard meter ruler. With international meters as the standard, the interferometer can be used to accurately measure the wavelength of light waves. The Fabry-Perot interferometer (etalon) has been used to determine the primary standard (cadmium red spectral line wavelength) and several secondary wavelength standards for wavelengths, thereby determining the wavelengths of other spectral lines by comparison.

Interferometer inspection optics

Tyman interferometers are commonly used to verify the quality of optical elements such as flat plates, prisms, and lenses. A flat plate or prism to be inspected is placed in one optical path of the Taman interferometer. Any unevenness in the refractive index or geometric size of the flat plate or prism will be reflected on the interference pattern. If a lens is placed in the optical path, the wavefront distortion caused by the lens can be understood based on the interference pattern, and the wave aberration of the lens can be evaluated.

Interferometer gravitational wave measurement

Interferometers can also be used for gravitational wave detection (Saulson, 1994) ^[4] . The concept of laser interferometer gravitational wave detector was proposed by former Soviet scientists Gertsenshtein and Pustovoit in 1962 (Gertsenshtein and Pustovoit 1962) ^[5] . In 1969, American scientists Weiss and Forward built preliminary test systems at MIT and Hughes laboratories in 1969 (Weiss 1972). ^[6] As of today, laser interferometer gravitational wave detectors have been developed for more than 40 years. At present, LIGO laser interferometer experiments claim that the first direct measurement of gravitational waves (LIGO collaboration 2016) ^[7] . LIGO can be considered as an interferometer with two channels of light, while in another type of gravitational wave detection experiment, a pulsar timing array can be considered as a multiple channel interferometer (Hellings and Downs, 1983) ^[8] .

Interferometer other

Used as a high-resolution spectrometer. Multi-beam interferometers such as Fabry-Perot interferometers have very sharp interference maxima and therefore have extremely high spectral resolution. They are often used for the analysis of fine and ultra-fine structures of spectra.

Historical role. In the 19th century, wave theorists believed that light or electromagnetic waves must propagate in an elastic medium. This hypothetical elastic medium is called ether. A series of experiments have been performed to verify the existence of ether and explore its properties. Experiments based on the principle of interference are the most accurate, the most famous of which are the Fizeau experiment and the Michelson-Morley experiment. In 1851, AHL Fizzo experimented with the specially designed interferometer on the speed of light in a moving medium to verify whether the moving medium was dragging the ether. In 1887, AA Michelson and EW Morey cooperated with the Michelson interferometer to try to detect the motion of the earth's relatively absolute still ether. The study of the ether provides evidence for A. Einstein's special theory of relativity.