An interferometer is a precision optical instrument that measures the difference in optical path using the principle of optical interference to measure the physical quantity. In general, an instrument that measures by the principle of optical interference can be called an interferometer.

      Any change in the optical path difference between the two coherent lights can very sensitively cause the movement of the interference fringes, and the optical path change of a certain coherent light is caused by the geometric path through which it passes or the change in the refractive index of the medium, so interference The change in the movement of the fringes can measure the geometric length or a small amount of change in the refractive index to measure other physical quantities associated therewith. The measurement accuracy is determined by the accuracy of measuring the optical path difference. Each time the interference fringe moves by one stripe pitch, the optical path difference changes by one wavelength. Therefore, the interferometer measures the optical path difference in units of light wave wavelength, and the measurement accuracy is high. The measurement method is unmatched.

      The interferometer is divided into two types: two-beam interferometer and multi-beam interferometer. The former has Rayleigh interferometer, Michelson interferometer and its variant Taiman interferometer, Mach-Qinte interferometer, etc. The latter has Fabri- Perot interferometer, etc.

The application of interferometers is extremely extensive, mainly in the following aspects:

Precision measurement of length

In the 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 geometrical path of the two-coherent light, and the length comparison or absolute measurement can be performed according to the number of movements of the stripe. The Michelson interferometer and the Fabry-Perot interferometer have been used to represent international meters at the wavelength of the cadmium red line.

Determination of refractive index

The geometrical path of the two beams remains the same, and the change in the refractive index of the medium can also cause a change in the optical path difference, causing the fringes to move. A Rayleigh interferometer is a typical interferometer that measures the refractive index by stripe movement. The Mach-Qinte interferometer applied to the wind tunnel was used to observe the change in the refractive index of the airflow in real time.

Wavelength measurement

Any method of measuring a standard meter in units of wavelength is a method of measuring the wavelength in units of a standard meter. Using international meters as the standard, the interferometer can accurately measure the wavelength of light waves. The Fabry-Perot interferometer (etalon) was used to determine the primary standard of wavelength (cadmium red line wavelength) and several secondary wavelength standards to determine the wavelength of other spectral lines by comparison.

Verify the quality of the optics

Taiman interferometers are commonly used to verify the quality of optical components such as plates, prisms and lenses. Place a plate or prism to be inspected in an optical path of the Tylman interferometer, and any non-uniformity in the refractive index or geometry of the plate or prism must be reflected on the interference pattern. If a lens is placed in the optical path, the wavefront distortion caused by the lens can be known from the interference pattern to evaluate the wave aberration of the lens.

other apps

Used as a high resolution spectrometer. Multi-beam interferometers such as Fabry-Perot interferometers have very sharp interference and thus have extremely high spectral resolution, and are often used for fine structure and hyperfine structure analysis of spectra.

     Wave theory in the 19th century believed that light waves or electromagnetic waves must be transmitted in elastic media. This hypothetical elastic medium is called ether. A series of experiments were done to verify the existence of the ether and to explore its properties. Experiments based on the principle of interference are the most accurate, the most famous of which are the Fizzo experiment and the Michelson-Morley experiment. In 1851, A.H.L. Fizzo experimented with the speed of light in a moving medium with a specially designed interferometer to determine whether the moving medium was dragging the ether. In 1887, A.A. Michelson and E.W. Morey collaborated to use the Michelson interferometer to attempt to detect the relatively static motion of the Earth's ether. The study of the ether provides evidence for A. Einstein's special theory of relativity.

Several typical optical interferometers

a Michelson interferometer

The Michelson interferometer is a precision optical instrument designed and manufactured by American physicist Michelson and Morey in 1883 to study the "Ether" drift. It uses the partial amplitude method to generate two beams to achieve interference. By adjusting the interferometer, it is possible to generate equal-thickness interference fringes and to generate isotropic interference fringes. It is mainly used for the measurement of length and refractive index. If one piece of interference fringe is observed, the amount of movement of the arm of M2 is λ/2, which is equivalent to the change of the air film thickness between M1 and M2 by λ/2. In modern physics and modern metrology techniques, such as the study of fine structure of spectral lines and the calibration of standard meters with light waves, there are important applications.

As shown in the figure, the optical schematic of the Michelson interferometer, the light reflected by M2 passes through the beam splitter three times, and the light reflected by M1 passes through the beam splitter only once. The compensation plate is set to eliminate this asymmetry. When using a monochromatic light source, it can be compensated by the air path length, and it is not necessary to compensate the plate; however, in the case of a complex color light source, the compensation plate is indispensable due to the difference in dispersion of glass and air.

If you want to observe the interference fringes of white light, it is basically completely symmetrical, that is, the optical path difference of the two-phase dry light is very small, and you can see the colored stripes at this time; if M1 or M2 has a slight tilt, you can get equal thickness. The interference fringes at the line (d = 0) are centrally symmetrical color straight stripes, and the central fringes are dark stripes due to the half-wave loss.

The most famous application of the Michelson interferometer is the zero result obtained from the observation of the etheric wind in the Michelson-Morley experiment. This dark cloud in the classic physics sky at the end of the 19th century provides the basic assumptions of the special theory of relativity. Experimental basis. In addition, since the laser interferometer can measure the optical path difference in interference very accurately, Michelson interferometer and other kinds of interferometers have been widely used in today's gravitational wave detection. The basic principle of laser interference gravitational wave observatory (LIGO) and many other ground laser interference gravitational wave detectors is to measure the optical path variation of laser caused by gravitational waves by Michelson interferometer, and the planned laser interference space antenna (LISA) The basic idea of ​​applying the Michelson interferometer principle has also been proposed. The Michelson interferometer has also been used in the search for extrasolar planets, although the Mach-Zehnder interferometer is more widely used in this type of detection. The Michelson interferometer is also used in the manufacture of delay interferometers, optical differential phase shift keying demodulator (Optical DPSK), which converts phase modulation into amplitude in a wavelength division multiplexing network. modulation.

Two Rayleigh interferometer

In 1896, in order to measure the refractive index of inert gas argon and helium, Rayleigh designed a special interferometer called the Rayleigh interferometer by using Young's double-slit interference principle.

Rayleigh interferometer is a high-precision measuring instrument that utilizes the principle of double-beam interference. It has a simple structure and is easy to use. Its optical principle is shown in the figure.

The height of the sample cell and the p1 and p2 compensators only occupy the upper half of the whole space. The compensator p1 has a fixed angle along the vertical axis. The compensator p2 can be rotated by the drum micrometer F to change the angle. L2 is Converging lens, L3 is a cylindrical mirror, the upper and lower rows of interference fringes are seen in the observation tube, and one column is formed by interference of two beams of light in the lower half of the slit, because the optical path difference of the lower half is constant, so the interference fringe is Fixed; the two beams of light passing through the upper half of the slit produce the upper half of the interference fringes after passing through the sample cell. When the optical path difference does not occur in the sample cell (the optical path difference originates from the chemical composition, temperature, pressure, etc. in the two chambers), and the other p1 and p2 do not add the optical path difference, they are aligned with the lower half interference fringes, otherwise The lower half of the interference fringes move, so that the lower half of the interference fringes in the interferometer is the fixed mark of the upper half of the interference fringes. When two different cells are loaded with different media, the refractive index is n1, n2 due to the difference in refractive index, the optical path difference is: △ = (n2 - n1) l = Kλ, where λ is the wavelength of the light source, K is the interference level corresponding to the optical path difference, and l is the length of the sample cell.

Three Fabry Perot interferometer

 The Fabry-Perot interferometer is a multi-beam interferometer consisting of two parallel glass plates, essentially the same as the interference principle of the parallel plane plates described in the previous section. The inner surfaces of the two glass plates have a relatively high reflectivity to ensure that interference fringes with sufficiently high fineness are obtained. Since the parallel plane plate has a transmission maximum only for light of a specific wavelength, the Fabry-Perot interferometer can transmit or reflect light whose frequency satisfies its resonance condition, and can achieve very high transmittance and reflectivity. It is therefore also known as the Fabry-Perot cavity or the Fabry-Perot etalon. Fabry-Perot interferometers are widely used in telecommunications, laser, spectroscopy, etc., and are mainly used to accurately measure and control the frequency and wavelength of light. For example, a combination of several Fabry-Perot interferometers is used in an optical wavelength meter, and their resonance frequencies are 10 times different from each other. After the incident light to be measured interferes with these interferometers, each measurement is generated. The wavelength of the light to be measured can be known by the pitch of the bright lines. In addition, the Fabry-Perot interferometer in the laser field is also used to suppress the broadening of the line to obtain a single-mode laser, and in the gravitational wave detection, the Fabry-Perot interferometer and the Michelson interferometer are used in combination. The effective length of the interference arm of the Michelson interferometer is increased by repeatedly oscillating the photons within the cavity.

The three important characteristic parameters of the Fabry-Perot interferometer are the fineness (the ratio of the free spectral range to the full width at half maximum of the transmission peak), the peak transmittance (the maximum ratio of the transmitted light intensity to the incident light intensity), Ratio factor (ratio of the maximum and minimum ratio of transmitted light intensity to incident light intensity), but the higher the reflectance, the higher the fineness, so the transmittance and fineness/lining factor cannot be both Very high.