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Interferometric technology is a kind of technology that uses the interference phenomenon of light to measure the tiny changes of some physical quantities, In general, it divides a beam of light into two beams through an optical element, one as a reference light and the other as a measuring light. The measuring light falls on the measured object or passes through the measured sample, and then the two beams are fitted again. By using the change of interference pattern, the slight change of a physical quantity of the target is checked out

The basic optical path structure of Michelson interferometer is shown in Figure 1, which is often used to measure the slight displacement change of objects. A beam of coherent light emitted from the light source 1 is divided into two beams by a beam splitter 2. A beam of transmitted light falls on the mirror M1, Another reflected light falls on the emitting mirror M2, M1 and M2 reflect these two beams of light back along the original path, After overlapping on the beam splitter mirror 1, it enters the beam expander mirror 3, Projected on the white screen 4, If we adjust the optical path properly, You will see a series of light and dark interference fringes on the white screen. These interference fringes will move with the movement of M1 or M2, and they are very sensitive. As long as the mirror moves half a wavelength, the interference fringes will move for one period, and the wavelength of light is generally on the order of microns, so it has high sensitivity and resolution.

The optical path structure of Mach-Zehnder interferometer is shown in fig. 2. A beam of coherent light emitted from light source 1 is divided into two beams by beam splitter 2. One beam of transmitted light falls on the reflecting mirror M1, and the other beam of reflected light falls on the emitting mirror M2. M1 and M2 respectively reflect these two beams to the beam splitter 3, and make these two beams coincide and enter the beam expander 4. If adjusted properly, we can see a series of light and dark interference fringes on the white screen 5 behind the beam expander. This kind of interferometer is mainly used to measure the change of refractive index of transparent materials, and most interferometers in optical fiber sensors adopt this kind of optical path structure.

The optical path structure of Sagnek interferometer is shown in Fig. 3. The optical path consists of a beam splitter 2 and three mirrors M. Its optical path is quite special, and the two beams of light propagate in reverse along the same path. Because the propagation paths of the two beams are strictly coincident, the influence of any actual sample acts on the two beams at the same time, and in most cases, the effects cancel each other out, so we can't observe the change, but this interferometer reflects the change of angle. Assuming that the interferometer rotates around an axis perpendicular to the plane of the optical path, one beam of light will propagate in the direction of rotation, while the other beam will propagate against the direction of rotation, which will cause the change of optical path difference and thus cause the interference fringes to move. At present, laser gyroscope and fiber optic gyroscope, which are widely used in aviation and aerospace fields, are based on this principle.

1 platform (400mm 600mm); One set of two-dimensional tunable semiconductor laser (635nm, 3mW); Two two-dimensional adjustable beam splitters; Three two-dimensional adjustable mirrors; One two-dimensional adjustable beam expander; 1 white screen; Air chamber (cavity length 100mm) + 1 set of pressure gauge; 9 magnetic watch seats with switch.

The main content of this experiment is to build three kinds of interferometers according to the optical paths shown in Figure 1, 2 and 3 on the optical experimental platform, and adjust the interference fringes with appropriate thickness. Then, an air chamber is added into the optical path, and the air chamber is pressurized to change the air pressure in the air chamber. Because the refractive index of the gas depends on the pressure of the gas, when this change only acts on a certain beam, it will bow up the change of the optical path difference between the two beams, thus causing the change of interference fringes. By reading out the relationship between air pressure and interference fringes, the curves of air pressure and interference fringes and the curves of air refractive index and pressure (the length of air chamber is 100mm) can be drawn. Comparing the measurement results of the three interferometers, the structure and characteristics of the three interferometers are understood.

1) In order to obtain interference fringes with appropriate thickness, the two re-fitted beams should be coincident as much as possible. The smaller the angle between two beams of light, the thicker the interference fringes, and vice versa. When adjusting the light path, two beams of light should fall in the same plane first. This can be determined by using a white screen fixed on the magnetic watch seat to observe whether the heights of each point of the two beams of light are the same, and then by making the two beams of light converge at the same point to ensure that the included angle in the horizontal direction is as small as possible.

2) The change of air pressure should be smooth and slow, which can be realized by the leakage of the air chamber itself, and the change of stripes can be counted by passing through a fixed point on the white screen.

Three kinds of classical interferometers can be built. Compared with the traditional Michelson interferometer, the experimental content is richer and more flexible. Making three optical paths by hand is beneficial to cultivate practical ability. Through comparison, we can understand the optical path characteristics and application methods of different interferometers and better understand the principle of optical interference.