Interference multibeam light filter (versions)

FIELD: physics, optics.

SUBSTANCE: light filter comprises translucent flat plate with thin-film coat applied on its one surface. In compliance with first version, light filter comprises radiation input optical prism with its flat face secured on said thin-film coat nearby plate end. Prism and film refractive indices are larger than that of the plate. In compliance with second version, plate end is skewed at acute angle to thin-film coat surface. Radiation is input to said film via said skewed end. Film refractive indices are larger than that of the plate. Radiation input in said film propagates therein at the angle to film surface nearby the plate smaller than that of full inner reflection but larger than that of the film second surface. Plate end remote from radiation input can be composed of cylindrical or spherical lens.

EFFECT: higher resolution and larger dispersion area.

6 cl, 3 dwg

 

The invention relates to optics, optical devices, based on the use of the phenomena of total internal reflection and interference of light beams, including devices, optical filters used in scientific research and technology for spectral analysis and monochromatization of light.

Solved the problem of creating filters with high resolution, i.e. a narrow bandwidth, while ensuring a wide free spectral range.

As taken similar known device type standards Fabry-Perot [Skokov IV Multibeam interferometers in measurement technology. - M.: Mashinostroenie, 1989], which contains a transparent flat plate, on the opposing surfaces of which are deposited semitransparent mirror coating. In principle, the standards are optical resonators with standing light wave. When passing through the Etalon collimated optical radiation between the mirror surfaces due to the multiple reflection occurs multibeam interference pattern, the radiation is standard only on the wavelengths of the spectrum, in between the plates which resonance occurs, i.e. fit an integer number of half-waves. Width generated by the reference spectrum band transmitted radiation to define what is largely a reflection coefficient of the mirror coatings; when using metallic coatings this coefficient is about 0.9 and less.

The advantage of the Fabry-Perot interferometers over the color filters of other types, such as diffraction, is their large aperture; lack of counterpart are significant losses of light energy on the mirrors, resulting in a significant width of spectral bandwidth at low orders of interference, i.e. the lack of a high resolution device when using it as a filter, and at large orders of interference, in which the filter has a consummate big resolution, dramatically decreases the dispersion region (free spectral range).

As the prototype was taken interferometric multibeam interferometer Lummer-Gerke [born M., wolf E. principles of optics. - M: CH. ed Phys-math. lit. The science. 1970 - 856]. The interferometer is a long plane-parallel plate of glass or quartz crystal. The beam of light from a source lying on the longitudinal axis of the plate, extends through the prism mounted on one end of the plate and impinges on the inner surface of the latter at an angle slightly less than the angle of total internal reflection. The trajectory of the beam inside the plate is a broken line, of the plate o is the CIO on its both sides, the number of light beams, starting in areas falling beam plate inside on its outer side. As the angle of incidence of the beam on the inner surface is slightly less than the angle of total internal reflection, the rays are refracted at the boundary surface and out into the air under the sliding angles to the surface. Reflected from the surface inside the rays continue to spread over the plate, like the propagation of light in the fiber. Released from the plates rays collected by the lens and form an interference pattern in the focal plane. Due to the large number of interfering beams resolution of the interferometer is very high.

The disadvantage of the prototype is a small free spectral range, because of the large ratio of the thickness of the plate to the wavelength of light and high orders of interference of light.

The problem solved by the present invention is the creation of an optical filter having a narrow spectral bandwidth and a wide free spectral region.

The task is solved in that in multibeam interference optical filter containing a flat transparent plate with the transparent thin-film coating one surface and the optical prism of the input radiation, in accordance with the invention, the optical prism fixed flat gr the new thin-film coating adjacent to the end plate, moreover, the refractive indices of the prism and the film more than the refractive index of the plate.

It is also proposed variant multipath interference filter containing a flat transparent plate with the transparent thin-film coating one surface, in which in accordance with the invention, one end of the plate is beveled at an acute angle to the surface of a thin-film coating, and the refractive index of the film more than the refractive index of the plate, while the radiation is introduced into the film through the beveled end of the plate.

It is also proposed that entered in the transparent film, the radiation is distributed in it at an angle to the film surface adjacent to the plate, the smaller the angle of total internal reflection, but a large angle of total internal reflection of the second surface of the plate.

It is also proposed that remote from the input radiation, the end plate is made in the form of a cylindrical or spherical lens.

The invention is illustrated using figure 1-3.

Figure 1 - scheme of the rays in the longitudinal section multibeam interference filter according to the invention. Here 1 - transparent thin-film coating of transparent flat plates 2, 3 - prism input radiation in thin-film coating, 4 - incoming beam of light, 5 - emerging beam of light, L - lens, f is the focal be the of the lens, 6 is a focal point, n0n1n2n3the indices of refraction of the environment, thin film coatings, plates, prisms, respectively; θ1and θ2- the angle of incidence of the beam on the surface of the transparent thin-film coating on the inside and the angle of refraction of the beam at the boundary between the transparent thin-film coating and plate.

Figure 2 - scheme of the rays in the longitudinal section of a variant multipath interference filter according to the invention. Here 7 is a transparent plate with a beveled edge, 8 - surface beveled edges, 9 - the first reflected beam.

Figure 3 - scheme of the rays in the longitudinal section multibeam interference filter that contains a built-in lens. Here 10 is transparent plate 11 is cylindrical or spherical lens.

In accordance with figure 1, the incoming collimated radiation falls on the face of the prism 3, is reflected from the second face and is held in the thin-layer coating 1. Next, the beam passes the floor, falling at an angle θ1on the boundary of the floor-plate, is reflected at the same angle and is refracted at an angle θ2:

sinθ2=n1n2sinθ1. (1)

The reflected beam in a broken trajectory is distributed in thin-film coatings, each falling on the boundary of the floor-plate, the beam is reflected and refracted. The reflection coefficient can be high due to the proximity of the angle of incidence to the angle of total internal reflection at the boundary. When ray falls on the border of the thin-film coating with the external environment, the beam experiences total internal reflection, as the condition:

θ1θKp=arcsin(n0/n1),(2)

where θkr- the angle of total internal reflection.

Refracted rays collected by the lens L and converges at the focal point, as they are mutually parallel and interfere. Under the condition of equality of the phase difference between adjacent rays size that is a multiple of 2π rad, there is a maximum in the interference. For light beams of different wavelength maxima of the interference are located in the focal plane of the lens at different distances from the optical the si lens, allowing, by placing the focal plane of the aperture with a slit or line of photodiodes, split light fluxes with different wavelengths.

In the case of the variant of the interferometer in figure 2 the radiation is introduced through the beveled at an acute angle to the edge of the plate 7. Surface 8 beveled edges should be flat and polished. Passing the plate, the incident light is similar to the prism 3 in figure 1, the light beam enters the thin-film coating 1, in the further course of rays similar to the one described in the description of figure 1. The difference is the presence reflected from the boundary of the beam 9 on the first pass through this border incident beam 4.

In the case shown in figure 3, remote from the input radiation end plate 10 is made in the form of a cylindrical or spherical positive lens 11, the refracted rays of this lens, as the lens L in figure 1, focus; the design of the interferometer optical circuit is simplified, there is no need in special lens L.

Considered the device differs from the prototype in that svetovida part of the device is made in two-part design; thin-film coating can have a thickness much smaller than in the case of the interferometer Lummer-Gerke, which allows to increase the free spectral range of the interferometer by several orders of magnitude. In addition to t the th, the radiation exits the cover through only one surface that doubles the intensity of the released radiation. Significantly reduced the length of the interferometer with the same values of resolution.

The phase difference δ between adjacent beams in thin film coating, if we neglect the phase shift experienced by the light in total internal reflection is determined by the formula:

δ=4πhλ0n12-n22sin2θ2(3)

The order of interference m is determined by the number of wavelengths in the optical path traversable by the beam during the step of trajectory in thin-film coating thickness h, and the formula:

m=δ2π=2hλ0n12-n22sin2θ2. (4)

Here λ0- the wavelength of light in free space. Given that θ2→0, it is possible according to the method proposed in [principles of optics, see above]to obtain a formula for the free spectral range:

Δλ0λ022hn12-n22|n12-n1λ0(n1-n2)λ0-n22|λ022hK,(5)

where K is weakly dependent on wavelength ratio, similar in magnitude to the refractive index of thin film coatings.

Resolving power of the interference filter is determined with reference to (5) by the formula:

λ0 δλ0,7lKλ0n12-n22,(6)

where l is the distance along the plate between the entrance and exit of the beam in thin-film coating, δλ is the spectral bandwidth of the filter. When using thin-film coating thickness of the order of the wavelength, as follows from (5), it is possible to have Δλ0≈λ0while the resolving power of the interferometer does not depend on the thickness of the coating and remains as high as in the interferometer Lummer-Gerke.

Feature multi-beam interferometer according to the invention is the fact that at small thickness of the thin film coatings are comparable to the wavelength of the light wave in it is distributed, as in the optical waveguide, and has a modal structure. This circumstance requires input radiation in thin-film coating at such an angle as to ensure the coincidence of the direction of the beam in the floor and fashion trends.

For manufacturing the device according to the invention uses materials and technology used in the optical industry. For prism, you can use sapphire galss) dial : is (n=1,75) or glass TF (n=1.7 to 1.9), for thin-film coating and plate - glass TF and fused silica. Important is the manufacture of optical surfaces of the prisms and plates with a high degree of flatness.

The above explanations show that the task can be achieved is described technical solutions.

The technical result of the invention is to provide a filter having a high resolution, large area variance.

The invention can be used in optics and optoelectronics as a narrowband optical filter, as a dispersing device monochromators and spectrometers.

1. The interference of multipath filter containing a flat transparent plate with the transparent thin-film coating one surface and the optical prism of the input radiation, characterized in that the optical prism fixed a planar thin-film coating adjacent to the end plate, and the refractive indices of the prism and the film more than the refractive index of the plate.

2. The device according to claim 1, characterized in that the input to the transparent film, the radiation is distributed in it at an angle to the film surface adjacent to the plate, the smaller the angle of total internal reflection, but a large angle of total internal reflection of the second surface of the film.

p> 3. The device according to claim 1, characterized in that the remote from the input radiation, the end plate is made in the form of a cylindrical or spherical lens.

4. The interference of multipath filter containing a flat transparent plate with the transparent thin-film coating one surface, wherein one end of the plate is beveled at an acute angle to the surface of a thin-film coating, and the refractive index of the film more than the refractive index of the plate, while the radiation is introduced into the film through the beveled end of the plate.

5. The device according to claim 4, characterized in that the input to the transparent film, the radiation is distributed in it at an angle to the film surface adjacent to the plate, the smaller the angle of total internal reflection, but a large angle of total internal reflection of the second surface of the film.

6. The device according to claim 4, characterized in that the remote from the input radiation, the end plate is made in the form of a cylindrical or spherical lens.



 

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