RU2515134C2 - Interference multibeam light filter (versions) - Google Patents

Abstract

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

Description

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

The problem of creating filters with high resolution, that is, a narrow passband, while providing a wide free spectral region, is being solved.

As an analogue, well-known devices of the Fabry-Perot type are taken [Skokov I.V. Multibeam interferometers in measurement technology. - M .: Mechanical Engineering, 1989], containing a transparent flat plate, on the opposing surfaces of which a translucent mirror coating is applied. According to the principle of operation, the standards are optical resonators with a standing light wave. When the collimated optical radiation is transmitted through the standard between mirror coatings due to multiple reflections, a multipath interference pattern arises, the radiation passes the standard only at those wavelengths of the spectrum at which resonance arises between the plates, i.e. an integer number of half-waves is stacked. The width of the spectrum band of the transmitted radiation formed by the standard is determined largely by the reflection coefficient of the mirror coatings; when using metal coatings, this coefficient is of the order of 0.9 or less.

The advantage of Fabry-Perot interferometers over other types of filters, for example diffraction ones, is their large aperture; A disadvantage of the analogue is the significant loss of light energy on the mirrors, which leads to a significant spectral bandwidth for small orders of interference, that is, insufficient resolution of the device when used as a filter, and for large orders of interference, in which the filter has an unrivaled large resolution, the dispersion region (free spectral region) decreases sharply.

As a prototype taken interference multi-beam interferometer Lummer-Gerke [Born M., Wolf E. Fundamentals of optics. - M .: Ch. ed. physical mat. lit. The science. 1970 - 856 s]. The interferometer is a long plane-parallel plate of glass or crystalline quartz. A beam of light from a source lying on the longitudinal axis of the plate enters it through a prism mounted on one of the ends of the plate and falls on the latter’s inner surface at an angle slightly smaller than the angle of total internal reflection. The beam path inside the plate is a broken line; a number of light beams emerge from the plate on both sides of the plate, starting at the places where the beam of the plate falls from the inside to its outer side. Since the angle of incidence of the beam on the inner surface is not much smaller than the angle of total internal reflection, the rays are refracted on the boundary surface and exit into the air at sliding angles to the surface. Rays reflected from the surface inward continue to propagate along the plate, similar to the propagation of light in a light guide. The rays emerging from the plate are collected by the lens and form an interference pattern in its focal plane. Due to the large number of interfering rays, the resolution of the interferometer is very high.

The disadvantage of the prototype is the small free spectral region, which is explained by the large ratio of the plate thickness to the wavelength of light and high orders of light interference.

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

The problem is solved in that in an interference multipath filter containing a flat transparent plate with a thin film transparent coating of one of its surfaces and an optical input prism, in accordance with the invention, the optical prism is fixed with a flat face on a thin film coating near the end of the plate, and the refractive indices of the prism and film are greater refractive index plate.

A variant of an interference multi-beam filter is also proposed, comprising a flat transparent plate with a thin film transparent coating of one of its surfaces, in which according to the invention one end of the plate is beveled at an acute angle to the surface of the thin film coating, the refractive index of the film being greater than the refractive index of the plate, while radiation is introduced into the film through the beveled end of the plate.

It is also proposed that radiation introduced into a transparent film propagates in it at an angle to the surface of the film adjacent to the plate, smaller than the angle of total internal reflection, but larger than the angle of total internal reflection of the second surface of the plate.

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

The invention is illustrated using figures 1-3.

Figure 1 - diagram of the path of the rays in a longitudinal section of an interference multi-beam filter according to the invention. Here 1 is a thin film transparent coating of a transparent flat plate 2, 3 is a prism for introducing radiation into a thin film coating, 4 is an incoming light beam, 5 is an output light beam, L is a lens, f is a focal length of a lens, 6 is a focal point, n ₀ , n ₁ , n ₂ , n ₃ are the refractive indices of the environment, thin-film coatings, plates, prisms, respectively; θ ₁ and θ ₂ are the angle of incidence of the beam on the surface of the thin film transparent coating from the inside and the angle of refraction of the beam at the interface between the thin film transparent coating and the plate.

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

Figure 3 is a diagram of the path of the rays in a longitudinal section of an interference multi-beam filter containing an integrated lens. Here 10 is a transparent plate, 11 is a 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 passes into a thin layer coating 1. Next, the beam passes through the coating, falls at an angle θ ₁ at the coating-plate interface, is reflected at the same angle and is refracted at an angle θ ₂ :

$$\sin {\mathrm{<figure-callout\; id="2"\; label="\theta "\; filenames="00000007.png"\; state="\{\{state\}\}">\theta </figure-callout>}}_2=\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2}\sin {\theta }_{\mathrm{one}}.\left(\mathrm{one}\right)$$

The reflected beam along a broken path propagates inside the thin-film coating; at each incident on the coating-plate interface, the beam is reflected and refracted. The reflection coefficient may be high due to the proximity of the angle of incidence to the angle of total internal reflection at the boundary. When the beam falls on the boundary of the thin-film coating with the external environment, the beam experiences total internal reflection, since the condition is satisfied:

$${\theta }_{\mathrm{one}}\succ {\theta }_{\mathrm{to}R}=\arcsin \left(n_0/n_{\mathrm{one}}\right),\left(2\right)$$

where θ _cr - the angle of total internal reflection.

The refracted rays are collected by the lens A and are focused at the focal point, since they are mutually parallel and interfere. If the phase difference between adjacent rays is equal to a multiple of 2π rad, a maximum is observed upon interference. For light rays of different wavelengths, interference maxima are located in the focal plane of the lens at different distances from the optical axis of the lens, which allows, by placing a diaphragm with a slit or a line of photodiodes in the focal plane, to separate 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 edge of the plate 7. The surface 8 of the beveled edge should be flat and polished. After passing through a plate, which for incident light is similar to prism 3 in FIG. 1, the light beam enters the thin-film coating 1, in the future the path of the rays is similar to that described in the description of FIG. 1. The difference is the presence of the beam 9 reflected from the interface at the first passage of this boundary by the incident beam 4.

In the case shown in FIG. 3, the end of the plate 10 remote from the radiation input site is made in the form of a cylindrical or spherical positive lens 11, the refracted rays by this lens, like lens A in FIG. 1, are focused; with this design of the interferometer, the optical scheme is simplified, there is no need for a special lens L.

The considered device differs from the prototype in that the light guide part of the device is made in the form of a two-component structure; a thin-film coating can have a thickness much less than in the case of the Lummer-Gercke interferometer, which allows increasing the free spectral range of the interferometer by several orders of magnitude. In addition, radiation leaves the coating only through one of its surfaces, which doubles the intensity of the emitted radiation. The length of the interferometer is substantially reduced at the same resolution values.

The phase difference δ between adjacent rays in a thin-film coating, if we neglect the phase shift experienced by the light at full internal reflection, is determined by the formula:

$$\delta =\frac{\mathrm{four}\pi h}{{\lambda }_0}\sqrt{n_{\mathrm{one}}^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2{\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="00000007.png"\; state="\{\{state\}\}">sin</figure-callout>}}^2{\theta }_2}\left(3\right)$$

The order of interference m is determined by the number of waves in the optical path traveled by the beam per step of kink of the trajectory in a thin-film coating of thickness h, and the formula:

$$m=\frac{\mathrm{<figure-callout\; id="2"\; label="\delta "\; filenames="00000007.png"\; state="\{\{state\}\}">\delta </figure-callout>}}{2\pi }=\frac{2h}{{\lambda }_0}\sqrt{n_{\mathrm{one}}^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2{\mathrm{<figure-callout\; id="2"\; label="sin"\; filenames="00000007.png"\; state="\{\{state\}\}">sin</figure-callout>}}^2{\mathrm{<figure-callout\; id="2"\; label="\theta "\; filenames="00000007.png"\; state="\{\{state\}\}">\theta </figure-callout>}}_2}.\left(\mathrm{four}\right)$$

Here λ ₀ is the wavelength of light in free space. Considering that θ ₂ → 0, it is possible by the method proposed in the book [Fundamentals of Optics, see above] to obtain the formula for the free spectral region:

$$\Delta {\lambda }_0\approx \frac{{\lambda }_0^2}{2h}\frac{\sqrt{n_{\mathrm{one}}^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2}}{|\; |\; |n_{\mathrm{one}}^2-n_{\mathrm{one}}{\lambda }_0\frac{\partial \left(n_{\mathrm{one}}-n_2\right)}{\partial {\lambda }_0}-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2|\; |\; |}\approx \frac{{\lambda }_0^2}{2hK},\left(5\right)$$

where K is a weakly wavelength-dependent coefficient close in magnitude to the refractive index of a thin-film coating.

The resolving power of the interference filter is determined taking into account (5) the formula:

$$\frac{{\lambda }_0}{\delta \lambda }\approx 0.7\frac{lK}{{\lambda }_0\sqrt{n_{\mathrm{one}}^2-{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="00000007.png"\; state="\{\{state\}\}">n</figure-callout>}}_2^2}},\left(6\right)$$

where l is the distance along the plate between the input and output of the beam into the thin-film coating, δλ is the spectral bandwidth of the filter. When using a thin-film coating with a thickness of the order of the wavelength, as follows from (5), it is possible to have Δλ ₀ ≈λ ₀ , while the resolving power of the interferometer does not depend on the coating thickness and remains as high as that of the Lummer-Gercke interferometer.

A feature of the multi-beam interferometer according to the invention is the fact that with a small thickness of a thin-film coating comparable to the wavelength of light, the wave propagates in it, as in an optical waveguide, and has a mode structure. This circumstance requires that the radiation be introduced into the thin film coating at an angle so that the beam direction in the coating coincides with the mode direction.

For the manufacture of the device according to the invention, materials and technology used in the optical industry are used. For a prism, you can use sapphire (n = 1.75) or TF glass (n = 1.7-1.9), for thin-film coatings and wafers - TF glass and fused silica. It is important to fabricate the optical surfaces of prisms and wafers with a high degree of flatness.

The above explanations show that the fulfillment of the task can be ensured by the described technical solutions.

The technical result of the invention is the creation of a high-resolution light filter with a large dispersion region.

The invention can be used in optics and optoelectronics - as a narrow-band filter, as a dispersing device of monochromators and spectrometers.

Claims (6)

1. An interference multi-beam filter containing a flat transparent plate with a thin film transparent coating of one of its surfaces and an optical input prism, characterized in that the optical prism is fixed with a flat face on the thin film coating near the end of the plate, and the refractive indices of the prism and film are greater than the refractive index of the plate. 2. The device according to claim 1, characterized in that the radiation introduced into the transparent film propagates in it at an angle to the surface of the film adjacent to the plate, smaller than the angle of total internal reflection, but greater than the angle of total internal reflection of the second surface of the film. 3. The device according to claim 1, characterized in that the end of the plate remote from the radiation input site is made in the form of a cylindrical or spherical lens. 4. An interference multi-beam filter containing a flat transparent plate with a thin film transparent coating of one of its surfaces, characterized in that one end of the plate is tapered at an acute angle to the surface of the thin film coating, and the refractive index of the film is greater than the refractive index of the plate, while radiation is introduced into the film through beveled end of the plate. 5. The device according to claim 4, characterized in that the radiation introduced into the transparent film propagates therein at an angle to the surface of the film adjacent to the plate, smaller than the angle of total internal reflection, but greater than the angle of total internal reflection of the second surface of the film. 6. The device according to claim 4, characterized in that the end of the plate remote from the radiation input site is made in the form of a cylindrical or spherical lens.

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