RU2548924C2 - Device to measure index of refraction of separate cross sections of investigated medium during non-stationary processes - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: device may be used to investigate high-speed processes in gases and other transparent media, for instance, in shock waves. The device comprises a source of monochromatic radiation, two transparent flat-parallel windows, between which there is the examined medium, a Foucault knife, a length-adjusted slot, perpendicular to the edge of the Foucault knife, a photodetector, a memory device. The angle of light beam fall at the inlet window is more than zero. The edge of the Foucault knife is arranged in parallel to the direction of heterogeneity motion or gradient of refraction index change gradient. By change of the photodetector signal they decide on the change of optical properties of the examined medium. The beam movement is registered in the direction perpendicular to the gradient of refraction index change, depending on the index of refraction of the medium in the cross section.

EFFECT: possibility to determine an index of refraction of the examined medium in an available cross section of a device.

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Description

The invention relates to optical methods and devices. In our consideration, the main measured quantity is the intensity of the light beam incident on the input of the photodetector. The measured value depends on the refractive index of the medium through which the beam passes, and therefore we can determine the changes in the refractive index of the medium.

We consider the case when the refractive index of the medium is uniform in the plane of the beam (see Fig. 1) and varies with time. Such a situation, in particular, arises if the front of the shock wave is flat and parallel to the plane of Figure 1, in which the light ray lies.

For cases when the refractive index varies in only one coordinate, a method has been developed that is described on page 35 of the book “Shadow Methods” L. Vasilieva with reference to the source: Ball GA 5 ^th symposium (internal.) on combustion, 1954. New York, 1955, p. 366.

In the study of non-stationary processes, in particular in shock tubes, in which an object sharply changes its characteristics, it is necessary to deploy the process in time, to obtain quantitative data throughout the experiment. The scheme of the method with photoelectric signal recording is described on page 61 of the book “Shadow Methods” L. Vasilieva with reference to the source: Vasiliev L.A., Galanin A.G., Ershov I.V. Suntsov G.N. Instruments and experimental equipment, vol. V-VII No. 3, 195 (1964). In the method described in this article, it is shown that the angle of light deviation in planar inhomogeneities is linearly related to the gradient of the logarithm of the refractive index of the gas, and the edge of the Foucault knife is perpendicular to the direction of motion of the inhomogeneity.

In the article "Experimental Investigation of Air Radiation from Behind a Strong Shock Wave" Jomal of Thermophysics and Heat Transfer Vol. 16, No. January 1-March 2002 (p.77-82), describes a device for detecting the position of the front of a shock wave in a shock tube, but this device allows you to determine the position of the front with an accuracy of not more than 1.1 mm and cannot provide information about the fine structure of the front of the shock the waves.

The objective of the invention is the creation of a device for producing an electrical signal correlated with the optical refractive index of the cross section of the medium under investigation during non-stationary processes.

The technical result is the possibility of obtaining an electrical signal correlated with the refractive index of the investigated medium 4 lying in the plane of Fig. 1.

The technical result is achieved due to the fact that in the device (Fig. 1), which is a light source 1, two transparent plane-parallel and parallel to each other windows 3 and 5, between which there is a medium 4 with a time-variable refractive index, Foucault knife 7 , a slit 8 perpendicular to the edge of the Foucault knife, a light recorder 9, a storage device 10, it is new that the angle _of incidence α ₁ of the light beam on the input window 3 is greater than zero and that the edge of the Foucault knife is parallel to the direction of motion of the inhomogeneity.

Fig. 1 shows a diagram of the device and the position of the light beam in the initial state.

In the initial state, the medium to the left of window 3 is homogeneous and has a light refractive index n ₁ , windows 3 and 5 are homogeneous, plane-parallel and have equal light refractive indices n ₂ , the medium 4 is homogeneous and has a light refractive index n ₃ , the medium to the right of window 5 homogeneous and has a light refractive index n ₁ . The incoming monochromatic uniform beam of light 2 lies in the plane of Figure 1.

Under these conditions, for the angles of incidence and the angles of refraction of the beam, the relations

,

,

,

,

it follows that sinα ₁ = sinα ₄ , and therefore, α ₁ = α ₄ and therefore the output beam is parallel to the incoming beam.

Part of the output beam 6 in height is delayed by a Foucault knife 7, and the rest passes further and falls on a slit 8, which limits the beam in width, then the beam enters the photodetector 9 (for example, PMT), then the signal is stored using memory device 10 (for example , oscilloscope). The edge of the Foucault knife is usually set in a position in which the knife delays half the output beam.

All deviation angles can be expressed in terms of the device parameters and the angle of incidence α ₁ :

, $${\mathrm{<figure-callout\; id="2"\; label="\alpha "\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}_2=\arcsin (\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}))$$

,

The output beam has a parallel displacement in the plane of Fig. 1 relative to the incoming beam. The magnitude of this offset depends on n ₂ , the thickness l of the windows, n ₃ and the distance L between the inner surfaces of the windows. Let us calculate the contribution to this displacement, which is made by medium 4. The deviation from the perpendicular to the window surface given by medium 4: $$\Delta y_{\mathrm{one}}=L\cdot tg\left({\alpha }_2\right)\Rightarrow \Delta y_{\mathrm{one}}=L\cdot tg(\cdot \arcsin (\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}))$$

The offset from the perpendicular to the surface of the window, given by the output window:

, $$\mathrm{<figure-callout\; id="2"\; label="\Delta \; y"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{y}_{}{}_2=l\cdot tg\left({\alpha }_3\right)\Rightarrow \mathrm{<figure-callout\; id="2"\; label="\Delta \; y"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{y}_{}{}_2=l\cdot tg\left(\arcsin \left(\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}\right)\right)$$

,

is a constant with constant α ₁ .

The total offset of the output beam relative to the incoming, provided that the thickness of the input and output windows are equal to:

$$\Delta y=\Delta y_{\mathrm{one}}+2\cdot \mathrm{<figure-callout\; id="2"\; label="\Delta \; y"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{y}_{}{}_2\Rightarrow \Delta y=L\cdot tg(\cdot \arcsin (\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}))$$

. $$\Delta y=L\cdot tg(\cdot \arcsin (\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}))+2\cdot l\cdot tg\left(\arcsin \left(\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}\right)\right)$$

.

The beam offset, depending on the change in n ₃ , is determined by the first term, the second term must be taken into account when designing the installation. Thus, the dependence of the beam offset on n ₃ is determined by the expression:

$$\Delta y=L\cdot tg(\cdot \arcsin (\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}))$$

Figure 2 shows a top view of the device diagram and the movement of the light beam.

Figure 3 shows the position of the beam with an increase in n ₃ , the refractive index of the medium being studied 4. In this case, the angle α _{2 of} the beam deflection in medium 4 decreased and most of the beam was delayed by the Foucault knife.

In the general case, when the beam is shifted, the Foucault knife will delay a larger or smaller part of the beam, and thus the photodetector signal will correlate with a change in the refractive index of the medium being studied 4. By changing the width of the slit, it is possible to adjust the thickness of the layer of the studied substance from which light enters the photodetector.

For small angles α ₁ we have: α ₁ ≈sinα ₁ ≈tgα ₁ ,

so

. $$\Delta y=L\cdot tg(\cdot \arcsin (\frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}\cdot \sin {\alpha }_{\mathrm{one}}))\approx L\cdot {\alpha }_{\mathrm{one}}\cdot \frac{n_{\mathrm{one}}}{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}_3}$$

.

With a change in n ₃ from n ₃₁ to n _{32, a} change in Δy will be:

, $$\Delta y_{\mathrm{one}}-\mathrm{<figure-callout\; id="2"\; label="\Delta \; y"\; filenames="00000014.png,00000015.png"\; state="\{\{state\}\}">\Delta \; </figure-callout>}{y}_{}{}_2=L{\alpha }_{\mathrm{one}}\frac{n_{\mathrm{one}}}{n_{31}}-L{\alpha }_{\mathrm{one}}\frac{n_{\mathrm{one}}}{n_{32}}=L{\alpha }_{\mathrm{one}}\cdot n_{\mathrm{one}}\left(\frac{n_{32}-n_{31}}{n_{31}\cdot n_{32}}\right)$$

,

and if the relative change in n _{3 is} small, we can write:

The above relations show that there is a correlation between the photodetector signal and the change in the refractive index in the medium.

The inventive device has four distinctive features that affect the achievement of a technical result.

First, the slit of the photodetector is perpendicular to the gradient of the refractive index of the medium under study.

The second - the edge of the Foucault knife is parallel to the gradient of the refractive index of the investigated medium.

Third, the beam displacement does not depend on the gradient of the refractive index, but on the refractive index of the medium under study.

Fourth, the device is very simple to manufacture and set up.

Claims (1)

A device for measuring the refractive index of individual cross sections of a test medium contains a monochromatic radiation source, two transparent plane-parallel windows between which the test medium is located, a Foucault knife, a slit width adjustable perpendicular to the edge of the Foucault knife, a photodetector, a storage device, characterized in that the angle of incidence of the beam the light on the input window is greater than zero and the edge of the Foucault knife is parallel to the direction of motion of the inhomogeneity or gradient of change in the refractive index.

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