SU1402850A1 - Method of determining diemensions of brownian particles - Google Patents

Abstract

The invention relates to optics of scattering media and can be used for the rapid determination of the particle size along the sounding path in macro-inhomogeneous media using backscattered radiation with its own inherent particle size. The goal is to expand the field of application by determining the size of particles in macro-inhomogeneous media. The method is to use the property of the limited temporal coherence of multimode lasers. The laser radiation is directed to a Michelson interferometer, in one of the arms of which, instead of a mirror, is the studied medium. The backward scattering of the light is mixed in the plane of reception with the reference wave, leading to the formation of intense intensities due to interference fluctuations. sivosti. However, due to the limited coherence length of ul. the latter are due to the interaction with the reference wave of only a part of the scattered radiation coming from the volume of the medium in the order of length L1 from a depth determined by the length of the reference plate. Measuring its magnitude and measuring each half the width of the power spectrum of fluctuations, the sizes of the particles at each depth of the macro-inhomogeneous medium are calculated with resolution equal to the coherence length of the radiation. 2 Il. i (lind 00 ate

Description

The invention relates to the optics of scattering media and can be used for the rapid determination of the particle size along the probing path in macro-inhomogeneous media with the presence of each macro-inhomogeneity by its size of the particles forming it using backscattered radiation. The purpose of the invention is to expand the scope of application of the method by determining the size of particles in macro-inhomogeneous media.

In the device for. The Michelson interferometer is used to carry out the proposed method, in one of the arms of which the mcr-inhomogeneous medium under study is located. A multimode continuous-action laser in the TEMddp mode is used as the radiation source. The coherence length D1 of its radiation at the center wavelength D is determined by the width LL of the spectral interval (the number q of excited axial modes) and has a value. By selecting the mode of operation of the source, the quantity / 5 L is chosen such that the length of the coherence 41 is equal to linear; the size of the smaller of the macroscopic inhomogeneities along the sounding path. This: the condition allows, if the length of the reference arm is equal to the optical depth

 about 4

I Z-center (j-th macroheterogeneity

j to register in the reception plane I intensity fluctuations due to the I interference with the reference wave of light scattered only by particles: the corresponding jth macroheterogeneity I and precisely determine their size

If the coherence length dl exceeds the linear size of any of the macroscopic inhomogeneities along the probing path, for example, if it is diagnosed (the equality of the length of the reference arm is equal to the optical depth Z °), its contribution to the recorded interference signal will not only particles of the diagnosable kth macroinhomogeneity, but also diffusers from adjacent layers of the medium, which B ultimately reduces the accuracy of determining the particle size at the optical depth of Z ,,

In the opposite case, when the length of YI coherence is significant., Smaller than the size of the macro-inhomogeneities of the medium, the informative fluctuations of the intensities due to the inter-

0

five

0

five

0

five

0

five

With the reference wave of scattered light from the diagnosable volume of the medium, length L.1, will be small compared to the intensity fluctuations (self-beating) of everything else that is incoherent with the reference field of scattered light and other interference. Therefore, the situation considered is unprofitable from an energy point because it leads to measurement errors due to low signal-to-noise ratios.

Therefore, for the realization of conditions that are optimal from the point of view of measurement accuracy and energy, the coherence length of the source used should be. correspond to the expected scale of macro-inhomogeneities of the medium.

Fig. 1 shows an apparatus for carrying out the proposed method; Fig. 2 shows the modulus of the function y (Z) of the temporal coherence as a function of the path difference Zct (where c is the speed of light at different output powers P; t is the time of propagation of the axial mode).

The method is carried out in the following way.

The radiation from source 1 is divided by beam splitter 2 into transmitted and reflected beams. The transmitted beam falls on the mirror 3 of the supporting arm, and the reflected beam is directed to the macro inhomogeneous medium 4 under study. These beams will be referred to below as the reference and. object. Reflected by mirror 3 and beam splitter 2, the supported beam is reduced in the receiving plane (diaphragm 5) with backscattered radiation and transmitted through beam splitter 2.

For convenience of reading the diagnosed depth of the medium, it is advisable to carry out an initial equalization of the length Zj of the reference arm (distance from the beam splitter 2 to the mirror 3) with the distance from the beam splitter 2 to the front boundary of the medium 4, in which case the subsequent increase in the length of the support arm by of the plane of pr.iema interference with the oporta wave of only radiation scattered by the volume of the medium over

- centered on optical depth

P about .7 / /

Zj; / п (. Where n is the refractive index of the medium; 2. j is the length of the reference iine at the depth

This interference pattern in the receiving plane (diaphragm 5) is a complex structure that changes with time due to Brownian motion of the particles. The fluctuations of the photocurrent i of the detector 6, proportional to the fluctuations of the illumination of the aperture of the diaphragm 5, will be called the information interference signal (IMS). The rest of the spreading light will add incoherently to the reference field and will not contribute to the IIS. In this case, it is necessary to fulfill the following conditions. First, the time Well of averaging the registration scheme should be significantly less than the time –C g of correlation of the Brownian motion of the particles, but much longer than the time — T of the radiation coherence of the source; secondly, the size g, diagrams 5 in front of the detector 6 must be smaller than the correlation radius g once scattered diagnosable: the volume of the pot, the indicated requirements are of easy radius size (gor 10 s) remote from the receiving plane by distance E / 1 m, the following:

city-10

b

-3

P

with -jp V2-10

 Arj

 g

 0.5 mm.

15

20

Simultaneously with the registration of informative, intense intensity fluctuations (due to the interference of the reference wave and coherent scattered ripping), detector 6 will also register weaker intensity fluctuations generated by the interference of the scattered particles of the fields between themselves (self-beating of scattered light - CPC), not Carrying information about the layered structure of the environment. These fluctuations have a wider spectrum and. in diagnosing the remote, they are combined in practice. For example, in low 2S environments, the results may be distorted.

The micron size (gor 10 s), remote from the receiving plane for a distance of E / 1 m, is as follows:

ten

-3

P

with -jp V2-10

 Arj

 g

 0.5 mm.

Simultaneously with the registration of informative, intense intensity fluctuations (due to the interference of the reference wave and coherent scattered ripping), detector 6 will also register weaker intensity fluctuations generated by the interference of the scattered particles of the fields between themselves (self-beating of scattered light - CPC), not Carrying information about the layered structure of the environment. These fluctuations have a wider spectrum and. in diagnosing remote volumes, environment may distort results

In the experiment described above, when using laser radiation with a coherence length equal to L 6 cm, / Z and s, and the diameter of the probing measurements of the half width of the IIS power spectrum, to estimate the contribution of CPC to the correlation function K (f, Z:} phototo | Ka i ( Fourier transform of which

A beam of 2 g o, 2 mm, condition 30 determines the power spectrum of the fluctuations and, accordingly, its half width) it is necessary to use the relation:

registration of the information interference signal from the j-ro of the diagnosed volume of the medium with particles

KU, zp i (t, z °) i (+. 2p

 ClV (z;) rj (z;) l-J E, (t. Gr i j. (2nZ - 2nzpjz H.

+ CI

(uB}

riH (Z))

(RO + z) 4

de C is constant;

I - source intensity

Sveta; .4 S - cross-sectional area

probe beam; D is the source radiation time; A and P - respectively, the coefficient

45

scattered particles and particle concentrations; R (, is the distance from the surface of the medium to the reception plane; is the modulus of the function of the time

source coherence; g (J, Z) is the correlation function of the Brownian particle motion at a depth Z of the medium; T (Z) is the transmission of the medium up to the depth Z, equal to exp- (| E (z) d

50

measurements of the half width of the power spectrum of the IMS, for estimating the contribution of the CDS to the correlation function K (f, Z:} photototal | Ka i (the Fourier transform of which

and, accordingly, its half-width), it is necessary to use the relation:

(one)

  I (2nZ - 2nZ) dZ JZ

45

-;,; attenuation index; directional radiation; n is the refractive index; Z is the optical depth; J is the level of heterogeneity.

Relationship (1) is calculated for a further reception zone under the assumptions of single scattering and smallness of the change in the optical characteristics of the medium at distances of the order of the coherence length D1. The first term determines the contribution to K (1, Zj) of the informative interference signal, the second one - the self-beating of scattered light (SRS). If the microphysical parameters of the medium are unknown, then the contribution of the CPC can be estimated experimentally with the reference beam overlapped. ,

The resulting relationship (Z) is represented graphically in FIG. 2, where curve 7 corresponds to (Z) at P 20 mW, curve 8 corresponds to v (Z) at P 130 mW, curve 9 corresponds to y (Z) at P 270 mW and curve 10 corresponds to v (Z) at P, 720 mW .

Claims (1)

Invention Formula The method of determining the size of Brownian particles, based on irradiating a medium with particles suspended in it by a beam of coherent laser radiation, recording the half width of the power spectrum of the intensity fluctuations of the backscattered medium radiation and calculating the particle size, which is different from expansion of the field of application by determining the size. ditch particles in macroheterogeneous media, coherence length 1 radiation j 0 5 20 5 Neither the source is chosen equal to the smaller size of macro-inhomogeneities along the probing path, the initial laser radiation is divided into object and reference beams, the medium under test is irradiated with the object beam, the reference beam and the back-scattered radiation is irradiated by the object beam. -shee to the plane of reception of the optical paths of the reference radiation and the light reflected by the front boundary of the medium, increase the optical path of the reference radiation by an amount Z equal to the optical depth of the medium on which The spatial resolution 1 determines the particle size and, for each Zi, the half-width of the power spectrum, fluctuations of intensity due to the interference of the reference wave and the radiation scattered by the diagnositous volume of the medium, length L1, centered at the depth Z °, 0