SU1078336A1 - Two-component laser anemometer - Google Patents

Abstract

A TWO-COMPONENT LASER ANEMOMETER containing an optically coupled laser, two splitters, two frequency shifters that focus and collect back-scattered light lenses, a swivel mirror 9 4 and a photodetector, as well as two measuring bodies of Doppler frequency, characterized in that In order to increase the signal-to-noise ratio, an optical delay line and two electric band-pass filters are introduced into it, and the first splitter is designed as an optical device for forming three laser beams lying in the same plane and wherein the electrical inputs of bandpass filters connected in parallel to the photodetectors, and the outputs - to E of the Doppler frequency meter, and the optical delay line is positioned between the first and second rasschepitei L E in the path of the central beam splitter .vogo lane. (L

Description

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The invention relates to measurement technology and can be used to measure the velocity vector of a two-dimensional liquid or gas flow by optical means. A two-component laser anemometer containing an optically coupled laser, two splitters, two frequency shifters, focusing and collecting back scattered light lenses, a rotating mirror and a photodetector, and a Doppler frequency meter 1 are known. However, splitters in a well-known anemometer are polarized , which entails the use of a variety of additional polarization optical elements. Therefore, the known anemometer has a low signal-to-shu ratio when operating in back scattered light. Theoretical studies of a differential scheme with backscattering based on the Mie scattering theory show that the magnitude of the Doppler signal is significantly dependent on the degree of polarization matching of the mixed waves. Moreover, the polarization agreement coefficient is equal to unity only when observing scattered radiation in a direction close to the optical axis of the circuit. Therefore, in the well-known scheme, in order to ensure effective polarization selection of optical signals, which precludes ambiguity in the measurement of orthogonal components of growth, it is necessary to receive scattered radiation in a small angular aperture, within which the coefficient of polarization matching is equal to unity. The aim of the invention is to increase the signal-to-noise ratio. The goal is achieved by the fact that a two-component laser anemometer containing an optically coupled laser, two splitters, two frequency shifters, focusing and collecting back-scattered light lenses, a turning mirror and a photodetector, and two Doppler frequency meters, introduced an optical line delays and two electric bandpass filters, and the first splitter is made in the form of an optical device forming three laser beams lying in the same plane, while the inputs of the electric strips O filters are connected in parallel with the photodetectors, and the outputs - to E of the Doppler frequency meter, and the optical delay line is positioned between the first and second splitters at the center beam path of the first splitter. The drawing shows the block diagram of the proposed anemometer. The anemometer contains a laser 1, an emitting beam 2, a prism splitter 3, dividing beam 2 into three parallel beams 4-6, a prism splitter 7 dividing beam 5 into two beams 8 and 9 of equal intensity, two frequency shifters 10 and 11, connected to electronic power supply units 12 and 13, an optical support line. 14, made, for example, from a section of a light guide of length L, focusing lens 15, measurement area 16, through which flow irregularities diffuse light 17 collected by lens 15, swivel mirror 18, lens 19, photodetector 20, two band-pass filters 21 and 22 and two Doppler meters of frequencies 23 and 24. A two-component laser anemometer works as follows. As a source of coherent measurement in the circuit, laser 1 is used, for which the function of the degree of temporal coherence (i) is known. Beam 2, having intensity IQ, is divided by splitter 3 into three parallel beams, two of which 4 and b have equal intensity K1-0.25, and 5 - Klo intensity 0.5 k - loss factor). Beam 4 is directed to the input of frequency shifter 10, with which its frequency is fixed by an amount f. Further, two beams 4 and 6, extending in the horizontal plane symmetrically relative to the optical axis of the circuit, are focused by the lens 15 into the measurement area of the two-dimensional flux 16. Beam 5 passes the optical delay line 14, by means of which the delay of beam 5 in time is provided for such an amount Jj in which the modulus of the degree of temporal coherence 2 (t) is zero. Beam 5 is then divided by splitter 7 into two beams 8 and 9 of equal intensity. Moreover, beam 8 passes a frequency shift device 10, in which it is shifted in frequency by a fixed value f ii. Next, the rays 8 and 9, propagating in a vertical width of 1010, symmetrically with respect to the axis of the circuit, are focused by a 15v lens, measurement area 16. Four beams intersecting at the focus of lens 15 create an interference field with horizontally and vertically arranged stripes in the measurement area. The formation of such an interference field structure is explained by the fact that rays 4 and 6, 8 and 9 are coherent, T.ei is (t) 1 for them, and rays 4, 8, 9, 6, 8, bi9 are not coherent, because the path difference for such rays is is L t, and at It) 0.

The scattered radiation 17 is collected by the lens 15 and, after reflection from the mirror 18, the line of the SOY 19 is directed to the photodetector 20. As a result of the optical heterodyning, the variable component of the signal is formed at the output of the photodetector 20.

In this case, the first component of the signal, incomplete information on the horizontal projection of the velocity vector, is extracted by a band-pass filter21 tuned to the frequency S2i. The second signal component, which carries information about the vertical projection of the velocity vector, is extracted by a band-pass filter 22 tuned to frequency H. Filtered from interference signals from the bandpass filter outputs are fed to the inputs of Doppler frequency meters 23 and 24, which provide information in analog or

digital form of the size and sign of two orthogonal projections of the vector

speed + UX and + Vij.

In the proposed meter, the scattered radiation is collected in the angular anepTjpe significantly larger than in the known scheme, since there are no restrictions in the scheme imposed by the condition of providing full polarization, separation of the scattered waves.

0 This leads to an increase in the power of the scattered radiation received by the photodetector, and, consequently, to an increase in the signal-to-noise ratio by an order of magnitude or more as compared to

5 famous. In addition, the proposed device is much simpler in design, it uses fewer optical elements, which increases the economic performance of the circuit. The scheme is also easier to use and to maintain. The economic efficiency from the introduction of a single sample meter is achieved by simplifying the design and maintenance during its operation.

Claims (1)

(5 4) (5 7) TWO-COMPONENT LASER ANEMOMETER, containing optically conjugated laser, two splitters, two frequency shifters, focusing and collecting backscattered light lenses, a rotary mirror and a photodetector, as well as two Doppler frequency meters, about t which, in order to increase the signal-to-noise ratio, an optical delay line and two electric bandpass filters are introduced into it, and the first splitter is made in the form of an optical device for the formation of three laser beams lying in one plane and wherein the electrical inputs of bandpass filters connected in parallel to the photodetectors, and the outputs - with calipers Doppler frequency, and the optical delay line is positioned between the first and second splitters at the center beam path of the first beam splitter.