SU1748071A1 - Laser doppler velocity meter - Google Patents

Abstract

Use: instrumentation, measuring the velocity of gas flows. SUMMARY OF THE INVENTION: The speed meter contains two optical branches intersecting each other, the first of which contains a laser sequential along the optical axis, a polarization-selective beam splitter at an angle to the optical axis, a first quarter-wave plate and a monostatic transceiver unit, and the second is sequentially installed one by one the side from the polarization of the butt-sets of the mirror and the second quarter-wave plate activating the splitter beam, and on the other side of it the lens and photodetector. An increase in the signal-to-noise ratio is achieved by inserting into the second optical branch between the polarization-selective beam splitter and the linear polarizer lens and rotating the splitter relative to the optical axis by 90 °. 2 Il. sl C

Description

The invention relates to laser measuring instruments and can be used in aviation, meteorology, atmospheric physics, ecology, and other fields of science, technology, and folk economics where it is necessary to measure the velocity of gas flows and other objects.

A known laser Doppler true airspeed and wind shear LATAS meter, consisting of an optical head including a laser, an optical heterodyne converter, a monostatic transceiver antenna unit, in which the receiving and transmitting optics are combined, and the processing unit of the Doppler signal, which includes a device for measuring and recording the frequency

Doppler signal and its corresponding speed value.

The disadvantages of the meter are the complexity of the design, adjustment and alignment of the optical heterodyne converter made according to the scheme of the Mach – Zander interferometer with the Brüster-type splitter, and the increased susceptibility to misalignment of the actual operating conditions.

The closest to the present invention is a laser Doppler velocity meter containing two optical branches intersecting each other, the first of which contains4 a polarization-selective beam splitter installed successively along the optical axis of the laser, the first

2

00

about

x |

a quarter-wave plate and a monostatic transceiver unit, and the second — a mirror and a second quarter-wave plate sequentially installed on one side of the polarization-selective beam splitter, and a lens and a photodetector connected to the processing unit Doppler on the other side of the polarization-selective splitter. - th signal

This laser Doppler meter greatly simplifies the setup and alignment of the optical heterodyne converter. However, such a meter has a significant disadvantage in the phase mismatch of the wave fronts of the signal and reference radiation, which leads to a decrease in the signal-to-noise ratio.

The aim of the invention is to increase the signal-to-noise ratio by geometrically compensating for the diffraction divergence of the signal beam and reducing the shot noise of the heterodyne photocurrent by reducing the parasitic polarization of the heterodyne radiation.

This goal is achieved due to the fact that a linear polarizer is inserted in the second branch between the polarization-selective beam splitter and the lens, and the polarization-selective beam splitter is rotated about the optical axis by 90 °.

Figure 1 shows the scheme of the optical part of the laser Doppler velocity meter; in fig. 2 shows the optical path in the wedge, which is used as a polarization selection splitter.

The meter contains two optical branches intersecting each other, the first of which contains a laser 1 successively installed along the optical axis, a polarization-selective beam splitter 2, the first quarter-wave plate 3 and the monostatic transceiver unit 4, and the second sequentially mounted on one side of the polarization-selective beam splitter, the mirror 5 and the second quarter-wave plate 6, and on the other side of the polarization-selective splitter, lens 7, fo the transceiver 8 and linear polarizer 9 and the processing unit of the Doppler signal 10, the input of which is connected to the output of the photodetector.

The meter works as follows.

The linearly polarized laser radiation 1 hits the polarization selection splitter 2, which reflects part of the radiation to form a reference beam (heterodyne), which through the linear polarizer 9, which cuts off the parasitic part of the polarization, the lens of the detector 7 enters the surface of the photodetector 8: the rest part of the radiation is transmitted in the forward direction and, passage through the quarter-wave plate 3,

0 where linear polarization passes into a circular, and monostatic antenna unit 4, enters the irradiated half-space; radiation scattered by aerosol particles is collected by the same antenna

5 and passes through the plate 3 again, where it changes the circular polarization to a linear, orthogonal plane of the pattern, after which it is reflected by the polarization selection splitter in

0 an additional channel, where it passes through a quarter-wave plate 6, is reflected from mirror 5 and again passes through plate b, thereby rotating the polarization plane by 90 ° (which now coincides

5 with the plane of polarization of the original beam) and, passing through the splitter, enters the linear polarizer, the detector lens and further to the detector surface, where the mixture is shined with the radiation of the local oscillator; A blocking device of the transformed Doppler signal, the input of which is connected to the output of the photoreceiver, produces the selection of the intermediate frequency signal and its further processing.

5 One of the main elements of the meter is a polarization-selection splitter, which should provide both spatial alignment of the signal and heterodyne beams,

0 and the formation of the reference beam. Consider its characteristics.

The reflection coefficient for polarization parallel to the plane of the figure (hereinafter E-polarization), in principle

5 can be brought to zero. Small rotations of the plate (wedge) make it possible to regulate the power of the local oscillator within fairly wide limits, and the use of a thick plate or wedge leads to a spatial separation of the beams reflected from the front and rear faces. In order to ensure the spatial coincidence of the signal and heterodyne beams, the heterodyne beam is formed

5 when reflected from the back face. The course of the rays in the wedge is shown in figure 2. Let us determine the transmittance Ks for this scheme. It will be determined by the transmission coefficients of radiation with E polarization through surfaces 1 and 2 during the direct passage of

the laser beam, the reflection coefficient for the H-polarization (orthogonal to the drawing plane) and the transmission for the E-polarization for the signal beam. In this way

Ks - О - К1еХ1 - К2е) К2н (1 - К2еХ1 - М

The coefficients Ke and Kn are determined by the Fresnel formulas for their angles of incidence 1, (pi, p (the angle p is not equal to the angle (p when working with a wedge).

The magnitude of the reference signal power will be determined by the transmittance Kg of face 1 and the reflectance of face 2 for radiation with E-polarization. The corresponding coefficient is

Kg (1 - Kie) (t - Kie) K2e

Since the radiation is not perfectly polarized, when reflected at the Brewster angle from the front and rear surfaces, the degree of depolarization of the radiation will increase, since the component orthogonal (the plane of FIG. 2) will be reflected by a large factor. The corresponding coefficient KP is

Kn (1 - K1n) K2n (1 - KiH)

Table 1 presents the calculated values of the coefficients Ks, Kg „Kp depending on the angle of incidence of radiation on a wedge from ZnSe with a solution angle of 1 °. The fourth column shows the angle at which the radiation leaves the plate.

The power of the parasitic illumination is determined not only by the laser power and the Kp coefficient, but also by the degree of depolarization of the laser radiation, which can vary from device to device. In general, in order to suppress the power of parasitic illumination, it is necessary to introduce a linear polarizer into arm 2-7.

The principle of operation of this speed meter, like the prototype, is based on the implementation of dependencies:

vA / Sigl (p, wo & L 3k

Q RESET C 9 (G + (1-6 &)# + a & / (), (2) where the following parameters are used:

rj is the quantum efficiency of the photodetector;

I is the probing radiation wavelength;

{fa is the volume backscatter coefficient in the atmosphere; RL - laser power;

L F - the width of the band of the received signal;

G is the mismatch function; h is the planken ns Planck ;.

 -)})

0

co-frequency of the probe radiation; y0 meter focus parameter:

Rn is the effective radius of the radiator field distribution at the transceiver aperture;

Rn Ro VI + 0 / L-UD G

I

Rr is the effective radius of distribution of the heterodyne field on the transceiver aperture;

 V 1 + (Ug-UDG

UL - emitter focusing parameter; d yg - heterody focusing parameter

on; yr-Po is the radius of the field distribution at the laser output;

FQ is the effective focusing distance of the meter; eleven

F0

one

+

one

F 2 | lul (1 + y) Ug (1 + U)

F is the focal distance of the monostatic transceiver unit; | l is the distance traveled by the probe radiation from the laser to the transceiver aperture; l And + s

1d is the equivalent distance traveled by the radiation of the local oscillator to the receiving-transmitting aperture; Ir h-li - 2I2

h is the distance from the polarization-selective splitter (2) to the laser (1);

12 — distance from the polarization-selective splitter (2) to the reflector (5);

h is the distance from the polarization-selective splitter (2) to the output aperture of the antenna unit (4);

1D - diffraction length; d:

DK - mismatch of the characteristic dimensions of the fields of the emitter and receiver;

А, - R R - У Ug (ul + ang) lA (S R + R2 fi + # + 2y№ А () is the mismatch of the focal lengths of the receiving and transmitting channels;

a-g

4ulug (1 + y2) (1 + y) 1-ulu d

(four)

Under the condition of strong focusing (y0 1), the arctg value rapidly moves

with to l / 2. From expressions (3) and (4) it can be seen that for a simultaneous equality of zero mismatch dk and 5i it is necessary and sufficient that the equality lg is fulfilled, which is equivalent to the condition AND g

The proposed meter has a higher signal-to-noise ratio with respect to the prototype, all other conditions being equal (the same laser power, backscatter, geometric dimensions, etc.). For convenience of comparison, we set the multiplier value

p ADGG RL ,,

TGShTch- equal to one. Usually

the distance between the optical elements in the meter has the same order of magnitude, therefore for the estimates we set: It 12 1 3 1 / |. Let us substitute in relation (1) the specific values of the parameters characteristic of the gauges used in practice:

z0 2 mm F 500 mm

The dependence of the signal-to-noise ratio (relative units) for the prototype and the claimed meter on the distance AND (in meters) is given in Table 2.

Thus, the meter has a Signal / Noise ratio of almost twice the magnitude of the prototype.

The tests carried out showed that the parameters of the claimed speed meter are close to theoretical values and lie within the experimental error. The signal-to-noise ratio for reflecting radiation from a rough disk at 4.5 W laser power was 0.9 of the theoretical estimate.

Claims (1)

The invention of the Laser Doppler velocity meter containing two intersecting optical branches, the first of which contains a laser successively mounted along the optical axis, and at an angle to the optical axis of the polarization-selective beam splitter, a quarter-wave plate and a monostatic transceiver unit, and the second — a mirror and a second quarter-wave plate sequentially mounted on one side of the polarization-selective beam splitter, and a lens and a photodetector, which is similar to the second quarter-wave plate. that, in order to increase noise immunity by increasing the signal-to-noise ratio, a linear polarizer was introduced into the second branch between the polarization-selective beam splitter and the lens, and e selectivity beam splitter rotated relative to the optical axis at 90 °. The results of the calculation of transmission and reflection for ZnSe (wedge with a solution angle of 1 °) Signal to noise ratio versus distance Table 1 table 2 ABOUT Fig 2.