RU1837332C - Acousto-optic spectrum analyzer with integration in time - Google Patents

Abstract

The invention relates to the field of optical information processing and may find application in radio engineering measurements. The spectrum analyzer contains a coherent light source, a collimator, acousto-optic modulators, projection lenses, aperture, an optical transparency, a matrix photodetector on charge-coupled devices, an optical image transformer consisting of two confocal spherical lenses. 1 ill.

Description

The invention relates to the field of optical information processing and may find application in radio engineering measurements, in radar to form a function of the uncertainty of radio signals in real time.

An object of the invention is to expand functionality by creating an uncertainty function.

The drawing shows a structural; circuit diagram of the device.

The device contains a source of coherence.,. a lot of light 1, collimator 2, the first acousto-epic modulator 3, the optical image transformer 4, the second acoustical modulator 5, the first projection lens 6, the aperture 7, the second three-way lens 8, the optical transparency 9, the photo sensor charge-coupled devices 10.

The device operates as follows. An electronic signal is received at the electrical input of the first acousto-epic-modulator

Ui (, t) bi (t)) i (t)) vn,

(1) where bi (t), v, (p (t) are the amplitude values.

frequency and phase of the signal, respectively,

t is the time

, is the coefficient of change in the time scale of the signal.

The transmittance of the first AOM in the presence of a signal in it is equal to

1

ti (x, t) 1 + i (Ј) Ki Ui (/ fi,

x (, t- rX7),

t 1

V

$ 4 | FC1 Ui- (2)

where is the x-coordinate along the direction of ultrasound propagation,

V is the speed of propagation of ultrasound,

1 - proportionality coefficient.

The modulator is illuminated by a beam of coherent light with a flat phase front parallel to the plane of the modulator and a uniform intensity distribution that is emitted by the laser (1} and the wide C /

WITH

ioo

00 I

CO CO

YU

with a collimator (2), the light field behind the first AOM is

Ei-Eoti (x, t), (3)

where Eo is the amplitude of the incident light.

The image of the first AOM is projected onto the second AOM (5) with a zooming effect, which is carried out using an optical transformer (4). The field illuminating the second modulator is equal to

Ei - $ Eo {1 + | IKiUi (PM. Ui (

i, t

X #,

V

(4)

where fh is the reciprocal of the magnification factor of the optical transformer, j-ja the second AOM receives a signal (i) (t) exp i2jzv & t, (5) (where the designations are similar to those used in formula (1).

The transmittance of the second modulator is equal to

tz (x, t) -lV (t + -T) + i IK2U2

x (t + -T).

where T is the propagation time of ultrasound in the modulator,

K2 is the coefficient of proportionality.

When the modulator is illuminated with coherent light, the amplitude of the light transmitted through the modulator will contain a component proportional to the transmittance t2 (x, t).

Using projection system lenses (6, 8) and a filtering aperture (7) with two holes located at the focusing points of the first diffraction orders, the images of the modulators are projected onto the surface of the photodetector. Assuming that both modulators operate in the Bragg mode, we will consider only the + 1st order of diffraction. An interference pattern is formed in the plane of the photodetector, the intensity of which in the case of a single increase in the projection system is

id (x, t) / KiUi (/ 8it--) + K2U2 (t +) / 2-Ki2 / Ui / 2 + K22 / U2 / 2 + 2Re x

(6)

{KiK2 Ui ((). U2 (t + $ -T)}.

(7)

For further consideration, we take into account the fact that the interference pattern is formed as a result of the addition of light beams propagating at an angle

equal to the sum of the angles of diffraction of light by ultrasonic waves in the first and second AOM.

,

fifteen

twenty

25

thirty

35

40

V

V

(8)

where A is the wavelength of light,

Au - the length of the ultrasonic wave in the 2nd AOM,

AI is the effective ultrasonic 0 wavelength in 1 AOM taking into account zooming.

The addition of two light waves at an angle modulates the interference pattern with a spatial frequency equal to

Fnp -VJ / A. (9)

Taking into account formulas (8), (9). as well as expressions for radio signals (1) and (5), the third term of the sum (7) can be written as

And t -) U2 (t + y - T). jrQ3i vi-V2) t -exp

(Prfavi + v $ xl (10)

A CCD array operating in the shift and sum mode is used as a photodetector, with the direction of charge movement along the matrix columns parallel to the direction of ultrasound propagation in the modulators. The transition in formula (10) to a moving coordinate system

 -ut

where and is the rate of charge movement along the columns of the CCD matrix, we obtain the following expression for the intensity of the interference pattern

A- Re {Ui ((Јi “Ј), - &) - U ((t + V) -exp ((/ 3ivi-V2) t- x

x (iffi # 2Vi + V2) t) -exp (l x 45x () xi)} (11)

Polaga / Si -fa -ft i - i i / u and you

complete algebraic transformations; we reduce expression (11.) to the form

50 A Re (Ui ((1 + / fe)) U ((1 + ф) (- i2jr tf vi-) t- x

55 x (vi +) t) -exp (

x .vi +%) xi)). (12)

npnUl U2 Uo. V1 V, V2 + AV

Up to terms of the second order of smallness, we can write

A Re {Uo ((1 + xUo ((1 +

) t-% V

and

vi v

T7) + V-T) x

1A

2l

V 1 V xexp () exp ()}. (thirteen)

At Av 0, integrating over time within the limits equal to the time of movement of charges along the matrix columns, we obtain the light distribution corresponding to the autocorrelation function of the signal Uo (t). This distribution modulated by the spatial frequency

2 v

F -G7-, there is a special case of the uncertainty function for the zero Doppler shift. The formation of the uncertainty function in the general case (for A v 0) requires the use of an optical transparency. As such a transparency, a lattice with strokes perpendicular to the columns of the matrix can be used, the transmittance of which is equal in intensity to

G- t-1 + cozy x. (14)

In a moving coordinate system, the presence of a transparency modulates the light intensity with a frequency

fM syU.

We obtain the required value of the spatial frequency of the banner by equating the frequency of this modulation to the value of the Doppler shift i Av. Hence,

y - # ... (I)

In order to form the uncertainty function for different Av values, it is necessary to have a set of gratings with different spatial frequencies defined by formula (15).

Given the need to form two quadrature components, each of the gratings should consist of two having the same spatial frequency and shifted relative to each other by a quarter of the grating step along the direction of movement of the charges.

The proposed device solves an additional problem associated with cash. in addition to the useful signal, a constant component. To filter this component and extract the desired signal, a method of subtracting two samples is used. For this purpose, four lattices with the same spatial frequencies and shifted relative to each other by a quarter of the lattice step are used for each value, similar to the way it was done in the prototype device. The general expression for the transmittance of the optical banner is similar to the expression for the transmittance of the prototype device:

n - - 1 .2,. + | T cos

PTOK 3

0.-.

 Ј1

T7 X W Ck 3 U U2 about. 0 l I 10 - (K) 2 J

0

5

0

5

0

5

0

5

x P

Yg (2K-1 + 8p)

1 }

(sixteen)

Ushtm

where Aw ™ is the band of analyzed frequencies, N is the number of elements in the matrix row, Ym is the size of the matrix row, and y is the coordinate along the matrix row. The function P (y) is defined by the expression

| B / y / 1;

RU)

LO ,.

However, unlike the prototype device, expression (14) describes the transmittance in intensity, since the transparency is located directly in front of the photodetector, the output signals of which are proportional to the light intensity.

The transparency transmission function allows to obtain four components in each frequency channel corresponding to the four phase values of the output signals of the photodetector

with

, y) which

can be calculated by the formula t + t

ft (x.y .t.VO / G -Adt (17)

t

where the values of A and G are defined by expressions (13) and (16), respectively.

Perform further calculations, for example, using

Dfy, y.t) Ф1 - Фй) 2 + (Фз -) 2 (18) we can find the squared modulus of the signal uncertainty function for different values of time delays and frequency shifts.

The number of resolvable points in frequency is determined by the number of elements in the line of the CCD photodetector. As follows from Kotelnikov’s theorem, in the simplest case this number is half the number of line elements. In this device, due to the applied processing algorithm, the difference reaches eight times. In this case, the frequency resolution is determined, as in conventional time-integrated spectrum analyzers, i.e. time of charge movement along the columns of the matrix.

As for the maximum achievable number of solvable points in range, it is half the number of elements in the matrix column.

The proposed device has significant advantages over analogues. The AQM bandwidth should not exceed the frequency bandwidth of the device. The analyzed signal is fed directly to the piezoelectric transducer AOM, and does not modulate the amplitude of the LFM signal. The absence of amplitude modulation in the electric path allows the maximum use of the dynamic range of the AOM, while expanding the dynamic range of the entire device.

Claims (1)

The claims Acousto-optic spectrum analyzer with integration over time, containing placed on the optical axis. coherent light, a collimator, a first ecousto-optic modulator, a first projection lens, an aperture, a second projection lens, an optical transparency and a photodetector array on charge-coupled devices, the aperture being located in the common focal plane of the projection lenses, characterized in that, for the purpose of expanding the functionality due to the formation of the uncertainty function, an optical image transformer placed behind the first acousto-optical modulator is introduced into it, consisting of two confocal spherical lenses arranged in series on the optical axis. and a second acousto-optic modulator, with the optical device placed directly in front of the array photodetector, the electrical input of the first acousto-optical modulator is an input for setting the time parameters of the spectrum analyzer. i i 9 7 3 f0