RU2349945C1 - Acoustooptical microwave surface ultrasound excitation deflector - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: acoustooptical deflector (AOD) accommodates parallelepiped piezoelectric crystal and piezoelectric transducer representing interdigital antiphase-excited electrode system. Herewith both signal injection buses are axially symmetric shaped with electrodes of maximum length dimension fitting the centre of piezoelectric transducer.

EFFECT: extended working frequency band.

7 dwg

Description

The present invention relates to devices for controlling the parameters of laser radiation, in particular to microwave acousto-optical deflectors based on volume acoustic waves (AOW), and can be used as a transparency in optical information processing systems.

A known design of the microwave acoustic acousto-optical deflector (AOD) (Fig. 1, a) based on a piezoelectric crystal in the form of a parallelepiped (1), in which acoustic waves are generated using a piezoelectric transducer (2) located on one of the side faces, and the absorber (3) is located on the opposite side of the parallelepiped. This analogue uses transducers made in the form of resonant rectangular plates of small thickness. The region of interaction of light and sound in the AOD is determined by the transverse dimensions of the acoustic column. In this analogue, the amplitude-frequency characteristic (AFC) of the AOD is controlled by changing the shape of the resonant plates of the piezoelectric transducer (in the form of a rhombus, hexagon, trapezoid, ellipse, etc.) [Voloshinov VB, Knyazev G.A. Acousto-optic cells with unequal interaction lengths in the light beam cross section. ZHTF. - 2003. - t. 73. - No. 11. - p.118-122].

The features of the chosen analogue common with the claimed invention are the following: an AOD based on a piezocrystal in the form of a parallelepiped, on the upper face of which a piezoelectric transducer of a complex configuration is applied (in the form of a rhombus, hexagon, trapezoid, ellipse, etc.), and on the opposite side of the piezocrystal the absorber is applied.

The reason that impedes the achievement of the claimed technical result is that the use of this analogue at frequencies above 1 GHz is not possible due to the technological difficulties of manufacturing a piezoelectric transducer, since the thickness of the resonant piezoelectric plates required for use in piezoelectric transducers operating at a frequency above 1 GHz, does not exceed 2-4 microns.

A known design of an acousto-optical microwave deflector (Fig. 1, b) based on a piezoelectric crystal in the form of a parallelepiped (1), in which acoustic waves are generated using a piezoelectric transducer (2) located on one of the side faces, and located on the opposite side of the parallelepiped absorber (3). In this analogue, a sectioned piezoelectric transducer (2) is used, which is made in the form of resonant rectangular plates of small thickness (Fig. 1, b). In this analogue, the frequency response of the AOD is controlled by changing the distribution of the amplitude of the ultrasound in the AOD along the direction of propagation of the optical beams by applying different levels of exciting voltage to the sections of the piezoelectric transducer [Bogomolov DV, Milkov MG, Parygin VN Control of the hardware function of acousto-optic cells with non-collinear interaction geometry. Radio engineering and electronics. - 2006. - t.51. - No. 1. - p. 100-106].

The features of the chosen analogue that are common with the claimed invention are the following: an AOD based on a piezocrystal in the form of a parallelepiped, on the upper face of which a piezoelectric transducer is applied, and an absorber is deposited on the opposite side of the piezocrystal, while the shape of the frequency response of the AOD is corrected by changing the distribution of the amplitude of ultrasound in the AOD .

The reason that impedes the achievement of the claimed technical result, as in the previous case, is that this analogue can work at frequencies not exceeding 1 GHz, therefore, the necessary increase in the range of operating frequencies cannot be achieved.

Of the known AODs, the closest in technical essence to the claimed object is an acousto-optical deflector with surface excitation of ultrasound using a phased array of transducers in the form of antiphase-energized electrodes (IDT) [Rozdobudko VV, Bakaryuk T.V. Acoustooptic microwave deflector with surface excitation of ultrasound. Instruments and experimental technique. - 2003. - No. 1, p.1-3].

The prototype device (figure 2) in its composition contains a piezoelectric crystal made in the form of a parallelepiped, playing the role of a light and sound conduit, on the upper face of which IDT is applied, an absorber is applied to the lower face of the parallelepiped to eliminate the influence of reflected sound waves. A modulated laser radiation is supplied to the front face of the piezocrystal perpendicularly to the piezoelectric transducer grating perpendicular to the top of the light and sound duct.

The features of the selected prototype, common with the claimed device, are as follows: the ANM includes a parallelepiped-shaped piezoelectric crystal, which acts as a light and sound conduit, with an IDT on its upper face and an absorber on its opposite face.

The principle of operation of the prototype deflector is illustrated in the drawing (figure 2), the positions in the drawing indicate: 1 - piezoelectric transducer, 2 - piezocrystal, 3 - absorber, 4 - modulated laser radiation.

The principle of operation is based on the well-known property of optical radiation - to diffract on a periodic structure arising as a result of the elasto-optical effect when an ultrasonic wave propagates in a solid. In this device, the excitation of elastic acoustic waves occurs directly from the upper face of the IDT piezocrystal.

To explain the principle of operation of the prototype, suppose that ultrasound is excited from the surface of a piezocrystal by a system of IDTs into a light and sound pipe of an acousto-optical deflector, having a total length L and consisting of m identical transducers of width l ₀ with a constant period of their location s. Note that in this case, the surface between the metal electrodes acts as an element of the IDT lattice; with the number of electrodes equal to N, the number of piezoelectric transducers proper will be m = N-1. An acoustic wave excited by such a flat phased piezoelectric transducer is described by the relation [Balakshiy V.I., Parygin V.N., Chirkov L.E. Physical foundations of acousto-optics. - M .: Radio and communication. - 1985. - 280 p.]:

- волновой вектор и ω - частота возбуждаемой акустической волны. Функция rect(x) такая, что:where y ₀ = sl ₀ is the gap between the transducers, and ₀ is the amplitude,

is the wave vector and ω is the frequency of the excited acoustic wave. The rect (x) function is such that:

.

.

The frequency response of the above AOD at low diffraction efficiency is determined by the ratio [Balakshiy V.I., Parygin V.N., Chirkov L.E. Physical foundations of acousto-optics. - M .: Radio and communication. - 1985. - 280 p.]:

угол падения света, а f₀ выбрана из условия наилучшей коррекции брэгговского угла:

; кроме того, входящий в (2) параметрin which Θ ₀ , in this case, normalized to

the angle of incidence of light, and f _{0 is} selected from the condition of the best correction of the Bragg angle:

; in addition, the parameter included in (2)

contains P _a0 - the sound power emitted by the grating, the radiation area of which is S = d × m · l ₀ . The operating frequency band is determined by the length of the acousto-optical interaction, the divergence and the cross-sectional shape of the acoustic column propagating in the piezocrystal.

при нормированном угле падения света Θ₀=-2 и

при

[Балакший В.И., Парыгин В.Н., Чирков Л.Е. Физические основы акустооптики. - М.: Радио и связь. - 1985. - 280 с.].The reason preventing the prototype from achieving the required technical result is its insignificant band of operating frequencies, the value of which is

at a normalized angle of incidence of light Θ ₀ = -2 and

at

[Balakshiy V.I., Parygin V.N., Chirkov L.E. Physical foundations of acousto-optics. - M .: Radio and communication. - 1985. - 280 p.].

The problem to which the invention is directed, is to expand the operating frequency band of the AOD, as well as reducing the unevenness of the frequency response.

The technical result consists in the fact that both of its signal supply lines are made in the form of an axisymmetric figure with a maximum size along the length of the electrodes per center of the piezoelectric transducer.

To achieve a technical result in an AOD containing a piezoelectric crystal in the form of a parallelepiped, which plays the role of a light and sound duct, on the upper face of which IDT is applied, and an absorber is applied to the opposite face, it is necessary that the piezoelectric transducer be a system of interdigital electrodes with antiphase excitation, and both of its signal supply lines are made in the form of an axisymmetric figure with a maximum size along the length of the electrodes falling on the center of the piezoelectric transducer.

б) полосы рабочих частот АОД от угла α₁ при углах падения света, соответствующих одногорбым и двугорбым АЧХ. На фиг.7 изображены возможные конфигурации электродов, обеспечивающие положительный эффект, который заключается в расширении полосы рабочих частот и уменьшении неравномерности АЧХ АОД.The invention is illustrated by drawings. Figure 3 presents the drawing of the inventive AOD, the positions in the drawing indicate: 1 - piezoelectric transducer, 2 - piezocrystal, 3 - absorber, 4 - modulated laser radiation. Figure 4 shows the normalized frequency response calculated for the case when the axisymmetric figure is a rhombus or a truncated rhombus (depending on the angle of apodization α) for two angles of incidence of light Θ ₀ = -2 and Θ ₀ = -2-1 / m and different apodization angles - α; the dependences shown in figure 4, a, correspond to one-humped frequency response, and in figure 4, b - two-humped. Figure 5 presents the same frequency response as in figure 4, but on a logarithmic scale (figure 5, a) - angle of incidence of light Θ ₀ = -2; b) is the angle of incidence of light Θ ₀ = -2-1 / m). Figure 6 presents the calculated dependences: a) the relative diffraction efficiency I (0) / I ₀ from the normalized angle of apodization

b) the frequency band of the AOD from the angle α ₁ at angles of incidence of light corresponding to one-hump and two-hump frequency response. Figure 7 shows the possible configurations of the electrodes, providing a positive effect, which consists in expanding the operating frequency band and reducing the uneven frequency response of the AOD.

The principle of operation of the inventive device is based on the well-known property of optical radiation to diffract on a periodic structure arising due to the elastic-optical effect when an ultrasonic wave propagates in a solid. In this device, the elastic acoustic waves are excited directly from the upper face of the piezocrystal (2) due to the inverse IDT piezoelectric effect (1), the ultrasonic wave propagates in the piezocrystal volume, and modulated laser radiation (4) is applied to the ultrasonic wave at one of the side faces at a Bragg angle diffracting through the opposite side face. On the face of the parallelepiped, opposite the one on which the IDT is located, to eliminate the influence of reflected sound waves, an absorber (3) is applied to eliminate the influence of reflected sound waves.

The use of a piezoelectric transducer, which is a system of interdigital electrodes with antiphase excitation, such that both signal supply lines to the electrodes are made in the form of an axisymmetric figure, leads to a decrease in the level of excited ultrasonic power to the edges of the aperture l, the length of the aperture of the piezoelectric transducer.

Let us consider some of the variants of interdigital piezoelectric transducers, in which both busbars for supplying a signal to the electrodes are made in the form of an axisymmetric figure with maximum dimensions in mutually perpendicular directions.

Figure 3 shows that the length of the interaction of light and ultrasound depends on the x coordinate if the light beam is directed along the Oy axis. For the lattice of converters in the form of a rhombus (Fig. 3), the length of the AO interaction within | x | ≤0.5d varies as l (x) = L-2 | x | ctg (α) = ms-2 | x | ctg (α ) In general, the angle α determines the shape of the lattice; depending on α, the relation for l (x) may be suitable for describing the converter in the form of a rhombus, hexagon, etc.

We denote the ratio of the electrode width to the lattice period as l ₀ / s = k, k is the metallization coefficient. We rewrite expression (2) as

or in a more simplified version:

а

Where

but

We write down the intensity of diffracted light for each partial component within the limits of the dimensions of the ultrasonic beam:

After integration, the last expression will take the following form:

substituting in which η (F) and q ₁ , we obtain the final expression for the frequency response:

.

.

.This expression makes sense when

.

Figure 4 shows the frequency response calculated in accordance with (3) for two angles of incidence of light Θ ₀ = -2 and Θ ₀ = -2-1 / m and different angles of apodization - α; the dependences shown in Fig. 3, a, correspond to one-humped frequency response, and in Fig. 3, b - two-humped. Figure 5 shows the same frequency response, built on a logarithmic scale. The calculations were carried out for the following data:

- the wavelength of laser radiation incident on the front face of the AOD, λ = 0.63 μm;

- the central frequency of the AOD bandwidth f ₀ = 1750 MHz;

- operating frequency band Δf = 500 MHz;

- the speed of ultrasound in a piezocrystal ν = 3.6 · 10 ³ m / s;

- indicator of acousto-optical quality M ₂ = 2.9 · 10 ^-15 s ³ / kg;

- refractive index n = 2.29;

- the width of the piezoelectric transducer d = 0.2 mm

до 90° относительная полоса ΔF, отсчитываемая по уровню 3 дБ, увеличивается на ≈16%. При этом дифракционная эффективность АОД падает с одновременным уменьшением пульсаций АЧХ вне основной полосы пропускания.From the analysis of relation (3) and consideration of figure 4. and 5, we can conclude that with a change in the angle α in the range from

up to 90 °, the relative band ΔF, measured at the level of 3 dB, increases by ≈16%. In this case, the diffraction efficiency of the AOD decreases with a simultaneous decrease in frequency response pulsations outside the main passband.

показан на фиг.6,а.The nature of the change η with variation of the normalized angle of appodization

shown in Fig.6, a.

Qualitatively, similar dependences are also characteristic of the case of AOD with bumpy frequency response with the difference that, at Θ ₀ = -2-1 / m with an increase in the angle α, a sharp decrease in the frequency response is observed in the ΔF band. Dependences ΔF = φ (α ₁ ) for both angles of incidence of light are shown in Fig.6, b; they differ from each other in the absolute value of ΔF: for Θ ₀ = -2-1 / m the band ΔF is on average 1.5 times higher than ΔF AOD at Θ ₀ = -2.

полоса рабочих частот увеличивается на 16%, а неравномерность уменьшается почти в два раза по сравнению с прототипом.As a result of the calculations at the angle of apodization

the operating frequency band is increased by 16%, and the unevenness is almost halved compared to the prototype.

Consequently, the use of piezoelectric transducers made in the form of an axisymmetric figure with maximum dimensions in mutually perpendicular directions, which are a system of interdigital electrodes with antiphase excitation, and both buses supplying a control signal to the electrodes are made in the form of an axisymmetric figure, which leads to an increase in the working band frequencies and reduce the frequency response of the acousto-optical deflector.

AOD can be made on the basis of a LiNbO ₃ piezocrystal, on the YOZ section of which a piezoelectric transducer is applied.

Comparable data for the prototype and the inventive device obtained for the AOD based on LiNbO ₃ with parameters: wavelength of laser radiation incident on the front face of the AOD, λ = 0.6328 μm; central frequency of the AOD bandwidth f ₀ = 1750 MHz; the speed of ultrasound in a piezocrystal ν = 3.6 · 10 ³ m / s; the indicator of acousto-optical quality M ₂ = 2.9 · 10 ^-15 s ³ / kg; refractive index n = 2.29; the width of the piezoelectric transducer d = 0.2 mm The prototype operates in the operating frequency band Δf = 500 MHz (at a level of 3 dB) with a maximum diffraction efficiency η = 5% / W.

The well-known photolithography technology used in the semiconductor industry can be used to make a piezoelectric transducer.

As the absorber 3 in the inventive device, it is advisable to use a substance with acoustic impedance close to the resistance of the crystal, to ensure matching of the sound duct and the absorber and to avoid re-reflection of the acoustic wave in the region of acousto-optic interaction.

Claims (1)

An acousto-optic deflector containing a piezoelectric crystal in the form of a parallelepiped, acting as a light and sound conduit, on the upper face of which an interdigital piezoelectric transducer is applied, and an absorber deposited on the opposite side, characterized in that the piezoelectric transducer is a system of interdigital electrodes with antiphase excitation, moreover the signal is made in the form of an axisymmetric figure with a maximum size along the length of the electrodes falling on the center of the piezoelectric transducer The gentleman.

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