RU2377702C1 - Electro-driven diffraction device - Google Patents

Abstract

FIELD: physics; acoustics. ^ SUBSTANCE: invention relates to optoelectronics, particularly to acoustooptical and acoustoelectronic devices, including micromechanical and microoptical devices. Periodic texture in the device is created by an elastic flexural wave in a strip with a mirror surface fixed by the ends over a substrate. The end of the strip is connected to the substrate through a piezoelectric transducer. On the substrate there is an array of identical parallel strips which form a one-dimensional matrix. Surfaces of substrates lie in a single plane. The piezoelectric element of the transducer is polarised perpendicular to surfaces of the electrodes, and its thickness is not greater than a quarter wavelength of the elastic wave in the piezoelectric element. Piezoelectric transducers at opposite ends of the strips are connected to two different alternating voltage sources, where the sources can tune output voltage phase. The piezoelectric element can consist of two parts lying at different sides of the longitudinal plane of symmetry of the strips. Said parts have opposite polarisation directions. ^ EFFECT: efficient excitation of elastic waves in an acoustic thin-film micromechanical waveguide, possibility of exciting different oscillation modes, possibility of electrical variation of the grating constant. ^ 6 cl, 3 dwg

Description

The invention relates to optical equipment, in particular to optoelectronics, acousto-optical and acoustoelectronic devices, including micromechanical and micro-optical devices.

An analogue of the proposed device is a well-known acousto-optical transducer [1] used for modulation and scanning of light. It can be a rod of transparent material in which a longitudinal acoustic wave is excited piezoelectric. A spatial periodic modulation of the refractive index of a substance occurs in the rod, which leads to diffraction of the light passing through the rod.

The disadvantages of the analogue are the significant weight and size parameters of the devices and the great importance of power consumption.

A prototype of the invention is a thin-film microwave for modulating light [2]. The prototype contains microwaves in the form of thin-film free strips fixed at the ends by ends in which elastic bending waves are excited.

However, in the publication [2] about the prototype there is no information about the method of excitation of elastic waves. The disadvantage of this device is the lack of an ordered structure inherent in the diffraction grating and structural elements that provide an aperture ratio acceptable in practical applications.

The problem solved by this invention is to overcome the disadvantages of the prototype, the development of an effective method of excitation of elastic waves in an acoustic thin-film micromechanical waveguide, the possibility of excitation of different modes of oscillations, the possibility of electrical changes in the constant diffraction grating.

The problem is solved in that in an electrically controlled diffraction device containing a periodic relief, which is created by an elastic bending wave in a strip with a mirror surface fixed by the ends above the substrate, an AC-to-elastic-voltage transducer connected by electrodes to an AC voltage source, according to the invention, one or both connect the end of the strip to the substrate through a piezoelectric transducer and fix on the substrate an array of identical parallel said strips forming a one-dimensional array, and their surfaces must be coplanar.

It is also proposed that the piezoelectric element of the transducer is polarized perpendicular to the surfaces of the electrodes, and its thickness is not more than a quarter of the length of the elastic wave in the piezoelectric element.

It is also proposed that said piezoelectric transducers at opposite ends of the strips are connected to two different AC voltage sources, said sources being able to phase-shift the output voltage.

It is proposed that the said piezoelectric element consists of two parts located on opposite sides of the longitudinal plane of symmetry of the strip, and the polarization directions of these parts are opposite.

New is the proposal that one or both ends of the strip be connected to the substrate through a piezoelectric transducer and an array of the same parallel said strips forming a one-dimensional matrix are fixed on the substrate, and their surfaces must lie in the same plane.

New is the proposal that the piezoelectric element of the transducer be polarized perpendicular to the surfaces of the electrodes, and its thickness should be no more than a quarter of the length of the elastic wave in the piezoelectric element.

New is the proposal that the said piezoelectric transducers at opposite ends of the strips be connected to two different sources of alternating voltage, and these sources have the ability to rearrange the phase of the output voltage.

New is the proposal that the aforementioned piezoelectric element consist of two parts located on different sides of the longitudinal plane of symmetry of the strip, with the polarization directions of these parts being opposite.

In Fig. 1, as an example, the device according to claim 1 is shown, in Fig. 2 is a side view of a single strip connected to a substrate by a piezoelectric transducer, in Fig. 3 is a side view of a single strip connected to a substrate piezoelectric transducer, consisting of two parts. Designations in the drawings: 1 - a strip with a mirror surface, 2 - the end of the strip mounted on the plate 3 through a piezoelectric transducer, which includes a piezoelectric element 4 and electrodes 5 and 6 from the side of one end of the strip and electrodes 7 and 8 from the other side. Electrodes 5 and 6 are electrically connected to an alternating voltage source U _{1 ~} , electrodes 7 and 8 are connected to an alternating voltage source U _{2 ~} . In figures 2 and 3, the arrows indicate the polarization directions of the piezoelectric element. In Fig.3, the piezoelectric element consists of parts 9 and 10 having opposite directions of polarization. In Fig. 1: l is the thickness of the piezoelectric element 4, Δl is the alternating change in the thickness of the piezoelectric element when voltage U _{~ is} turned on.

The device operates as follows. When you turn on the alternating voltage U _{1 ~ the} piezoelectric element (figure 1) periodically changes its thickness with the frequency of the voltage, the location in the space of the end of the strip attached to the element. A running elastic wave is excited in the strip, the direction of which is shown by a horizontal arrow. The oscillation amplitude in the wave is approximately equal to Δl, the circular frequency ω is equal to the frequency of the alternating voltage, the wavelength in the strip is determined by the expression

where c _in is the speed of the wave in the strip.

If resonance phenomena are not taken into account, then the wave amplitude can be determined by the expression

(пьезополимер пористый полипропилен),

; получим Δl=0.18 мкм. Такое значение амплитуды обеспечит эффективное применение устройства во многих приложениях. Граничную частоту работы устройства можно найти из условия, что по толщине пьезоэлемента должно расположиться не более четверти продольной длины волны в его материалеwhere d ₃₃ is the piezoelectric module of the material of the piezoelectric element, E is the amplitude value of the electric field in the piezoelectric element, U ₀ is the amplitude of the alternating voltage. Take: l = 200 μm,

(piezopolymer porous polypropylene),

; we obtain Δl = 0.18 μm. This amplitude value will ensure the effective use of the device in many applications. The boundary frequency of the device can be found from the condition that the thickness of the piezoelectric element should be located no more than a quarter of the longitudinal wavelength in its material

where c _p is the speed of sound in a piezopolymer. Common value for sound velocity polymers

,

,

we get:

f ₀ = 2.5 · 10 ⁶ Hz and Λ = 4 · 10 ^-5 m (at an achievable value

).

)

When you turn on the voltage between the electrodes of the piezoelectric element in figure 3, a torsional elastic wave is excited in the strip. In a first approximation, the amplitude of the torsion angle can be calculated by the formula

where a is the width of the strip. Assuming a = 50 μm, we obtain: α = ± 7.2 mrad = ± 0.4 °.

If the traveling wave is reflected from the second end of the strip, then the ordered picture of the relief of the bending wave will be disturbed. To reduce the amplitude of the reflected wave, it is necessary to apply an alternating voltage U _{2 ~} to the electrodes 7 and 8 of the piezoelectric elements with such amplitude and phase at which the piezoelectric transducer excites an elastic wave in the strip, equal in amplitude to the incoming wave and opposite in phase. As a result of the interference of the waves, they cancel each other out, and the traveling wave mode is realized in the stripes. Such an inclusion of the indicated voltages is also possible at which a standing wave is excited in the strip, the amplitude of which, as is known, is 4 times the amplitude of the traveling wave with equal wave intensities.

The traveling wave mode makes it possible to smoothly change its length in the strip by continuously and smoothly changing the frequency of the alternating voltage, that is, smoothly electrically controlled changes in the constant diffraction grating, the function of which is performed by this device. The possibility of smooth frequency tuning is also facilitated by the piezoelectric excitation of the elastic wave directly at the end of the strip, since this does not cause resonant effects when transmitting the oscillation power from an alternating voltage source to an elastic wave in the entire frequency range from zero to that in which the standing mode does not occur in the piezoelectric element waves, that is, until a quarter of the wavelength begins to fit along the thickness of the piezoelectric element. The factors listed in this paragraph will enable the device to provide a smooth and wide-range wavelength of diffracted light.

The relief that appears on the strip when a traveling bending wave appears is sinusoidal. The diffraction of light on a sinusoidal relief has a peculiar character. Under normal incidence of light on such a relief, only two diffraction maxima are observed in the diffraction pattern - +1 and -1, which simplifies the use of such a diffraction grating as a monochromator - there are usually no overlaps of various spectral orders.

A torsional wave gives a peculiar relief pattern in the strip matrix, which is also periodic and diffractively active. A feature of torsion-mode strips is the possibility of obtaining low elastic wave velocities with greater mechanical stiffness and strength of the strips.

The use of an array of separate relatively narrow strips to increase the aperture of the diffraction grating instead of, for example, one wide strip is dictated by the fact that when the operating temperature changes due to various thermal expansions of the strips and the substrate, lateral warping of the strip surfaces occurs, leading to a violation of their flatness and poor diffraction properties of the device. This effect does not occur in the longitudinal direction of the strips, since there are possibilities to keep the tightness of the strips constant.

The results of the analysis and calculations, thus, confirm the solution of the tasks posed during the creation of the invention.

For the manufacture of the device, for example, the following materials can be applied: a substrate - glass, a strip - metal, made of tantalum or titanium, electrodes - from aluminum, a piezoelectric element - from piezoceramics or piezopolymer. The manufacture of the device can be carried out by methods used in the technology of microcircuits and in the technology of micromechanical devices.

The device can find application, for example, in micromechanical acousto-optical devices for optical information processing, in various measuring transducers.

Information sources

1. Physical encyclopedia. / Ch. ed. A.M. Prokhorov. - M .: Owls. encyclopedia. T.1. 1988.

2. Chesnokov VV, Chesnokov DV The use of elastic waves in thin-film microwaves to modulate light. // International conf. "Applied Optics-96" (September 17-20, 1996, Russia, St. Petersburg): Abstract. doc. - St. Petersburg: GOI, 1996. - P.246.

Claims (4)

1. An electrically controlled diffraction device containing a periodic relief, which is created by an elastic bending wave in a strip with a mirror surface fixed by the ends above the substrate, an AC-to-elastic-voltage transducer connected by electrodes to an AC voltage source, characterized in that one or both ends of the strip connected to the substrate through a piezoelectric transducer, and on the substrate is fixed an array of identical parallel said strips forming a one-dimensional matrix y, and their surfaces lie in the same plane. 2. The device according to claim 1, characterized in that the piezoelectric element of the transducer is polarized perpendicular to the surfaces of the electrodes, and its thickness is not more than a quarter of the length of the elastic wave in the piezoelectric element. 3. The device according to claim 1, characterized in that said piezoelectric transducers at opposite ends of the strips are connected to two different AC voltage sources, said sources being able to phase-shift the output voltage. 4. The device according to any one of paragraphs, characterized in that the said piezoelectric element consists of two parts located on opposite sides of the longitudinal plane of symmetry of the strip, and the polarization directions of these parts are opposite.

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