JP3101445B2 - Surface acoustic wave convolver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave convolver for extracting a convolution signal of two surface acoustic wave signals propagating in opposite directions by utilizing the physical nonlinear effect of a piezoelectric substrate.

[0002]

2. Description of the Related Art A surface acoustic wave convolver for extracting a convolution signal of two surface acoustic wave signals has been increasing in importance in recent years as a key device for performing spread spectrum communication, and has been actively studied. I have.

FIG. 7 is a schematic diagram showing a first conventional example of a surface acoustic wave elastic convolver. In the figure, 1 is Y
Piezoelectric substrate such as cut (Z-propagation) lithium niobate,
2, 3 are comb-shaped input electrodes formed on the surface of the piezoelectric substrate 1,
Reference numeral 4 denotes an output electrode formed on the surface of the piezoelectric substrate 1. These electrodes are made of a conductive material such as aluminum,
Usually, it is formed directly on the surface of the piezoelectric substrate 1 using a photolithography technique.

In the surface acoustic wave device having such a configuration, when an electric signal having the carrier angular frequency ω is input to the comb-shaped input electrode 2, the surface acoustic wave is excited by the piezoelectric effect of the substrate. Similarly, when an electric signal having the carrier angular frequency ω is input to the comb-shaped input electrode 3, a surface acoustic wave is excited. These two
Two surface acoustic waves, the output electrode 4 acts as a waveguide,
While confined in the output electrode, they propagate on the piezoelectric substrate 1 in opposite directions. This waveguide is known as a Δv / v waveguide. The Δv / v waveguide reduces the propagation speed of surface acoustic waves compared to the free surface by electrically shorting the substrate surface, and conducts by totally reflecting the surface acoustic waves at the boundary between the free surface and the short-circuited portion. It is confined in the wave path. The Δv / v waveguide is described in detail in “Ohm Co., Ltd., Handbook of Elastic Wave Device Technology, P180-181” and the like. The confinement effect in a surface acoustic wave waveguide is
The effect is larger as the difference between the speeds of the surface acoustic waves in the waveguide portion and the outer portion of the waveguide is larger.

[0005] The surface acoustic waves hit on the output electrode 4 as described above are caused by the physical nonlinear effect of the piezoelectric substrate 1, causing the surface acoustic waves to fall on the output electrode 4.
It is extracted from the output electrode 4 as a convolution signal of two input signals (carrier angular frequency 2ω). That is,
^Assuming that two surface acoustic waves are F (tx / v) e ^{j (-kx + wt)} and G (t + x / v) e ^{j (kx + wt)} , the nonlinear interaction on the piezoelectric substrate 1 A surface acoustic wave of the product F (tx / v) · G (t + x / v) e ^2jwt is generated. This signal is integrated within the electrode length region by providing a uniform output electrode,

[0006]

[Outside 1] Is extracted as a signal represented by Here, the integration range is substantially ± ∞ when the interaction length is sufficiently larger than the signal length.
Assuming that τ = tx / v, equation (1) becomes

[0007]

[Outside 2] And the signal is a convolution of two surface acoustic wave signals. Such a convolution mechanism is described in, for example, “Shibayama,“ Application of Surface Acoustic Wave ”,
Television, 30, 457 (1976)].

The convolution efficiency (ratio of input to output) of the elastic convolver is 1) a figure of merit M indicating the degree of nonlinearity of the material of the piezoelectric substrate 2) the surface acoustic wave at the output electrode (waveguide) It is known that it depends on the size of the confinement effect. However, Li
The nonlinearity of the material of the piezoelectric substrate such as NbO ₃ is smaller than the nonlinearity of an AE surface acoustic wave convolver using a semiconductor.
Therefore, in order to improve the non-linearity of the LiNbO _3, the surface layer of the so-called "proton exchange process" of the substrate, and the surface acoustic wave Elastic obtained by emphasizing the nonlinear effect of the substrate by replacing the lithium atom and protons Convolva [Kakio, Matsuoka, Nakagawa, proton exchange 128 Y-cut LiNbO ₃
SAW characteristics on a substrate, '92 Autumn Proceedings of the Acoustical Society of Japan, p1087 (1992).

FIG. 8 is a schematic view showing such a second conventional example. In the figure, the same members as those shown in FIG. 7 are denoted by the same reference numerals, and detailed description is omitted. In the drawing, reference numeral 5 denotes a proton exchange layer formed on the surface of the piezoelectric substrate. With such a configuration, the nonlinearity of the LiNbO ₃ substrate can be increased eightfold.

Further, when the proton exchange treatment the LiNbO ₃ substrate, such as a Y-cut plate, the speed of the LiNbO ₃ SAW substrate is changed in comparison with the LiNbO ₃ substrate untreated, the amount of change is up to 40% , And that the velocity may increase or decrease depending on the proton exchange processing conditions in the Z propagation direction of the Y-cut, according to V. Hinkov, Pr.
oton exchanged waveguides for surface acoustic wav
e on LiNbO ₃ , J. Appl. Phys. 62 3573 (1987)].

[0011]

However, when only the .DELTA.v / v effect of the output electrode is used as shown in the first conventional example, the waveguide portion (short-circuit portion) and its outer portion (free surface) are used. ), The velocity difference of the surface acoustic wave is not so large, and the confinement effect (waveguide effect) in the waveguide is not sufficient, so that the surface acoustic wave excited from the input electrode does not enter the output electrode (waveguide). There is a problem that there are leaking materials and radiation mode surface acoustic waves that leak to the outside of the output electrode while propagating on the output electrode (waveguide), and convolution efficiency is small accordingly.

As a countermeasure, a method of increasing the thickness of the output electrode or increasing the mass of the output electrode (waveguide) by using a high-density electrode material such as Au (mass load effect) to provide a further speed difference. However, there are problems such as difficulty in controlling the film thickness, frequency dispersion of the propagation velocity between the modes of the surface acoustic wave, and distortion of the convolution signal waveform.

The problem of the confinement effect of such a surface acoustic wave in a waveguide is that a single-mode surface acoustic wave propagates, for example, when the output electrode width is less than about three wavelengths of the input surface acoustic wave. This is particularly problematic. Furthermore, when a beam focusing means such as an arc-shaped input electrode or a parabolic horn waveguide is used, the elasticity leaks (extracts) without entering the waveguide at the entrance of the output electrode (waveguide). Since there are more surface waves, the angle of incidence of the surface acoustic wave on the waveguide is limited, and the shape of the focusing means is limited.

In order to increase the non-linearity of the substrate, Li
When proton exchange is formed on the surface layer of the NbO ₃ substrate, the nonlinearity of the material is improved, but on the piezoelectric substrate, an index indicating the conversion efficiency of converting an electric signal into a surface acoustic wave signal (or vice versa) k ² (Electro-mechanical coupling constant) is reported to be small. This is a decrease in piezoelectricity,
It is believed due to the effect of ionic conductivity on the input electrode.

[0015] In addition, the propagation loss of surface acoustic waves increases in the proton-exchanged region as compared to the untreated region, as described in PJ Burnett and GADBriggs, Acoustic prope
rtiesof proton-exchanged LiNbO ₃ studied using the
acoustic microscopy V (z) techniqe, J. Appl. Phys. 60
(7), 2517 (1986)]. Therefore, when a proton exchange layer is formed on the entire surface of the substrate as in the surface acoustic wave convolver shown in the second embodiment,
Since k ² (electric-mechanical coupling constant) decreases and the propagation loss increases, there is a drawback that convolution efficiency decreases as a result.

[0016]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface acoustic wave convolver in which the effect of confining a surface acoustic wave at an output electrode (waveguide) is improved and the convolution output is large. It is to be.

An object of the present invention is to provide a surface acoustic wave convolver in which an output electrode is formed or a region where an output electrode is formed such that the speed of a surface acoustic wave in an output electrode forming region is lower than the speed of a region on both sides thereof. This is achieved by selectively performing proton exchange on the surface layer portion of the lithium niobate substrate under any condition that changes the desired surface acoustic wave velocity in any one of the regions other than the region where is formed.

According to the above means of the present invention, the surface acoustic wave velocity of the lithium niobate substrate in the region subjected to the proton exchange treatment is made faster or slower than the surface acoustic wave velocity in the untreated region depending on the proton exchange treatment conditions. Therefore, in addition to the Δv / v effect, the velocity difference of the surface acoustic wave between the output electrode (waveguide) portion and the outer portion thereof can be further increased, so that the effect of confining the surface acoustic wave can be improved. It is possible to obtain high convolution efficiency.

[0019]

Embodiment 1 FIG. 1 is a schematic perspective view showing a first embodiment of a surface acoustic wave convolver according to the present invention. FIG.
FIG. 2 is a sectional view showing an AA ′ section in FIG. 1. In the figure, 1
Denotes a Y-cut lithium niobate substrate, and 2 and 3 denote comb-shaped input electrodes formed on the surface of the lithium niobate substrate 1 so that surface acoustic waves are excited in the Z direction. An output electrode formed on the surface so that surface acoustic waves propagate in the Z direction. Reference numeral 5 denotes a proton exchange layer formed by replacing protons with lithium atoms on the surface layer of the output electrode 4 forming region of the lithium niobate substrate 1.

The electrodes 2, 3, and 4 are made of a conductive material such as aluminum, and are usually formed directly on the surface of the piezoelectric substrate 1 using a photolithography technique. Before the output electrode 4 is formed, the proton exchange layer 5 is a mask for preventing proton exchange in a region other than the region on the surface of the piezoelectric substrate 1 where proton exchange is desired, that is, a portion excluding the output electrode 4 formation region,
After selectively forming Au / Ti or the like, it was formed by immersion in a benzoic acid solution heated to 250 ° C. for about 10 hours, for example. By the proton exchange treatment under the above conditions, the velocity of the surface acoustic wave propagating in the proton-exchanged part is lower than the velocity of the untreated part.

In the surface acoustic wave convolver having such a configuration, when an electric signal having the carrier angular frequency ω is input to the comb-shaped input electrode 2, the surface acoustic wave is excited by the piezoelectric effect of the substrate. Similarly, when an electric signal having the carrier angular frequency ω is input to the comb-shaped input electrode 3, a surface acoustic wave is excited. These two surface acoustic waves propagate on the piezoelectric substrate 1 in opposite directions. Here, in the region where the output electrode 4 is formed, the velocity of the surface acoustic wave is lower than that of the non-proton-exchanged region on both sides of the output electrode 4 due to the proton exchange treatment, and the output electrode itself acts as a Δv / v waveguide. The surface acoustic waves are strongly confined in the output electrode 4. The Δv / v effect is to electrically short the substrate surface to lower the propagation speed of the surface acoustic wave than the free surface, and to confine the surface acoustic wave to the short-circuited portion. The two surface acoustic waves that collide on the output electrode 4 generate a convolution signal (carrier angular frequency 2) due to the nonlinear effect of the piezoelectric substrate.
ω) from the output electrode 4.

By adopting such a configuration, a substrate using only the Δv / v effect shown in the first conventional example and a substrate obtained by subjecting the entire surface of the piezoelectric substrate to proton exchange processing shown in the second conventional example can be used. Since the velocity difference between the surface acoustic wave between the output electrode 4 (waveguide) portion and the portion outside the output electrode 4 is increased, the effect of confining the surface acoustic wave in the waveguide is improved, and a large convolution efficiency is obtained. In addition, since the proton exchange treatment is performed only on the output electrode 4 formation region of the substrate, k ² is smaller than that of the surface acoustic wave convolver shown in the second conventional example in which the proton exchange treatment is performed over the entire piezoelectric substrate.
(Electro-mechanical coupling constant) and an increase in propagation loss of surface acoustic waves can be suppressed.

The effect of the present embodiment is obtained when the width of the output electrode is, for example, three or less wavelengths of the input surface acoustic wave such that a single-mode surface acoustic wave propagates on the output electrode (waveguide). Particularly effective, the effect can be obtained even in the case of a waveguide width such that a multi-mode surface acoustic wave propagates.

In the first embodiment, before the proton exchange treatment, titanium ions are implanted (pre-diffusion) by performing titanium diffusion treatment at least in the output electrode forming region of the lithium niobate substrate surface layer where proton exchange is performed. Alternatively, it is possible to suppress damage to the lithium niobate substrate due to the proton exchange treatment and to control the amount of change in the surface acoustic wave velocity.
In this embodiment, a regular ID is used as an input electrode.
Although an example using T has been described, an apodized or chirped IDT may be used.

[Second Embodiment] FIG. 3 is a schematic diagram showing a second embodiment of the surface acoustic wave convolver of the present invention. FIG.
FIG. 4 is a sectional view showing an AA ′ section in FIG. 3. 3 and 4, the same members as those in FIGS. 1 and 2 are denoted by the same reference numerals. In the figure, 1 is a Y-cut lithium niobate substrate, 2 and 3 are comb-shaped input electrodes formed on the surface of the lithium niobate substrate 1 so that surface acoustic waves are excited in the Z direction, and 4 is lithium niobate. An output electrode formed on the surface of the substrate 1 so that a surface acoustic wave propagates in the Z direction. Reference numeral 6 denotes a proton exchange layer formed by replacing protons with lithium atoms on the surface layer in a region outside the region where the output electrode 4 is formed on the lithium niobate substrate 1.

The electrodes 2, 3, and 4 are made of a conductive material such as aluminum, and are usually formed directly on the surface of the piezoelectric substrate 1 using a photolithography technique. Before the formation of the output electrode, the proton exchange layer 6 has a portion on the surface of the piezoelectric substrate 1 excluding a region where proton exchange is desired, that is, the output electrode 4.
Masks to prevent proton exchange in both outer regions of, for example, Au / T
After selectively forming i and the like, the film was formed by immersion in a benzoic acid solution containing lithium benzoate as a diluent, which was heated to 250 ° C., for about 10 hours. Under the above conditions, the velocity of the surface acoustic wave propagating through the proton-exchanged portion by the proton exchange treatment with the dilute solution is equal to the first speed.
Contrary to the example shown in the embodiment, the velocity becomes larger than the surface acoustic wave velocity of the untreated portion.

In the surface acoustic wave convolver having such a configuration, when an electric signal having the carrier angular frequency ω is input to the comb-shaped input electrode 2, the surface acoustic wave is excited by the piezoelectric effect of the substrate. Similarly, when an electric signal having the carrier angular frequency ω is input to the comb-shaped input electrode 3, a surface acoustic wave is excited. These two surface acoustic waves propagate on the piezoelectric substrate 1 in opposite directions. Here, both sides of the output electrode 4 forming region subjected to the proton exchange treatment are connected to the output electrodes 4 not subjected to the proton exchange.
Since the speed of the surface acoustic wave is higher than that of the formation region, the surface acoustic wave is confined in the output electrode formation region. The output electrode portion has a surface acoustic wave velocity smaller than that of the free surface due to the Δv / v effect due to an electric short circuit, but the increase in velocity due to proton exchange is sufficiently large. More strongly. Where Δv / v
The effect is that the surface of the substrate is electrically short-circuited so that the propagation speed of the surface acoustic wave is lower than that of the free surface, and the surface acoustic wave is confined in the short-circuited portion. The two surface acoustic waves that collide on the output electrode 4 are extracted from the output electrode 4 as a convolution signal (carrier angular frequency 2ω) due to the nonlinear effect of the piezoelectric substrate.

By adopting such a configuration, the substrate using only the Δv / v effect shown in the first conventional example and the substrate obtained by subjecting the entire surface of the piezoelectric substrate to proton exchange processing shown in the second conventional example are compared with those of the first conventional example. Since the velocity difference between the surface acoustic wave between the output electrode 4 (waveguide) portion and the portion outside the output electrode 4 is increased, the effect of confining the surface acoustic wave in the waveguide is improved, and a large convolution efficiency is obtained. Further, since the proton exchange treatment is performed only on the region outside the region where the output electrode 4 is formed on the substrate, the proton exchange treatment is performed as compared with the case where the proton exchange treatment is performed on the entire piezoelectric substrate such as the surface acoustic wave convolver shown in the second conventional example. A reduction in k ² (electro-mechanical coupling constant) and an increase in propagation loss of surface acoustic waves can be suppressed.

The effect of this embodiment is that the width of the output electrode is, for example, three or less wavelengths of the input surface acoustic wave such that a single-mode surface acoustic wave propagates on the output electrode (waveguide). Particularly effective, the effect can be obtained even in the case of a waveguide width such that a multi-mode surface acoustic wave propagates.
In this embodiment, a regular ID is used as an input electrode.
Although an example using T has been described, an apodized or chirped IDT may be used.

Third Embodiment FIG. 5 is a schematic view showing a third embodiment of the surface acoustic wave of the present invention. In this figure,
The same members as those in FIG. 1 are denoted by the same reference numerals, and detailed description will be omitted.

This embodiment has a surface acoustic wave focusing means such that the comb-shaped input electrode has an arc shape, and the width of the output electrode is the width of the output electrode through which the single mode surface acoustic wave propagates. In the surface acoustic wave convolver, the piezoelectric substrate 1
In this example, a proton exchange layer is formed on the surface layer of the output electrode 4 forming region.

By adopting such a structure, as shown in the figure, conventionally, depending on the expected angle of the arc-shaped input electrode, the surface acoustic wave excited from the arc-shaped electrode can output the output electrode (waveguide) at the entrance of the output electrode (waveguide). The surface acoustic wave that has leaked (leaved as it is) without entering the waveguide can be confined in the output electrode (waveguide). That is, since the incident angle of the surface acoustic wave confined in the output electrode (waveguide) can be made larger than in the past, the degree of freedom in designing the input electrode is improved.

In the present embodiment, an example in which the input electrode is arc-shaped has been described. However, the input electrode may have a shape having a plurality of curved surfaces, or may have any shape as long as the surface acoustic wave is focused on the entrance of the output electrode. Further, in the present embodiment, the arc-shaped input electrode is shown as the surface acoustic wave focusing means, but it is also possible to provide a focusing means such as a parabolic horn waveguide, a multistrip coupler between the input electrode and the output electrode. Similar effects can be obtained.

[Fourth Embodiment] FIG. 6 is a schematic diagram showing a fourth embodiment of the surface acoustic wave of the present invention. In this figure,
The same members as those in FIG. 1 are denoted by the same reference numerals, and detailed description will be omitted.

This embodiment has a surface acoustic wave focusing means such that the comb-shaped input electrode has an arc shape, and the width of the output electrode is the width of the output electrode through which a single-mode surface acoustic wave propagates. In the surface acoustic wave convolver, the piezoelectric substrate 1
This is an example in which a proton exchange layer is formed on the surface layer in a region outside the region where the output electrode 4 is formed.

With such a configuration, as shown in the figure, conventionally, depending on the expected angle of the arc-shaped input electrode, the surface acoustic wave excited from the arc-shaped electrode can output the output electrode (waveguide) at the entrance of the output electrode (waveguide). The surface acoustic wave that has leaked (leaved as it is) without entering the waveguide can be confined in the output electrode (waveguide). That is, since the incident angle of the surface acoustic wave confined in the output electrode (waveguide) can be made larger than in the past, the degree of freedom in designing the input electrode is improved.

In the present embodiment, an example in which the input electrode is arc-shaped has been described. However, the input electrode may have a shape having a plurality of curved surfaces, or may have any shape as long as the surface acoustic wave is focused on the entrance of the output electrode. Further, in the present embodiment, the arc-shaped input electrode is shown as the surface acoustic wave focusing means, but it is also possible to provide a focusing means such as a parabolic horn waveguide, a multistrip coupler between the input electrode and the output electrode. Similar effects can be obtained.

In this embodiment, an example is shown in which only the outside of the region where the output electrode is formed is subjected to the proton exchange treatment, but both the propagation path of the surface acoustic wave excited from the input electrode and the region where the output electrode 4 is formed. The proton exchange may be performed if it is a part other than the above.In particular, by performing proton exchange treatment on both sides of the propagation path of the surface acoustic wave excited from the input electrode,
The focusing effect of the surface acoustic wave can be further enhanced.

In the first to fourth embodiments described above,
Although an example using benzoic acid as a means for proton exchange has been described, a strong acid such as phosphoric acid may be used, or a solution may be diluted with a lithium salt such as lithium benzoate. Further, the proton exchange conditions (temperature, time, etc.) shown in the present embodiment are merely examples, and the present invention is not limited to these conditions. In addition, the first
In the second to second embodiments, the example of the lithium niobate substrate of the Y-cut plate has been described, but the 128-degree rotated Y-cut plate may be used. In the first to fourth embodiments, before the proton exchange treatment, titanium ions are implanted (pre-diffusion) by performing titanium diffusion treatment on at least the output electrode forming region of the lithium niobate substrate surface layer where proton exchange is performed. Alternatively, it is possible to suppress damage to the lithium niobate substrate due to the proton exchange treatment and to control the amount of change in the surface acoustic wave velocity.

Further, in the above-mentioned first to fourth embodiments, after the proton exchange treatment, the lithium niobate substrate may be subjected to an annealing treatment, so that the change in the surface acoustic wave velocity can be controlled. As the annealing treatment conditions, for example, 400 ° C. in an oxygen atmosphere is suitable, but other conditions may be used. Further, by making the comb-shaped input electrodes 2 and 3 in the first to fourth embodiments double electrodes (split electrodes), it is possible to suppress the reflection of surface acoustic waves at the input electrodes,
The characteristics of the element can be further improved.

[0041]

As described above, according to the present invention, at least two input electrodes for exciting the first and second surface acoustic waves on the piezoelectric substrate and the nonlinearity of the piezoelectric substrate are utilized. In a surface acoustic wave convolver having an output electrode for taking out a convolution signal of two surface acoustic wave signals, a surface layer of a lithium niobate substrate is subjected to proton exchange under specific conditions to produce a surface acoustic wave. By making use of the fact that the speed increases and decreases, a surface acoustic wave velocity difference is created between the output electrode forming area and the area outside the area, so that the surface acoustic wave can be confined in the output electrode (waveguide), and a large convolution output Can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a first embodiment of a surface acoustic wave convolver of the present invention.

FIG. 2 is a sectional view showing an AA ′ section of the embodiment of FIG. 1;

FIG. 3 is a schematic view showing a surface acoustic wave convolver according to a second embodiment of the present invention.

FIG. 4 is a sectional view showing an AA ′ section of the embodiment of FIG. 3;

FIG. 5 is a schematic view showing a third embodiment of the surface acoustic wave convolver of the present invention.

FIG. 6 is a schematic diagram showing a fourth embodiment of the surface acoustic wave convolver of the present invention.

FIG. 7 is a schematic diagram showing a first conventional example of a surface acoustic wave convolver.

FIG. 8 is a schematic diagram showing a second conventional example of a surface acoustic wave convolver.

[Explanation of symbols]

 Reference Signs List 1 piezoelectric substrate 2 comb-shaped input electrode 3 comb-shaped input electrode 4 output electrode 5 proton exchange layer

Continued on the front page (72) Inventor Tadashi Eguchi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Norihiro Mochizuki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) The inventor, Takahiro Honey, 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A-3-227110 (JP, A) JP-A-64-64382 (JP, A (58) Fields surveyed (Int.Cl. ⁷ , DB name) H03H 9/72 H03H 9/145 H03H 9/25

Claims (11)

(57) [Claims] At least one of a first surface of a lithium niobate substrate and at least two surfaces for exciting first and second surface acoustic waves are provided.
A surface acoustic wave convolver comprising: one input electrode; and an output electrode for extracting a convolution signal of the first and second surface acoustic waves by utilizing the nonlinearity of the substrate. By replacing at least a part or all of the lithium atoms in a part of the surface layer of the main surface with protons, the propagation speed of the surface acoustic wave in the region where the output electrode is formed is increased in the propagation direction of the surface acoustic wave. A surface acoustic wave convolver characterized in that the surface acoustic wave propagation velocity is smaller than the propagation speed of surface acoustic waves in the region on both sides of the attached output electrode. 2. The surface acoustic wave convolver according to claim 1, wherein at least a part or all of lithium atoms in a surface layer in a region where the output electrode is formed are replaced with protons. 3. The surface acoustic wave convolver according to claim 1, wherein at least a part or all of the lithium atoms in the surface layer on both sides of the output electrode are replaced with protons. 4. The surface acoustic wave convolver according to claim 1, wherein said lithium niobate substrate is a Y-cut plate. 5. The surface acoustic wave convolver according to claim 4, wherein the propagation direction of the surface acoustic wave is the Z direction. 6. The surface acoustic wave convolver according to claim 1, wherein the lithium niobate substrate is a 128-degree rotated Y-cut plate. 7. The surface acoustic wave convolver according to claim 6, wherein a propagation direction of the surface acoustic wave is an X direction. 8. The surface acoustic wave convolver according to claim 1, wherein the width of the output electrode is a width in which a multi-mode surface acoustic wave propagates. 9. The surface acoustic wave convolver according to claim 1, wherein the width of the output electrode is set to a width through which a single mode surface acoustic wave propagates. 10. The surface acoustic wave convolver according to claim 1, wherein the input electrode is formed in a shape that focuses the surface acoustic wave on an entrance end face of the output electrode. 11. The surface acoustic wave convolver according to claim 1, further comprising means for focusing a surface acoustic wave on an entrance end face of the output electrode, between the input electrode and the output electrode.