JP2937799B2 - Acousto-optic filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acousto-optic filter, and more particularly to an improvement in an optical wavelength filter utilizing an acousto-optic effect by collinear coupling.

[0002]

2. Description of the Related Art An optical wavelength filter utilizing this type of acousto-optic effect has features such as high-speed operation, a large tuning variable width, and simultaneous selection of a plurality of wavelengths. This type of optical wavelength filter transmits a single linearly polarized light formed on a predetermined substrate and excited at an input end of the optical waveguide through the optical waveguide by a surface acoustic wave generated by an excitation electrode loaded on the optical waveguide. This is an acousto-optic filter that converts only a specific wavelength component of guided light to linearly polarized light orthogonal to the above-mentioned polarized light.

FIG. 5 shows an example of such an acousto-optic filter, which is disclosed in Electronics Letter Vol.
98-399 pages (ELECTRONICS LETTE
RSVol. 25, no. 6, pp398-399,1
989).

The filter is formed on a lithium niobate substrate 41, and a channel type optical waveguide 42 is formed on the substrate 41. Surface acoustic wave excitation electrodes 43a and 43b loaded on the optical waveguide 42 are formed. Reference numerals 45a to 45d denote surface acoustic wave absorbers, and 46 at the center of the substrate is a photometric element.

An interaction including an optical waveguide (titanium diffusion optical waveguide) 42 on the surface of the substrate 41 is caused by the surface acoustic waves excited by the surface acoustic wave excitation electrodes 43a and 43b loaded on the lithium niobate substrate 41. Region 44
The periodic changes in the refractive index occur at a and 44b.

[0006] Of the single linearly polarized light excited at the input end of the optical transmission line 42, for example, linearly polarized light having an electric field component horizontal to the substrate 41 (hereinafter referred to as TE polarized light), it is within the interaction regions 44a and 44b. Only TE polarized light of a specific light wavelength that satisfies the phase matching condition due to the periodic change in the refractive index is converted into linearly polarized light (hereinafter referred to as TM polarized light) having an electric field component perpendicular to the substrate.

The TM-polarized light is transmitted by the TM-polarized light analyzing element 46 formed of a Z-cut lithium niobate wafer piece provided near the center of the element, and the TE-polarized light is emitted to select a specific wavelength. .

The second surface acoustic wave excitation electrode 43b
As a result, the TM-polarized light having the specific light wavelength is converted into the TE-polarized light again. The two-stage polarization conversion makes it possible to reduce the side lobe in the filter characteristics.

FIG. 6 shows the configuration of another conventional acousto-optic filter. Journal of Light Wave Technology, Vol. 12, No. 6, pp. 977 to 982 (JOURNAL
OFLIGHTWAVE TECHNOLOGY Vo
l. 12, No. 6, pp977 to 982, 1994)
It has been disclosed.

The surface acoustic wave excited by the surface acoustic wave exciting electrode 53 loaded on the lithium niobate substrate 51 has the titanium diffusion regions 57a, 57b, 57c formed on the surface of the lithium niobate substrate 51 as cladding. The light propagates along the surface acoustic wave waveguide.

The surface acoustic wave propagates between two adjacent surface acoustic wave waveguides in a so-called power flow, so that the surface acoustic wave starts at the surface acoustic wave absorber 55a on the optical waveguide 52b and ends at the surface acoustic wave absorber 55b. Interacting area 5
4, an intensity distribution of the surface acoustic wave is generated. The conversion of the specific light wavelength from TE polarized light to TM polarized light in the interaction region 54 is determined by the distribution of the coupling coefficient between the light wave and the surface acoustic wave, and the coupling coefficient corresponds to the intensity distribution of the surface acoustic wave. I have.

When the coupling coefficient is uniform over the interaction region, the upper limit of the side lobe level in the filter characteristic is limited to -9 dB. However, by weighting the coupling coefficient, a further lower side lobe can be obtained. Can be achieved.

The acousto-optic filter shown in FIG. 6 has a structure in which two surface acoustic wave waveguides are adjacent to each other, thereby realizing a weighted coupling between the surface acoustic wave and the light wave, and a low side lobe. It is trying to make it.

[0014]

As described above, an acousto-optic filter utilizes the interaction between a surface acoustic wave and a light wave, and in a conventional structure in which the coupling coefficient between the surface acoustic wave and the light wave is uniform, The side lobe level in the filter characteristics is logically -9 dB, and it is not possible to further reduce the side lobe. To further reduce the side lobe,
In the acousto-optic filter of FIG. 5, two TE / TM polarization conversion elements are connected in tandem, but there are problems such as an increase in the size of the elements and an increase in power consumption.

In the acousto-optic filter of FIG. 6, the side lobe is reduced by weighting the coupling coefficient between the surface acoustic wave and the light wave. However, since two surface acoustic wave waveguides need to be newly provided, There are problems such as an increase in the size of the element and a complicated manufacturing process.

An object of the present invention is to provide an acousto-optic filter capable of achieving a low side lobe with a simple configuration without causing an increase in size, an increase in power consumption, and the like.

[0017]

According to the present invention, an elastic table is provided.
A single linearly polarized light formed on a substrate having a plane wave polarization and excited at the input end of the optical waveguide is generated by an excitation electrode loaded on the optical waveguide and propagated on the substrate. An acousto-optic filter that converts only a specific wavelength component of the guided light propagating through the optical waveguide into a linearly polarized light orthogonal to the polarized light, wherein the excitation electrode is orthogonal to a direction in which the optical waveguide is formed. An acousto-optic filter is obtained which is formed to be inclined at a predetermined angle with respect to the direction, and wherein the optical waveguide is formed so as to be shifted from the center position of the opening width of the excitation electrode.

[0018]

According to the present invention, a surface acoustic wave propagating in a direction deviated from the propagation direction of a light wave is excited by a surface acoustic wave excitation electrode formed to be inclined from a substrate crystal axis, and the surface acoustic wave is propagated in the propagation direction of the light wave. Is generated. By adjusting the distribution of the coupling coefficient between the surface acoustic wave and the light wave based on the intensity distribution by shifting the position of the optical waveguide from the center position of the opening of the surface acoustic wave excitation electrode, the surface acoustic wave is designed to reduce the side lobe. And bring the weighted combination of light waves.

[0019]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a plan view for explaining an embodiment of the present invention. An example in which an optical waveguide 12 is formed by a titanium diffusion method using a lithium niobate substrate 11 will be described. X
A titanium stripe having a width of 6 to 10 μm and a thickness of 500 to 1500 ° is formed on a lithium niobate substrate 11 having a cut Y-axis propagation direction, and thermally diffused at 950 to 1100 ° C. to provide a single mode titanium diffused optical waveguide. 12 is manufactured.

The interaction region 14 between the surface acoustic wave and the guided light starts from the surface acoustic wave excitation electrode 13 and ends at the surface acoustic wave absorber 15b.

As the surface acoustic wave excitation electrode 13, for example,
Using a comb-like shape, the electrode finger cycle is 10-1
Fabrication is performed at a cycle that satisfies the exchange condition between TE polarized light and TM polarized light at a central light wavelength of 5 μm. For example, when the light wavelength is 1.55 μm, the electrode finger cycle takes a value near 21 μm.

The surface acoustic wave excitation electrode 13 has a Z axis (a direction in which the optical waveguide 12 is formed: a direction orthogonal to the Y direction on the substrate surface).
It is formed so as to be shifted by a predetermined angle θ with respect to the direction. A surface acoustic wave is excited from the opening of the excitation electrode 13 and, as shown by a broken line in FIG.
5b. The surface acoustic wave in the opposite direction (−Y axis direction) from the opening of the excitation electrode 13 is absorbed by the surface acoustic wave absorber 15a.

The optical waveguide 12 is formed so as to be shifted from the center position of the opening width of the surface acoustic wave excitation electrode 13 by Δz.

FIG. 2 is a characteristic diagram for explaining an uneven coupling coefficient distribution generated in the acousto-optic filter of the present invention. On the lithium niobate substrate having the X-cut Y-axis propagation direction, the surface acoustic wave is deflected (the direction of the wavefront normal and the direction of energy propagation do not match), and the surface acoustic wave and the propagation direction of the guided light are different from each other. Non-uniformity (weighting) occurs in the coupling coefficient of the light wave. The uneven coupling coefficient between the surface acoustic wave and the light wave changes the side lobe level and the full width at half maximum in the filter characteristics. The distribution of the coupling coefficient depends on the inclination angle (θ) of the surface acoustic wave excitation electrode 13 and also on the relative position (△ z) of the optical waveguide 12 with respect to the opening direction of the surface acoustic wave excitation electrode 13. Δz is set at a position within the electrode opening width, and θ is set between 0 and −10 ° for the + X cut lithium niobate substrate, and between 0 and + 10 ° for the −X cut substrate.

FIG. 3 is a region diagram showing an example of the condition ranges of θ and Δz for realizing a low side lobe level of −9 dB or less in the present invention. The electrode opening width is 75 ° (Λ is the wavelength of the surface acoustic wave), and the length of the interaction region 14 is 1000
This is the case of Λ. By setting the position of the optical waveguide in a region forming a node in the intensity profile of the surface acoustic wave in the electrode opening direction, weighted coupling between the surface acoustic wave and the light wave for reducing the side lobe is realized. In this embodiment, the optical waveguide is formed at a position of -40 to -25 from the center of the electrode opening.
And -20 to -4 and +10 to +20.

Here, the acousto-optic crystal substrate 1 shown in FIG.
The acoustic distribution of the surface acoustic wave on the YZ plane 1 is devised with reference to FIG. The acoustic distribution (amplitude and phase) on the y1 plane at the position of the surface acoustic wave excitation electrode 13 is represented by U1
(Y1, z1), the sound wave distribution U2 (y2, z2) at an arbitrary point (y2, z2) in the interaction area 14 in FIG. 1 is expressed as follows: U2 (y2, z2) = j / Λ∫∫U1 (y1 , ^Z1 ) · (e− ^jkr / r) dy1 · dz1 (1)

Here, r is the distance from the y1 plane to the y2 plane (see FIG. 4), and k is the wave number vector.

By solving the above equation (1) for an arbitrary point (y2, z2) in the interaction area 14, the acoustic wave distribution of the surface acoustic wave can be obtained. The above-described coupling coefficient is proportional to the power of the surface acoustic wave, and can be obtained from the sound wave distribution. Therefore, the distribution of the coupling coefficient can be calculated from the above equation (1). Therefore, the filter characteristic of FIG. 1 can be calculated based on the distribution of the coupling coefficient.

FIG. 3 is a region diagram showing an example of the condition ranges of θ and Δz for realizing the low side lobe shown in FIG.

[0031]

As described above, in the present invention,
The surface acoustic wave propagating in a direction deviated from the propagation direction of the light wave excited by the inclined surface acoustic wave excitation electrode generates an intensity distribution of the surface acoustic wave in the propagation direction of the light wave, and the surface acoustic wave by the intensity distribution By adjusting the distribution of the coupling coefficient between the surface acoustic wave and the center of the aperture of the surface acoustic wave excitation electrode by adjusting the distribution of the coupling coefficient between the surface acoustic wave and the light wave, weighted coupling between the surface acoustic wave and the light wave is realized, and a low sidelobe level of -9 dB or less is achieved. ing.

For this reason, it is possible to realize a low side lobe level with a single element without using a structure in which a TE / TM polarization conversion element is connected in two stages, and to make a special fabrication such as providing a surface acoustic wave waveguide. The side lobes can be easily reduced by a manufacturing procedure similar to the conventional one without adding a process.

As described above, in an optical integrated circuit that requires an optical wavelength selection function, it can be said that the effect of supplying a small-size, low-power acousto-optic filter with good S / N is extremely large.

[Brief description of the drawings]

FIG. 1 is a plan view showing an embodiment of an acousto-optic filter of the present invention.

FIG. 2 is a characteristic diagram for explaining a non-uniform distribution of a coupling coefficient generated in the acousto-optic filter of the present invention.

FIG. 3 is a region diagram showing an example of a conditional range of θ and Δz for realizing a low side lobe level of −9 dB or less in the present invention.

FIG. 4 is a diagram for explaining a sound wave distribution of a surface acoustic wave in the acousto-optic filter of the present invention.

FIG. 5 is a perspective view illustrating an example of a conventional technique.

FIG. 6 is a perspective view for explaining another example of the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Lithium niobate 12 Optical waveguide 13 Surface acoustic wave excitation electrode 14 Interaction area 15a, 15b Surface acoustic wave absorber

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References Proceedings of the 42nd Joint Lecture on Applied Physics, Spring 1995 3 (issued March 28, 1995) p. 1046 Toru Hosoi, Yutaka Nishimoto, "28a-ZF-8 LiNbO (3) Examination on transmission characteristics of waveguide type acousto-optic filter-Examination of side lobe level ((II))-" 1991 (1991) ) IEICE Autumn Meeting Lecture Paper Volume 4, Communication and Electronics (Issued August 15, 1991) p. 4-225 Nobuo Goto, Yasumitsu Miyazaki "Improvement of Filter Characteristics by Inclined SAW Waveguide in C-195 Collinear Acoustooptic Device"

Claims (1)

(57) [Claims] 1. A single linearly polarized light formed on a substrate having a surface acoustic wave bias and excited at an input end of an optical waveguide is generated by an excitation electrode loaded on the optical waveguide and propagates on the substrate. An acousto-optic filter that converts only a specific wavelength component of the guided light propagating through the optical waveguide into linearly polarized light orthogonal to the polarized light by the surface acoustic wave, wherein the excitation electrode is orthogonal to the direction in which the optical waveguide is formed. And the optical waveguide is formed so as to be shifted from the center position of the opening width of the excitation electrode, and the crystal surface in which the surface acoustic wave is deflected is formed.
Acousto-optic filter in which the optical waveguide is formed in
In the filter, the deviation amount from the center position is within the opening width of the excitation electrode.
And the opening width ratio: -0.53 to -0.33,-
0.27 to -0.053, +0.13 to +0.27
The predetermined substrate is an X-cut lithium niobate substrate,
The predetermined angle is in a range larger than 0 ° and smaller than ± 5 °.
Acoustooptic filter, characterized in that it is set to.