JP2985457B2 - Acousto-optic tunable wavelength 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 tunable wavelength filter, and more particularly to an acousto-optic tunable wavelength filter using a surface acoustic wave.

[0002]

2. Description of the Related Art A conventional acousto-optic tunable wavelength filter is shown in FIG.
As shown in FIG. 1, an optical waveguide 2 for guiding light on a dielectric substrate 1, an interdigital electrode 3 for exciting a surface acoustic wave, and an elasticity for confining a surface acoustic wave beam. And a surface wave waveguide 4.

The optical waveguide 2 is formed by Ti (titanium) diffusion or the like. In this optical waveguide, the light wave propagates as a TE mode or a TM mode. The TE mode is a mode in which the electric field component of light is in the horizontal direction with respect to the substrate 1, and the TM mode is a mode in which the component is in the vertical direction.

The surface acoustic wave excited in the interdigital electrode 3 propagates between the surface acoustic wave waveguides 4. However, if an anisotropic material (for example, LiNbO ₃ ) is used for the substrate, a well-known acousto-optic The effect occurs. That is, the refractive index of light changes due to the stress and strain generated by the surface acoustic wave. That is, by causing light and surface acoustic waves to propagate in parallel, a conversion action occurs from the TE mode to the TM mode (or vice versa).

Here, the speed of the surface acoustic wave is five orders of magnitude slower than the speed of the light, so that the amplitude of the surface acoustic wave appears to stop when viewed from the light. In this case, the acousto-optic effect is generated according to the period of the surface acoustic wave, so that the above-described mode conversion occurs between the TE mode and the TM mode of light satisfying a certain condition with respect to the wave number of the surface acoustic wave.

That is, since the TE mode light of a certain wavelength is converted into the TM mode only by the surface acoustic wave of a certain frequency f0, it is possible to change the wavelength of the mode-converted light by changing the frequency of the surface acoustic wave. You can. Therefore, various characteristics can be provided by changing the arrangement of the interdigital electrodes 3.

In this conventional acousto-optic tunable wavelength filter, a surface acoustic wave waveguide 4 is provided on the propagation path in order to avoid the diffraction loss of the surface acoustic wave beam, and the waveguide width G is the cross width W of the interdigital electrode. Was matched.

However, since the cross width W usually has several tens to several hundreds of wavelengths due to the problem of impedance, if it is made to match the waveguide width G as it is, the waveguide mode becomes a multi-mode.
Then, due to the influence of the higher-order mode, the wavelength filter characteristic has a drawback that a large spurious (Sprious) occurs in a range higher than the center wavelength λ0 as shown in FIG.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks, and an object of the present invention is to improve the filter characteristics.

[0010]

An acousto-optic tunable wavelength filter according to the present invention comprises: an optical waveguide provided on a dielectric substrate for transmitting light;
A cross finger-like electrode that generates a surface acoustic wave that converts the mode of the transmitted light, and a surface acoustic wave waveguide that has a width larger than the maximum cross width of the finger-like electrode and transmits the surface acoustic wave. It is characterized by including.

[0011]

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

FIG. 1 is a plan view showing a schematic configuration of one embodiment of an acousto-optic tunable wavelength filter according to the present invention, and the same parts as those in FIG. 2 are indicated by the same reference numerals. In the figure,
The acousto-optic tunable wavelength filter of the present embodiment differs from that of FIG. 2 in that the width G of the surface acoustic wave waveguide 4 is wider than the maximum cross width W of the interdigital electrodes 3.

As described above, the width G of the surface acoustic wave waveguide 4
Is larger than the maximum cross width W of the interdigital electrode 3, the intensity of each of the multimodes propagating in the waveguide can be controlled.

FIG. 5 shows the dependence of the coupling efficiencies of the zero-order and second-order modes, which most affect the filter characteristics, on the ratio (W / G) between the maximum cross width W and the waveguide width G. It is shown. In the figure, even if the ratio (W / G) is changed from 100 to 50%, the efficiency 5 in the 0th-order mode does not change much, whereas the efficiency 6 in the second-order mode changes greatly.
%, The efficiency becomes minimum.

Therefore, if a condition near this is selected, the secondary mode is hardly propagated, and only the zero-order mode is propagated. As a result, it is possible to reduce the deterioration of the wavelength filter characteristic due to the higher-order mode, which is a problem in the conventional filter.

That is, the optical waveguide 2, the interdigital electrode 3, and the surface acoustic wave waveguide 4 are formed on the dielectric substrate 1, and the width of the surface acoustic wave waveguide 4 is set larger than the maximum intersecting width of the interdigital electrode 3. Also to be wide. In particular, if the dimensional ratio (cross width W / waveguide width G) is selected to be about 70%, as described above, the second mode has a low coupling efficiency and becomes difficult to propagate.

FIG. 3 shows the wavelength filter characteristics when 70% is selected. In the characteristic of FIG. 3, the side lobe level in a high frequency band is reduced as compared with the conventional filter characteristic shown in FIG. 4, and therefore, according to the present invention, the performance of the filter can be improved.

[0018]

As described above, the present invention has an effect that the filter characteristics can be improved by setting the width of the surface acoustic wave waveguide to be larger than the maximum cross width of the interdigital electrodes.

[Brief description of the drawings]

FIG. 1 is a plan view showing a schematic configuration of an acousto-optic tunable wavelength filter according to an embodiment of the present invention.

FIG. 2 is a plan view showing a schematic configuration of a conventional acousto-optic tunable wavelength filter.

FIG. 3 is a wavelength filter characteristic diagram of the filter of FIG. 1;

FIG. 4 is a wavelength filter characteristic diagram of the filter of FIG. 2;

FIG. 5 is a characteristic diagram showing the dependence of the waveguide mode coupling efficiency on the ratio of the crossover width to the waveguide width.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Dielectric substrate 2 Optical waveguide 3 Interdigital electrode 4 Surface acoustic wave waveguide

──────────────────────────────────────────────────続 き Continued on the front page (56) References Electronics Letters, Vol. 25, No. 23 pp. 1583-1584 (9th November 1989) IEEE 1990 Ultrasonics Symposium Proceedings Vol. 2 pp. 605 -608 (58) Fields surveyed (Int. Cl. ⁶ , DB name) G02F 1/11-1/125 G02F 1/33-1/335 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. An optical waveguide provided on a dielectric substrate for transmitting light, an interdigitated electrode for generating a surface acoustic wave for converting a mode of the transmitted light, and a maximum intersecting width of the interdigital electrode. An acousto-optic tunable wavelength filter comprising: a surface acoustic wave waveguide having a larger width and transmitting the surface acoustic wave.