KR19990059954A - Acousto-optic filter having a low side lobe and a method of manufacturing the same - Google Patents

Abstract

The acoustooptic filter according to the present invention includes a substrate formed of lithium niobate (LiNbO ₃ ), an optical waveguide formed by depositing and diffusing a titanium (Ti) film on a substrate, and formed in a strip shape in a central portion of the substrate, SAW generator formed of gold or aluminum in the center portion and generating surface acoustic wave (SAW), SAW attenuation layer formed in two areas separated from the optical waveguide, and transmitted while attenuating the SAW, and the amount of optical waveguide It is formed at the end and includes a SAW absorbing layer to prevent the SAW from being reflected. Here, the optical waveguide is formed in a strip shape near the center of the substrate, and the SAW attenuation layer is formed by depositing with antimony nitride (InSb).

Description

Acousto-optic filter having a low side lobe and a method of manufacturing the same

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an acoustic-optic filter used as a filter or switch in an optical communication system of wavelength division multiplexing (WDM), and more particularly, to an acoustic optical filter having a low side lobe. It is about.

In general, an acoustooptic filter consists of an optical waveguide formed on a substrate and a SAW transducer that generates a surface acoustic wave (hereinafter referred to as SAW).

Such an acoustooptic filter selects one wavelength from light having multiple wavelengths in the following manner.

Multiwavelength light having the same polarization state, for example TE mode, is incident on one end of the optical waveguide, and the light having this multiwavelength interacts with the SAW generated by the SAW converter. By this interaction, only one wavelength of light having multiple wavelengths changes its polarization state (e.g., TM mode) and only the light which has changed this polarization state is output to the outside of the acoustooptic filter. That is, by interacting with SAW of a suitable frequency to light having a plurality of wavelengths incident on the optical waveguide, a specific wavelength can be selected from the plurality of incident wavelengths.

1 is a plan view of a conventional acoustooptic filter.

As shown in FIG. 1, the conventional acoustooptic filter includes a SAW barrier layer 10, an optical waveguide 20, a SAW waveguide 30, a SAW IDT (surface acoustic wave inter digital transducer) electrode 40 and a SAW absorbing layer. It consists of 50.

In FIG. 1, a single optical waveguide 20 is formed in the center of a lithium niobate (LiNbO ₃ ) substrate (not shown), and a SAW waveguide 30 for guiding SAW is also formed in the substrate.

The SAW IDT electrode 40 is formed at one end of the optical waveguide to generate SAW, and the SAW barrier layer 10 is formed on both sides of the optical waveguide so that the SAW is transmitted without attenuation along the SAW waveguide 30. . The SAW absorbing layer 50 is formed at the input / output ends of the optical waveguide 20 and the SAW waveguide 30 to prevent the SAW from being reflected.

The acoustooptic filter shown in Fig. 1 is formed by the following manufacturing method.

A titanium (Ti) film for forming the SAW barrier layer 10 is deposited on the upper and lower portions of the substrate on a substrate made of LiNbO ₃ , and the Ti film is patterned using a mask or the like to form a strip pattern. The Ti film is then heated to 1050 ° C. for about 16 hours to be diffused.

Next, a Ti film for forming the optical waveguide 20 is deposited, patterned into a strip shape in the center of the substrate, and then diffused for 8 hours. At this time, not only the Ti film for the optical waveguide but also the SAW barrier layer 10 heated for 16 hours are simultaneously diffused. Therefore, the Ti film for the SAW barrier layer is diffused for about 24 hours in total, and the Ti film for the optical waveguide is diffused for about 8 hours.

Next, gold or aluminum is deposited on one end of the optical waveguide 20 and patterned to form a SAW IDT electrode 40, and SAW absorbing layers 50 are formed on both sides of the substrate using a rubber adhesive or the like.

As described above, in the manufacturing process of the acoustooptic filter of FIG. 1, the Ti diffusion time is usually about 24 hours in the SAW barrier layer (SAW waveguide) fabrication process formed through the internal diffusion of Ti, and the optical waveguide and the SAW barrier layer ( SAW waveguide) is a very complicated process.

In FIG. 1, the SAW affects the optical waveguide to a constant size without attenuation in the acoustooptic filter, and has little effect since it is absorbed by the SAW absorbing layer 50 outside the filter.

FIG. 2 is a diagram showing a function (transmission function) of the response distribution of the SAW traveling in the z direction on the optical waveguide in the acoustooptic filter of FIG.

In Fig. 2, the horizontal axis normalizes the advancing direction of the SAW (z direction), and the length L of the filter (the distance from the SAW IDT electrode 40 to the SAW absorption layer 50) is 1, based on this L. The remaining lengths are shown. The vertical axis represents the response distribution value q (z) with respect to the normalized z value, and q (z) represents the maximum value of the response of the SAW to the optical waveguide as 1, and the relative value thereof.

As shown in Fig. 2, since the SAW is transmitted without attenuation in the acoustooptic filter, the distribution function q (z) becomes 1 within the z value of 1, and 0 elsewhere.

FIG. 3 is a diagram illustrating a frequency response to the response distribution function of FIG. 2.

In Fig. 3, the horizontal axis represents the frequency f, and the vertical axis represents the magnitude of the frequency response to the frequency f. In this case, the magnitude of the frequency response is defined as the size of the main lobe as 0 decibels (dB), and the remaining values are expressed as relative values with respect to the main lobe.

As shown in Fig. 3, since the value of the side lobe is large, the difference between the main lobe and the maximum size of the adjacent side lobes is only about 15 dB. As described above, when the side lobe value is increased, signal interference between wavelengths occurs when broadband light having a plurality of wavelengths is incident, resulting in noise between channels.

The present invention is to solve the above problems, to shorten the manufacturing time of the acoustooptic filter and to simplify the manufacturing method, and also to reduce the size of the side lobe value to reduce the inter-channel noise due to signal interference of adjacent wavelengths. Is to reduce.

1 is a plan view of a conventional acoustooptic filter.

2 is a diagram showing a function of the response distribution of surface acoustic wave (SAW) on an optical waveguide.

3 shows a frequency response to the function of FIG.

4 is a plan view of an acoustooptic filter according to an embodiment of the present invention.

5 is a diagram showing a function of a response distribution of surface acoustic waves on an optical waveguide.

FIG. 6 shows a frequency response to the function of FIG.

Acousto-optic filter according to the present invention to achieve this object is

A substrate, an optical waveguide formed in a portion of the substrate and guiding light having a plurality of wavelengths, a SAW generator formed in a central portion of the optical waveguide and generating surface acoustic wave (SAW), and two separated around the optical waveguide SAW attenuation layers each formed in the region and allowing SAWs to be attenuated and transmitted, and SAW absorbing layers formed at both ends of the optical waveguide and preventing SAWs from being reflected.

Here, the substrate is formed of lithium niobate (LiNbO ₃ ), and the optical waveguide is formed by depositing and diffusing a titanium (Ti) film on the substrate. The SAW generator is also formed of gold or aluminum and the SAW absorber layer is formed of rubber adhesive, wax or cellophane tape.

At this time, the optical waveguide is preferably formed in a strip shape near the center of the substrate.

In addition, the SAW attenuation layer is preferably formed by depositing with antimony nitride (InSb).

On the other hand, the acoustooptic filter manufacturing method according to the present invention

Depositing and patterning a first metal film on a substrate; diffusing the patterned first metal film to form an optical waveguide; depositing and patterning a second metal film on the optical waveguide to form a surface acoustic wave (SAW) near the center of the optical waveguide. ) Forming a generator, depositing and patterning a third metal film on the substrate to form a SAW attenuation layer in each of two regions separated from the optical waveguide, and forming SAW absorbing layers at both ends of the optical waveguide. It includes.

Here, the substrate uses lithium niobate (LiNbO ₃ ), and the SAW absorbing layer is formed of a rubber adhesive, wax or cellophane tape. In addition, the first metal film is made of titanium, and the second metal is made of gold or aluminum. In this case, the third metal film is preferably formed of antimony nitride (InSb).

Hereinafter, with reference to the drawings will be described an embodiment of the present invention;

4 is a plan view of an acoustooptic filter according to an embodiment of the present invention.

As shown in Fig. 4, the acoustooptic filter of the present invention comprises a substrate 100, an optical waveguide 200, a SAW attenuation layer 300, a SAW IDT electrode 400 and a SAW absorption layer 500.

In FIG. 4, a single optical waveguide 200 is formed in a strip shape in the center of the substrate 100 formed of LiNbO _3, and a SAW IDT electrode 400 is formed near the center of the optical waveguide 200. Unlike in FIG. 1, the SAW IDT electrode 400 delivers SAW in both directions.

SAW attenuation layer 300 is formed on both sides of the optical waveguide so that the propagation of SAW is monotonically reduced in both directions to be transmitted. The SAW absorbing layer 500 is formed at the input / output terminal of the optical waveguide 200 to prevent the SAW from being reflected.

The acoustooptic filter shown in Fig. 4 is formed by the following manufacturing method.

First, a titanium (Ti) film is deposited on a substrate 100 made of LiNbO ₃ , and the Ti film is patterned by using a mask to form a strip-shaped pattern. The Ti film is then diffused at 1050 ° C. for about 8 hours to form the optical waveguide 200. .

Next, antimony nitride (InSb) is deposited and patterned using a mask to form a strip-shaped SAW attenuation layer 300. At this time, the SAW attenuation layer 300 is formed in each of the two areas separated from the optical waveguide 200 as a center.

Subsequently, after depositing with gold or aluminum, the SAW IDT electrode 400 is formed near the center of the optical waveguide, and the SAW absorption layer 300 is formed using a rubber adhesive, wax, cellophane tape, or the like.

As described above, according to the exemplary embodiment of the present invention, instead of the SAW barrier layer requiring the diffusion process of 24 hours or more, the SAW attenuation layer is formed by the deposition and the patterning using the mask without the diffusion process. It is advantageous in that manufacturing is simplified.

The operation of the acoustooptic filter of the present invention shown in Fig. 4 is as follows.

Light having a wide wavelength having a plurality of wavelengths is input to the input end of the optical waveguide 200 in the TE mode (horizontal polarization). That is, broadband light having a plurality of wavelengths is incident on a polarizing plate (not shown) having a polarization direction in the TE mode, and light passing through the polarizing plate is input to an input terminal of the optical waveguide 200.

At this time, when a radio frequency (RF) signal having a specific frequency is applied to the SAW IDT electrode 400, SAW is generated before and after the SAW IDT electrode 400 to propagate in both directions to change the refractive index of the optical waveguide. In this case, SAW does not occur directly below the SAW electrode, but a slight change in refractive index occurs due to the electro-optic effect.

Changes in refractive index due to SAW and electro-optic effects have the effect of grating the refractive index over the entire length, so that only one wavelength of the incident TE mode light corresponding to a particular RF goes into TM mode (vertical polarization). Will change.

That is, according to the change in refractive index caused by SAW or the like, only wavelengths that meet the following phase matching conditions among the incident light of the TE mode change to the polarization state of the TM mode in response to the SAW, and light of the other wavelengths maintains the TE polarization state.

Where λ _o is the wavelength of incident light, n _o is the normal refractive index of the optical waveguide, n _e is the ideal refractive index of the optical waveguide, λ _s is the wavelength of the SAW, v _s is the phase velocity of the SAW, and f _s is the frequency of the SAW, respectively. Indicates. As can be seen from the above equation, by appropriately selecting the frequency f _s of the SAW, the wavelength λ _o corresponding to the matching condition can be selected.

Therefore, when a polarizing plate (not shown) having a polarization direction of TM is provided at the output end of the optical waveguide, only a wavelength satisfying the matching condition can be output through the output end of the optical waveguide.

FIG. 5 is a diagram showing a function of a response distribution of an SAW traveling in a z direction on an optical waveguide in the acoustooptic filter of FIG. 4.

In FIG. 5, the horizontal axis normalizes the advancing direction (z direction) of the SAW. The distance D from the SAW IDT electrode 400 to the SAW absorbing layer 500 is 1, and the remaining values are represented based on this D. will be. The vertical axis represents the response distribution value q (z) with respect to the normalized z value, where q (z) is the maximum value of the response of the SAW to the optical waveguide, and is represented by a relative value thereof.

In Fig. 5, the portion having a value of 1 on the horizontal axis represents the SAW IDT electrode 400, and the portion having a value of 0 and 2 on the horizontal axis represents the SAW absorbing layer 500 on the left and right sides, respectively. In addition, in FIG. 5, a represents a response value of the optical waveguide by SAW, and b represents a response value of the optical waveguide due to the electro-optic effect generated in the SAW IDT electrode.

In Fig. 4, when SAW is transmitted in both directions from the SAW IDT 400, the size of the SAW is reduced because the SAW is transmitted through the SAW attenuation layer. Therefore, the response value of the optical waveguide by the SAW decreases as it moves away from the SAW IDT, and thus appears as shown in a of FIG. 5.

FIG. 6 is a diagram illustrating a frequency response to the response distribution function of FIG. 5. FIG.

In Fig. 6, the horizontal axis represents the frequency f, and the vertical axis represents the magnitude of the frequency response to the frequency f. In this case, the magnitude of the frequency response is defined as 0 decibel (dB) of the main lobe, and the remaining values are expressed as relative values to the main lobe.

As shown in Fig. 6, the side lobe according to the present invention is smaller than the conventional acoustooptic filter shown in Fig. 3, and the difference between the main lobe and the maximum size of the adjacent side lobes is about 25 dB. Becomes Therefore, since the side lobe value is relatively smaller than that of the conventional acoustooptic filter as shown in FIG. 3, when the broadband light having a plurality of wavelengths is incident, the signal interference is less affected and the noise between the channels is reduced. There is this.

As described above, according to the present invention, the manufacturing time of the acoustooptic filter can be shortened and the manufacturing method can be simplified, and the size of the side lobe value can be reduced to reduce the inter-channel noise due to signal interference of adjacent wavelengths. have.

Claims (9)

Substrate, An optical waveguide formed in a portion of the substrate and guiding light having a plurality of wavelengths; A SAW generator formed at a central portion of the optical waveguide and generating surface acoustic wave (SAW); A SAW attenuation layer formed in each of two areas separated from the optical waveguide, and transmitting the SAW while being attenuated; An acoustooptic filter formed at both ends of the optical waveguide and including an SAW absorbing layer for preventing the SAW from being reflected. In claim 1, The optical waveguide is formed in a strip shape near the center of the substrate. In claim 1, The substrate is formed of lithium niobate (LiNbO ₃ ), The optical waveguide is formed by depositing and diffusing a titanium (Ti) film on the substrate. In claim 3, The SAW generator is formed of gold or aluminum, The SAW absorbent layer is formed of a rubber adhesive, wax or cellophane tape. The method according to any one of claims 1 to 4, The SAW attenuation layer is an acoustooptic filter formed of antimony nitride (InSb). Depositing a first metal film on a substrate, and patterning the first metal film; Diffusing the patterned first metal film to form an optical waveguide; Depositing and patterning a second metal film on the optical waveguide to form a surface acoustic wave (SAW) generator near the center of the optical waveguide; Depositing and patterning a third metal film on the substrate to form SAW attenuation layers in two regions separated from the optical waveguide; Forming SAW absorbing layers at both ends of the optical waveguide. In claim 6, And the optical waveguide is formed in a strip shape near the center of the substrate. In claim 6, The substrate is formed of lithium niobate (LiNbO ₃ ), The SAW absorbent layer is formed of rubber adhesive, wax or cellophane tape, The first metal film is made of titanium, And the second metal film is made of gold or aluminum. The method according to any one of claims 6 to 8, The third metal film is a method for manufacturing an acoustooptic filter made of antimony nitride (InSb).