JPH0346603A - Waveguide type optical demultiplexer and its manufacture - Google Patents

Abstract

PURPOSE:To obtain stability to temperature and to facilitate the manufacture by providing an optical directional coupler consisting of two optical waveguides which has a specific difference in equivalent refractive index and a grating pattern which is installed nearby at least one optical waveguide and generates periodic equivalent bending rate variation in a light transmission direction. CONSTITUTION:The optical waveguides 22 whose equivalent refractive indexes n1 and n2 are shown by an in equality I are put close to each other to constitute the optical directional coupler 24. Further, the grating pattern 25 whose is period LAMBDA is installed for matching and the optical directional coupler 24 couples only light with wavelength lambdai satisftying an equation II with the other optical waveguide 22. Namely, separated wavelength components are outputted in the same direction with incident light 26 (provided that the light is outputted to the optical waveguide 22 which is separated spatially). Consequently, the constitution is simple, the device is stable to temperature, and the manufacture is easy.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical demultiplexer that selectively separates specific wavelengths of light waves, and in particular to a waveguide optical demultiplexer that uses an optical waveguide installed on a substrate. This article relates to corrugated equipment and its manufacturing method.

(Conventional technology) Optical communication technology is used for subscriber systems, data transmission between computers,
Their introduction into various fields, such as intelligent in-building LANs, is progressing, and the demand for higher capacity and multiple channels is increasing. Wavelength multiplexing transmission technology is a transmission method that takes full advantage of the characteristics of light, transmitting different information on multiple wavelengths independently using a single optical fiber, and it meets the requirements mentioned above. This is a powerful method that can meet the requirements. In wavelength multiplexing transmission, the receiver and L
A branching section such as an AN requires an optical demultiplexer that selectively extracts a specific wavelength. Conventionally, optical demultiplexers have been put into practical use using dielectric multilayer interference filters or diffraction gratings, but optical waveguides have been developed to enable further miniaturization, higher resolution, and multiple channels. A waveguide optical demultiplexer has been reported. FIGS. 5 and 6 are perspective views showing conventional waveguide branching filters.

Figure 5 shows an example of a polarization conversion type optical demultiplexer, and details can be found in Optics Letters, Vol. 5, No. 11, pp. 473-475 (OPTIC 8 LETTER 8, Vol.
11 pp473-475). In FIG. 5, an optical waveguide 2 formed by diffusing titanium (Ti) is installed on the surface of a lithium niobate (LiNb03) crystal substrate l, and a control electrode 3 having a length L and having periodic holes in its second part is installed. is installed. The incident light 4 is linearly polarized light in the direction parallel to the substrate (Z-axis direction), that is, the TE mode, and due to the voltage applied to the control electrode 3, the following equation (1) is not satisfied and the wavelength λ
Only i is converted into TM mode (polarized light component in the X-axis direction).

Here, nTEi and nTMi are the isolateral refractive indexes of the TE and TM modes of the wavelength input i component propagating through the optical waveguide 2, respectively. Also, the spectral width Δλ of the wavelength to be polarized is
is the formula (2).

Δλ = input i
(2) Figure 6 is an example of an optical demultiplexer using Blup reflection.
Optics Vol. 19, No. 16, pp. 2848-2855 (APLIED 0PTIC8, Vol. 19° No.
16. pp. 2848-2855).

In FIG. 6, two optical waveguides 12 and 13 having mutually different cross-sectional shapes are formed adjacent to each other on the surface of a substrate 110 to constitute an optical directional coupler 14. Furthermore, a reciprocal grating 15 having a length L and having periodic holes is formed between the optical waveguides 12 and 13 of the optical directional coupler 14. Only the wavelength component λi that is incident on the optical waveguide 12 and satisfies the following formula (3) is reflected by the reflection grating 15 and coupled to the optical waveguide 15 to become the output light 17.

Here, n1iq n2i is the equal-side refractive index of the wave λi component of the light wave propagating through the optical waveguides 12 and 13. Further, the wavelength spectrum width Δλ diffracted by the reflection grating 15 is determined by equation (2) as a rough approximation.

(Problems to be Solved by the Invention) In the conventional polarization conversion type optical demultiplexer shown in FIG. It becomes very complicated. In addition, in equation (1), the isolateral refractive indexes nTEi and nTMi are related to the optical refractive index n8 and the ordinary refractive index nQ, respectively, and the difference 1ne -nol is very sensitive to temperature. Therefore, there is a drawback that a temperature control I/roll is always required.

On the other hand, in order to realize the Bragg reflection type optical demultiplexer shown in Figure 6, it is necessary to form a reciprocal grating with periodic holes that satisfy equation (3).
For NbO3 crystals and GaAs crystals, the normal value is 1000 to 3000 A around the wavelength of 1.3 to 1.5 pm.
It is extremely difficult to manufacture as it will only reach a certain level.

In addition to the optical demultiplexers shown in Figures 5 and 6 above, for example, there are optical demultiplexers that utilize the wavelength fraction of the propagation constant of an asymmetric optical directional coupler.
', Vol. 33, No. 2, pp. 161-163 (Appl.

Phy, Lett, Vol. 33. No,
2. pp. 161-163), but this case has the disadvantage that the selectable wavelength spectrum width Δλ is large (several hundred holes).

An object of the present invention is to provide a waveguide optical demultiplexer that is simple in structure, stable over temperature, easy to manufacture, and has a relatively narrow wavelength spectrum width, and a method for manufacturing the same.

(Means for Solving the Problems) The waveguide type optical demultiplexer of the present invention is provided with a difference 1n1- in equilateral refractive indexes that are arranged in close proximity to each other on a substrate and aligned with each other in the lowest-order waveguide mode. n21 is 5.6X10-3≦”n
An optical directional coupler constituted by two optical waveguides with □−n2|≦1.3, and a periodic isolateral refractive index change in the light transmission direction, which is installed near at least one of the optical waveguides. It is constructed by installing a grid pattern that gives

Furthermore, according to the present invention, an optical directional coupler is constructed by placing a first optical waveguide formed by metal diffusion and a second optical waveguide formed by ion exchange close to each other on an optical crystal substrate; There is obtained a method for manufacturing a waveguide optical demultiplexer, characterized in that a periodic lattice-like pattern is installed in the light transmission direction within the first or second optical waveguide or in the vicinity of the optical waveguide, and further comprises: A method for manufacturing a waveguide optical demultiplexer is obtained, characterized in that the periodic lattice pattern is composed of either periodic metal diffusion regions or periodic ion exchange regions. Further, on the substrate having an electro-optic effect, the difference Int -n21 in the isolateral refractive index of the lowest-order waveguide modes juxtaposed in close proximity to each other is 5°6x10 |n1-n21 < 1 .3, an optical directional coupler configured by two optical waveguides, and installed near at least one of the optical waveguides,
A waveguide optical demultiplexer is obtained, which is characterized by a lattice pattern that provides periodic isolateral refractive index changes in the light transmission direction, and at least one row of control electrodes installed along the optical directional coupler. .

(Function) In the waveguide type optical demultiplexer of the present invention, the isolateral refractive index n1. An optical directional coupler is constructed by placing optical waveguides having n2 in close proximity. Further, a lattice pattern having synchronization A satisfying the following equation (4) is installed near the optical waveguide.

In the present invention, the coupling between two optical waveguides with significantly different propagation constants is matched by installing the above-mentioned lattice pattern with a period A, and the optical directional coupler is used to match only the wavelength of λi that satisfies the above equation (4). It is used for coupling to other optical waveguides. In this case, instead of using polarization conversion as in the conventional example shown in Fig. 5, we use spatial coupling between the same polarized lights, and instead of Bragg reflection by a grating as in the conventional example shown in Fig. 6. This differs from the conventional example in that the separated wavelength components are output in the same direction as the incident light (but to separate spatially separated optical waveguides).

Here, if you want to obtain a narrow wavelength spectrum width of 100 A or less, set the incident wavelength λi to 1.3 pm and the length L<30 m.
Assuming m, from equation (2), A<201 pm, and from (4), it can be seen that In1-n21 and 5.6X10-3 are sufficient. On the other hand, considering ease of manufacture, it is necessary that A>lpm, and for this purpose, 1nx-n21 and 1.3 are required from equation (4).

The two waveguides having different equilateral refractive indices can be obtained by using different manufacturing methods, or can be made of different materials. However, when two optical waveguides are formed using the same means, for example, when they are formed using only the Ti diffusion method or only the ion exchange method so that only the cross-sectional shapes of the optical waveguides are different, the above (4)
) The value of 1n1-n21 in the equation becomes a small value of about 10-3 orders of magnitude, and the spectral width Δλ is 100A for a value of L that is usually easy to manufacture (about 1 to 30 mm), which is narrow. The value of Δλ is not available.

In the manufacturing method of the present invention, the two optical waveguides are created using completely different methods, for example, the Ti diffusion method and the ion exchange method, so that the values of nl and n2 are generally very different, and the value of A is from several to several + 11 m, and the value of Δλ is We are getting several to dozens of holes as a value.

(Embodiment) FIG. 1 is a perspective view showing an embodiment of a waveguide optical demultiplexer according to the present invention.

A first optical waveguide 22 formed by Ti diffusion method and a second optical waveguide 23 formed by proton ion exchange are formed on a LiNbO3 crystal substrate 21, and they are located close to each other within several pm at the center of the substrate. It constitutes an optical directional coupler 24. In addition, a dielectric film such as a niobium oxide film is formed on the top of the optical directional coupler 24 to form a grid pattern 2.
5 is formed. Here, the refractive index of the optical waveguide 22 is T
Due to i-diffusion, the substrate height has increased by about 10-2,
Its equivalent refractive index is n1=ns+Δn1 (however, n8 is L
Refractive index of iNbO3 crystal substrate 21, Δn1=5X10
”). Furthermore, the refractive index of the optical waveguide 23 is approximately 10-1 larger than the substrate due to proton exchange, and its equivalent refractive index is n2=n8+Δn2 (however, Δn2=5.5
X10'').The period A of the grid pattern 25 is set to 26 pm so as not to satisfy equation (4).Also, if the length L of the grid pattern 25 is 20 mm, then (2
) The wavelength spectrum width obtained from the formula is λi = 1.3pm
On the other hand, Δλ=17A.

(11) In this embodiment, the optical waveguide 23 with a large refractive index may be a multimode optical waveguide, but the conditions must be selected so that equation (4) holds true for n2 for the zero-order mode.
It is possible to excite only the secondary light. In addition, the optical waveguide 23
An ion implantation method can be used as a means for creating the refractive index, which also provides a large change in refractive index. Furthermore,
Only one optical waveguide is made of LiNbO using another dielectric film.
It can also be formed on the tricrystalline substrate 21, in which case the values of |n1 n21 can be made even larger.

Further, as a method for forming the grid pattern 25, L
A method of directly etching the iNbO3 crystal substrate 21 by ion beam processing or the like is also possible.

Instead of the LiNbO3 crystal substrate 21, it is also possible to use a semiconductor substrate made of lithium tantalate, GaAs, or the like. In particular, when a III+V group compound semiconductor substrate is used, the optical demultiplexer of the present invention can be obtained by a method such as forming the optical waveguides 22 and 23 by independently growing them so that they have different compositions.

(12) FIG. 2 is a perspective view showing another embodiment of the waveguide optical demultiplexer according to the present invention. LiNbO as in the embodiment shown in FIG.
An optical waveguide 22 formed by a Ti diffusion method and an optical waveguide 23 formed by a proton exchange method are formed on a tricrystalline substrate 21 to constitute an optical directional coupler 24. However, in this embodiment, the lattice pattern 30 is formed in the optical waveguide 23 by the Ti diffusion method.

The operation of this embodiment is similar to the example shown in FIG. 1, and the optical waveguide 2
Only the component of wavelength λi in the incident light 26 to the optical waveguide 23 is coupled to the optical waveguide 23 and output.

FIG. 3 is a diagram showing an embodiment of the method for manufacturing a waveguide optical demultiplexer according to the present invention.

FIG. 3 is a diagram (perspective view) showing only a part of the optical directional coupler in the waveguide optical demultiplexer cut out for explanation.

In FIG. 3, first, a Ti film of several hundred amps coated on a LiNbO3 crystal substrate by sputtering or the like is patterned by photolithography to form a first optical waveguide pattern 32 and a lattice pattern 33 (see FIG. 3(a). )
). Next, by leaving the substrate in an electric furnace at 1000 to 11000C for several hours, Ti diffuses into the substrate and T
An i-diffused optical waveguide 34 and a Ti-diffused grating pattern 35 are formed (FIG. 3(b)). Next, a second optical waveguide 36 is formed in a region including the grating pattern 35 close to the first optical waveguide by a proton exchange method (FIG. 3(C)).

Here, as an example of the proton exchange method, a Ti film mask having an opening in the shape of the second optical waveguide 36 is placed on a substrate, and the substrate is placed in benzoic acid at 200 to 250°C for several tens of minutes. Optical waveguides are obtained by soaking for several hours.

Another example of the method for manufacturing an optical demultiplexer according to the present invention is as follows:
There is a method in which only the first optical waveguide is first formed by Ti diffusion, and then proton exchange is performed using a second optical waveguide pattern and a mask having lattice pattern openings.

FIG. 4 shows one embodiment of the present invention, and is a perspective view of a waveguide optical splitter that can be controlled by voltage.

In FIG. 4, an optical waveguide 22 formed by the Ti diffusion method and an optical waveguide 23 formed by the proton exchange method are formed on a LiNbO3 crystal substrate 21 having an electro-optic effect, as in the embodiment shown in FIG. It consists of

In this embodiment, the grid pattern 40 is the optical directional coupler 24.
The optical waveguides 22 and 23 are formed by an ion exchange method or a Ti diffusion method. Further, the optical directional coupler 24
In this embodiment, in which a pair of control electrodes 41 are installed above the optical waveguides 22 and 23, when the voltage applied to the control electrodes is O, the incidence on the optical waveguide 22 is similar to the embodiment shown in FIG. 26 (4) is not satisfied and the component of wavelength λi is in the optical waveguide 2.
However, the refractive index under the control electrode changes, and as a result, the value of |nl n21 in equation (4) changes, so the wavelength λi extracted from the optical waveguide 23 changes in accordance with the voltage. That is, in this embodiment, the wavelength to be separated can be adjusted by the magnitude of the voltage applied to the control electrode.

(Effects of the Invention) As described above, the present invention provides a waveguide optical demultiplexer that has a simple groove bottom, is stable with respect to temperature, is easy to manufacture, and has a relatively narrow wavelength spectrum width, and a method for manufacturing the same. can get.

[Brief explanation of drawings]

(15) FIGS. 1, 2, and 4 are diagrams showing an embodiment of a waveguide optical splitter according to the present invention, and FIG. 3 is a diagram showing an embodiment of a method for manufacturing a waveguide optical splitter according to the present invention. 5 and 6 are diagrams showing examples of conventional waveguide type optical demultiplexers. In the figure, 1
．． 11.21.31 is lithium niobate crystal, 2.22
．． 34 is an optical waveguide by Ti diffusion, 23.36 is an ion exchange optical waveguide, 25.30.35.40 is a grating pattern, and 41 is a control electrode.

Claims (4)

[Claims] (1) Difference in equivalent refractive index for the lowest-order waveguide modes juxtaposed close to each other on the substrate | n_1−
n_2| is 5.6×10^-^3≦|n_1-n_2
|≦1.3; an optical directional coupler configured with two optical waveguides; and a lattice-shaped coupler that is installed near at least one of the optical waveguides and provides a periodic change in equivalent refractive index in the light transmission direction. A waveguide optical demultiplexer characterized by having a pattern. (2) An optical directional coupler is constructed by placing a first optical waveguide formed by metal diffusion and a second optical waveguide formed by ion exchange close to each other on an optical crystal substrate, and A method for manufacturing a waveguide optical demultiplexer, characterized in that a periodic lattice pattern is installed in the light transmission direction within or close to the optical waveguide of No. 2. (3) The waveguide optical demultiplexer according to claim 2, wherein the periodic lattice pattern is composed of either periodic metal diffusion regions or periodic ion exchange regions. Production method. (4) On a substrate having an electro-optic effect, the difference in equivalent refractive index for the lowest-order waveguide modes juxtaposed in close proximity to each other |n_1-n_2| is 5.6×10^-^
3≦|n_1−n_2|≦1.3, and an optical directional coupler that is installed near at least one of the optical waveguides and has periodic equivalent refraction in the light transmission direction. What is claimed is: 1. A waveguide optical demultiplexer, comprising: a grid pattern that provides a rate change; and at least one pair of control electrodes installed along the optical directional coupler.