JPS5937486B2 - light switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical switch that electrically controls the intensity of light waves guided in an optical integrated circuit and the selection of the guided light path.

A typical conventional element of this type is a balance bridge type optical switch.

This basic structure is shown in FIG. Two identical linear optical waveguides 2 and 3 are formed on the surface of a substrate 1 having an electro-optic effect. The area where the optical waveguides 2 and 3 are parallel and close is 2
There are 3 dB coupling parts 4 and 5 at these parts. Further, an optical modulation section 6 is present in the middle, and electrodes T, 8, and 9 are provided for providing a phase difference φ between the optical waveguides 2 and 3 by the electro-optic effect. The substrate 1 is a LiNb03 single crystal, which is an electro-optic crystal, and the polarization axis 10 (indicated by an arrow) is inside the substrate and is parallel to the direction of the applied electric field concentrated in the gap between each electrode. ing. In an element having such a structure, the input end 11 of the optical waveguide 2 is
When light of unit intensity is incident on the 3 dB coupling section 4, 50% of the optical energy is transferred to the optical waveguide 3. When no voltage is applied to the optical modulator 6, that is, when the phase difference between both waveguides is φ■0, the following 3.
Further optical coupling occurs in the dB coupling section 5, resulting in 100%
The optical energy of the waveguide 3 is transferred to the waveguide 3. That is, the light intensity at the output end 12 is o, and the light intensity at the output end 13 is 1. On the other hand, when a voltage is applied to the optical modulator 6 so that the phase difference φ=π, 50% of the light wave transferred to the waveguide 3 at the 3dB coupling unit 4 is transferred to the waveguide 2 at the 3dB coupling unit 5.
Returning to , the light intensity at the output end 12 becomes 1 and the light intensity at the output end 13 becomes 0.

Therefore, by utilizing the electro-optic effect, optical switching becomes possible. In the optical modulator 6, two conditions are essential: no optical coupling occurs between one waveguide and only two electrically controllable phase differences are provided.

For this reason, in the structure shown in FIG. 1, it is necessary to increase the distance between both waveguides. For this purpose, a curved or polygonal waveguide is used to connect the two 3 dB coupling sections 4 and 5 and the optical modulation section 6.
However, in an optical waveguide created by impurity diffusion or the like using, for example, an LlNbO3 single crystal as a substrate, the difference in refractive index is small, so the optical loss due to bending is large.

In addition, this curved portion or folded line portion only serves to connect the 3 dB coupling portion and the optical modulation portion, and does not contribute to the optical switching function, and may cause optical propagation loss or increase the overall length of the device. Ta. Furthermore, when creating a 3 dB coupling section, it was extremely difficult to achieve 3 dB coupling as designed. Conventionally, in order to eliminate this curved portion, there is a structure in which two straight waveguides are used and two 3 dB coupling portions and optical modulation portions are provided.

This basic configuration diagram is shown in FIG. Figure 3 is A- in Figure 2.
FIG. 2 and 3, 1 is a substrate made of electro-optic crystal, 2 and 3 are substrates 1
These are two identical straight optical waveguides fabricated on the surface of .

Both 4 and 5 are 3 dB coupling sections, and 6 is an optical modulation section.

As mentioned above, two conditions are essential in the optical modulation section: no optical coupling occurs between the 12 waveguides and an electrically controllable phase difference is provided. The latter may be achieved by utilizing the electro-optic effect of the substrate. In FIGS. 2 and 3, electrodes are omitted. In order to satisfy the former condition, there is a method of shaving off the gap 14 between the waveguides 2 and 3 in the optical modulator 6 to increase the difference in refractive index and weaken the degree of coupling. There are techniques such as ion milling and sputter etching that can be used to shave the substrate 1, but since the gap size required for coupling between waveguides is usually several μm, this minute gap is It is extremely difficult to scrape off with precision. In order to solve the above-mentioned problems, the present invention integrates two 3dB coupling parts and an optical modulation part into a set of optical waveguides consisting of only straight parts, and makes it easier to create the optical modulation part. This is what I did.

The present invention will be explained in detail below with reference to the drawings. The basic configuration of an embodiment of the present invention is shown in FIG. Figure 5 is B- of Figure 4.
It is a sectional view at W. In Figures 4 and 5,
1 is a substrate made of a substance having an electro-optic effect, and 2 and 3 are two identical straight optical waveguides formed near the surface of the substrate 1. For example, when using an LlNbO3 single crystal as a substrate, if Ti metal is evaporated to a thickness of about 500λ, patterned, and thermally diffused in the atmosphere at 1000°C for 10 hours, when the width of the Ti metal is 8 μm, the wavelength 1.15
It is possible to create an optical waveguide that allows propagation of only a single mode for μm laser light. The gap between these optical waveguides is several microns.
By setting m, optical coupling occurs between both optical waveguides, and a light wave propagating through one waveguide gradually moves to the other waveguide. When the gap is set to 5 μm, 1
Coupling length T required to transfer 00% of light waves. is 5-1
The number will be 0. The coupling coefficient in this case is defined as K=π/2t0. In FIG. 4, 4 and 5 are the first and third regions obtained by dividing the coupling region (total length T of the device in the structure of the device according to the present invention) into three, where two waveguides are parallel and close to each other. Both area lengths T2 and t3 are 10
Set to half the 0% bond length, ie, TO/2.

In this case, the 100% coupling length T of the created device. It is necessary to clarify the value of One method is to load a thin film or a metal film having a refractive index higher than the refractive index of the waveguide in the form of a strip onto one of the waveguides.

T by sequentially removing the film from the edge and measuring the degree of optical coupling each time. seek. In this method, loading a thin film causes a shift in the propagation constants of the two waveguides.
It takes advantage of the fact that the bond disappears. Therefore, only the region where the thin film is removed can be used as an effective bond length. As a result, both the lengths of the first region 4 and the third region 5 are T. /2 can be easily achieved, and a 3 dB coupling section can be created. 6 is the remaining second region, which acts as a light modulation section.

This will be explained in detail later. Note that 7 and 8 are electrodes, and 11
is the entrance end, 12 and 13 are the exit ends, 15 is the deletion area, 16
indicates a buffer layer created over the entire substrate surface. As the thin film to be loaded, a thin film having a refractive index smaller than that of the waveguide, such as a SlO2 or At2O3 film, is used.

Since the purpose of the buffer layer 16 is to prevent light absorption by the metal electrodes 7 and 8, it is sufficient that it is loaded only on the light modulation section 6 at least. The electrodes 7 and 8 are a pair of planar electrodes created directly above the two waveguides, and their spacing is approximately equal to the spacing between the optical waveguides, and their length matches the length T4 of the optical modulation section 6. do. In this case, the polarization axis 10 of the electro-optic crystal indicated by an arrow is perpendicular to the substrate surface. When a voltage is applied to the electrodes 7 and 8, an electric field in the depth direction is concentrated under the electrodes, and the signs of the electric fields directly under the electrodes 7 and 8 are different. Since the waveguides 2 and 3 exist in the region where this electric field is concentrated, a phase difference φ9 occurs between the waveguides. LiNb substrate
If O3, the phase difference φ9 for TM mode light is given by:

Here, λ is the laser beam wavelength in vacuum, Ne is the refractive index of the waveguide, R33 is the electro-optic coefficient, r is the correction coefficient, V is the applied voltage, and 9 is the electrode spacing. On the other hand, in the optical modulation section 6, since the region 15 in contact with the outside of the waveguide 3 is shaved off, the propagation coefficient of the waveguide 3 changes significantly compared to that of the waveguide 2.

The difference in propagation constant is Δβ, and the phase difference between the propagating lights in the waveguides 2 and 3 after passing through the optical modulator 6 is φ3−AΔβ. A is a constant. The propagation constant difference Δβ can be made sufficiently larger than the original coupling coefficient K between the two waveguides. As a result, a phase mismatch occurs in the optical modulation section 6 where optical coupling should normally occur between both waveguides, and no transition of light waves occurs. For example, 100% bond length T. If it is 101gt, then from the coupling coefficient K=π/2t0, K=1.5X104μ
It becomes m-1. In addition, the difference Δβ in the propagation constant caused by scraping off the region in contact with one side of the LiNbO3 diffused waveguide is calculated by assuming that the waveguide has a square cross section of 8 μm and the wavelength is 1 μm, as shown in FIG. Since the refractive index of the region in contact with one side of the waveguide is changed from the refractive index of the substrate N2 = 2.2 to the refractive index of air n1
By setting = 1.0, Δβ happiness 1.2X10-3μ
m-1, which is one order of magnitude larger than the coupling coefficient K.

Therefore, the optical coupling between the waveguides in this region is negligibly small. The deletion region 15 can be easily scraped off using sputter etching or ion milling. Also, as shown in Figure 5,
It is not necessary to cut it down to the same depth as the waveguide; it is sufficient that the difference Δβ in propagation constants is sufficiently large. As a result of the above, the total phase difference φ due to the electro-optic effect and scraping off the region in contact with one side of the waveguide is as follows.

On the other hand, when light of unit intensity is incident on the waveguide 2, the light intensities P and Q at the output end faces 12 and 13 of the waveguides 2 and 3 are given by the coupling theory.

Figure 7 shows how P and Q change. Therefore, after φ3 reaches a constant value by scraping off the region 15, P and Q can be changed from O to 1 by changing φ9 only within a range of 1φVKπ, making optical switching possible.

For example, when φ3=π, the required applied voltage V.
is given by >>Nle-00-beak.

Therefore 9=5μM. t4=10 days, λ=1.15μ
M. ne=2.2, R33=30.8X10-6μm/
V. When r=0.5, optical switching operation becomes possible at VO==1.8V. In this embodiment, the case where the polarization axis 10 is perpendicular to the substrate is shown, but the case where the polarization axis 10 is inside the substrate is also the same as described above. A cross-sectional view of the light modulation section in this case is shown in FIG. The operation is the same as that of the embodiment shown in FIG. 4, so a description thereof will be omitted. The difference from FIG. 4 is that an electric field parallel to the substrate is used, so electrodes 7 and 8 are connected to one waveguide 2.
It is set up to sandwich the. FIG. 9 shows a cross section of a light modulating section according to another embodiment of the present invention, with electrodes and polarization axes omitted.

One of the conditions in the optical modulation section is "two waveguides 2,
In order to achieve the phase mismatch required to eliminate optical coupling between the waveguides 3, the surface of one of the waveguides 3 is shaved off. The cross-sectional shape of the waveguide is obtained by cutting off the region 15,
By changing the dimensions, a sufficient change in the propagation constant can be induced. For example, the cross-sectional dimension of the waveguide is 8 μm square, the refractive index difference between the substrate and the optical waveguide is 0.006, and the wavelength is 1.
µm, and if the waveguide dimensions are made 6 µm x 8 µm by scraping the waveguide surface to a depth of 2 µm, the resulting change in propagation constant Δβ is approximately 2.88 x 10-3 µm.
-1. 100% bond length T.

If 10 pieces, the coupling coefficient K = 1.5 x 10-4 μm
-1, which means that a change in the propagation constant that is one order of magnitude larger than the coupling coefficient K has occurred, and optical coupling between the waveguides can be ignored. 10 and 11 are cross-sectional views of an embodiment in which electrodes 7 and 8 are formed in the light modulation section shown in FIG.
The figure shows the case where the polarization axis 10 of the substrate material 1 is perpendicular to the surface of the substrate 1, and FIG. 11 shows the case where the polarization axis 10 is parallel to the surface of the substrate 1.

16 is a buffer layer.

The 3 dB coupling portions in the first region and the third region are as described above, and the optical switching operation is also the same, so a description thereof will be omitted.

Note that although the substrate has been described using LiNbO3 single crystal as an example, other materials may be used as long as it has an electrochemical effect, and semiconductor crystals such as GaAs may also be used.

As explained above, the optical switch of the present invention integrates two three-curve coupling sections and an optical modulation section into a set of optical waveguides consisting of only straight sections, and also performs optical coupling in the optical modulation section. In order to eliminate this problem, the size and shape of the two waveguides are changed, which makes it extremely easy to create an optical modulation section (this has the advantage of being extremely easy to create).

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of a conventional balanced bridge type optical switch, and Figure 2 is a basic configuration diagram of another conventional optical switch.
3 is a sectional view taken along A-N in FIG. 2, FIG. 4 is a basic configuration diagram of an embodiment of the present invention, FIG. 5 is a sectional view taken along B-W in FIG. 4, and FIG. 6 is a propagation FIG. 7 is a cross-sectional view of the optical waveguide used in the calculation of the constant. FIG. 7 is a diagram showing changes in light intensity at the output end of the waveguide. FIG. FIG. 9 is a schematic cross-sectional view of a light modulation section according to another embodiment of the present invention, and FIG. 10 is a light modulation section in which an electrode is formed on the light modulation section shown in FIG. 9 (the polarization axis of the substrate material is perpendicular to the surface of the substrate). FIG. 11 is a cross-sectional view of the light modulation section shown in FIG. 9 in which electrodes are formed in the light modulation section (in the case where the polarization axis of the base material is parallel to the surface of the substrate). 1...Substrate, 2...Optical waveguide, 3...
...Optical waveguide, 4...3dB coupling section, 5.
...3dB coupling section, 6... Optical modulation section, 7
...electrode, 8...electrode, 9...
・Electrode, 10...Polarization axis, 11...Incidence end, 12...Output end, 13...Output end, 14... Gap portion, 15... Deletion area, 16... Buffer layer.

Claims (1)

[Scope of Claims] 1. A coupling region of two parallel and adjacent linear optical waveguides created near the surface of a substrate made of a substance having an electro-optic effect is divided into three, and the first region and the third region are divided into three regions. The degree of optical coupling between the optical waveguides in the second area is set to 50%, and the phase difference between the optical waveguides in the second area is created by scraping off the area that contacts the outside of one of the optical waveguides in the second area. In order to provide matching, reduce the optical coupling between the optical waveguides in the second region to zero, and electrically change the phase difference between the optical waveguides in the second region,
An optical switch characterized in that a set of electrodes is provided in the second region. 2 The coupling region of two parallel and adjacent linear optical waveguides created near the surface of a substrate made of a substance with an electro-optic effect is divided into three parts, and the light between the optical waveguides in the first and third regions is divided into three parts. By making the coupling degree 50%, and by scraping off the surface of one of the optical waveguides in the second region and changing the cross-sectional shape and dimensions of this optical waveguide, the distance between the optical waveguides in the second region is A set of electrodes is provided in the second region in order to provide a phase mismatch to the optical waveguides, to make the optical coupling between the optical waveguides in the second region zero, and to electrically change the phase difference between the optical waveguides in the second region. An optical switch characterized by the following: