JPH0480362B2 - - Google Patents

Description

[Detailed description of the invention] This invention splits a light beam into two,
On the other hand, the present invention relates to a waveguide optical beam splitter used to overlap two optical beams.

Optical integrated circuit technology is essential for the practical application of optical communications and optical information processing. Among these technologies, waveguide optical circuit technology has become a particularly important basic technology. There are many attempts to construct various optical circuits using waveguides, one of which is the optical beam splitter. Conventionally, directional couplers have been used to function as optical beam splitters in waveguide optical circuits. This consists of two parallel optical waveguides that are close to each other, and the optical power is transferred between the optical waveguides by coupling between the optical waveguides. If set appropriately, it is possible to achieve the same functionality as a beam splitter. However, in order to do this, three parameters, namely the shape, spacing, and coupling length of the optical waveguide, must be precisely set to appropriate values, which is a drawback.

As a waveguide type optical transmission line, there is one described in Japanese Patent Publication No. 43304/1983. This optical transmission line is so-called Y-shaped and has three ports. This optical transmission line has a second branch at one end of the first branch,
It is constructed by connecting two third branches in a Y-shape, and the propagation constants of these three branches are different from each other. Then, separation, mode conversion, or mode selection of the light introduced into the first branch is performed.

However, in order to put optical information processing into practical use, it is necessary to be able to combine not only light but also light. For example, in the field of optical measurement using light interference, there are a total of four ports: an input port for the incident light, a separate output port for the output light, and two ports for the two lights to be interfered. Optical waveguides with ports are desired.

SUMMARY OF THE INVENTION An object of the present invention is to provide a waveguide optical beam splitter that does not require complicated and precise parameter settings, is easy to configure, and has four ports.

The waveguide optical beam splitter according to the present invention has a symmetrical optical waveguide pair constituted by two single mode optical waveguides having the same phase constant crossing each other at one end, and two single mode optical waveguides having mutually different phase constants. A pair of asymmetric optical waveguides formed by one-mode optical waveguides crossing each other at one end are formed on a substrate and are coupled to each other at each intersection to form an X-shaped optical waveguide. , when an in-phase light wave is input from one of the optical waveguide pairs on the symmetric side or the asymmetric side, an output light wave is obtained from one optical waveguide of the other optical waveguide pair, and an out-of-phase optical wave is obtained from one of the optical waveguide pairs. When a light wave is input, an output light wave is obtained from the other optical waveguide of the other pair of optical waveguides. The optical waveguide is realized by forming a portion having a higher refractive index than the surrounding area on a substrate made of an optical material.

According to this invention, the pair of symmetrical and asymmetrical optical waveguides, each composed of two single mode optical waveguides and crossing each other at one end, can be combined in an X shape on a substrate, making it easy to manufacture. be. Moreover, the waveguided light beam according to this invention
The splitter has a total of four ports, two ports for each of the symmetric optical waveguide pair and the asymmetric optical waveguide pair. When an in-phase light wave is input from one of the optical waveguide pairs on the symmetrical side or the asymmetric side, an output optical wave is obtained from one optical waveguide of the other optical waveguide pair, and an outgoing optical wave is obtained from one optical waveguide pair on the above-mentioned one optical waveguide pair. When a light wave is inputted, an output light wave is obtained from the other optical waveguide of the other pair of optical waveguides, so that the function of combining the light beams can be achieved. Even if they are replaced, the same function can be achieved, so the function of splitting the light beam can also be achieved. According to the present invention, such optical multiplexing and demultiplexing functions can be realized in the form of an optical waveguide formed on a substrate, so that significant progress in optical integrated circuit technology can be expected.

First, the operating principle of the optical beam splitter according to the present invention will be explained. In Figure 1, 1
A pair of single mode optical waveguides 1 and 2 intersect at one end at a small angle θ1. These optical waveguides 1 and 2 have equal widths W1 and W2,
Therefore, the phase constants are set equal. There is another pair of single mode optical waveguides 3 and 4, which also have a small angle θ at one end.
It intersects at 2. Widths W3 and W of optical waveguides 3 and 4
4, the width W4 of the optical waveguide 4 is different from that of the optical waveguide 3.
width W3. Therefore, the phase constants of optical waveguides 3 and 4 are different, and optical waveguide 3 is larger. Although the widths W1 and W2 of the optical waveguides 1 and 2 and the width W3 of the optical waveguide 3 are set equal in FIG. 1, they do not necessarily have to be equal. Also, the intersection angle is set to θ1>θ2, but θ
1≦θ2 may be satisfied. The optical waveguides 1 and 2 and the optical waveguides 3 and 4 are coupled at their respective intersections so that these optical waveguides are substantially straight. This joint portion is designated by the reference numeral 5. For convenience of explanation, the direction from the optical waveguides 1 and 2 to the optical waveguides 3 and 4 is the Z axis, and the direction perpendicular to the plane of the paper is the X axis.
Take the XYZ coordinate axes as the axes. Moreover, the optical waveguides 1 and 2 are called the symmetric side, and the optical waveguides 3 and 4 are called the asymmetric side.

For simplicity, consider a two-dimensional structure with no change in the X direction. Furthermore, both of the two crossing angles θ1 and θ2 are sufficiently small, the light wave travels approximately in the Z direction, and the distance between the optical waveguides 1 and 2 and the distance between the optical waveguides 3 and 4 changes with respect to minute changes in the Z direction. Changes are assumed to be negligible. That is, when considering a minute section excluding the coupling portion 5, it can be considered that there are two parallel optical waveguides and a uniform five-layer structure is formed in the Y direction. In such a case, a local normal mode analysis method can be applied.

As is well known, the eigenmodes of a five-layer optical waveguide consisting of two single mode optical waveguides are:
There are two types: even mode and odd mode. Figure 2a,
Part b shows the propagation states of even mode and odd mode in this five-layer optical waveguide structure.
FIG. 2c shows how the phase constants of the even and odd modes of this five-layer optical waveguide structure change.
On the symmetric side consisting of the optical waveguides 1 and 2, at a position sufficiently far from the coupling part 5 and where the distance between the optical waveguides 1 and 2 is wide, the coupling between the optical waveguides 1 and 2 can be ignored, so the two eigenmodes are degenerated. However, the phase constants of both modes are equal. As the coupling portion 5 is approached, the degeneracy is broken and the difference in phase constants of both modes becomes larger.
In the coupling part 5, the two optical waveguides become one, and the 3
Since it has a layered optical waveguide structure, the even mode shifts to the fundamental mode (the one with the larger phase constant) of the three-layered optical waveguide, and the odd mode shifts to the primary mode (the one with the smaller phase constant). After passing through the coupling part 5, the optical waveguide 3
When the optical waveguides 3 and 4 enter the asymmetric side consisting of The phase constants of each asymptote to different values. In this example, since the width of the optical waveguide 3 is wider than the width of the optical waveguide 4, the phase constant of the optical waveguide 3 is larger. Therefore,
Even mode light wave power is concentrated in the optical waveguide 3, and odd mode light wave power is concentrated in the optical waveguide 4.

The above explanation is for the case where the light propagates from the symmetric side to the asymmetric side, but when the light propagates from the asymmetric side to the symmetric side, the above explanation can be followed in reverse.

FIG. 3 shows the light wave output obtained when various light waves are input into the above-mentioned optical waveguide from the symmetrical side. FIG. 3a shows two optical waveguides 1 on the symmetric side,
This is a case where light waves of the same phase are input to 2. The even mode is excited and propagates on the symmetrical side, and changes to the fundamental mode at the coupling portion 5, and to the even mode again on the asymmetrical side. Since the optical wave power of the even mode on the asymmetric side is concentrated in the optical waveguide 3, the output optical wave is obtained from the optical waveguide 3. FIG. 3b shows a case where light waves having mutually opposite phases are input into two optical waveguides 1 and 2 on the symmetrical side. The odd mode is excited and propagates on the symmetrical side, and changes to the primary mode at the coupling portion 5, and to the odd mode again on the asymmetrical side. Since the light wave power of the odd mode on the asymmetric side is concentrated in the optical waveguide 4, the output light wave is obtained from the optical waveguide 4. FIG. 3c shows a case where a light wave is input only to the optical waveguide 2. In FIG. In this case, it is considered that the even mode and the odd mode are excited with equal power on the symmetric side, so the superposition of a and b in Fig. 3 occurs, and light waves with equal power are output to optical waveguides 3 and 4. .

It is also possible to input light waves from the asymmetric side, in which case the reverse process described above is followed. for example,
When a light wave is input to the optical waveguide 3, light waves in the same phase are outputted from both optical waveguides 1 and 2 on the symmetric side (indicated by broken line arrows in FIG. 3a). Furthermore, by inputting in-phase or anti-phase light waves from both optical waveguides 3 and 4 on the asymmetric side, it is also possible to output light waves from either one of the optical waveguides 1 and 2 on the symmetric side (as shown in Figure 3c). (see dashed arrow). From the above considerations,
It will be understood that this optical waveguide structure has a function equivalent to a normal optical beam splitter (for example, a half mirror).

FIG. 4 shows an embodiment. By thermally diffusing Ti onto one surface of the LiNbO ₃ crystal substrate 10,
As shown in FIG. 1, a pair of optical waveguides 1 and 2, a pair of optical waveguides 3 and 4, and a coupling portion 5 at the intersection of these are formed. At the ends of these optical waveguides 1 to 4 opposite to the coupling part 5, parallel optical waveguides 11,
12, 13, and 14 are consecutive. It is also possible to use the optical waveguides 11 and 12 as the input side and the optical waveguides 13 and 14 as the output side, or conversely, the optical waveguide 1
It is also possible to use the optical waveguides 3 and 14 on the input side and the optical waveguides 11 and 12 on the output side. In any case, the various light branching or superpositions described above under the principle of operation can be performed. Intersection angle between optical waveguides 1 and 2, intersection angle between optical waveguides 3 and 4 θ1, θ2
are all taken to be small values of 1.2° or less. If the crossing angle θ2 between the optical waveguides 3 and 4 is made smaller, the asymmetry between the two optical waveguides 3 and 4 will be emphasized, so the crossing angle θ2 on the asymmetric side is changed to the crossing angle θ1 on the symmetric side.
It is advantageous to make it smaller.

Besides LiNbO ₃ many other optical materials can be used as the substrate. For example, an optical waveguide can be formed by using glass as a substrate and diffusing silver ions. Furthermore, it is possible to construct the optical beam splitter according to the present invention using optical fibers without forming an optical waveguide on the substrate.

FIG. 5 shows an application example in which a waveguide type optical beam splitter according to the present invention is used to construct a differential optical modulation element with an inverted output. In FIG. 5a, by diffusing Ti on a LiNbO ₃ substrate 10, in addition to the above-mentioned optical waveguides 1 to 4, coupling portion 5, and optical waveguides 11 to 14, a Y-shaped structure consisting of optical waveguides 21 to 23 is formed. An optical branch path is formed. The optical waveguides 21 and 22 are continuous with the optical waveguides 11 and 12, respectively. In addition, a pair of electrodes 3 are placed on the parallel optical waveguides 11 and 12 via a SiO ₂ buffer layer 32.
1 is created, and a voltage V is applied to this electrode 31 by a power source 33. The buffer layer 32 prevents light from being absorbed by the electrode 31. The electrode 31 may cover either part or all of the optical waveguides 11 and 12.

FIG. 5b shows a plan view of the optical waveguide, and with reference to this figure, an example of the dimensions of each part of the differential light modulation element is as follows.

Width W1 of optical waveguides 1 and 11, width W2 of optical waveguides 2 and 12, and optical waveguides 21 and 22
Width W5 of optical waveguides 3 and 23 is 3 μm Width W3 of optical waveguides 3 and 13 is 3 μm Width W4 of optical waveguides 4 and 14 is 2.5 μm Intersection angle θ1 of optical waveguides 1 and 2 is 0.02 rad Intersection angle of optical waveguides 3 and 4 θ2 is 0.005 rad Crossing angle θ3 of optical waveguides 21 and 22 is 0.02 rad Length L1 of optical waveguides 1 and 2 is 1 mm Length L2 of optical waveguides 3 and 4 is 8 mm Length L5 of coupling part 5 is 40 μm Parallel optical waveguide Length L3 of optical waveguides 11, 12 and electrode 31 is 10 mm. Length L4 of optical waveguides 21 and 22 is 1 mm. Distance D1 between optical waveguides 11 and 12 is 20 μm. Distance D2 between optical waveguides 13 and 14 is 40 μm. A pair of electrodes 31 The distance D3 is 22 μm. The light wave enters from the optical waveguide 23 and is split into two by the optical waveguides 21 and 22. When a voltage is applied between the pair of electrodes 31, a phase difference corresponding to the applied voltage V occurs between the two branched light waves as they propagate through the optical waveguides 11 and 12. Therefore, as explained with reference to FIGS. 3a and 3b, light waves are output from either or both of the optical waveguides 13 and 14 at an appropriate rate depending on this phase difference.
The relative intensities of the light output from the optical waveguides 13 and 14 are shown in FIG. As can be seen from this figure, the output light from the optical waveguide 13 and the output light from the optical waveguide 14 are inverted from each other.

In this optical modulation element, it is also possible to input light from the optical waveguide 13 or 14. This corresponds to the case where light is incident in the direction shown by the broken lines in FIGS. 3a and 3b. Output light modulated according to the voltage V applied to the electrode 31 is obtained from the optical waveguide 23, and the output light obtained when inputting to the optical waveguide 13 and when inputting to the optical waveguide 14 is reversed. are doing.

Figure 7 shows yet another application example,
This is a case where a reflective differential optical modulator is constructed using a waveguide optical beam splitter. This element is
It is constructed in the same manner as the device shown in FIG. 5, but without the Y-shaped optical branch. The ends of the parallel optical waveguides 11 and 12 opposite to the connection side with the optical waveguides 1 and 2 serve as terminations, and at this termination, aluminum is deposited on the end surface of the substrate 10 by a method such as vacuum evaporation. A reflective film 34 is formed by.

One optical waveguide on the asymmetric side, for example, optical waveguide 1
The light input to 3 is divided equally into optical waveguides 1 and 2 when it travels from the coupling part 5 toward the symmetrical side, passes through parallel optical waveguides 11 and 12, and is reflected by the reflective film 34, and returns to the optical waveguide 11. , 12. Optical waveguide 1
In the process of reciprocating between the waveguides 11 and 12, the light in both waveguides 11 and 12 is modulated by the voltage applied to the electrode 31.
A phase change occurs. And both optical waveguides 11,1
Depending on the phase difference between the two lights, the optical waveguide 4 or 3
The light beam travels in either direction, or branches into both directions at an appropriate ratio and exits from the optical waveguides 14 and 13. When the light in both optical waveguides 11 and 12 has exactly the opposite phase, most of the light incident on the optical waveguide 13 is output from the optical waveguide 14. FIG. 8 shows the relationship between the relative intensity of the light that has entered one of the optical waveguides 13 or 14 and exits from the other optical waveguide and the voltage applied to the electrode 31.

In the application example shown in Figure 7, a phase difference is electrically given to the light propagating in two parallel optical waveguides, but a phase difference can also be given to the two lights by other methods. , it becomes possible to develop various applications.

In FIG. 9, the light from the parallel optical waveguides 11 and 12 is emitted to the outside of the substrate 10, is reflected by reflective surfaces 41 and 42 such as reflective mirrors or objects placed outside the substrate 10, and is reflected again. Optical waveguide 1
I am guided by 1 and 12. The intensity of the output light changes depending on the relative positional relationship between the reflective surfaces 41 and 42.
For example, assuming the wavelength of light is λ, the reflecting surfaces 41 and 4
When the distance from the optical waveguide 2 is λ/4, most of the light incident on the optical waveguide 13 is output from the optical waveguide 14. This configuration can be applied, for example, to optical reading of digital audio discs. For inputting and outputting light from the optical waveguides 11 and 12, an optical fiber or a lens system may be used.

In the application example shown in FIG.
2 is guided to separate optical fibers 43 and 44, and these optical fibers 4
The light reflected at the tips of 3 and 44 returns to the optical waveguide 1.
It has been returned to 1 and 12. Two optical fibers 43
Differences in various physical quantities, such as the lengths of the optical fibers 43 and 44, and the temperatures at which the two optical fibers 43 and 44 are placed, are converted into changes in the intensity of the output light from the optical waveguide 14. Applicable.

In FIG. 11, a reflective film 45 is provided at the end of one optical waveguide 12, and no phase change occurs in the light of this optical waveguide 12. Light is extracted to the outside only from the other optical waveguide 11 and is returned to the optical waveguide 11 again by some means. Factors that cause a phase change to the light in the optical waveguide 11, such as the length of the optical fiber and the temperature of the optical fiber as described above, are converted into changes in the intensity of the output light from the optical waveguide 14.

In each of FIGS. 9 to 11, light is input from the optical waveguide 13 and output from the optical waveguide 14, but conversely, light is input from the optical waveguide 14,
The light may be output from the optical waveguide 13.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the operating principle of a waveguided optical beam splitter, Figures 2a and b are diagrams showing how the eigenmode propagates in this beam splitter, and Figure 2c is a diagram showing the phase constant. FIG. 3 is a graph showing the changes, and FIG. 3 is a diagram showing various relationships between the input and output of light waves to this beam splitter. FIG. 4 is a perspective view showing an embodiment of the present invention, and FIGS. Fig. 6 shows an example of application of the beam splitter according to the present invention, in which Fig. 5a is a perspective view;
FIG. 5b is a configuration diagram of an optical waveguide, FIG. 6 is a graph showing the relationship between applied voltage and relative intensity of output light, and FIGS. 7 and 8 show other application examples. FIG. 7 is a perspective view, FIG. 8 is a graph showing the relationship between applied voltage and relative intensity of output light, and FIGS. 9 to 11 are configuration diagrams showing still other application examples. 1, 2, 3, 4...single mode optical waveguide, 5...
...Joining portion, 10...Substrate, θ1, θ2...Intersection angle.

Claims (1)

[Claims] 1 A symmetrical optical waveguide pair constituted by two single-mode optical waveguides with equal phase constants crossing each other at one end, and two single-mode optical waveguides having mutually different phase constants crossing each other at one end. A pair of asymmetric side optical waveguides constituted by When in-phase light waves are input from one optical waveguide pair, an output light wave is obtained from one optical waveguide of the other optical waveguide pair, and when an opposite-phase optical wave is input from one of the optical waveguide pairs, an output optical wave is obtained from the other optical waveguide pair. an output light wave is obtained from the other optical waveguide of the pair;
Waveguide optical beam splitter.