JPH0554090B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to optical logic circuits. 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 optical logic circuits. An object of the present invention is to provide an optical logic circuit that directly performs logical operations on light and is therefore capable of high-speed processing. The optical logic circuit according to the present invention includes a pair of symmetrical optical waveguides constituted by two single-mode optical waveguides having the same phase constant crossing each other at one end, and a pair of symmetrical optical waveguides having two single-mode optical waveguides having mutually different phase constants. A pair of asymmetric optical waveguides constituted by 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 (asymmetrical X-branch optical waveguide). One pair of optical waveguides on the symmetric side or the asymmetric side is used as the optical input side, and the other side is used as the optical output side. Incident light having a predetermined phase relationship is introduced, and a phase shifter is provided on the substrate to control the phase of this incident light according to a logical value input signal, and a logical value is output from the optical waveguide pair on the optical output side. It is characterized by being able to obtain output light representing . A phase shifter is provided on the optical waveguide pair on the optical input side, and according to a certain logic, depending on the phase shift state of the incident light by this phase shifter,
Output light is obtained from one or both optical waveguides of the pair of optical output side optical waveguides. Since logical operations are performed directly using light, extremely high-speed optical logical operations can be realized. Further, in this invention, since an asymmetrical X-branch optical waveguide is used, the extinction ratio is good, and the phase relationship between the incident light and the output light is highly accurate. First, the operating principles of the two optical waveguide pairs used in this invention will be explained. In FIG. 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, and therefore have equal phase constants. There is another pair of single mode optical waveguides 3 and 4, which also intersect at one end at a small angle θ2. Unlike the widths W3 and W4 of the optical waveguides 3 and 4, the width W4 of the optical waveguide 4 is narrower than the width W3 of the optical waveguide 3. Therefore, the phase constants of optical waveguides 3 and 4 are different, and optical waveguide 3 is larger. Width W1 of optical waveguides 1 and 2,
Although W2 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,
θ1≦θ2 may be satisfied. Such an optical waveguide 1,
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, optical waveguides 1 and 2 to optical waveguides 3,
Take the XYZ coordinate axes, with the direction facing 4 as the Z axis and the direction perpendicular to the paper as the X axis. In addition, optical waveguides 1 and 2
is 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. In this figure, a light wave incident on the symmetrical side with a phase indicated by a solid line propagates through the optical waveguide 3 with a phase indicated by a solid line. Furthermore, the light wave that has entered with the phase indicated by the broken line exits from the optical waveguide 3 with the phase indicated by the broken line. The light waves indicated by the solid line and the light waves indicated by the broken line are in opposite phase to each other. FIG. 3b shows a case where light waves having opposite phases to each other (for example, light waves shown by solid lines or light waves shown by chain lines) are input into two optical waveguides 1 and 2 on the symmetric side. On the symmetric side, the odd mode is excited and propagates,
At the coupling portion 5, the mode changes to the first-order mode, and on the asymmetric side, the mode changes again to the odd mode. 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. An output light wave caused by an input light wave shown by a solid line is shown by a solid line, and an output light wave caused by an input light wave shown by a chain line is shown by a chain line. Note that it is also possible to input the light wave from the asymmetric side, and in this case, the reverse process described above is followed. Now, as an example of a logic circuit, Ex−NOR
Consider a (exclusive NOR) circuit. The symbol of the two-input Ex-NOR circuit is as shown in FIG. 4, and the truth table of the output X for the two inputs A and B is shown in the following table. Input A, B
If only one of them is "1", the output X becomes "0".

[Table] In the above two pairs of optical waveguides, optical waveguide 1,
Let us consider the output from the optical waveguide 3 when optical signals having the same phase or 180° out of phase (opposite phase) are input to the optical waveguide 2. It is assumed that the logic "0" and "1" of the input optical signals to the optical waveguides 1 and 2 are expressed by their phases. That is, in FIG. 3a, if the input optical signal has a phase shown by a solid line, it is set as "1", and if it has a phase shown by a broken line, it is set as "0".
The input side optical waveguides 1 and 2 are inputs A and B shown in FIG.
correspond to It is assumed that the logic "0" or "1" of the optical signal obtained from the optical waveguide 3 is expressed by the presence or absence of optical power. That is, as shown in FIG. 3a, if there is a light wave output from the optical waveguide 3, it is set as "1", and as shown in FIG. This is set as "0". The output of the optical waveguide 3 is represented by X1.
If the logic of each optical signal is determined in this way, the output X1
It will be understood that is Ex-NOR of inputs A and B. That is, as shown in Fig. 3a, when inputs A and B are both "0" or "1", output X1 is "1", and as shown in Fig. 3b, when inputs A and B are both "0" or "1", output If either one is "1", the output X1 becomes "0". In such an optical waveguide, a complementary output to the output X1 is obtained from the optical waveguide 4. If the output of the optical waveguide 4 is represented by X2, the output X2 is the output of the inputs A and B.
Ex−NOR. The relationship between inputs A and B and outputs X1 and X2 is shown in the truth table described above. In this way, a predetermined optical logic operation is achieved by representing logical "0" and "1" according to the phase of the light wave and the presence or absence of the light wave. FIG. 5 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. Parallel optical waveguides 11 and 1 are provided at the ends of these optical waveguides 1 to 4 opposite to the coupling portion 5.
2, 13, and 14 are consecutive. Further, a Y-shaped optical branch path consisting of optical waveguides 21 to 23 is formed. The optical waveguides 21 and 22 are continuous with the optical waveguides 11 and 12, respectively. Furthermore, electrodes 31 and 32 are provided on both sides of the optical waveguides 11 and 12, respectively, so as to sandwich these optical waveguides. These electrodes 31 and 32 constitute a phase shifter. Appropriate voltages are applied to each electrode 31, 32 via switches 33, 34 of a control circuit 35, respectively. When a voltage is applied to the electrodes 31 and 32, the phases of the lights propagating through the optical waveguides 11 and 12 are shifted by 180 degrees, respectively. The optical waveguide 23 side is the input side, and light from a light emitter 41, such as a semiconductor laser, enters the optical waveguide 23. The logic output is obtained from the optical waveguide 13 or 14, and these output lights are received by photodetectors 43 and 44, respectively, and converted into electrical signals before being sent to the processing circuit 4.
Enter 5. The light incident on the optical waveguide 23 is split into two parts with equal optical power and the same phase by the Y-shaped optical branch path, and travels to the optical waveguides 11 and 12 via the optical waveguides 21 and 22, respectively. When the switches 33 and 34 are off (in this case, both inputs A and B are set to "1"), the light propagating through the optical waveguides 11 and 12 is in phase with each other, so that only the optical waveguide 13 Output light is obtained (output X1 is "1" and output X2 is "0"). Both switches 33,
34 are both on (both inputs A and B are "0"), the phases of the lights traveling through the optical waveguides 11 and 12 are shifted by 180 degrees, so in this case as well, they are in phase with each other, and the optical waveguides 13 and 13 are in phase with each other. Output light can be obtained only from the When either switch 33 or 34 is on (either input A or B is "0"),
Since the phase of one of the lights propagating through the optical waveguides 11 and 12 is inverted, the lights propagating through these optical waveguides 11 and 12 have opposite phases to each other, and output light can be obtained only from the optical waveguide 14. (Output X1
is “0” and X2 is “1”). In this way, Ex-NOR and Ex-OR logic outputs are obtained from the photodetectors 43 and 44, realizing a very high-speed optical logic circuit. It goes without saying that many other optical materials besides LiNbO ₃ can be used as the substrate. For example, an optical waveguide can be formed by using glass as a substrate and diffusing silver ions. Let us briefly touch on the fact that optical logic operations can be realized in the same way even when light waves are input from the asymmetric side. FIGS. 6a and 6b show extracted light having the phase indicated by the solid line in FIGS. 3a and 3b. FIG. 6c 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 light waves in the optical waveguide 1 are superposed on each other in Figure 6 a and b (in Figure 6 a and b, the light waves in the optical waveguide 1 (because they are out of phase, they cancel each other out), and light waves with equal power and the same phase are output to the optical waveguides 3 and 4. 6th
Figure d shows the case where the light wave is input only to the optical waveguide 1, and in this case, the phases of the light waves in Figure 6a and Figure 6b are reversed (shown by the chain line in Figure 3b). light wave), and optical waveguides 3 and 4
A light wave with a power equal to and with an opposite phase is output. As described above, when light waves are input from the asymmetric side, the process opposite to the above is followed. That is, when a light wave is input to the optical waveguide 3, light waves in the same phase are output from both optical waveguides 1 and 2 on the symmetric side. (Indicated by the dashed arrow in Figure 6a). When light waves are input to optical waveguide 4, optical waveguides 1 and 2
Light waves having mutually opposite phases are output from the two (indicated by broken line arrows in FIG. 6b). Furthermore, when optical waves of the same phase are introduced into the optical waveguides 3 and 4 on the asymmetric side, the optical wave is output from only one optical waveguide 2 on the symmetric side, and when optical waves of opposite phase are introduced into the optical waveguides 3 and 4, An output light wave is obtained only in the other optical waveguide 1 on the symmetrical side (indicated by broken arrows in FIGS. 6c and d, respectively).
From the above, it can be seen that the Ex-NOR or Ex-OR logical operation can be similarly realized even when light waves are input from the asymmetric side.

[Brief explanation of drawings]

Figure 1 is a configuration diagram showing the operating principle of two optical waveguide pairs, Figure 2 a and b are diagrams showing how the eigenmode propagates in this optical waveguide pair, and Figure 2 c is a diagram showing the change in phase constant. 3 is a diagram showing various relationships between the input and output of light waves to this optical waveguide pair, FIG. 4 is an Ex-NOR circuit diagram, and FIG. 5 is a graph showing the following:
A perspective view showing an embodiment of the invention, FIGS. 6a and 6b,
c and d collectively show the relationship between the input and output of light waves in a pair of optical waveguides for all cases in which light waves are introduced into the symmetrical side and the asymmetrical side, respectively. 1, 2, 3, 4... optical waveguide, 5... coupling part,
10... Substrate, 31, 32... Electrode, θ1, θ2
...intersection angle.

Claims (1)

[Claims] 1. A pair of symmetrical optical waveguides formed 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 optical waveguides on the asymmetric side are formed on the substrate and are configured by crossing each other at one end, and are connected to each other at each intersection to form an X-shaped optical waveguide, and the optical waveguides on the symmetric side or One of the optical waveguide pairs on the asymmetric side is used as the optical input side, and the other is used as the optical output side. When light is introduced, a phase shifter is provided on the substrate to control the phase of this incident light according to a logic value input signal, and output light representing a logic value output is obtained from the optical waveguide pair on the light output side. Optical logic circuit.