JPS6123529B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a circuit element that can electrically control the degree of coupling of light guided by one optical waveguide to another adjacent optical waveguide. As is well known, when two optical waveguides with equal phase velocities are arranged at an extremely narrow interval, guided light propagating through one optical waveguide gradually transfers to the other optical waveguide. This is called "optical coupling." 1 like this
A set of optical waveguides is formed in a material that has an electro-optic effect, and the refractive index of each optical waveguide is changed by an external electric field, causing a shift in phase velocity between the two optical waveguides. It is possible to control the optical coupling efficiency and modulate the intensity of light propagating through the optical waveguide. By the way, this type of electrode structure conventionally considered was composed of only two striped electrodes, as shown in FIG. 1 is a plate-shaped substrate made of a substance that has an electro-optic effect, and is usually
Electro-optic crystals such as LiNbO ₃ and LiTaO ₃ are used. 2 indicates the direction of the polarization axis of the substance, which is perpendicular to the substrate surface. 3-1, 3-
Reference numeral 2 denotes an optical waveguide having the same phase velocity, which is formed near the surface of the substrate, and the gap therebetween is formed to be extremely small. 4-1 and 4-2 are two stripe electrodes constituting a planar electrode, and are provided along a part or all of the optical waveguides 3-1 and 3-2, respectively.
Moreover, it is created almost directly above. When a voltage is applied to these two striped electrodes, electric field components in the direction of the polarization axis are concentrated near the ends of the two electrodes, and by utilizing the fact that their signs are different, the light propagating through the optical waveguide can be detected. By increasing one phase velocity and decreasing the other, the optical coupling efficiency is changed and optical modulation is performed. That is, now the phase velocity β of both optical waveguides is
₁ , β ₂ is β ₁ = β ₂ when no voltage is applied.
For example, if the length L ₀ of the coupling portion of both optical waveguides is set to obtain 100% coupling, then the light incident on the optical waveguide 3-1 is, for example,
It will be emitted from the optical waveguide 3-2. If a voltage is applied in this state, a deviation Δβ=β ₁ −β ₂ will occur between the phase velocities β ₁ and β ₂ . Mirror (Bell
SE of Syst.Tech J magazine vol.33, p661 (1954)
According to Mr. Miller's argument, when light of unit intensity is incident on the optical waveguide 3-1, the optical intensity of both optical waveguides at the exit is I ₁ = 1-sin ² (DOL ₀ )/D ² I ₂ = sin ² (DCL ₀ )/D ² However,

It is given by [Formula]. Figure 2 shows the relationship between I ₁ , I ₂ and Δβ/C. Therefore, if Δβ increases due to voltage application, I ₁ gradually increases from O, and conversely I ₂ decreases, so that an optical modulation operation is performed. In particular, by changing Δβ from O to √12C, 100% modulation can be achieved. By the way, optical waveguides with electro-optic effects are usually made by using Cu or Cu on a substrate crystal such as LiNbO ₃ or LiTaO ₃ .
Although it is created by thermally diffusing impurities such as TiO ₂ in the form of a line, it is extremely difficult to create two optical waveguides with exactly the same dimensions and refractive index distribution. Therefore, it is almost impossible to completely match the phase velocities of the two optical waveguides. In this way, there is a slight deviation △ in the phase velocity of the optical waveguide.
When β ₀ is present, for example, if we focus on I ₁ , even in an electric field of 0, there is a leakage I ₁ (△β _{0 /}
C) will be emitted from the optical waveguide 3-1.
(Indicated by A in Figure 2) When a voltage is applied in this state, if a phase velocity shift of the same sign as △β ₀ occurs, the intensity of I ₁ will not be from 0 but at a certain intensity.
Since it changes from I ₁ (△β ₀ /C), the modulation S/
N gets worse. Also, in the case of △β ₀ and the opposite sign,
The intensity of I ₁ decreases, and if |△β ₀ |<|△β|, the inconvenience arises that it becomes 0 once on the way. For this reason, even in a directional optical coupler that is theoretically capable of 100% modulation, in practice the modulation efficiency decreases due to the phase velocity shift that inevitably occurs, making it questionable whether it can be a practical circuit element. It was said that In order to solve this drawback, the present invention provides a state in which the third stripe electrode is installed parallel to and close to the optical modulation electrode, so that the phase velocity of the optical waveguide is adjusted to a predetermined value, and no signal voltage is applied. The phase velocities of both optical waveguides are made to match completely, and the drawings will be described in detail below. FIG. 3 shows an embodiment of the present invention, in which 1 is a plate-shaped substrate made of a substance having an electro-optic effect;
2 indicates the direction of the polarization axis of the substance, which is perpendicular to the substrate surface. 3-1 and 3-2 are almost identical optical waveguides created near the surface of the substrate, and the gap between them is extremely small. The phase velocities of the two do not need to match completely. 4-1 and 4-2 are two stripe electrodes that constitute a planar electrode, and each of them is connected to the optical waveguide 3-
It is created along part or all of 1 and 3-2 and almost directly above it. Reference numeral 5 denotes a stripe electrode for phase velocity adjustment, which is formed close to and parallel to the stripe electrode for optical modulation. FIG. 4 shows the electric field distribution when voltage is applied to two stripe electrodes attached to the substrate surface. The figure shows the results calculated using the "sequential relaxation method." Typical examples of materials used as substrates are
It is an electro-optic crystal such as LiNbO ₃ or LiTaO ₃ and can induce a change in refractive index with high efficiency even with an electric field parallel to the polarization axis. Therefore, in the figure, only the electric field component (absolute value |Ez|) in the direction of the polarization axis is shown. The maximum value of the electric field component is indicated by "*", and the following 9,
An 11-level display from 8 to 0 is used. This shows that the electric field is concentrated near the electrode end, resulting in a bimodal distribution. FIG. 5 similarly shows Ez using the depth from the substrate surface as a parameter. The vertical axis is the electric field Ez normalized by the electric field E ₀ =V ₀ /g when the electrode gap is g and the applied voltage is V ₀ . It is clear from the figure that the directions of the electric fields generated in the vicinity of the two striped electrodes are opposite to each other. When LiNbO ₃ is used as the substrate, the amount of change in refractive index for an extraordinary ray having a polarization plane in the direction of the polarization axis is given by Δne=−1/2n ⁸ _e γ ₃₃ Ez. ne is the refractive index for extraordinary rays, and _γ33 is the electro-optic constant. Therefore, the sign of the refractive index change induced beneath the electrode is different. In the figure, the stripe electrode width W
In contrast, the case where the electrode gap g is W:g=2:3 is shown, but even if this ratio changes, electric field concentration will still occur near the electrode ends. FIG. 6 shows a sectional view of this embodiment. Consider a case where the phase velocity β ₁ of the optical waveguide 3-1 is slightly smaller than the phase velocity β ₂ of the optical waveguide 3-2 in an initial state in which no voltage is applied to the modulation electrodes 4-1 and 4-2. In order to match both phase velocities, it is necessary to increase β ₁ or decrease β ₂ .
According to optical waveguide theory, this can be achieved by increasing the refractive index of the optical waveguide 3-1 or decreasing the refractive index of the optical waveguide 3-2. The sign of the refractive index change when a voltage is applied between the stripe electrodes is negative when the electric field Ez matches the direction of the polarization axis. Therefore, if the modulation electrode 4-2 is used as a common ground electrode and the phase adjustment voltage 6-2 is applied between the electrodes 4-2 and 5, the refractive index of the optical waveguide 3-2 will decrease. death,
The phase velocity shift Δβ can be set to zero. In this state, a voltage 6-1 for optical modulation may be applied between the electrodes 4-1 and 4-2. Next, consider the case where β ₁ >β ₂ . In this case as well, it is necessary to increase β ₂ as before. In other words, the refractive index of the optical waveguide 3-2 may be increased. That is, in the optical waveguide 3-2, the electric field Ez only needs to be in the opposite direction to the polarization axis, so a positive voltage can be applied to the electrode 4-2 as shown in FIG. In this embodiment, a case is shown in which an optical waveguide is formed on the substrate surface in the positive direction of the polarization axis, but the same can be considered for the case in which the optical waveguide is formed on the substrate surface in the negative direction. Another embodiment of the invention is shown in FIG. Components that are the same as those in FIG. 3 are given the same numbers. The difference from the embodiment shown in FIG. 3 is that the modulation electrodes 4-1, 4-2
Two stripe electrodes are applied in parallel and close to each other on the outside of both sides. These are the cases of more practical electrode configurations for phase velocity adjustment. Generally, the magnitude relationship between the phase velocities of the two optical waveguides produced may be reversed even if they are produced under the same conditions, and the ground electrode for adjusting the phase velocity must also be changed. Correspondingly, the electrode position for phase velocity adjustment also changes. FIG. 8 shows a case in which the phase velocity can be adjusted in any case. The magnitude relationship is determined by measurement after the waveguide is fabricated. These operating principles are the same as those shown in FIGS. 6 and 7, so a description thereof will be omitted. As explained above, according to the present invention, the phase velocities of the two optical waveguides for optical coupling do not necessarily have to be completely matched when the optical waveguides are created, and the third stripe electrode Since it is possible to adjust the phase velocity by using this method, it is possible to solve the technical problem of having to create exactly the same optical waveguide. Therefore, the present invention has great advantages not only in optical modulation in optical transmission, but also in increasing its practicality as an integrated optical switch element.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a conventional directional optical coupler;
3 is a diagram showing the configuration of an embodiment of the present invention. FIG. 4 is a diagram showing an example of the calculation results of the electric field distribution of the planar electrode structure according to the present invention. Figure 5 is a characteristic diagram showing an example of the calculation results of the electric field distribution in the same planar electrode structure.
The figure is a side view of FIG. 3 and is an explanatory diagram showing an example of voltage application. FIG. 7 is a side view of FIG. 3 and is an explanatory diagram showing another example of voltage application. FIG. It is a block diagram of an Example. 1... Plate-shaped substrate, 2... Polarization axis direction, 3-1,
3-2... Optical waveguide, 4... Stripe electrode (for optical modulation), 5... Stripe electrode (for phase velocity adjustment), 6-1, 6-2... Power supply voltage.

Claims (1)

[Claims] 1. Two almost identical optical waveguides close to each other near the same surface of a plate-shaped substrate having an electro-optic effect and having a surface perpendicular to the direction of the polarization axis, and a part or part of the optical waveguide on the surface of the substrate. a directional optical coupler comprising two stripe electrodes along the entire length thereof, a third stripe electrode on the surface of the substrate, parallel to and adjacent to the stripe electrode, on the outside of one or both of the electrodes; , and a voltage source connected between the stripe electrode and the third stripe electrode.