JPH0364048B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a waveguide type optical switch element, and particularly to a waveguide type optical switch element that can electrically control the traveling direction of light traveling in a waveguide.

(Prior Art) Conventionally, an optical switch element using a directional coupler and having no polarization dependence has been proposed (Reference:
Appl.Phys.Lett. 35 (10), 15 November 1979,
p748−750 and literature: IEEE JOURNAL OF
QUANTUM ELECTRONICS, QE−18 (10),
(1982), p1772−1779).

The optical switch element disclosed here is
This type has a waveguide formed by diffusing Ti into a LiNbO ₃ substrate. As schematically shown in a plan view in FIG. 5, this structure has a first waveguide 1 in the form of straight stripes with a constant width and a second waveguide 2 in the form of curved stripes with a constant width on a substrate. Further, a first electrode 3 is provided on the first waveguide 1, and a second electrode 4 and a third electrode 5 are provided on the second waveguide 2. The first electrode 3 is used as a ground side electrode, and the second and third electrodes 4 and 5 are used as control electrodes.

In this element structure, the direction from the upper edge of the electrode 3 to the lower edge is the waveguiding direction, and the waveguiding distance from this upper edge is z, and the waveguiding distance to the lower edge of the electrode 3 (element length) is L. In addition, the spacing between the first and second waveguides 1 and 2 is d, and the structure is such that the spacing d sequentially changes according to the distance z so that it becomes minimum between the second and third electrodes 4 and 5. It is becoming. In this structure, when equal voltages are applied variably to the second and third electrodes 4 and 5 on the second waveguide 2, a high extinction ratio (i.e., a ratio value) is obtained over a wide range above a certain threshold voltage. A bar state of (large) is obtained.

The optical switch element of the conventional structure shown in FIG. 5 utilizes the phenomenon of obtaining a bar state having such properties (having a threshold value).

Generally, in an optical switch element using a directional coupler, first, the coupling length is made to match the element length, and + and - voltages are applied to the second and third electrodes 4 and 5, which are divided into two. The light input to the input port (z=0) of one waveguide 1 or 2 is transferred to the other waveguide 2 or 1 by applying a
A so-called cross state is obtained in which the signal is transferred to the output port (z=L) and output from the output port (z=L).

On the other hand, in the so-called bar state, in which light input to the input port of one waveguide 1 or 2 is made to travel straight and output from the output port of the same waveguide 1 or 2, the second and third electrodes 4 and 5 Obtained by uniformly applying the same voltage.

By the way, LiNbO ₃ which is usually used for the substrate
The electro-optic effect for the extraordinary light is three times larger than for the ordinary light, so the equivalent refractive index difference of the extraordinary light is three times larger than that of the ordinary light. Therefore, if a voltage is applied to obtain a lever state for ordinary light, a bar state with a high extinction ratio can be obtained for extraordinary light.

In addition, in order to obtain a cross state for ordinary and extraordinary light, the design and manufacturing conditions must be determined so that the coupling length for both ordinary and extraordinary light is the same, and this coupling length is approximately equal to the element length. Then, by applying + and - voltages to the two-part electrodes 4 and 5, respectively, and finely adjusting the bond length, the element length and the bond length may be perfectly matched.

(Problems to be Solved by the Invention) With the method described above, a bar state with a high extinction ratio for both ordinary light and extraordinary light can be easily obtained.

However, as mentioned above, due to the fact that the magnitude of the electro-optic effect differs depending on the type of polarization, when trying to control both lights when obtaining a cross state, compared to controlling only one light, The allowable range of manufacturing conditions such as the difference in equivalent refractive index and the value of the coupling coefficient has become significantly narrower, making it difficult to obtain a high extinction ratio.

It is an object of the present invention to provide a waveguide type optical switch element which relaxes the manufacturing conditions for obtaining a cross state, obtains a high extinction ratio, and operates by simple voltage control.

(Means for solving the problem) In order to achieve this objective, according to the present invention,
By sequentially changing the mutual spacing according to the waveguide distance from the starting point to the end point of the waveguide of the optical switch, the mutual coupling coefficient can be changed sequentially according to the waveguide distance. In a waveguide type optical switch element comprising first and second waveguides provided on a substrate and electrodes provided in relation to these waveguides, the equivalent refraction of these first and second waveguides is The index difference becomes zero at the midpoint of the waveguiding distance and increases toward the starting point and ending point of the waveguiding, and at the starting point of the waveguiding, the equivalent refractive index of the first waveguide becomes the second waveguide. is larger than the equivalent refractive index of the waveguide, and conversely, at the end point of the waveguide, the equivalent refractive index of the second waveguide is larger than the equivalent refractive index of the first waveguide.
It is characterized in that the width of both or one of the first and second waveguides is changed according to this distance.

Furthermore, in carrying out the present invention, the electrodes are used as a control electrode and a ground side electrode, the control electrode is provided on the first waveguide, and the ground side electrode is provided outside of the first and second waveguides. is suitable.

(Function) According to the present invention, in addition to weighting the coupling coefficient by changing the interval between two waveguides according to the waveguide distance in the waveguide direction, the width of the waveguide is By changing the , the equivalent refractive index difference between the waveguides is weighted so that it becomes zero at the center of the element and increases toward both edges, so the equivalent refractive index difference to obtain the cross state is The range of the refractive index difference is widened, and therefore, when an optical switch element without polarization dependence is constructed, a cross state can be easily obtained.

Furthermore, since the conditions for the value of the coupling coefficient are relaxed, when an optical switch element without polarization dependence is constructed, it is not necessary to match the coupling coefficients of ordinary light and extraordinary light.

Furthermore, for a bar state, a high extinction ratio can be obtained regardless of polarization.

Furthermore, since voltage is applied only in the bar state, voltage control becomes simple and easy.

(Embodiments) Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1A is a diagram schematically showing an embodiment of the waveguide type optical switch element of the present invention. In the figure, 21 and 22 are first and second waveguides, which are formed on the substrate. Note that this substrate is made of a material such as LiNbO ₃ -z, which has a large change in refractive index due to the Pockels effect with respect to an electric field in a direction perpendicular to the plane of the drawing. 23 and 24 are these waveguides 21 and 2
first and second electrodes respectively provided on 2;
One edge of these electrodes is taken as the starting point of waveguide of the optical switch element (the point where z=0), and the other edge is taken as the end point of the waveguide (the point where z=L).

In this example, the width of 21 is sequentially increased as the waveguide distance (also called propagation distance) increases from the point z=0, and by doing this, the equivalent refractive index increases with the waveguide distance. It is designed as such. Generally, the equivalent refractive index increases as the width increases.

Further, the second waveguide 22 is curved with a certain curvature relative to the first waveguide 21, so that the distance d between the waveguides 21 and 22 is minimized at the midpoint of the element, and the increase in z Thus, the coupling coefficient between 21 and 22 changes as the waveguide distance z increases. Generally, as the spacing increases, the coupling coefficient decreases.

Furthermore, in this embodiment, unlike the conventional case, a perfect crossing state is obtained by the waveguide structure, so there is no need to use a two-part electrode as a control electrode.

Next, with reference to FIG. 2, the operating principle of the optical switch element having the structure shown in FIG. 1A will be explained.

In such a structure, the first and second electrodes 2
When no voltage is applied to 3 and 24 and 〓 and ±
FIG. 2 shows the equivalent refractive indices n _a , n _b and _na ′, n _b ′ of each waveguide 21 and 22 when a voltage of is applied. In the figure, the horizontal axis shows the propagation distance (waveguide distance) z, and the vertical axis shows the equivalent refractive index.
The n _eff is plotted and shown.

First, the cross state will be explained. In order to obtain a crossed state, it is assumed that no voltage is applied to the first and second electrodes 23 and 24. In this case, as shown in Figure 2, the equivalent refractive indices n _a and n _b are n _a < n _b where the propagation distance z is small, and at the midpoint of the element, z = 1/2L. n _a =n _b . Furthermore, where z is large, n _a >n _b .

In order to obtain this state, in the structure shown in FIG. 1A, the width of the waveguide 21 is tapered from the point z=0 to the point z=L, and At this point, the widths of both waveguides 21 and 22 are made equal.

By the way, the conditions for obtaining a cross state are that n _a is a linear function of the propagation distance z, and the coupling coefficient is Ko
It is determined from the coupling equation as exp{z-L/2) ² /r}. However, L is the element length, and r and Ko are constants determined by the waveguide spacing. in this case,
As shown in FIG. 5, when the equivalent refractive index of the first waveguide 21 does not change with respect to the propagation distance z (△n=0), the structure shown in FIG. It is determined how many times light input from the input port (z=0) of one of the waveguides passes between these two waveguides 21 and 22 while traveling to the output port (z=L). (Literature: Reference).

Here, the equivalent refractive index of the first waveguide 21 is the fifth
As in the case shown in the figure, there is no change (△n=
0), Ko and r are set so that the number of times the light moves between both waveguides 21 and 22 is 3 or more times.
(A in Figure 3), A in Figure 1
As shown in FIG.
If the difference △n between the equivalent refractive indices n _a and n _b at L) is made sufficiently large, a cross state (extinction ratio - 20dB). In addition, here, Figure 3 A~
C is a characteristic curve diagram showing the relationship between propagation distance z and optical power, where r = 500 and Ko = 0.15.
When A is △n/Ko=0, B is △
When n/Ko=1.3/(2π/λ) and C is △n/
In these figures, the dotted line indicates the optical power of the waveguide into which light is input, and the solid line indicates the optical power of the waveguide into which no light is input. ing.

In comparison with the conventional example, in the case of the conventional optical switch element shown in FIG. 5, if the element length and the coupling length do not match as shown in FIG. 3A, the second and third electrodes 4 and 5, respectively, to match the element length and the bond length to obtain a cross state. In this case, as shown in FIG.
In the state shown in , the tolerance range for the deviation from the optimal voltage value is narrow (-20 dB), so the tolerance range for the uniform equivalent refractive index change △n _e caused by voltage is also narrow; It is impossible to simultaneously obtain a cross state for both ordinary and extraordinary polarized light, which have different electro-optic effects.

However, as in this embodiment, if the width of the waveguide 21 is varied in a tapered manner along the waveguide direction, a cross state can be obtained within a structurally wide range of equivalent refractive index change △n _e . Therefore, the manufacturing conditions of the optical switch element can be greatly relaxed.

Next, the bar state will be explained with reference to FIGS. 4A and 4B. FIGS. 4A and 4B are characteristic curve diagrams showing the relationship between propagation distance and optical power similar to FIGS. 3A to 3C. These are shown under the conditions of r = 500, Ko = 0.15, and △n/Ko = 1.3/(2π/λ), and in the case of A, △n'/Ko = 1.7/(2π/
λ) and B, △n′/Ko=6/(2π/λ)
It is. In the figure, the dotted line represents the optical power of the waveguide into which light is input, and the solid line represents the optical power of the waveguide into which light is not input.

To obtain the bar state, + and - voltages are applied to the electrodes 23 and 24, respectively. In this case, an equivalent refractive index difference Δn' occurs between the waveguides 21 and 22 as shown in FIG. At this time, due to the properties of this type of directional coupler, the light input from the input port (z = 0) of the waveguide 21 hardly transfers to the waveguide 22, and the light input from the output port (z = 0) of the waveguide 21 L).

In this embodiment, as with the conventional optical switch element, the extinction ratio does not decrease below -18 dB above a certain Δn' (voltage) (FIGS. 4A and 4B).

FIG. 1B shows another embodiment of the invention, in which the widths of both the first and second waveguides 21 and 22 are tapered, and in the first waveguide 21 the waveguide distance is Therefore, the second waveguide 22 is made wider, and the second waveguide 22 is made narrower, and the waveguide distance L is set for both waveguides 21 and 22.
A curvature is provided so that the interval is minimized at the midpoint of the waveguide to prevent variations in characteristics caused by the input waveguide.

FIG. 1C shows still another embodiment of the present invention, in which the second electrode 24 is provided only on the first waveguide 21, and the first electrode 23 is placed on the left side from the first waveguide 21. The second waveguide 22 is provided separately, and has a structure in which no electrode is provided on the second waveguide 22.
By doing this, an optical switch element structure is obtained that operates on materials exhibiting the Kerr effect, such as KTM and PLZT.

(Effects of the Invention) As is clear from the above explanation, the waveguide type optical switch element of the present invention has a wide range of equivalent refractive index difference for obtaining a cross state, so that it can produce light without polarization dependence. Even when a switch is configured, a cross state can be easily obtained.

Furthermore, according to the present invention, the conditions for the value of the coupling coefficient are relaxed, so even if an optical switch without polarization dependence is constructed, it is not necessary to match the coupling coefficients of both the ordinary and extraordinary polarized lights.

Further, according to the present invention, a bar state with a high extinction ratio can be obtained regardless of polarization.

Furthermore, according to the present invention, voltage is applied to the electrodes only when a bar state is obtained, making voltage control simpler and easier than in the past.

[Brief explanation of drawings]

1A to 1C are schematic plan views for explaining the structure of the waveguide type optical switch element of the present invention, and FIG. 2 is a characteristic showing changes in the equivalent refractive index during operation, for explaining the present invention. Curve diagrams, FIGS. 3 A to C and FIGS. 4 A and B are characteristic curve diagrams showing the relationship between propagation distance and optical power for explaining the operation of the waveguide type optical switch element of the present invention, and FIG. 1 is a schematic plan view for explaining a conventional waveguide type optical switch element. 21...First waveguide, 22...Second waveguide, 2
3...first electrode, 24...second electrode.

Claims (1)

[Claims] 1. The mutual coupling coefficient is determined by changing the mutual spacing according to the waveguide distance from the starting point to the end point of the waveguide of the optical switch. In a waveguide optical switch element comprising, on a substrate, first and second waveguides provided to vary sequentially according to the waveguides, and electrodes provided in association with these waveguides, the distance being It is large at the starting point and end point of the waveguide and is minimum at the midpoint of the waveguiding distance, and the equivalent refractive index difference between the first and second waveguides is at the midpoint of the waveguiding distance. The refractive index of the first waveguide becomes zero and increases toward the starting point and ending point of the waveguide, and at the starting point of the waveguide, the equivalent refractive index of the first waveguide is lower than the equivalent refractive index of the second waveguide. conversely, the equivalent refractive index of the second waveguide is larger than the equivalent refractive index of the first waveguide. A waveguide type optical switch element characterized in that the width of one or both of the elements is changed according to the distance. 2. In the waveguide optical switch element according to claim 1, the electrode is a control electrode and a ground side electrode, the control electrode is provided on the first waveguide, and the ground side electrode is provided on the first waveguide. and a waveguide type optical switch element, characterized in that it is provided outside the second waveguide.