JPS6211831A - Waveguide type optical switching element - Google Patents

Abstract

PURPOSE:To relieve the preparing conditions for obtaining a cross state and to obtain a high extinction ratio by changing the widths of the 1st and 2nd waveguides according to the distance in the waveguide direction of an optical switch so as to maintain the adequate difference in the equiv. refractive index between the 1st and 2nd waveguides. CONSTITUTION:The 1st and 2nd waveguides 21, 22 are formed to a substrate consisting of LiNbO3-z or the like. Electrodes 23, 24 are respectively provided on the waveguides 21, 22. The width of the wave guide 21 is successively increased according to the increase of the waveguide distance z from the start point (z=0) of the guided wave of the optical switching element which is the one end edge of the electrodes 23, 24 to increase the equiv. refractive index together with the waveguide distance. The 2nd waveguide 22 is disposed to curve with respect to the 1st waveguide 21 to change the coupling coefft. between the waveguides 21 and 22 according to the increase of the waveguide distance.

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 in which the traveling direction of light traveling through a waveguide is electrically controlled 111.

(Prior Art) Conventionally, an optical switch element using a directional coupler and having no polarization dependence has been proposed (Reference I: App 1. Ph
ys, Lett, 35(10), 15 Novem
ber 1979, p74B-750 and literature IF
: IEEE JO RNAL OFQUANTUM
ELECTRONIC3, QE-18 (10), (1
982), p +772-177!3).

The optical switch element disclosed herein is of a type in which a waveguide is formed by diffusing Ti into a LiNb0 substrate. As schematically shown in a plan view in FIG. 5, this structure consists of a first waveguide l in the form of a straight stripe with a constant width and a second waveguide in the form of a curved stripe with a constant width and a constant curvature. 2 is provided on the substrate L, a first electrode 3 is provided on the first waveguide 1'2, 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 waveguide direction is from the -L side edge to the lower edge of the electrode 3, and the waveguiding distance from the -L side edge is 2, and the waveguide direction from the -L side edge to the lower edge of the electrode 3 is set to 2. Let L be the wave distance (element length). Also, the distance d between the first and second waveguides l and 2 is defined as the distance d between the second and third electrodes 4 and 5.
The structure is such that the distance changes sequentially according to the distance #2 so that the distance becomes the minimum in the middle of the distance #2. In this structure, when equal voltages are variably applied to the second and third electrodes 4 and 5 on the second waveguide 2, a high extinction ratio (i.e., ratio value) is obtained over a wide range of -1- above a certain threshold voltage. is large) of par (
Bar) state is obtained.

The conventional optical switch element 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 a voltage of 10 and - is applied to the second and third electrodes 4 and 5, which are divided into two. was applied to finely adjust the coupling length, and one waveguide 1
Alternatively, the light input to the input port of 2 (z = O) can be transferred to the other waveguide 2 or 1 and the output port (Z
=L), the so-called cross
) is getting the state.

On the other hand, the 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 so-called bar state is the second and third electrode 4
and 5 by uniformly applying the same voltage.

By the way, the electro-optic effect of LiNbO3 used in the substrate for extraordinary light is usually three times greater than for ordinary light, and therefore the equivalent refractive index difference of extraordinary light is three times greater than that of ordinary light. Therefore, if a voltage is applied to obtain a bar state for ordinary light, an eight-state with a high extinction ratio can be obtained for extraordinary light.

Also, to obtain a cross state for ordinary light and extraordinary light,
The design and manufacturing conditions are determined so that the coupling lengths match for both ordinary light and extraordinary light, and this coupling length almost matches the element length, and voltages of 0 and - are applied to the two-part electrodes 4 and 5. The element length and the bond length may be perfectly matched by applying each voltage and finely adjusting the bond length.

(Problems to be Solved by the Invention) In method V 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, this is due to the fact that the magnitude of the electro-optic effect differs depending on the type of polarized light.

Therefore, when trying to control both lights to obtain a cross state, the tolerance range for manufacturing conditions such as the equivalent refractive index difference and the value of the coupling coefficient becomes significantly narrower than when controlling only one light. For this reason, it has not been easy to obtain a high extinction ratio.

An object of the present invention is to provide a waveguide type optical switch element which can ease the manufacturing conditions for obtaining a cross state, obtain a high extinction ratio, and operate by simple voltage control.

(Means for Solving the Problems) In order to achieve this object, according to the present invention, the mutual coupling coefficient is adjusted so that the spacing between the phase buds is sequentially adjusted according to the distance in the waveguide direction of the optical switch. A waveguide type optical switch comprising, on a substrate, first and second waveguides provided so as to change sequentially according to the distance, and electrodes provided in relation to these waveguides. In the element, the first and second waveguides are arranged such that the equivalent refractive index difference between the first and second waveguides changes according to the above-mentioned distance and becomes zero at the midpoint of the waveguide distance of the optical switch element. It is characterized in that the width of both or one of the two is changed according to this distance.

Furthermore, in carrying out the present invention, the width of one or both of these waveguides is changed so that the equivalent refractive index difference between the first and second waveguides is changed at a constant rate depending on the distance between the first and second waveguides. It is preferable to let it.

Furthermore, the width of both or one of these first and second waveguides is varied at a constant rate, and these waveguides are arranged so that the above-mentioned spacing between the waveguides is the minimum at the above-mentioned midpoint and this waveguide is at a point. It is preferable to arrange them symmetrically.

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 this 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 value, 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. The range is wide, and therefore, a cross state can be easily obtained when an optical switch element without polarization dependence is constructed.

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 the Par 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. 1(A) 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. Incidentally, as this substrate, for example, LiNbO3-Z
S made of a material that has a large change in refractive index due to the Pockerne effect in response to an electric field in a direction perpendicular to the paper plane of the figure, such as
It uses a polished board. 23 and 24 are these waveguides 2
1 and 22, respectively, one edge of these electrodes is used as the starting point for waveguide of the optical switch element (point Z-0), and the other edge is used as the waveguide point. end point (z
= point L).

In this example, the width of 21 is sequentially widened from the point z=0 as the waveguide distance (also referred to as propagation distance) 2 increases, 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 arranged so as to be curved with a certain curvature relative to the first waveguide 21, so that the distance d between these waveguides 21 and 22 is the minimum at the midpoint of the element, and The coupling coefficient between 21 and 22 is configured to change as the waveguide distance Z increases. Generally, as the spacing increases, the coupling coefficient decreases.

Furthermore, in this embodiment, unlike the conventional case,
Since the waveguide structure provides a completely lossy state, 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. 1(A) will be explained.

In such a structure, the first and second electrodes 23 and 2
Isometric refractive index n, 1 of each waveguide 21 and 22 when no voltage is applied to 4 and when voltages of 10 and - are applied
, nl, and n, A, l, nbl are shown in FIG. 2, respectively. In the figure, the horizontal axis plots the propagation distance (waveguide length M) Z, and the vertical axis shows the equilateral refractive index "ef".
f 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 FIG. 2, the isolateral refractive indexes na, nb are n
, <nl, , and at the point z=1/2L, which is the midpoint of the element, na =rrl). Furthermore, where 2 is large, na > nl).

In order to obtain this state, in the structure shown in FIG. 1(A),
The width of the waveguide 21 widens in a tapered manner from the point z=0 toward the point z=L, and the widths of both waveguides 21 and 22 are made equal at the midpoint of the element.

By the way, the conditions for obtaining a cross state are that n is a linear function of propagation distance 2, and the coupling coefficient is Ko exp(-(
z−L/2) 2/ r) is obtained from the bond equation. However, L is the element length, and r and KO are constants determined by the waveguide spacing. In this case, when the isolateral refractive index of the first waveguide 21 does not change with respect to the propagation distance 2 (Δn=0) as shown in FIG. The light input from the input port (z=0) of one waveguide in the structure shown in Figure (A) is transmitted to the output port (z=L).
It is determined how many times these two waveguides 21 and 22 are to be changed over the course of the process (see Reference 2I).

Here, when it is assumed that the isolateral refractive index of the first waveguide 21 does not change (Δn=0) as in the case shown in FIG.
The number of times the light moves between both waveguides 21 and 22 is 3 times.
K so that it becomes 1-. and r is set (third
As shown in FIG. 1(A), the expansion angle of the width of the first waveguide 21 is increased to create equal sides at both ends of the element (z=0.z=L). If the difference △n between the refractive indices n and n is made sufficiently large, (△n≧Ko/(2π
/in)), a loss state (extinction ratio -2 odB) as shown in FIGS. 3(B) and 3(C) is obtained. Note that FIGS. 3(A) to 3(C) are characteristic curve diagrams showing the relationship between propagation distance 2 and optical power, in which r = 500, and
K.

-〇, when 15, (A) is Δn/, when = 0,
(B) is when △n/K o ” 1.3/(271/in) and (C) is when △n/K o = 4/
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.

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. 3(A), the second and third electrodes 4 and 5
By applying voltages of 10 and -, respectively, the element length and the bond length are matched to obtain a cross state. In this case, the state shown in Figure 3 (A) is optimal. Since the tolerance range for deviation from the voltage value is narrow (-20dB),
The tolerance range for the uniform isolateral refractive index change Δne caused by voltage is also narrow, and for this reason, it is impossible to create a cross state at the same time for both ordinary and extraordinary polarized light, which have different electro-optic effects. It is possible.

However, if the width of the waveguide 21 is varied in a tapered manner along the waveguide direction as in this embodiment, a cross state can be obtained within a structurally wide range of isolateral refractive index change Δn8. Therefore, the manufacturing conditions of the optical switch element can be greatly relaxed.

Next, the eight-state will be explained with reference to FIGS. 4(A) and CB). Figure 4 (A) and (B) are Figure 3 (A)
It is a characteristic curve diagram showing the relationship between propagation distance and optical power similar to (C). These are r = 500, KO = O
, I5 and △n/K o = 1.37 (2π/
In the case of <A), △
n'/Ko = 1. ? / (27j /in) and in the case of (B), Δn' /Ko = 6/
(contains C27Li). 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, the electrodes 23 and 24 must be
Apply a voltage of . In this case, the waveguide 21
As shown in FIG. 2, an isolateral refractive index difference Δn′ occurs between and 22. At this time, due to the properties of this type of directional coupler, the light input from the input capacitor (Z-) of the waveguide 21 hardly transfers to the waveguide 22;
is output from the output port (Z=L).

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

FIG. 1(B) shows another embodiment of the present invention, in which the widths of both the first and second waveguides 21 and 22 are tapered, and the first waveguide 21 has a waveguide It is widened according to the distance, narrowed in the second waveguide 22, and both waveguides 2
1 and 22 are provided with a curvature such that the interval is minimum at the midpoint of the waveguide distance to prevent variations in characteristics due to the input waveguide.

FIG. 1(C) shows still another embodiment of the present invention, in which the second electrode 24 is provided only in the first waveguide 21-1-.
is provided, and the first electrode 23 is provided on the left side thereof apart from the first waveguide 21, and in addition, no electrode is provided above the second waveguide 22. By doing this,
This provides an optical switch element structure that operates on substances exhibiting the Kerr effect, such as KTN and PLZT.

(Effects of the Invention) (1) As is clear from the above explanation, according to the waveguide type optical switch element of the present invention, the range of isolateral refractive index difference for obtaining a cross state is wide, so polarization dependence is reduced. Even if an optical switch is configured without any cross state, a cross state can be easily obtained.

Further, 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 configured, it is not necessary to match the coupling coefficients of the ordinary light and the extraordinary light on both sides.

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 the drawing]

FIGS. 1(A) to (C) are schematic plan views for explaining the structure of the waveguide type optical switch element of the present invention, and FIG. 2 is an isolateral refractive index during operation for explaining the present invention. Characteristic curve diagrams showing changes in , Figures 3 (A) to (G) and Figure 4 (
A) and (B) are characteristic curve diagrams showing the relationship between propagation distance and optical power to explain the operation of the waveguide type optical switch element of the present invention. FIG. 2 is a schematic plan view for explanation. 21...First waveguide, 22...Second waveguide 2
3...first electrode, 24...second electrode. I C- to -d
-mu, \t1-muJ ('M--mu x/J do-do) -1 locoichino \no' Procedural Amendment June 1986 411

Claims (4)

[Claims] (1) The mutual coupling coefficient can be changed by sequentially changing the mutual spacing according to the distance in the waveguide direction of the optical switch.
A waveguide optical switch element comprising, on a substrate, first and second waveguides provided to vary sequentially according to the distance, and electrodes provided in association with these waveguides, Both or one of the first and second waveguides is adjusted such that the equivalent refractive index difference between the first and second waveguides changes according to the distance and becomes zero at the midpoint of the waveguide distance of the optical switch element. A waveguide type optical switch element characterized in that the width is changed according to the distance. (2) In the waveguide type optical switch element according to claim 1, the waveguide is configured such that the equivalent refractive index difference between the first and second waveguides is changed at a constant rate according to the distance. A waveguide type optical switch element characterized in that the width of one or both of the two is changed. (3) In the waveguide type optical switch element according to claim 2, the width of both or one of the first and second waveguides is changed at a constant rate, and these waveguides are A waveguide type optical switch element, characterized in that the waveguide is arranged so that the distance between the two is minimum at the midpoint and the waveguide is point symmetrical. (4) In the waveguide type 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 connected to the ground side electrode. A waveguide type optical switch element, characterized in that it is provided outside the first and second waveguides.