JPS60182424A - Optical control circuit - Google Patents

Abstract

PURPOSE:To enable coupling to an optical fiber with a low loss and to enable low-voltage operation by making the refractive index of the optical waveguides constituting an optical control element larger than the refractive index of input and output optical waveguides. CONSTITUTION:Optical waveguides 2, 3 are installed on an LiNbO3 substrate and a pair of electrodes 4 are formed thereon to constitute a directional coupler. An input optical waveguide 7 having incident ends 15, 16 and optical waveguide 8 having exit ends 17, 18 are manufactured at the end faces of the substrate 1 by thermal diffusion of a thin Ti film pattern. The refractive index in the parts of the waveguides 2, 3 has a distribution in the depth direction and the max. refractive index thereof is made n1. The waveguides 5-8 have the max. refractive index n2. The refractive indices are made n1>n2 and the depth of the distribution is about the same. The connecting part of both optical waveguides is so formed that the refractive indices change gradually in order to decrease the loss by the mode conversion of the propagated light.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a light control element that modulates light waves, switches light paths, etc., and particularly relates to a waveguide type light control device that performs control using an optical waveguide provided in a substrate. 0 Regarding Circuits (Prior Art and Its Problems) As the practical use of optical communication systems progresses, advanced systems with even higher capacity and multiple functions are being sought. There is a need to add new functions such as generation of high-speed optical signals and switching and exchanging optical transmission lines. In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers or light emitting diodes, but with direct modulation, high-speed modulation of several GHz or more is difficult due to effects such as relaxation oscillation, and wavelength fluctuations Since this occurs, the coherent optical transmission method has drawbacks such as difficulty in application.

One way to solve this problem is to use an external optical modulator. In particular, a waveguide type optical modulator constructed using an optical waveguide formed in a substrate has the features of being small, highly efficient, and fast. On the other hand, optical switches are used to switch optical transmission lines and provide network switching functions. Optical switches currently in use mechanically move prisms, mirrors, fibers, etc., and have drawbacks such as slow speed, insufficient reliability, and large size. There is. What is currently being developed as a means to solve this problem is a waveguide-type optical switch that uses a leading waveguide, which is capable of high speed and multi-element integration.
It has features such as high reliability. In particular, materials using ferroelectric materials such as LrNb0B crystals have features such as low light absorption, low loss, and high efficiency due to large electro-optic effects. Various types of optical control elements such as coupled optical modulators or switches and total internal reflection optical switches have been reported. When applying such a waveguide type optical control element to an actual optical communication system, 0LiNbO, which requires low loss and high speed at the same time.
An optical waveguide formed by diffusing Tt into a crystal has a small propagation loss of 0.1 to 0.2 d- for a wavelength of 1.3 μm. When applied to an actual optical fiber system, it is necessary to sufficiently reduce the coupling loss with the optical fiber, and for this reason, the optical energy distribution of the propagation mode of the optical waveguide should be as similar to the optical energy distribution of the propagation mode of the original fiber. Optical waveguides are being created to bring them closer together. By the above means, a loss value of 1 to 2 dB has been obtained when an optical waveguide is inserted between original fibers. On the other hand, when a light control element using the electro-optic effect as described above is used in an actual system, its operating speed is usually determined by the performance of the drive circuit. In order to increase the speed, reduce power consumption, and downsize the drive circuit, it is practically important to reduce the operating voltage of the light control element as much as possible. However, in order to reduce the voltage of the optical control element, it is necessary to concentrate the optical energy of the propagation mode near the electrode where the applied electric field is strong, and the conditions for this voltage reduction are generally the same as those of the optical fiber mentioned above. This is different from the conditions for reducing coupling loss.

The optical energy distribution of a commonly used single mode optical fiber has a width of about 6 to 8 μm at which the intensity is 14, so if low-loss coupling is intended, the optical energy distribution of the optical waveguide should also be selected to be about the above value. It will be done. This condition is, for example, V Ramaswamy (V, Rama swam
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eatronicsLetters) Volume 18, 1
Issue, pages 30 to 31. On the other hand, in order to reduce the voltage, it is necessary to select the width of the optical energy distribution of the propagation mode of the optical waveguide to be smaller than the width suitable for low-loss coupling with the optical fiber. Regarding the trade-off between this low voltage condition and the condition to reduce the coupling loss with the optical fiber, see El Riviere (L, Rivie).
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As stated in Technical Digest No. 29C4-4 (pages 362-363) of National Institute of Technology (Ni Co., Ltd.), as mentioned above, with conventional optical control devices, both low-loss coupling and low-voltage operation cannot be obtained at the same time. There was a drawback that there was no

(Objective of the present invention) The object of the present invention is to eliminate the drawbacks of the conventional light control element described above,
An object of the present invention is to provide an optical control circuit that can be coupled to an optical fiber with low loss and that can operate at low voltage. an input/output optical waveguide connecting the optical control element and an optical input/output end provided on the end surface of the substrate; The light control element is configured such that the equivalent refractive index of the light wave mode propagating through the optical waveguide is larger than the equivalent refractive index of the light wave mode propagating through the input/output optical waveguide. (Detailed description of the configuration) The present invention As described above, by making the equivalent refractive index of the light wave mode propagating between the optical waveguide and the input/output optical waveguide that constitute the optical control element different, the optical waveguide of the optical fiber can be The equivalent refractive index is set to give a propagation light energy distribution close to the energy distribution, and independently of this, the equivalent refractive index of the light wave mode propagating through the optical waveguide that constitutes the light control element is set so that the energy distribution of the light wave mode is the same as the electrode. By setting it so that it is sufficiently confined in the vicinity, an optical control circuit that is capable of low-loss coupling and low-voltage operation can be obtained.

As mentioned above, means for making the equivalent refractive indexes of the two leading waveguides different include, for example, making the refractive index of the ill optical waveguide itself different; (2) making the shapes of the optical waveguides, such as width and depth, different; (3) Of the OF notation methods that can vary the refractive index of the region in contact with the leading waveguide (method 11 is particularly suitable for adding impurities to ferroelectric crystals such as LiNbO3 crystals, as described in later examples). When creating an optical waveguide by diffusing phosphor, it is a relatively easy and practical method to manufacture.Method (2) is also easy to apply when creating an optical waveguide using the commonly used phosphorography technique. As a means of realizing 0 [3), which is applicable to a fairly wide range of base materials and guiding waveguide materials, for example, coating only one of the two optical waveguides with another substance with a different refractive index from that of the optical waveguide. There are ways to do this. In this case, in order to achieve the object of the present invention, it is necessary to increase the equivalent refractive index by coating the leading waveguide constituting the light control element with another substance. As described above, in the present invention, the object of the present invention can be fully achieved regardless of the structure as long as the equivalent refractive index is different f.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view showing an embodiment of a light control circuit according to the present invention. In Figure 1, on the %L i N b Os substrate l, there are several to several tens of micrometers of width adjacent to each other at intervals of several micrometers.
Optical waveguides 2 and 3 with a length of several meters to several orders of magnitude are installed, and a pair of electrodes 4 are formed on them via a SiO□ film (not shown in the figure) provided to prevent light absorption. The optical waveguides 2 and 3 and the electrode 4 constitute a directional coupling type optical control element. In addition, an incident end 15.
Input optical waveguides 5, 6 with 16 are connected to the input sides of optical waveguides 2, 3 and to the output sides of optical waveguides 2, 3, respectively.
Therefore, the optical waveguides 2, 3, 5, 6, 7.8 are all connected to the substrate 1.
The Ti thin film baton formed on the surface of the substrate is thermally diffused into the substrate. The refractive index of the optical waveguides 2, 3 has a distribution as shown in FIG. 2 (al) in the depth direction, and the maximum refractive index is n.
．． The refractive index of 7,8 has the distribution in the depth direction as shown in Fig. 2(b), and the maximum refractive index is n, and %"l > n!
(nl and n2 are about 10-3 to]O''2). Also, the depth of the distribution of (al and (bl) is about the same.

Connection part 9.10 between optical waveguides 2, 3 and 5.6, 7.8
In order to reduce loss due to mode conversion of propagating light, the refractive index is formed so that it gradually changes from the distribution of rat in Figure 2 to the distribution of (b) over several orders of 100 μm. .

The above-mentioned embodiments can be obtained by, for example, the following manufacturing method. First, an optical waveguide pattern is created on an L+NbO substrate using a normal film lithography technique. That is, a photoresist is uniformly applied onto the LINbO substrate, and the photoresist is exposed through a photomask having the same shape as the optical waveguide portion shown in FIG. 1. To form 1, a Ti film is deposited over the entire surface with a thickness of about 500 to 700 Å.

Next, portions corresponding to the input/output optical waveguides 5, 6, and 7.8 K are covered with a shielding plate, and a TI film of about 200 to 400 λ is further deposited. After this, by dissolving the photoresist (Ill), the light control element part as shown in FIG.
,'z at the input/output optical waveguide (however, 1, >1.)
An optical waveguide pattern of Ti film having a film thickness of .'310 is formed. The boundary between the film thicknesses t and t can be formed into an arbitrary tapered shape by adjusting the distance between the shielding plate and the substrate, or by gradually moving the shielding plate during vapor deposition. can. The substrate on which the Ti film pattern is installed as shown in Figure 3 is 1000 S.
By heating in a furnace at -1100 DEG C. for about 5 to 10 hours, an optical waveguide as shown in FIG. 1 is formed. This manufacturing method utilizes the fact that the refractive index of the optical waveguide depends on the thickness of the Ti film before diffusion. A transparent and highly insulating buffer γ such as Sin is used in the light control element.
A film is formed, and an electrode pattern is formed thereon by photolithography techniques.

Next, the operation of the optical control circuit shown in FIG. 1 will be explained. The incident light 11 to the input end 15 passes through the input optical waveguide 5 and is guided to the optical waveguide 2 of the directional coupling type optical control element.The optical waveguides 2 and 3 are close to each other and form a directional coupler. The energy of the propagating light in the optical waveguide 2 is transferred to the optical waveguide 3 in four stages.

Here, the lengths of the optical waveguides 2 and 3 are such that when no voltage is applied to the electrode 4, almost 100% of the energy of the incident light is transferred to the optical waveguide 3, that is, equal to the complete coupling length. In the state where the applied voltage is O, the incident light 11 passes through the optical waveguide 3 and the leading waveguide 8 and is emitted from the output end 18. On the other hand, when a voltage is applied to the electrode 4, the matching of the phase constants of the propagating lights in the optical waveguides 2 and 3 is lost due to the refractive index change due to the electro-optic effect, and at a certain voltage value, the coupling of the incident light to the optical waveguide 3 is lost. The light becomes O, passes through the optical waveguide 2 and the output optical waveguide 7, and is emitted from the output end 17. As described above, the optical path of the incident light is switched depending on the presence or absence of the applied voltage. Further, if attention is focused only on the emitted light 12 from the emitting end 18, the emitted light 12 will be modulated by the voltage waveform applied to the electrode 4. The voltage required for the above-mentioned optical path switching or the voltage required for 100% modulation is smaller as the energy of the propagating light is smaller and confined under the electrode.

In this example, the refractive index of the optical waveguides 2 and 3 is shown in FIG.
The energy distribution of the propagating light is as shown in Figure 2 (
As shown in el, it becomes smaller and is strongly confined within the optical waveguide. On the other hand, the input/output optical waveguides 5, 6, 7,
8, the refractive index is small as shown in Figure 2 (bl), so the energy distribution of the propagating light is widened as shown in Figure 2 (dl), and the optical fiber has a low It is possible to combine losses.

In this example, a manufacturing method was used in which the Ti film thickness was made different between the optical control element and the input/output optical waveguide as shown in Figure 3, but a uniform Ti film thickness was used for both, and thermal diffusion In some cases, the optical control circuit according to the present invention can be manufactured even if the temperature of the portion where the input/output optical waveguide is formed is higher than the temperature of the portion where the control element is formed. FIG. 4 shows an example of the above-mentioned manufacturing method. FIG. 4 is a cross-sectional view showing the thermal diffusion process, where 20 is the cross section of the diffusion furnace, l is L i N
A BOS substrate, 13 a T+ film, and 21 an additional heater installed close to a portion where an input/output leading waveguide is to be formed. The light is diffused by making the temperature of the inlet/output leading waveguide portion several tens of degrees higher than that of the light control element portion. Other methods for locally heating a part of the substrate as described above include local high-frequency heating, annealing using a wax flash lamp, and the like.

In this case, the refractive index distribution of the input/output optical waveguide extends deeper into the substrate than the light control element portion, and the maximum refractive index can also be lowered by about 10 orders of magnitude.

FIG. 5 is a plan view showing a total reflection type 2×2 matrix optical switch which is a second embodiment of the optical control circuit according to the present invention. Electrodes are formed on each intersection of two parallel optical waveguides 31 and 32 formed on a LiNbO3 substrate l and two optical waveguides 33 and 34 that intersect with the optical waveguides at a small angle, and a total reflection optical switch 24 is formed. , 25 , 26 ,
The optical waveguide 31.32♂6 has an incident end 35. The input optical waveguides 37 and 38 having the same diameter are connected to form optical waveguides 33 and 34.
Output optical waveguides 4], 42 having output ends on end faces 39, 40 opposite to the above of the TJ i N b Os substrate l are installed here. Here, the optical waveguides 31, 32, 33, .
34 has a large refractive index so that the propagating light energy is strongly confined near the substrate surface and the total internal reflection type optical switches 24, 25, 26, and 27 operate at low voltage.
The input/output optical waveguides 37, 38, 41.42 are single mode optical fibers 43, 44, 45 connected to their respective end faces.
．． 46, the refractive index is selected so that the propagation light energy distribution is comparable to the light energy distribution in the optical fiber. In addition, the optical waveguide 31
, 32, 33, 34 and input/output optical waveguides 37, 38, 41
．． The connecting portion 42 is formed so that the refractive index changes in a tapered manner in the light transmission direction. The operating principle of this embodiment is that by applying a voltage to the electrodes of the total reflection optical switch at the intersection, the propagating light energy passing through one optical waveguide is reflected and transferred to the other intersecting leading waveguide. , which switches the optical path of the optical signal between the single mode optical fibers 43 and 44 and the single mode optical fibers 45 and 46. In this embodiment, the optical waveguides 31, 32, 33, 34
The refractive index of the input/output optical waveguides 37.3B and 41.42 was made larger than that of the optical waveguides 31, 32, and 33.34.
The effect of the present invention can be obtained even if the width of the input/output optical waveguide is made wider by 1 to several μm and both optical waveguides are connected in a tapered manner. According to the invention, it is possible to obtain an optical control circuit that can be coupled with optical fibers with low loss and that can operate at low voltage. For switches, etc., both low operating voltage characteristics and low loss optical fiber coupling characteristics, which were conventionally obtained with separate devices, can be obtained with a single device.
The substrate material, optical waveguide shape, electrode shape, etc. of the optical control circuit of the present invention are not limited to the above embodiments, and LiTa0. Ferroelectric crystals such as crystals, IV group semiconductor crystals such as InP, etc. are used as optical waveguides, such as optical waveguides created by ion exchange or crystal acid receptors, and electrode shapes that are more suitable for high speed. A traveling waveform electrode or the like can be used.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of the optical control circuit according to the present invention, and FIG. 5 is a 2X embodiment of the optical control circuit according to the present invention.
A plan view of a two-matrix optical switch. FIGS. 3 and 4 are diagrams for explaining the method of manufacturing an optical control circuit according to the present invention.
FIG. 2 is a diagram for explaining the present invention in detail.
are optical waveguides constituting a light control element, 5, 6, 7, 8°3
7, 38, 41, and 42 are input/output optical waveguides, 4 is an electrode, and 13 is a Ti thin film that is a diffuser. llO! Figure 4

Claims (1)

[Claims] fil at least one light control element constituted by a guide waveguide installed on a substrate and an electrode installed near the optical waveguide; In an optical control circuit consisting of an input/output optical waveguide that connects optical input/output ends, the isolateral refractive index of the optical wave mode propagating through the optical waveguide constituting the optical control element is defined as the isolateral refraction index of the optical wave mode propagating through the input/output optical waveguide. (2) A light control circuit characterized in that the refractive index of the optical waveguide constituting the light control element is also made larger than the refractive index of the input/output optical waveguide. The light control circuit described in the first item of the scope. (3) The optical control circuit 0 according to claim 1, characterized in that the width of the optical waveguide constituting the optical control element is made larger than the width of the input/output leading waveguide.