JPH0785150B2 - Integrated optical circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type integrated optical circuit having excellent stability and easy configuration of other channels.

[0002]

2. Description of the Related Art A method for implementing a broadband digital service integrated system including an HDTV by applying an optical fiber transmission technique to a subscriber end is being actively studied. An integrated optical circuit such as an optical filter or an optical switch is one of the optical components indispensable for realizing such a system. The definition of an optical switch usually refers to both a type that spatially switches the optical path and a type that simply turns on / off the light intensity. Since the requirements that an integrated optical circuit must have are high performance and low cost, integrated optical circuits constructed by constructing an optical waveguide on an optical crystal substrate have been mainly studied. As the physical optical effect used, the electro-optical effect of crystals is mainly selected from the viewpoint of operating speed.
As a typical unit optical switch, the light guided from the optical fiber is divided into two channel waveguides with equal light intensity, and a phase difference is given to the light transmitted through each waveguide by the electro-optic effect, and the interference of There is a structure in which the strength is changed by the effect. This optical switch has a drawback that it is sensitive to changes in the operating point because changes in the intensity of interference light are periodic with respect to the phase difference. Factors that change the operating point include variations in waveguide precision, residual strain during process processing, pyroelectric effects in crystals, impurities in crystals, ion drift and changes in space charge accumulation due to holes, etc. However, it has not yet been possible to realize a sufficiently stable and highly reliable element.

Further, it has been considered to realize an optical wavelength filter by a directional coupler or a Mach-Zehnder interferometer composed of a waveguide, but these integrated optical circuits utilizing the interference effect have been described above. I have the same problem.

[0004]

The fundamental cause of the instability of the conventional integrated optical circuit is that it utilizes the optical interference effect as described above. One of the useful optical phenomena that can be used to construct an integrated optical circuit such as an optical switch is diffraction. A planar waveguide and an interdigital electrode are formed on the electro-optic crystal, and the electric field applied between the electrodes forms a periodic grating of refractive index change near the crystal surface via the electro-optic effect. An optical switch based on the principle of diffracting guided light by a grating has already been proposed and the principle has been demonstrated. The diffraction angle is inversely proportional to the electrode period length, and the diffraction efficiency is larger when the penetration depth of the electric field into the substrate is deeper, and these two conditions contradict each other. For this reason, the element length becomes long, and since the interdigital electrodes are used, it is difficult to form another channel.

It is an object of the present invention to provide an integrated optical circuit including a waveguide type optical switch which eliminates all of the above drawbacks.

[0006]

In order to achieve the above object, optical power is not shifted due to interference between adjacent channel waveguides, but the channel waveguides are parallelized by changing the refractive index period. However, the basic concept of the present invention is to transfer light between channel waveguides that are spatially separated.

Therefore, as a basic constitution, an electro-optic crystal is used as a substrate, two channel waveguides parallel to each other and separated from each other are formed on this substrate, and a planar waveguide is filled between them. When an electric field is applied to the channel waveguide of, the pointing vector becomes a mode of a plane waveguide having an angle from the channel waveguide direction, that is, a plane waveguide mode having a propagation wave number in a direction orthogonal to the channel waveguide direction. It is converted and propagated, and an electric field is also applied to the other channel waveguide, and this planar waveguide mode is converted into the waveguide mode of this channel by the principle of reciprocity. Without the application of an electric field, the planar guided mode passes across the channel waveguide. Then, an electric field is applied to the channel waveguide to convert it into a planar waveguide mode having a propagation wave number in a direction orthogonal to the channel direction. A means for producing a cycle of electro-optical effect by a uniform electric field by the electrodes of. Therefore, it is also a feature that an inversion lattice of spontaneous polarization is previously formed on the crystal surface. Specifically, in the case of an optical switch composed of a waveguide for each input and output,
An electro-optic crystal is used as a substrate, and an inversion period of a crystal orientation having a period along the surface and two channel waveguides of an input and an output, which are on the surface of the crystal and are spaced apart from each other so as not to be optically coupled, and these two waveguides. Other channels can be configured by forming a planar waveguide having an equivalent refractive index smaller than that of the channel waveguides and an electrode for applying an electric field to the channel waveguides. Thus, an electro-optical optical switch that is stable and easy to manufacture can be obtained.
Furthermore, if the planar waveguide has a wavelength selection function (determines the thickness of the planar waveguide according to the propagation wavelength), it also functions as an optical filter.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing an embodiment of the present invention, showing the configuration of an example of a unit optical switch. Crystals having a large electro-optical effect, for example, C plate lithium tantalate crystal 1
Spontaneous polarization is periodically inverted along the surface of the substrate by electron beam irradiation to form a spontaneous polarization inversion periodic structure 2, and two parallel channel waveguides 3 and 3 are formed by a proton exchange method. 4 are formed, and the planar waveguide 5 is formed between them. At this time, the depth of the waveguide and the amount of protons are adjusted so that the equivalent refractive index of the planar waveguide 5 becomes smaller than the equivalent refractive index of the channel waveguides 3 and 4.
In this embodiment, the equivalent refractive index of the channel waveguide is 2.2.
2 and the equivalent refractive index of the planar waveguide was 2.218.
The reason why the equivalent refractive index of the planar waveguide is made smaller than the equivalent refractive index of the channel waveguide is to confine and propagate light in the channel waveguide (to prevent leakage to the planar waveguide). . Further, electrodes 6A and 7A for applying an electric field to the channel waveguides are provided on the respective channel waveguides, and electrodes 6B and 7B are provided in regions near the respective channel waveguides. In order to prevent absorption of light by the electrode, a material having a smaller refractive index than the substrate, for example, an optical buffer layer 11 such as SiO ₂ is thinly provided between the crystal plane and the electrode. As shown in FIG. 1, the stripe-shaped spontaneous polarization inversion region 2a forming the spontaneous polarization inversion periodic structure 2 and the channel waveguide are diagonally crossed.

The operating principle of this optical switch is as follows. TM incident from one end of the input channel waveguide 3
When no voltage is applied between the electrodes 6A and 6B provided on and near the input channel waveguide 3 for the mode light 8, for the TM mode light, even if there is a site of spontaneous polarization reversal in the waveguide. Since the refractive index does not change spatially, it travels through the waveguide without being affected by the propagation characteristics and is output from the output end of the input channel waveguide 3. Electrode 6
When a voltage is applied between A-6B, an applied electric field component perpendicular to the substrate causes an electro-optic constant tensor component r ₃₃ ,
A change in the refractive index is given to the TM mode light which is an optical electric field perpendicular to the substrate. At this time, since the spontaneous polarization inversion periodic structure 2 is formed in the vicinity of the surface of the substrate in advance, the positive / negative of the refractive index change is inverted at the positive / negative positions in the polarization direction. Therefore, TM mode light is equivalent to passing through the phase grating and is diffracted in the Bragg angle direction determined by the grating pitch. As shown in FIG. 2, this diffraction direction has two vectors, that is, a fraction vector K _CH of incident channel TM mode light and a spatial lattice vector K _a formed by a spontaneous polarization inversion periodic structure, and T of a planar waveguide.
The fraction vector K _{p1 of the} M-plane waveguide mode light is closed and forms a triangle. This phase matching condition is similar to the anomalous Bragg diffraction effect in the acousto-optic effect which is well known. That is, the incident channel T
Since the wavenumber vector K _CH of M-mode light and the spatial lattice vector K _a formed by the spontaneous polarization inversion periodic structure are not orthogonal,
The TM plane waveguide mode light to be converted does not go out in both directions across the channel waveguide, but is in one direction, which is efficient. Unlike acoustic traveling waves, the wave vector of a stationary spatial phase grating has both positive and negative directions. When the wave number vector K _CH of the channel TM mode light and the spatial lattice vector K _a formed by the spontaneous polarization inversion periodic structure are orthogonal to each other, a fractional vector K _P1 of the planar waveguide mode light that matches in both directions with the waveguide sandwiched is obtained. Although it may exist, since K _CH and K _a are inclined, the wave number vector K _p1 of the planar waveguide mode light is a vector satisfying the matching condition only in one diagonal direction of the waveguide as shown in FIG. Is convenient because there can be no.

The TM-plane waveguide mode light 9 diffracted is
Since there is a planar waveguide between the channel waveguides, the propagation in the depth direction of the substrate is blocked, and a propagating wave in the direction parallel to the surface is propagated through the planar waveguide and an output parallel to the input channel waveguide 3 is generated. Arrives at the channel waveguide 4. When a voltage is applied between the electrodes 7A-7B provided in the vicinity of the output channel waveguide, it is converted into the guided TM mode of the channel waveguide 4 due to the effect of being contrary to that which occurs in the channel waveguide 3. The light travels through this waveguide and becomes the emitted TM mode light 10 and is emitted from the output channel waveguide 4. Electrode 7A-
When the electric field is not applied between 7B, the TM plane waveguide mode light travels across the channel waveguide 4 because the phase grating due to the refractive index change is not formed in the vicinity of the channel waveguide 4, It leaves no energy in the waveguide.

Thus, the electrodes 6A-6B of the input side channel waveguide and the electrodes 7A- of the output side channel waveguide are provided.
Only when voltage is applied to both electrode pairs of 7B,
Light emerges from the output channel waveguide and does not appear unless a voltage is applied to either electrode pair.

When a large number of channel waveguides and electrodes on the output side are provided in parallel, light is emitted from an arbitrary channel waveguide, or voltages are adjusted and applied to electrode pairs near a plurality of emission waveguides. For example, light is emitted from a plurality of channel waveguides at an arbitrary intensity ratio, so-called 1 to n.
An integrated optical circuit of an optical switch can be constructed. Further, if a plurality of input side channel waveguides are provided and one output side channel waveguide is provided, any input light can be connected to the output channel waveguide. Also, an integrated optical circuit of an n-to-1 optical switch that merges a plurality of input lights into one output is possible.

Under the matching conditions described above, even if the TM mode equivalent refractive index of the channel waveguide and the planar waveguide, and the direction and pitch of the spontaneous polarization inversion grating are correctly designed, variations in manufacturing and variations in incident wavelength are caused. The characteristics of the integrated optical circuit cannot be stably obtained unless the margin for is taken into consideration. In this case, if the relative angle formed by the direction of the lattice vector formed by the spontaneous polarization inversion periodic structure and the light transmission direction of the channel waveguide is gradually changed along the light transmission direction of the channel waveguide, the TM The above phase matching condition will be satisfied somewhere in the transmission direction of the mode,
A strong channel / planar mode conversion occurs on the spot, variations in the equivalent refractive index at the time of manufacture, variations and variations in the incident wavelength can be absorbed, and a stable integrated optical circuit can be obtained.

It has been known that, for a lithium tantalate crystal, a lithium niobate crystal or the like, an inversion pattern of spontaneous polarization can be formed on the surface of a crystal plate by subjecting a grown single crystal wafer to an appropriate treatment such as heat treatment or electron beam drawing. ing. Further, it is known that the inversion of the spontaneous polarization can be formed in the entire crystal ingot by reversing the inrush current and controlling the growth temperature periodically during the crystal pulling and growing. Therefore, the spontaneous polarization inversion periodic structure can be formed by these methods.

In the above-mentioned embodiment, the case where the ferroelectric lithium tantalate crystal is used as the substrate material and the waveguide forming method by the proton exchange method is described, but other known methods such as the Ti diffusion method are known. A similar function can be realized by a waveguide forming method, another ferroelectric substance (for example, BaTiO ₃ ), another paraelectric substance having an electro-optical effect (for example, BSO), or a semiconductor.

The basic configuration of the present invention is to perform optical coupling between independent channel waveguides that are spatially separated by a plane mode of a plane waveguide that fills the gap through diffraction by a phase grating. In the optical switch of the first embodiment, the phase grating uses the period of the electro-optical effect based on the inversion period of the crystal orientation.

Another useful element is possible by modifying the constituent elements in the above embodiment. For example, a wavelength selecting function is given to a planar waveguide that fills the space between independent channel waveguides, that is, for example, 1.3 μm light is planar mode guided and 1.5 μm light is cut off in a plane. Set the waveguide thickness. The thickness of the planar waveguide at this time is, for example, when using a lithium niobate crystal for the substrate and forming the high refractive index layer by the Ti diffusion method,
The thickness may be about 1.8 μm.

The phase grating has a structure in which a spontaneous polarization inversion periodic structure is formed on the substrate, and a grating is formed on the channel waveguide with a dielectric film or the like (for example, SiO ₂ ), and the grating is attached to the phase grating. The phase matching condition of FIG. 2 is set so as to be satisfied with 1.3 μm light. That is, K
_{When the} angle between _CH and K _p1 is θ _i and the angle between K _CH and K _a is θ _a , K _CH = K _p1 · COS (θ _i ) + Ka · COS (θ _a ) K _p1 · SIN (Θ _i ) = K _a · SIN (θ _a ), the values of θ _i and θ _a such that 1.3 μm light is satisfied,
The period of the diffraction grating, the thickness of the channel waveguide and the planar waveguide, etc. are determined. With this configuration, for example, an optical filter that can combine or demultiplex two types of wavelengths can be formed. By further adding a channel waveguide and appropriately determining the wavelength selection function of the planar waveguide, it is possible to combine and demultiplex another wave, and it is structurally easy to expand the function. The material used for the element having this function is not particularly required to have a physical optical effect, and may be any ordinary dielectric such as optically transparent glass.

[0019]

According to the present invention, it is possible to obtain an integrated optical circuit such as a waveguide electro-optical optical switch which is excellent in electro-optical stability and can be easily configured for multi-channel output. In addition, an integrated optical circuit that can absorb variations in the equivalent refractive index due to ambient temperature changes and wavelength variations and variations in incident light can be realized, and a wavelength filter function with a simple configuration can also be realized. By developing the basic concept of the present invention, useful integrated optical circuits can be created.

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of a unit optical switch that is an embodiment of the present invention.

FIG. 2 is a diagram showing matching conditions for lightwave mode conversion in an optical switch.

[Explanation of symbols]

1 LiTaO ₃ crystal 2 spontaneous polarization inversion periodic structure 3 input channel waveguide 4 output channel waveguide 5 planar waveguide 6A, 6B, 7A, 7B electrode 8 incident TM mode light 9 TM planar waveguide mode light 10 outgoing TM mode light 11 Optical buffer layer

Claims (3)

[Claims] 1. A plurality of channel optical waveguides which have the same equivalent refractive index and are separated from each other so as not to be optically coupled, and a planar waveguide which is located between the channel waveguides and whose equivalent refractive index is lower than that of the channel optical waveguide. , The plurality of channel guides
An integrated optical circuit having means for imparting the same refractive index period change to the waveguide . 2. An electro-optic crystal is used as a substrate, and an inversion periodic structure of a crystal orientation having a periodic arrangement direction along a surface thereof is separated from the same conductive surface so as not to be optically coupled to the crystal surface. A plurality of channel waveguides having wave characteristics, a planar waveguide formed between these channel waveguides having an equivalent refractive index smaller than that of the channel waveguides, and an electric field is applied to the plurality of channel optical waveguides. And an integrated optical circuit. 3. A plurality of channel optical waveguides, which are separated from each other to the extent that they are not optically coupled, have the same waveguide characteristics, and guide light of a plurality of wavelengths, and are formed between these channel waveguides. Integrated light having a smaller equivalent refractive index than that of the channel optical waveguide and having a planar waveguide for guiding light of a specific wavelength, and means for changing the refractive index period of the channel waveguide. circuit.