JPH0262519A - Optical switch for directional coupler type waveguide - Google Patents

Abstract

PURPOSE:To obtain the optical switch which is small in the emission loss of a waveguide while the characteristics do not depend upon the polarizing state of incident light by providing linear optical waveguides to upper and lower layers among three layers of semiconductor crystal at a fine crossing angle. CONSTITUTION:The semiconductor crystal layers 3 and 5 are stacked on a semiconductor substrate 1 and the linear optical waveguides 4 and 2 are formed on the upper and lower layers while having a slight angle difference. Counter electrodes 6 and 7 are provided on the surface of the semiconductor crystal layer 5 as the clad part of the liner optical waveguide 4 nearby the intersection. When neither a voltage nor current is applied between the counter electrodes 6 and 7, light entering one linear optical waveguide 2 is projected from the linear optical waveguide 4 with full power. When a voltage or current is applied between the counter electrodes 6 and 7, the light entering the linear optical waveguide 2 travels in the linear optical waveguide 2 as it is and is projected. Thus, switching which has redundancy to a deviation in the wavelength of the incident light is carried out without polarized dependency.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a directional coupler type waveguide switch, and in particular,
A light control element for optical communication, in which the characteristics of the element do not depend on the polarization state of incident light, and furthermore, even when a matrix-shaped optical integrated circuit is configured, optical loss due to radiation in a curved optical waveguide is small. This invention relates to a directional coupler type waveguide optical switch.

[Conventional technology]

Optical communication systems are being put into practical use, and more advanced network systems with large capacity and multiple functions are being developed. New functions such as switching functions of optical transmission networks and high-speed connections and switching between terminals in optical data buses are required, and the need for optical switching networks that enable these functions is increasing.

Optical switches currently in use include prisms, mirrors,
It mechanically moves optical fibers, etc., and has drawbacks such as low switching speed and insufficient reliability, and large size that makes it unsuitable for matrix formation. A waveguide-type switch that uses optical waveguides installed on a substrate is being developed as a means to solve this problem, and has features such as high speed, high-speed integration of multiple elements, and high reliability. . In particular, those using ferroelectric materials such as lithium niobate (LiNb03) crystals have the following characteristics: low light absorption and low loss, high efficiency due to large electro-optic effect, and planar structure. Therefore, the production process is simple and batch processing allows for high productivity.

Many device methods have been invented for waveguide switches, but the operating voltage is relatively low, and since the channel waveguide is constructed from beginning to end in the light transmission direction, the radiation loss of light is relatively low. Directional coupler type optical switches are often used because of their small size and other advantages.

The structure and operating principle of a directional coupler type optical switch are as follows.

Two channel optical waveguides that are close to each other and parallel to each other are formed on the same plane of an electro-optic crystal plate such as lithium niobate, and electrolytic application electrodes are installed along these two channel optical waveguides. .

The phase is adjusted so that when no voltage is applied to the directional coupler, all the light in one waveguide transfers to the other waveguide (this state is generally called a cross state and is represented using the symbol ■). If the length of the coupling part of two optical waveguides with the same constant is placed close to each other is set, and an appropriate voltage is applied, the phase constants of the two waveguides will be different. No coupling of light to the other occurs, and the light travels as it is through one waveguide into which it entered (this state is generally referred to as a bar state and is represented using the symbol O).
Switching is realized by selecting two states: the ■ state and the O state.

[Problem to be solved by the invention]

Although the conventional directional coupler type optical switch described above has the above-mentioned characteristics, it also has the following drawbacks.

The first drawback is that it is polarization dependent.

- In high-speed, large-capacity optical communication systems, an optical switch is inserted into an optical transmission line and is often used to switch the optical path of an optical signal transmitted through an optical fiber. One mode optical fiber is used.

A light wave transmitted through a single mode fiber is generally elliptically polarized light, and its polarization state also changes over time. In conventional waveguide type optical switches, switch characteristics such as voltage and crosstalk depend largely on the polarization state of incident light. Therefore, it is difficult to insert this waveguide type optical switch in the middle of an optical fiber transmission line.

A second drawback is that when a matrix is constructed using directional coupler type optical switches, optical radiation loss occurs in curved waveguides connecting switch elements. When considering a large switch matrix that switches optical paths between inputs and outputs of multiple channels, the number of switch stages increases, and the number of curved waveguides connecting them increases.
The total amount of light emitted and lost at each of these locations is enormous.

A third drawback is that the properties are sensitive to wavelength. As is well known, the oscillation wavelength of a semiconductor laser varies from lot to lot and depending on temperature. On the other hand, since the mode distribution of the channel waveguide has dispersion characteristics with respect to wavelength, the coupling length between the waveguides changes.

That is, as the wavelength becomes shorter, the optical confinement effect of the waveguide increases and the coupling length becomes longer, and as the wavelength becomes longer, the coupling length becomes shorter. Therefore, the voltage value of the two switch states of the cross bar (■・O) changes depending on the input wavelength. Adjusting and applying different voltage values is difficult and inconvenient.

As an optical switch method that avoids the drawbacks of the directional coupler type described above, a semiconductor material is used, a cross-type optical waveguide is formed on the same surface of the semiconductor material, and a cross-type optical waveguide is formed using a large refractive index reduction due to current injection. Methods using the principle of light transmission or total internal reflection are being considered. In principle, this method is based on the principle that the refractive index decrease caused by current injection has no polarization dependence and is not very sensitive to wavelength dependence.Also, since a mesh-like switch matrix can be constructed using only straight waveguides, the waveguide radiation Losses are small. Contrary to these advantages, at the intersection, the low refractive index layer generated by current injection does not necessarily form a clear refractive index step in terms of geometric shape, and the low refractive index layer alone does not form a clear refractive index step with respect to the total reflection surface. There is a drawback that high crosstalk characteristics cannot be obtained because good total reflection conditions cannot be formed for the entire incident angle spectrum having a waveguide mode.

As mentioned above, conventional waveguide type optical switches have a number of drawbacks such as polarization dependence, increased radiation loss in curved waveguides, sensitivity of wavelength characteristics, and low crosstalk characteristics.

The purpose of the present invention is to eliminate the drawbacks of the conventional waveguide type optical switch described above, and to provide a switch whose characteristics do not depend on the polarization state of incident light, less radiation loss in the waveguide, and a wide wavelength tolerance range.
Moreover, it is an object of the present invention to provide a waveguide type optical switch that has excellent crosstalk characteristics.

[Means to solve the problem]

The directionally coupled waveguide optical switch of the present invention has a straight channel optical waveguide provided in each of two upper and lower semiconductor crystal layers that are parallel to each other through a first semiconductor crystal layer. It is configured to include means for changing the refractive index of the cladding portion of the upper layer channel optical waveguide by imparting a slight angular difference in the direction of the transmission axis.

〔Example〕

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a perspective view showing the configuration of a first embodiment of a directional coupler type waveguide optical switch of the present invention, FIG. 2 is an AA' cross-sectional view (a) of the embodiment of FIG. 1, They are a BB' cross-sectional view (b) and a c-c' cross-sectional view (c).

In FIG. 1, a semiconductor crystal substrate 1 serving as a third semiconductor crystal layer has one groove formed therein for forming a linear optical waveguide 2. As shown in FIG.

The direction of this groove is, for example, the y-axis.

A groove-shaped projection for forming a straight optical waveguide 4 on the surface of a semiconductor crystal layer 3 as a first semiconductor crystal layer having a composition higher in refractive index than that of the semiconductor crystal substrate by epitaxial growth on the semiconductor crystal substrate. A semiconductor crystal layer 5 as a second semiconductor crystal layer having the same composition as the semiconductor crystal substrate is further formed on the semiconductor crystal layer 3.

The width and height of the groove-like protrusion to form the straight optical waveguide 4 as a channel optical waveguide are the same as the groove for forming the straight optical waveguide 2, and the direction of the groove-like protrusion is the same as that of the groove for forming the straight optical waveguide 2. The direction is rotated by a slight angle, approximately 1°, from the y-axis selected as the direction of the groove for forming the waveguide 2, and moreover, the groove in which the straight optical waveguide 2 is to be formed is straddled near the center of the semiconductor crystal layer 3. It is formed like this. The region of the crystal layer containing the protrusions forming the linear optical waveguide 4 forms a waveguide. In the vicinity where the straight optical waveguides 2 and 4 intersect vertically, an electric field or current is applied to the surface of the semiconductor crystal layer 5, which is the cladding part of the straight optical waveguide 4 provided above, to create a cross-section of the clot part of the straight optical waveguide 4. Opposing electrodes 6 and 7 are provided along this optical waveguide for changing the refractive index. FIG. 2 is a longitudinal sectional view taken along lines AA', O-○', and BB' of the embodiment shown in FIG.

Next, the operation of the directional coupler type optical switch of this embodiment will be explained. The light incident on one of the straight optical waveguides 2 is transmitted to the opposite electric field 8i! When no voltage or current is applied between 6 and 7, the optical power gradually transfers to the straight optical waveguide 4 as it travels through the straight optical waveguide 2, and upon exit, the optical power gradually transfers to the straight optical waveguide 4.
Full power is emitted from. On the other hand, when a voltage or current is applied between the opposing electrodes 6 and 7, the light incident on the linear optical waveguide 2 continues along its own linear optical waveguide 2 from beginning to end and exits as it is. That is, when no voltage is applied, the path is switched from the straight optical waveguide 4, and when the voltage is applied, from the straight optical waveguide 2. In addition, this optical switching characteristic using the straight optical waveguides 2 and 4 as optical switching elements is independent of polarization, and has redundancy against wavelength deviations of the incident light.The reason why the above-mentioned operation is realized is as follows. It is understood as follows. In other words, in this structure in which two straight optical waveguides with the same phase constant intersect vertically, the strength of the optical coupling between the two waveguides is not uniform, and the third waveguide is weighted in the light transmission direction. This is effectively equivalent to configuring a directional coupler having the coupling coefficient between optical waveguides as shown in FIG.

Generally, when the coupling constant between two optical waveguides with the same phase constant is uniform in the light traveling direction, the amount of coupled light is 5in2
Proportional to (to 2). Here, is the coordinate 2 of the light traveling direction.
is a constant binding constant independent of . That is, the amount of combined light becomes a periodic function of 2. perfect coupling for a specific wavelength and polarization mode, i.e. 5in2
Even if the coupling length of the directional coupler is set to 2° so that = 1, the coupling constant will change if the wavelength or polarization mode differs slightly, so perfect coupling will not be achieved in these cases. (sin2 (Zo)≠On the other hand, the change in the amount of coupled light of a directional coupler weighted by the coupling constant in the light transmission direction with respect to 2 is in a stepwise manner as shown by the solid line in Fig. 3(b). , and is not a periodic function.In addition, the curve shown in Figure 3 (b
Although the inclination of the slope is different, as shown by the broken line in ), it still becomes a step-like curve, so perfect coupling is ensured.

Next, when a voltage or current is applied between the directional electrodes 6 and 7 provided on the surface of the semiconductor crystal layer 3, the linear optical waveguide 4
Since the refractive index of the cladding portion of the straight optical waveguide 4 changes and the phase constant of the straight optical waveguide 4 no longer matches the phase constant of the waveguide 2, direct coupling between the optical waveguides no longer occurs.

An example of a directional coupler in which the optical coupling constant between two straight optical waveguides is weighted along the light transmission direction using a curved waveguide on a single crystal plane uses a Li NbO3 crystal. Examples are known. The spacing between waveguides on a plane is changed in the direction in which light travels, and it has been confirmed that the switching characteristics are polarization independent and redundant with respect to the wavelength of incident light.

In this known example, a curved waveguide must be used.

Therefore, radiation loss at the curved portion is unavoidable. On the other hand, the present invention has the feature that it can be constructed using only straight waveguides. This is sometimes noticeable when optical switch elements using straight optical waveguides are configured in a 7-trix configuration as shown in FIG.

Note that the structure and manufacturing method of the optical waveguide are not limited to those shown in the above embodiments, and many known methods can be used.

For example, FIG. 5 is a perspective view showing the structure of a second embodiment of the directional coupler type waveguide optical switch of the present invention. This structure can be created by repeatedly using the etching technique and crystal growth technique used in manufacturing semiconductor lasers.

〔Effect of the invention〕

As explained above, according to the present invention, there is a slight difference in the optical transmission axis of the channel waveguide provided in each of the second and third semiconductor crystal layers of two upper and lower layers parallel to each other via the first semiconductor crystal layer. By changing the refractive index of the cladding part of the channel waveguide in the upper layer by giving an angular difference of This has the effect of realizing a directional coupler type waveguide optical switch that has a wide range of characteristics and excellent crosstalk characteristics.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the structure of a first embodiment of the directional coupler type waveguide optical switch of the present invention, FIG. 2 is an AA' cross-sectional view (a) of the embodiment of FIG. 1, B-B' cross-sectional view (b), CC' cross-sectional view (C), and Figure 3 (a) show the spatial distribution characteristics of the coupling coefficient between the optical waveguides in the light transmission direction of the embodiment of Figure 1. 3(b) is a characteristic diagram of the amount of coupled light between the leading waveguides of the embodiment of FIG. 1, and FIG. 4 is a perspective view showing an example of a structure in which the optical waveguides of the embodiment of FIG. 1 are formed into a matrix. FIG. 5 is a perspective view showing the structure of a second embodiment of the directional coupler type waveguide optical switch of the present invention. 1... Semiconductor crystal substrate, 2... Straight optical waveguide,...
- Semiconductor crystal layer, 4... Direct optical waveguide, 5... Semiconductor crystal layer, 6, 7... Counter electrode, 8, 9... Straight optical conductive path, 10.11... Optical waveguide core part .

Claims (1)

[Claims] A slight angular difference is imparted to the light transmission axis direction of the straight channel optical waveguide provided in each of the upper and lower second and third semiconductor crystal layers that are parallel to each other through the first semiconductor crystal layer. A directional coupler type waveguide optical switch, comprising means for changing the refractive index of a cladding portion of a channel optical waveguide.