JPH0550436U - Directional coupler type optical control device - Google Patents

Abstract

(57) [Abstract] [Purpose] An object of the present invention is to obtain a directional coupler type optical control device having desired characteristics irrespective of both TE and TM modes with a certain yield or more. [Structure] Formed on the same substrate by the same process,
A plurality of directional couplers having slightly different optical waveguide widths and slightly different optical waveguide gaps at the coupling part, and optical coupling of these directional couplers whose coupling length is closest to the perfect coupling length. A directional coupler type optical control device comprising an optical fiber connected to a waveguide.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an optical control device for modulating a light wave, switching an optical path, etc., and more particularly to a waveguide type optical control device for controlling using an optical waveguide formed in an electro-optic crystal substrate.

[0002]

[Prior Art]

 With the practical use of optical communication systems, high-capacity, multifunctional, advanced systems are required, and new functions such as generation of higher-speed optical signals, switching of optical transmission lines, and replacement are required. Has been done. In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers and light emitting diodes. However, direct modulation has drawbacks such as high-speed modulation of several GHz or more is difficult due to effects such as relaxation oscillation, and it is difficult to apply to a coherent optical transmission system because it causes wavelength variation. As a means to solve this, there is a method of using an external optical modulator. In particular, a waveguide type optical modulator composed of an optical waveguide formed in an electro-optic crystal substrate is small, highly efficient, It has the feature of high speed.

On the other hand, an optical switch is used as a means for obtaining a function of switching an optical transmission line and a function of exchanging a network. The optical switch currently in practical use switches the optical path by mechanically moving a prism, a mirror, a fiber, etc., and has a drawback that it has a low speed, a large top shape, and is not suitable for matrix formation. As a means to solve this, a waveguide type optical switch using an optical waveguide is being developed, and features such as high speed, multi-element integration, and high reliability can be obtained. In particular, the one using a ferroelectric material such as lithium niobate (LiNbO ₃ ) crystal is characterized by low light absorption and low loss and high efficiency because it has a large electro-optical effect. Therefore, various types of optical control devices such as a directional coupling type optical modulator or an optical switch and a total reflection type optical switch have been reported. When such a waveguide-type optical control device is applied to an actual optical communication system, basic performance such as low loss and high speed, as well as operational stability and long-term reliability are practically indispensable. is there.

As an example of a conventional optical control device, FIG. 2 is a plan view (a) of a directional coupling type optical switch and a cross-sectional view (b) of this figure cut along a CD with the CD direction left and right. Show. In FIG. 2 (a), titanium is thermally diffused on a lithium niobate crystal substrate (hereinafter, simply referred to as a substrate) 1 cut out perpendicularly to the optical axis to form two strips having a refractive index larger than that of the substrate 1. Optical waveguides 2 and 3 are formed, and they are close to each other in the central portion of the substrate 1 by several μm to form a directional coupler 4. A control electrode 5 is formed on the optical waveguide forming the directional coupler 4 via a buffer film 6 for preventing light absorption by the electrode. FIG. 2B shows a sectional view of the directional coupler 4 perpendicular to the optical waveguides 2 and 3.

In FIG. 2, the incident light 7 incident on the optical waveguide 2 gradually propagates through the portion of the directional coupler 4 so that the optical energy gradually shifts to the adjacent optical waveguide 3, and after passing through the directional coupler 4, Almost 100% of the energy is transferred to the optical waveguide 3 and becomes emitted light 8. On the other hand, when a voltage is applied to the control electrode 5, the refractive index of the optical waveguide under the control electrode 5 changes due to the electro-optic effect, and the phase velocity mismatch between the waveguide modes propagating in the optical waveguides 2 and 3. Occurs, the coupling state between the two changes, and emitted light 9 is generated.

[0006]

[Problems to be solved by the device]

 In the conventional directional coupler type optical control device described above, a metal (eg, titanium) is thermally diffused on a ferroelectric crystal (eg, lithium niobate) substrate to form an optical waveguide type directional coupler. In doing so, the metal film thickness that satisfies the perfect bond length differs depending on the crystal growth lot. Therefore, a part of the same crystal growth lot was extracted in advance, the metal film thickness that satisfied the complete bond length was determined, and based on that, the directional coupler type optical control device was formed on the remaining crystal substrate. There is. As a result, a directional coupler-type optical control device that satisfies the perfect bond length can be obtained if the metal film thickness varies depending on the film formation lot of the same crystal growth lot or the polishing lot of the crystal substrate differs even in the same crystal growth lot. There was a drawback that I could not do it. In particular, when trying to obtain a directional coupler type optical control device in which the complete coupling length is the same in both TE and TM modes, the metal film thickness can be controlled very accurately (± 5 to 10 A for the titanium film). There was a problem that I had to do.

[0007]

[Means for Solving the Problems]

 According to the present invention, a plurality of directional couplers having slightly different optical waveguide widths and slightly different optical waveguide gaps in a coupling portion, which are formed on the same substrate by the same process, and these directional couplers. A directional coupler type optical control device is obtained, which comprises an optical fiber connected to an optical waveguide of a directional coupler whose coupling length is the closest to the complete coupling length.

Further, according to the present invention, a plurality of directional couplers, which are formed on the same substrate by the same process and have slightly different optical waveguide widths and slightly different optical waveguide gaps at their coupling portions, Among these directional couplers, the control electrode formed via the buffer layer on the top of the directional coupler whose coupling length is the closest to the full coupling length, and the directional coupling whose coupling length is the closest to the full coupling length. And an optical fiber connected to an optical waveguide of the optical device.

[0009]

【Example】

 FIG. 1 is a plan view (a) showing an embodiment of a directional coupler type optical control device according to the present invention and a sectional view (b) of a directional coupler. The relationship between the sectional view (b) and (a) is the same as in Fig. 2. A titanium film is formed on the substrate 1 of lithium niobate, and is thermally diffused at about 1000 ° C. for several hours, and the optical waveguides 2, 3, 20, 30, 21, 21, 31, 22 having a depth of 3 to 10 μm are formed. 32, 23, 33 are formed. These waveguides form a set of two and form directional couplers 4, 40, 41, 42, 43 in the central portion of the substrate 1 in which the respective optical waveguides are close to each other by several μm. .. The optical waveguide width is changed by 0.5 μm for each optical waveguide forming each directional coupler. The optical waveguide gap at the coupling portion of the directional coupler is also changed by 0.5 μm.

A directional coupler using an optical waveguide formed by thermally diffusing titanium into a desired pattern on the substrate 1 can change the complete coupling length by setting the optical waveguide gap of the coupling portion as a variable parameter. Moreover, by making the optical waveguide width a variable parameter, even if the titanium film thickness is slightly changed, a perfect coupling length that matches both TE and TM polarized light can be obtained. In other words, if directional couplers with different optical waveguide widths and optical waveguide gaps are formed on the same substrate, even if the titanium film thickness changes slightly due to the control accuracy of the film forming apparatus, it will be In this directional coupler, the directional coupler having the most perfect coupling length in both TE and TM modes can be selected by measurement.

If a control electrode 5 is formed on this directional coupler via a buffer layer 6 made of silicon dioxide and the optical fibers 10, 11, 12, and 13 are coupled to an optical waveguide, both TE and TM can be obtained. It is possible to stably obtain a directional coupler type optical control device having good crosstalk characteristics when a voltage is not applied to the control electrode for each mode.

[0012]

[Effect of the device]

 As described above, in the present invention, a plurality of directional couplers having slightly different optical waveguide widths and optical waveguide gaps of the directional coupler coupling portion are simultaneously formed on the same substrate, and the most complete coupling length among them is formed. By selecting a directional coupler whose λ is equal to the coupling length, it is possible to obtain a directional coupler type optical control device having desired characteristics with a certain yield or more.

[Brief description of drawings]

FIG. 1 is a plan view (a) and a sectional view (b) showing an embodiment of a directional coupler type optical control device according to the present invention.

FIG. 2 is a plan view (a) and a cross-sectional view (b) showing an example of a conventional directional coupler type optical control device.

[Explanation of symbols]

1 substrate (lithium niobate crystal substrate) 2,3,20,30,21,31,22,32,23,
33 Optical Waveguide 4, 40, 41, 42, 43 Directional Coupler 5 Control Electrode 6 Buffer Layer 7 Incident Light 8, 9 Outgoing Light 10, 11, 12, 13 Optical Fiber

Claims (2)

[Scope of utility model registration request] 1. A plurality of directional couplers, which are formed on the same substrate by the same process and have slightly different optical waveguide widths and slightly different optical waveguide gaps at their coupling portions, and these directional couplers. A directional coupler-type optical control device comprising: an optical fiber connected to an optical waveguide of a directional coupler whose coupling length is closest to the complete coupling length. 2. A plurality of directional couplers, which are formed on the same substrate by the same process and have slightly different optical waveguide widths and slightly different optical waveguide gaps at their coupling portions, and these directional couplers. Of the directional coupler whose coupling length is closest to the full coupling length among the control electrodes formed through the buffer layer and the optical waveguide of the directional coupler whose coupling length is the closest to the full coupling length. And a directional coupler type optical control device.