JPH0380228A - Light control device - Google Patents

Abstract

PURPOSE:To always stably obtain designed characteristics at a high yield by providing a film equal to or larger than the mass of control electrodes on the upper or lower layer or the same plane of the control electrodes. CONSTITUTION:The mass loading film 10 is equal to or larger than the mass of the control electrodes 5. For example, the specific gravities of aluminum and SiO2 are respectively 2.69, 2.22 and, therefore, the film of the SiO2 is formed to the thickness of about >=1.2 times the thickness of the aluminum when the SiO2 is used as the mass loading film 10. The mass loading film 10 is easily deposited by a sputtering method, vapor deposition method, etc. As a result, the concentration of strains near to the control electrodes 5 is decreased and the change in the refractive index of the dielectric crystal near optical waveguides 2, 3 is suppressed. The designed characteristic are always stably obtd. in this way at the good yield.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an optical control device that performs modulation of light waves, optical switching, etc., and in particular, a waveguide type optical control device that performs control using an optical waveguide provided in a substrate. Regarding devices.

(Conventional technology) As the practical use of optical communication systems progresses, advanced systems with even higher capacity and multi-functions are required. It is necessary to add additional functions. 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, it is difficult to achieve high-speed modulation of around 10 GHz or higher due to effects such as relaxation oscillation, and wavelength fluctuations. The coherent optical transmission method has drawbacks such as difficulty in application due to the occurrence of

As a means to solve this problem, there is a method of using an external modulator. In particular, a waveguide-type optical modulator constituted by an optical waveguide formed in a substrate has the advantage of being small, highly efficient, and fast. On the other hand, an optical switch is used as a means for switching optical transmission lines and obtaining network switching functions. Optical switches currently in practical use mechanically move prisms, mirror fibers, etc., and have the drawbacks of slow speed, insufficient reliability, and large size making them unsuitable for matrix formation. A waveguide type optical switch using an optical waveguide is currently being developed as a means to solve this problem, and has features such as high speed, ability to integrate multiple elements, and high reliability. Especially lithium niobate (LINbO)
a) Materials using ferroelectric materials such as crystals have features such as low light absorption and low loss, and high efficiency due to large electro-optic effect, and they have the advantage of being highly efficient even though they are conventional Various types of light control elements have been reported, such as a coupler type optical modulator/switch, a total reflection type optical switch, and a Matsuhatsu Enda type optical modulator. When applying such a waveguide-type optical control element to an actual optical communication system, in addition to basic performance such as low loss and high speed, reproducibility of operating characteristics, that is, high device yield, is essential for practical purposes. It is.

(Problems to be Solved by the Invention) However, with conventional waveguide type optical control devices, sufficient characteristics are not obtained in terms of reproducibility of operating characteristics. 6th
The figure shows a plan view (a) and a cross-sectional view (b) of a directional coupler type optical switch as an example of a conventional optical control device. In FIG. 5(a), band-shaped optical waveguides 2 and 3 are formed on a lithium niobate crystal substrate 1 cut perpendicularly to the Z-axis by diffusing titanium to make the refractive index larger than that of the substrate. The optical waveguides 2 and 3 are close to each other within a few pm at the center of the substrate, forming a directional coupler 4. Further, a control electrode 5 is formed on the optical waveguide constituting the directional coupler 4 via a buffer layer 6 for preventing light absorption by the electrode. FIG. 6(b) shows a sectional view of the directional coupler 4 perpendicular to the optical waveguide 2.3.

In FIG. 6, as the incident light 7 entering the optical waveguide 2 propagates through the directional coupler 4, the light energy gradually transfers to the adjacent optical waveguide 3, and the directional coupler 4
After passing through, almost 100% of the energy is transferred to the optical waveguide 3 and becomes the output 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 changes due to the electro-optic effect, causing phase velocity mismatch between the waveguide modes propagating in the optical waveguides 2 and 3. , the coupling state between the two changes.

The control electrode 5 is formed by etching the control electrode film other than the control electrode using a mask or by lithography after forming the control electrode film. It is known that during the formation of buffer layer films and control electrode films, distortion occurs in the substrate due to differences in thermal expansion coefficients between each deposited film and the underlying material, and differences in elastic constants such as Poisson's ratio. In this state, the amount of strain that was present during film formation is generally uniformly distributed over the entire dielectric crystal substrate, so even if the absolute value of the refractive index of the optical waveguide and dielectric crystal substrate changes, the dielectric strength of the optical waveguide The refractive index difference with respect to the body crystal does not change before and after film formation. Therefore, the optical waveguide characteristics retain the characteristics when the optical waveguide is formed on the dielectric crystal substrate, and the coupling state of the directional coupler is not changed in any way. However, since the control electrode portion formed by subsequently etching the control electrode film is elastically discontinuous, the strain that fluctuates at the time of forming the control electrode is unevenly concentrated and localized in the vicinity of the control electrode. This strain causes changes in the refractive index of the ferroelectric crystal substrate due to piezoelectric effects, photoelastic effects, and the like. Therefore, this refractive index fluctuation also affects the vicinity of the optical waveguide formed near the control electrode,
This will change the optical waveguide characteristics. As a result, the coupling state of the directional coupler changes, resulting in a problem that the optical waveguide coupling state as designed cannot be obtained with good reproducibility.

Note that the amount of change in this bonding state varies from batch to batch of buffer layer film and control electrode film formation.

An object of the present invention is to provide a light control device that eliminates the drawbacks of the conventional light control devices described above and can always stably obtain designed characteristics at a high yield.

(Means for Solving the Problem) An optical control device according to the present invention includes an optical waveguide formed on a dielectric crystal substrate having an electro-optic effect, a control electrode provided near the optical waveguide, and a control electrode provided in the vicinity of the optical waveguide. It is composed of a film that is equal to or larger than the mass of the control electrode disposed on the upper layer or the lower layer or on the same plane.

(Function) In the optical control device of the present invention, a film that is equal to or heavier than the mass of the control electrode is coated on the upper layer, the lower layer, or the same plane of the control electrode (hereinafter referred to as a mass loading film). According to the inventor's experiments, the directional coupler generated after forming the control electrode by forming a film on the upper or lower layer or on the same plane as the control electrode, which has a mass equal to or heavier than the mass of the electrode, It becomes possible to suppress changes in the bonding state. This is because, as mentioned above, the strain applied to the dielectric crystal substrate during the deposition of the control electrode film and the buffer layer film causes the elastic discontinuous region formed near the optical waveguide after the control electrode is formed near the control electrode. This is because the area near the control electrode is prevented from becoming an elastic discontinuous region by coating the mass loading film on the upper layer, the lower layer, or on the same plane as the control electrode. As a result, the concentration of strain near the control electrode is reduced, so that changes in the refractive index of the dielectric crystal near the optical waveguide are suppressed. Therefore, the optical waveguide characteristics retain the characteristics when the optical waveguide is formed on the dielectric crystal substrate, and the coupling state of the directional coupler is not changed in any way.

From the above, the optical control device of the present invention can always stably obtain the characteristics as designed with a higher yield than in the past.

(Example) FIG. 1 is a plan view (a) and a cross-sectional view of a directional coupler type optical switch in which a mass loading film is formed on the upper layer and the same plane as the control electrode, which is an example of the optical control device according to the present invention. (b
) is shown. Similar to the example shown in FIG. 6, optical waveguides 2 and 3 with a thickness of about 3 to 10 pm are formed by thermally diffusing titanium at about 900 to 1100°C for several hours on a two-plate lithium niobate crystal substrate 1. In the center of the substrate, the optical waveguides are close to each other within several pm to form a directional coupler 4. A control electrode 5 is formed thereon with a buffer layer 6 interposed therebetween. This embodiment further includes a mass loading membrane 10. In this example, the material of the control electrode 5 is as follows:
In addition to various metals such as gold, aluminum, molybdenum, and titanium, various conductive materials such as silicide, ITO, zinc oxide, and tank oxide are used as the mass loading film 10, as shown in FIG. When the film 10 is in direct contact with the control electrode 5, Al2O3, SiO2゜5i
ON, carbon, Si3N4. Si, Fe2032MgF2
Those with a volume resistivity of 10Ω・0m or more are used, such as
On the other hand, if the mass loading membrane 10 does not directly contact the control electrode 5, any material may be used, including the material of the control electrode described above.

Furthermore, the thickness is set such that the mass of the mass loading film IO is equal to or greater than the mass of the control electrode 5. For example, when aluminum is used as the material for the control electrode 5 and SiO2 is used as the mass loading film 10, the specific gravity of aluminum and 5102 is 2.69.2.22, respectively, so the thickness of SiO is about 1.5 mm thick than that of aluminum. It may be twice or more. Note that the mass loading film 10 can be easily deposited by sputtering, vapor deposition, or the like.

FIG. 2 is a plan view of another embodiment of the optical control device according to the present invention, a directional coupler type optical switch in which a mass loading film is formed below the control electrode and the control electrode is formed on the mass loading film. (a) and a cross-sectional view (b) are shown. In this embodiment as well, the mass loading membrane 10 is in direct contact with the control electrode 5, and the first
The same effect as the embodiment shown in the figure can be obtained.

FIG. 3 is a plan view (a) and a cross-sectional view (b) of a directional coupler type optical switch in which a mass loading film is formed only on the same plane as the control electrode, which is another embodiment of the optical control device according to the present invention. ) is shown. In this embodiment, the mass loading membrane 10 and the control electrode 5 are in direct contact, and the same effect as the embodiment shown in FIG. 1 can be obtained.

FIG. 4 shows a plan view (a) and a cross-sectional view (b) of a directional coupler type optical switch in which a mass loading film is formed on the upper layer of the control electrode, which is another embodiment of the optical control device according to the present invention. . In this embodiment, an electrically insulating layer 11 is formed between the mass loading film 10 and the control electrode 5, and although the mass loading film 10 and the control electrode 5 are not in direct contact with each other, as shown in FIG. The same effect as the embodiment described above can be obtained.

Next, FIG. 5 shows the coupling state of the directional coupler before and after the formation of the control electrode according to this example, that is, the change in the branching ratio of the directional coupler and the same result in the conventional optical control device. Note that this result relates to TE polarized light. The basic operation of the directional coupler type optical switch of this example is the same as that of the conventional example shown in FIG. 6, but as shown in FIG. The change in branching ratio is significantly smaller than in conventional devices.

It is clear that the present invention is effective not only for optical control devices using directional couplers but also for all optical control devices such as Matsuhatsu Enda structure and crossed waveguide structure.

(Effects of the Invention) As described above, the optical control device of the present invention can achieve the designed characteristics with high yield compared to conventional optical control devices.
Always obtained stably.

[Brief explanation of drawings]

FIGS. 1(a), (b) to 4(a), (b) are a plan view and a sectional view showing an example of a light control device according to the present invention, and FIG. 5 is a characteristic diagram showing the effects of the present invention. , Figure 6 (a
) and (b) are a plan view and a sectional view showing an example of a conventional light control device. In the figure, 1... lithium niobate crystal substrate, 2° 3... optical waveguide, 4... directional coupler, 5... control electrode, 6... buffer layer, 7... incident light, 8,9
. . . Emitted light, 10 . . . A film that is equal to or larger than the mass of the control electrode and loaded on the upper or lower layer of the control electrode or on the same plane, 11 . . . Electrical insulating layer.

Claims (1)

[Claims] In an optical control device including an optical waveguide formed on a dielectric crystal substrate having an electro-optic effect and a control electrode provided in the vicinity of the optical waveguide, the control electrode may be formed in an upper layer, a lower layer, or on the same plane as the control electrode. A light control device comprising a film having a mass equal to or greater than the mass of the control electrode.