JPS6396626A - Waveguide type light control element - Google Patents

Abstract

PURPOSE:To realize a low-loss light control element which is driven at a low voltage by forming a buffer layer only between an optical waveguide and an electrode right above the optical waveguide. CONSTITUTION:The waveguide type light control element consists of a Ti diffused optical waveguide 103 formed by diffusing titanium in a beltlike shape and a couple of electrodes 104 and 105 on an LiNbO3 substrate 101 made of lithium niobate crystal. Then the SiO2 buffer layer 102 is formed only between the electrode 104 right above the optical waveguide and the optical waveguide 103 and not formed between the other electrode 105 and substrate 101. Consequently, a light wave is prevented from being absorbed by the electrode parts and the operating voltage is reducible, so the low-loss, low-voltage light control element is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical circuit that modulates light waves, and more particularly to a waveguide type optical circuit that performs control using a guide waveguide provided in a substrate.

[Conventional technology and its problems]

BACKGROUND ART In recent years, as optical communication systems have become more practical, systems with higher capacity and higher functionality are required, and optical control elements such as faster light wave modulators and optical switches are required. In such an optical control element, the insertion loss may limit the transmission distance of the optical signal, so low loss as well as high speed are important.

As a high-speed optical control element, a waveguide is formed by thermally diffusing Ti into a substrate such as LiNbC)+ and lithium oxide crystal, which have a large electro-optic effect, and the refractive index distribution of the waveguide is controlled by electro-optic control. There are optical modulators that are controlled by changing the optical field using an electric field, and there have been reports on optical phase modulators, cut-off optical intensity modulators, single-interference interference optical modulators, and the like. In such a waveguide optical modulator, the electro-optic effect itself is very fast, but in order to operate at high speed in an actual system, the modulation voltage of the element must be low. Ru. This is because the higher the speed, the more difficult it becomes to obtain an electric circuit that generates high voltage.

Further, when the above-mentioned optical modulator is applied to an actual optical fiber transmission system, low loss properties are also required. Ti diffusion Li
The insertion loss between optical fibers of the N b O3 waveguide optical modulator is mainly caused by coupling loss with the optical fiber and waveguide loss when the light wave propagates in the waveguide. Regarding coupling loss, after forming the Ti diffusion waveguide, Mg, which is an ion that lowers the refractive index of the substrate, is further diffused on the waveguide surface to make the energy distribution of the light emitted from the warm waveguide match the energy distribution of the optical fiber. 0.15 per end face
The present inventors demonstrated that the coupling loss can be reduced to about dB at the 1986 Conference on Integrated Optics and Waveguide Optics (
Topical Meeting on Integration
ated and Guided-Have 0pti
cs) Post Deadline Papers PDP-2. On the other hand, regarding the waveguide loss, the Ti diffusion waveguide itself has a waveguide loss of 0.1 to 0.2 for a wavelength of 1.3 μm.
Although it is a small value of dB/cm, it is known that forming an electrode thereon increases waveguide loss. This is due to the fact that -i has the largest γ4. This is because a 1N BOS substrate is used in the modulator so as to be able to take advantage of this, and the TM mode is used as the guided light, so that the electric field distribution of the light wave seeps out, causing absorption of the light wave at the electrode portion. In order to prevent this, as shown in FIG.
L I N b O3 substrate 101 and electrode 1 formed with
It was formed between 04.105. However, Si
Forming a buffer layer such as n between the electrode and the substrate reduces waveguide loss, but the effective electric field applied to the waveguide 103 decreases compared to when the buffer layer 102 is not formed, resulting in poor operation. There is a problem of increased voltage. As the thickness of the buffer layer 102 increases, the waveguide loss decreases, but the operating voltage increases.

As described above, the present invention aims to reduce the operating voltage of a Ti-diffused LiNb0:+ waveguide type optical control element without increasing the waveguide loss.

Means for Solving Problem C] The present invention provides an optical waveguide formed by diffusing band-shaped titanium into a lithium niobate crystal, and a light guide provided on the optical waveguide.
In a waveguide type optical control element having a pair of electrodes and a buffer layer formed between the electrode and the optical waveguide, the buffer layer is formed only between the electrode and the optical waveguide directly above the waveguide. It is characterized by

[Effect]

In the present invention, a sufficiently thick buffer layer is formed between the electrode directly above the waveguide and the waveguide to prevent absorption of light waves by the electrode. Li
No buffer layer is formed between the NbO3 substrates. Therefore, the electric field strength applied to the waveguide is increased compared to the conventional one, and the operating voltage can be reduced even if the buffer layer is sufficiently thick, making it possible to realize a low-loss, low-voltage optical control element.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1(a) is a perspective view showing the structure of a Ti-diffused LiNbo3 optical phase modulator according to the present invention, and FIG. 1(b) is a sectional view thereof.

In FIG. 1, a Ti diffused waveguide 10 is formed by diffusing Ti strips into a LiN b Os substrate 101.
3. S i O□ buffer layer 102 (omitted in 102a)), a pair of electrodes 104 <s
(wavelength electrode) and 105 constitute an optical phase modulator.

In the optical phase modulator whose configuration is shown in FIG. 1, the light wave incident from the input side optical waveguide end face 106 is diffused by Ti due to the electro-optic effect according to the voltage applied between the pair of electrodes 104 and 105. The refractive index of the waveguide 103 changes and undergoes phase modulation, and the light is emitted from the output photometering blade waveguide end face 107.

A method of manufacturing the optical phase modulator shown in FIG. 1 will be briefly described. First, an optical waveguide pattern made of a Ti film is formed on the LiNb0z substrate 101 using a normal photolithography technique. That is, a waveguide pattern of a Ti film having a thickness of about 500 to 2000 μm and a width of several to 10 μm is formed by the lift-off method or etching. The substrate on which the waveguide pattern of Ti is formed is 1000 to 1100.
By heating in a diffusion furnace for about 5 to 10 hours, Ti is diffused into the L i N b Oz substrate, and the refractive index of that portion increases slightly to form the optical waveguide 103 . Thereafter, a Si0g film of 2000 Å or more is formed only on a portion corresponding to the electrode 104 on the waveguide in FIG.

In order to form the SiO2 film only on the part corresponding to the electrode 104 on the waveguide in FIG.
Either remove S i O in other parts by etching the film, or remove S i O! Electrode 1 on the waveguide during film formation
This may be carried out by a so-called lift-off method in which a region other than the portion corresponding to 04 is covered with a photoresist and the photoresist film is melted after forming the SiO□ film. After forming the StO□ film, the first
A pair of electrodes 104 and 105 as shown in the figure is formed. Thereafter, the leading waveguide end faces 106 and 107 on the incident side and outgoing side are formed by polishing or cleaving the leading waveway 103 in the vertical direction. The above is the method for manufacturing the optical phase modulator shown in FIG.

Next, the principle of obtaining an optical phase modulator capable of low loss and low voltage operation according to the present invention will be explained.

As mentioned above, Ti diffused L using 7M mode guided light
iNb0. In an i-wave type optical control element, if the buffer layer does not have a certain thickness, waveguide loss will increase due to light absorption at the electrodes. The inventors have determined that the electrode spacing is 5 μm.
According to an experimental study on the optical phase modulator shown in Fig. 2 in which a Si0g film is formed on the entire surface of the substrate, the thickness of the Sing buffer is 3000 μm for a wavelength of 1.3 μm.
An increase in waveguide loss due to light absorption was observed when there was no chromogen to the extent of humans.

On the other hand, regarding the operating voltage, when the electrode length is 16 mm in the optical phase modulator shown in FIG. 6■, but when the thickness of the SiO□ film was 3000 mm, the half-wave voltage increased to 7V, about IV. When the wavelength is 1.5 μm, the increase in half-wavelength voltage becomes even larger, and when the Si 02 film thickness is 1,500 people, it is 8.4μ, but when it is 3,000 people, it becomes IO.
It became V. In this way, in a Ti diffused waveguide type optical control element that uses guided light in the 7M mode, in order to prevent an increase in waveguide loss due to light absorption at the electrodes, the Sin! Film thickness 3000
It is necessary to provide color dispersion on a human level, which results in an increase in operating voltage.

However, the only electrode that causes light absorption is the electrode 104 directly above the waveguide, and almost no light absorption occurs at the other electrode 105. Therefore, as in the conventional example shown in FIG. It is not necessarily necessary that the SiO□ film be formed below.

This is because under the normally used Tf diffusion conditions, the electric field distribution of guided light in the horizontal direction to the substrate surface is almost attenuated at a position 5 μm away from the center of the waveguide, and when the waveguide width is several μm or more, the electrode spacing is about 5 μm. If so, electrode 1 directly above the waveguide
Approximately 3000 chromogens are installed between waveguide 04 and waveguide 103.
, if a film is formed, the other electrode 105 and the substrate 1
No buffer layer is required between 01 and 01.

In the present invention, as shown in FIG. 1(b), the electrode 104 directly above the waveguide 103 and the waveguide 103
Only about 3000 chromogen in between, buffer layer 102
The structure has been adopted. In this configuration, even if the thickness of the Si○2 buffer layer is about 3000 mm,
The thickness of the SiO□ buffer layer in the conventional example shown in Fig. 2 is 150 mm.
The electric field strength is equivalent to that in the case of 0 people, and the half-wave voltage can be reduced compared to the conventional example where the SiOz buffer layer thickness is about 3000 people, and the chrominance. Therefore, by employing the structure of the optical phase modulator according to the present invention shown in FIG. 1(b), a low-loss, low-voltage optical phase modulator can be obtained.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment.

It has the same structure as the optical phase modulator shown in the example, and is made of Ti.
The present invention can also be applied to cutoff type optical intensity modulators having different diffusion conditions. Further, the present invention is applied to a branching interference type optical modulator and a phase modulator section of an optical switch to provide a low-loss and low-voltage optical modulator.

It goes without saying that an optical switch can realize this.

Furthermore, as for the electrode shape, traveling wave electrodes etc. which are more suitable for high speed can also be used, and as a buffer layer, it is possible to use Sin,
Needless to say, this is not limited to.

〔Effect of the invention〕

As described above, according to the present invention, Ti-diffused LiNbo
:1 It is possible to reduce the operating voltage of the light control element without increasing its waveguide loss, and it is possible to realize a waveguide type light control element with low loss and low voltage.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining an optical phase modulator which is an embodiment of the present invention, and FIG. 2 is a diagram for explaining problems in the prior art. 101 ・ ・ ・ ・ ・ L 1Nb()+
Basic layer 102...5i02 buffer layer 103
...Ti diffusion waveguide 104 ... Electrode on waveguide 105 ... Electrode 106 ... Incident side optical waveguide end surface 107
... Output side optical waveguide end face (b) Figure 1 Figure 2 Procedure amendment

Claims (1)

[Claims] (1) It has an optical waveguide formed by diffusing strip-shaped titanium in lithium niobate crystal, a pair of electrodes provided on the optical waveguide, and a buffer layer formed between the electrode and the optical waveguide. 1. A waveguide type light control element, characterized in that a buffer layer is formed only between an electrode directly above the waveguide and the optical waveguide.