JPH05241115A - Waveguide type optical device - Google Patents

Abstract

PURPOSE:To enable wide-band modulation by simple constitution by forming a groove in the opposite surface of a waveguide substrate from its electrode formed surface along an electrode. CONSTITUTION:A waveguide 11 is formed on the waveguide substrate 10 and a couple of electrodes 12 and 13 for applying an electric field to the waveguide 11 are formed on the waveguide 11. A lithium niobate substrate which has relatively high electrooptic effect among ferroelectric materials is employed as the waveguide substrate 10. In the opposite surface 15 of the waveguide substrate 10 from the electrode formed surface 14, one groove 15 is formed locally. The modulation band of a Mach-Zehnder type optical modulator which has the groove 16 formed at a part of the waveguide substrate 10 is measured and thin piece formation is performed by using constitution whose modulation band is 6GHz when the thickness (t) of the waveguide substrate is 0.8mm to obtain a 20GHz modulation band when the thickness (t) is 0.3mm, thereby greatly widening the modulation band.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical device, and more particularly to a waveguide type optical device suitable for use as a broadband waveguide type optical modulator.

[0002]

2. Description of the Related Art Generally, there are an electro-optical effect, a magneto-optical effect, an acousto-optical effect, etc. as a physical phenomenon that causes a change in light. Among them, the electro-optic effect is a phenomenon in which the refractive index or the dielectric constant of the medium is changed by changing the electric field.

In a waveguide type optical device using a substrate having an electro-optical effect, a portion having a high refractive index is formed as an optical waveguide in the substrate, and a voltage is applied to the upper portion or the vicinity of this waveguide. Electrodes are formed. By applying an electric field to this electrode and changing its refractive index, the phase and intensity of light can be modulated, or the optical path can be switched.

Among ferroelectric materials, a lithium niobate substrate which exhibits a relatively high electro-optical effect is generally used as a substrate. In this waveguide type optical device using lithium niobate, a titanium film is formed on a substrate, patterned into a desired waveguide pattern, and then thermally diffused at a high temperature of about 1000 ° C. for several hours to form a waveguide. Then, a silicon dioxide buffer layer is formed on the upper portion, and an electrode is formed on the upper portion with a metal film. Such a waveguide type optical device can integrate a function of modulating light and a function of switching an optical path on a substrate. Moreover, since the above-mentioned function is high-speed, an external modulator for large-capacity optical communication or OTDR (Optical Time Domain Refle
Development is in progress as a switch for switching the optical path in a ctometer).

FIG. 4 shows a Mach-Zehnder type optical modulator which has been widely researched and developed, and is being studied for practical use. The structure and operation principle will be briefly described with reference to this drawing. The waveguide substrate 1 is made of a lithium niobate substrate, and a Mach-Zehnder type optical interferometer consisting of two branch portions and two central parallel waveguides 2 is formed on the waveguide substrate 1. A silicon dioxide buffer layer 3 (see FIG. 5) formed on the entire surface of the waveguide substrate 1 is used to form an electrode 4 of a metal layer made of chromium and gold on the waveguide 2 in two parallel portions. And the electrode 5 is formed. An input side optical fiber 6 and an output side optical fiber 7 are provided on both end faces of the waveguide substrate 1 having the waveguide 2 and the electrodes 4 and 5, respectively.
Are optically coupled. Further, the output ends of the drive circuit 8 for applying a voltage and inputting a signal are connected to the electrodes 4 and 5.

When a voltage is externally applied to the electrodes 4 and 5, a vertical electric field (indicated by an arrow A) is generated in the waveguide 2 formed in the waveguide substrate 1 as shown in FIG. The refractive index of the waveguide 2 changes due to the electro-optic effect of lithium niobate. Reference numeral 9 indicates a line of electric force.

The relationship between the applied voltage and the light output is shown in FIG.
You When voltage is not applied to the electrodes 4 and 5, the light is
It is diverted and joins again. In the Mach-Zehnder type optical interferometer
If the two arms have the same structure,
Since there is no phase difference between the
Is output from the output side optical fiber 7. However, the applied voltage
When the pressure is increased, the phases of the branched lights differ from each other,
The phase difference becomes π at a certain voltage. Two minutes like this
When divergence merges again, light output is observed due to interference
It will not be done. In this way, light is changed depending on the presence or absence of voltage application.
It is a Mach-Zehnder type optical modulator that adjusts. This light modulation
The modulator has a very high modulation band (tens of GH ₂Above), large
Currently, research is actively underway to realize capacity transmission.
It

The modulation band is one of the indexes indicating the performance of the optical modulator. As a factor for limiting the widening of the modulation band, (1) the microwave resonates with the waveguide substrate,
Generating a dip in the low frequency range (5~10GH _{2), (2)}
Examples include the difference between the traveling speeds of the microwave and the guided light, and (3) the size of the loss of the microwave propagating in the electrode.
Countermeasures for the above three problems are (i), and a method of thinning the shape of the lithium niobate substrate (for example,
"Ti: LiNbO ₃ modulator frequency characteristic improvement": Kiyono other, IEICE Spring Zendai C227 1990 years), (ii), a method of the electrode top covered with the shield electrode (e.g., "shield velocity matching Ti: LiNbO ₃ Optical Modulator Design and Prototyping ":
Kono et al., IEEJ Transactions 1991/11 Vol. J74
-C NO. 11), (iii), a method of increasing the thickness of the electrode formed on the substrate (for example, M. Seino).
et al ,: Technical Digest of ECOC 199
0, p999-1002).

[0009]

However, each of the above-mentioned three methods has the following problems. That is, with regard to (i), since it is thin, it is not easy to manufacture, and there is a problem in handling. Also, (ii)
For example, the structure is complicated, and regarding the thickening of the electrode in (iii), there is an upper limit in terms of production and stability.

An object of the present invention is to solve the above-mentioned problems, and it is an object of the present invention to provide a waveguide type optical device which can easily realize a wide band optical modulator.

[0011]

The invention according to claim 1 is
A waveguide substrate having a waveguide formed near the surface and having an electro-optical effect, at least a pair of electrodes for applying an electric field to the waveguide, and at least one groove formed along the electrodes on the back surface of the waveguide substrate. It is composed of and.

The invention according to claim 2 is characterized in that the waveguide substrate is made of a lithium niobate substrate.

[0013]

According to the present invention, since the groove is formed on the surface of the waveguide substrate having the electro-optic effect opposite to the surface on which the electrode is formed, the groove can be easily formed with a simple structure. Further, since only the thickness of the substrate near the lower portion of the electrode is reduced and the thickness of the other portions is thick, the waveguide substrate can be easily manufactured and handled.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a Mach-Zehnder type optical modulator which is an example of a waveguide type optical device according to the present invention. A waveguide 11 is formed on the waveguide substrate 10, and a pair of electrodes 12 and 13 for applying an electric field to the waveguide 11 are formed on the waveguide 11. The waveguide substrate 10 is a lithium niobate substrate that exhibits a relatively high electro-optical effect among ferroelectric materials.

A groove 16 is locally formed along the electrodes 12 and 13 on the surface 15 of the waveguide substrate 10 opposite to the electrode formation surface 14.
Are formed. In addition, the groove 16 is locally formed 1
As one method, as shown in FIG. 2, an excimer laser beam machine is used and a mask 18 having a pattern similar to the desired etching pattern to be thinned is arranged in the optical path of the laser beam 17. Then, the laser light that has passed through the mask 18 is reduced and projected onto the waveguide substrate 10 by the condenser lens 19 to perform ablation etching (for example,
"Laser Processing" pp. 506-511, Nikkei Technical Book Co., Ltd.). By such a method, the groove 16 having an arbitrary shape can be formed in the waveguide substrate 10.
The output ends of the drive circuit 20 are connected to the electrodes 12 and 13.

The electrode 1 having the above-described structure and construction method is used.
The thickness of the waveguide substrate 10 directly below 2 and 13 is 0.8
It has become possible to reduce the thickness from mm to 0.3 mm.
The width dimension of the waveguide substrate 10 is 5 mm, and the groove 16
Has a width of 2 mm.

As described above, the groove 1 is formed in a part of the waveguide substrate 10.
When the modulation band of the Mach-Zehnder type optical modulator formed with No. 6 is measured and thinning is performed using a configuration in which the modulation band is 6 GHZ when the thickness t of the waveguide substrate is 0.8 mm, t is 0.3 mm. At that time, the modulation band became 20 GHz, and a wide band of the modulation band could be realized.

FIG. 3 shows an example of changes in the modulation band when the thickness of the waveguide substrate 10 is changed. As is clear from FIG. 3, the modulation band is widened by reducing the thickness of the waveguide substrate 10.

In the present embodiment, the ablation etching using the excimer laser is adopted as the method for forming the groove 16, but the method is not limited to this, and a mechanical groove forming method using a dicing saw or the like is used. Can also be applied.

[0020]

As described above, the waveguide type optical device according to the present invention has a simple structure because the groove is formed along the electrode on the surface opposite to the electrode forming surface of the waveguide substrate. Wideband modulation is now possible and easy to handle. In addition, productivity is dramatically improved because no complicated manufacturing process is required.

[Brief description of drawings]

FIG. 1 is a perspective view of a Mach-Zehnder type optical modulator which is an embodiment of a waveguide type optical device according to the present invention.

FIG. 2 is an explanatory diagram showing a method of forming a groove by an excimer laser.

FIG. 3 is a diagram showing a relationship between a thickness of a waveguide substrate and a modulation band.

FIG. 4 is a perspective view showing an example of a conventional Mach-Zehnder interferometer type optical modulator.

FIG. 5 is a sectional view of a conventional Mach-Zehnder interferometer type optical modulator.

FIG. 6 is an electric field distribution diagram of a conventional Mach-Zehnder type optical modulator.

FIG. 7 is a diagram showing the relationship between applied voltage and optical output.

[Explanation of symbols]

 10 Waveguide Substrate 11 Waveguide 12, 13 Electrode 14 Electrode Forming Surface 16 Groove

Claims (2)

[Claims] 1. A waveguide substrate having a waveguide formed near the surface and having an electro-optical effect, at least a pair of electrodes for applying an electric field to the waveguide, and a waveguide substrate formed on the back surface of the waveguide substrate along the electrodes. And at least one groove formed therein. 2. The waveguide type optical device according to claim 1, wherein the waveguide substrate is made of a lithium niobate substrate.