JPH01298313A - Optical modulator - Google Patents

Abstract

PURPOSE:To widen the modulation band of the above modulator without deteriorating the modulation efficiency thereof by forming a waveguide substrate near the part right under a light guide more thickly than in the peripheral part thereof. CONSTITUTION:The thickness in the part right under the region between a 2nd electrode 4 and the light guide 2 is larger than the thickness in the other part. Of the electric fields impressed to the electrodes 3, 4 passing an LiNbO3 substrate 1, the electric lines of force passing the light guide 2 do not leak to the outside of the LiNbO3 substrate and the other electric lines of force pass the air once and advance again into the LiNbO3 substrate 1. The speed of the modulation waves impressed to the electrodes is, therefore, increased without decreasing the electric fields applied on the light guide 2. The modulation band is widened in this way without deteriorating the modulation efficiency.

Description

[Detailed Description of the Invention] [Summary] Regarding an optical modulator, for the purpose of expanding the modulation band, an electrode is provided on an optical waveguide provided on a waveguide substrate, and a modulation voltage is applied to the electrode to In the optical modulator that modulates light passing through the optical waveguide, the waveguide substrate in the vicinity immediately below the optical waveguide is configured to be thicker than the peripheral portion thereof.

(Industrial Application Field) The present invention relates to an optical modulator that utilizes an electro-optic effect, and particularly to a traveling waveform modulator.

[Conventional technology]

In light modulation devices that utilize electro-optic effects, the time taken for light to pass through the device is said to be negligible compared to the speed at which the modulation voltage changes. However, if the electro-optic crystal is lengthened to improve modulation efficiency, the time required for light to pass through the electro-optic crystal cannot be ignored, and the degree of modulation decreases. Therefore, a traveling waveform modulator that propagates not only light waves but also modulated waves in the same direction realizes a wide modulation band.

An example of this traveling waveform modulator is shown in FIG.
LiNb0:+substrate 11, optical waveguide 12 and electrode 13
, 14, and phase modulation is performed by applying a high frequency voltage to the electrodes 13 and 14 to change the refractive index of the optical waveguide 12.

To make the traveling waveform modulator wider band, electrode 1
3.14 It is necessary to bring the speed of the modulated wave (microwave) that propagates closer to the speed of light. One method for this is to make the waveguide substrate (LiNbO, substrate 11) thinner.

This is explained as follows. That is, Fig. 4 (a
) as shown in LiNb0. When the substrate 11 is thicker than a certain level, most of the microwave electric lines of force passing through the LiNb0i substrate 11 pass only through the LiNb0z substrate 11. However, when the LiNbO3 substrate 11 is made thinner as shown in FIG. 2(b), the LiNb0. A portion of the electric lines of force passing through the substrate 11 once passes through the air. If this happens, the microwave will be confused and the dielectric constant will be the same ID (
It is smaller than in case a).

As a result, the speed of the microwave propagating through the electrodes 13 and 14 increases, and a wide modulation band is obtained.

In FIG. 4, the same members as in FIG. 3 are given the same reference numerals.

[Problem to be solved by the invention]

However, if the LiNbO3 substrate 11 is made too thin, the following problem will occur. That is, as shown in FIG.
When the NbO3 substrate 11 is made extremely thin, the electric lines of force passing through the waveguide 12 also pass through the air, and the equivalent circuit becomes as shown in FIG. 6(a). Here, Cs' is the capacitance of the substrate, Cg is the capacitance of the waveguide section, and C
a1r is the volume in air.

On the other hand, the equivalent circuit of the conventional (thick board) is as shown in Fig. 6(b) (Cs is the capacitance of the board), so Cs in Fig. 6(b) is replaced by the series capacitance of Cs' and Ca1r,
The series capacitance between Cs' and Ca1r is smaller than Cs. Therefore, when a voltage is applied to the electrodes, the magnitude of the electric field effectively applied to the waveguide becomes smaller, and the modulation efficiency deteriorates.

The present invention does not deteriorate the modulation efficiency of the optical modulator, and
The purpose is to widen the modulation band.

[Means for Solving the Problems] A first optical modulator of the present invention includes an electrode on an optical waveguide provided on a waveguide substrate, and applies a modulation voltage to the electrode to pass through the optical waveguide. A second optical modulator of the present invention is characterized in that the waveguide substrate in the vicinity directly below the optical waveguide is thicker than the peripheral portion thereof, and the second optical modulator of the present invention is characterized in that: The above object is achieved by filling a thin portion of the waveguide substrate of the first optical modulator with a material having a low dielectric constant.

[Effect]

In the first optical modulator of the present invention, the lines of electric force passing through the optical waveguide do not leak to the outside of the LiNbO5 substrate, and the other lines of electric force temporarily leak to the outside of the substrate.

Therefore, the speed of the microwave (modulated wave) applied to the electrode can be increased without effectively reducing the electric field applied to the optical waveguide.

In the second optical modulator of the present invention, the speed of the modulated wave is increased as in the first optical modulator, and the thin portion of the waveguide substrate is filled and reinforced with a low dielectric constant material, so that the waveguide substrate is strength increases.

[Embodiment] FIG. 1 shows the configuration of an optical modulator according to an embodiment of the present invention, in which (a) is a perspective view and (b) is a sectional view.

In the figure, 1 is a LiNbO5 substrate, 2 is a LiN
After forming a band-shaped Ti film on the bO3 substrate l, the cough Ti
An optical waveguide with a diameter of about 10 μm is formed by diffusing

3 is LiNb0 so as to cover the optical waveguide 2 at its end.
．． A first electrode in the form of a flat plate is formed on the substrate l, and 4 is a second electrode in the form of a strip with a width of 10 to 20 μm formed parallel to the optical waveguide 2 by about 157zmj+1. (A high frequency voltage is applied to the first electrode (13 and second electrode 4) to modulate light.

As shown in FIG. 1(b), the second electrode 4 and the optical waveguide 2
The thickness of the part directly below the area between is about 30 μm, and the thickness of the other part (
(approximately 15 μm). In addition,! , 1N
The unevenness of the b01 substrate 1 is formed by selectively grinding the LiNb0:+ substrate 1 using a blade saw used for machining grooves in magnetic heads.

Therefore, of the electric field applied to the electrodes 3 and 4 passing through the LiNb(1+ substrate 1), the lines of electric force passing through the optical waveguide 2 are L
The other lines of electric force do not leak to the outside of the iNb0z substrate 1, and after passing once through the air, they enter the LiNb01 substrate 1 again. Therefore, the speed of the modulated wave applied to the electric class 1 increases without reducing the electric field applied to the optical waveguide 2, so that the modulation band is expanded.

FIG. 2 is a configuration diagram of an optical modulator according to another embodiment of the present invention, in which (a) is a perspective view and (b) is a sectional view. In the figure, 5 is a LiNb0□ group (reverse), 6 is an optical waveguide, 7,
8 is an electrode for applying a modulation voltage, and is made of LiNb0
A thin portion of the r-substrate 5 is filled and reinforced with a low dielectric constant filler 9 (for example, epoxy resin).

According to this embodiment, it is possible to provide two optical modulators having the same effect as the first embodiment, that is, a wider modulation band, and the low dielectric constant filler 9 makes it possible to provide LiNb0.
Since the substrate 5 is reinforced, reliability is also improved.

[Effects of the Invention] According to the present invention, the speed of the modulated wave can be increased without reducing the electric field effectively applied to the optical waveguide, so optical modulation with a wider modulation band can be achieved. It becomes possible to provide equipment.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical modulator according to an embodiment of the present invention, FIG. 2 is a configuration diagram of an optical modulator according to another embodiment of the invention, and FIG. 3 is a configuration diagram of an optical modulator according to a conventional example. 4 is a sectional view of a conventional optical modulator, FIG. 5 is an explanatory diagram of problems in the conventional optical modulator, and 617 is an equivalent circuit diagram of a conventional optical modulator. It is. (Explanation of symbols) 1, 5...LiNbO3 base, 2.6...
Leading waveguide, 3... First electrode, 4... Second electrode, 7.8... Electrode, 9... Low dielectric constant filler.

Claims (2)

[Claims] (1) In an optical modulator that includes an electrode on an optical waveguide provided on a waveguide substrate and modulates light passing through the optical waveguide by applying a modulation voltage to the electrode, the An optical modulator characterized in that a waveguide substrate is thicker than its peripheral portion. (2) The optical modulator according to claim 1, wherein a thin portion of the waveguide substrate is filled with a material having a low dielectric constant.