JPS59107324A - Optical modulator - Google Patents

Abstract

PURPOSE:To obtain an optical modulator which is not influenced by a plane of polarization of a light by separating a leading-in light to a TE mode and a TM mode by a branching element, modulating it by an intermediate Mach-Zender type modulating element, and thereafter, combining both light by a combining element. CONSTITUTION:Two waveguides 2, 3 are formed on an electro-optical crystal plate, and crossed at a point B and a point C, and the point B and the point C become a branching element and a combining element, respectively. The waveguides 2, 3 are made to branch into two on the way, and Mach-Zender type waveguides M1, M2 are formed on 2B and 3B by a voltage applying electrode 5 and an earth electrode 6. The led-in light is separated into a TE mode which goes straight on and a TM mode which turns and advances, by the branching element, they are modulated by the waveguides M1, M2, respectively, the TM mode light is curved by the combining element, and both light are combined and outputted to the outside. Therefore, it is possible to obtain an optical modulator which is not influenced by a plane of polarization of a leading-in light and has no polarization dependability.

Description

[Detailed description of the invention] (a) Technical field of the invention The present invention relates to improvements in modulators.

(b) Background of the technology When transmitting an input signal through optical communication using a semiconductor laser device, etc., an optical modulator is used to widen the frequency band of the input signal and transmit the signal at high speed. .

As such an optical modulator, for example, T is applied to a substrate of an electro-optic crystal such as lithium niopate (LiNbO3).
A leading waveguide is used in which a material such as titanium i (titanium) is diffused into a thin line to change the refractive index of the substrate.

Optical modulators using such optical waveguides are compact and the light is confined within the narrow waveguide area, so they have the advantage of requiring only a low voltage for modulation, so they are widely used as optical circuit elements. It is being

(C) Conventional technology and problems Although such a LINb O3 crystal has a large electro-optic effect and can easily form a waveguide that can transmit optical signals with low loss, the crystal has anisotropy in the refractive index. Also, depending on whether the polarization plane of the introduced light is in TM mode or TE mode, the degree of variation in the refractive index value of the crystal when an electric field is applied to the crystal differs, that is, there is a difference in the electro-optic coefficient. Therefore, there is a drawback that the signal output from the optical modulator is affected by the polarization direction of the light introduced into the optical modulator.

A waveguide optical modulator using such LiNbO1 as a crystal substrate uses two waveguides formed by linearly diffusing Ti on a LiNbO3 substrate in close proximity to each other to vary the refractive index of the substrate. , a directional coupler that modulates the light transmitted to the waveguide, or a single waveguide branched into two directions, and an insulating film placed on one of the branched waveguides.
4i, there is a modulation element such as a Mach-Zender interferometer that forms a pole and modulates the introduced light by applying a voltage to this electrode, but these all apply an electric field to the substrate depending on the polarization plane of the introduced light. This results in the drawback of polarization dependence, such as a difference in the rate at which the refractive index changes when the polarization changes.

Therefore, in order to eliminate such polarization dependence, it is necessary to consider the cutting direction when cutting the electro-optic crystal such as LiNbO3 that forms the waveguide type optical modulator, or to change the shape of the electrode. Various considerations were made, such as giving the electrode cyclicity, but all of them required time and effort to cut the crystal, took too much time to form the electrode, and made the shape of the electrode complicated. I don't like it.

(W. Purpose of the Invention The present invention eliminates the above-mentioned problems. When forming a waveguide type optical modulator using an electro-optic crystal, the polarization plane of light introduced into the modulator has a TM mode force. ＞Depending on the TE mode, when an electric field is applied to the crystal, the degree of variation in the refractive index of the crystal does not change, that is, the optical signal output from the optical modulator is affected by the polarization direction of the introduced light. It is an object of the present invention to provide a novel optical modulator that is free from the irradiation.

(e) Structure of the Invention In order to achieve the above object, the optical modulator of the present invention has two waveguides provided on an electro-optic crystal substrate, and the TE mode of the introduced light is set by intersecting the waveguides. of the crystal when a separated single beam of light and TM mode light is formed, and then the two waveguides are branched to introduce and modulate T'E mode light and T to I mode light into the crystal. A Manno-1 Zehnder modulator was formed with electrodes having a length corresponding to the ratio of the value of the upper optical constant to each mode light, and then 14iJ
The present invention is characterized in that a merging element is formed for merging the branched waveguides and merging TE mode light and TM mode light.

(f) Practical Example of the Invention An example of the present invention will be described in detail below with reference to the drawings. ', j51 is a plan view showing the purchase of the optical modulator of the present invention, and FIG. 2 is a sectional view taken along the line AA'.

LiNbO3 cut perpendicular to the C-axis direction as shown
After T1 is deposited on the crystal substrate 1''2, it is diffused to a predetermined depth to form two optical waveguides 2.3. This .sigma wave path is made to intersect at point B and point 0, and the part including point B is used as a branching element, and the part including point 0 is used as a merging element. And each waveguide 2 from branch point B to confluence point C
．． 3 branches into 2A, 2B and 3A, 3B in the middle.

Next, after forming an insulating film 4 such as a silicon dioxide (SiO2) film on this substrate l, branched waveguides 2B and 3Bl are formed.
Second, aluminum (At) or the like is deposited through the insulating film 4 to form a voltage applying positive electrode 5, and a grounding electrode 6 is formed opposite to this electrode in a separate comb shape.

The length Ll of the electrodes 5 and 6 formed on the branched waveguide 2B and facing each other is about 100% compared to the length t3 of MkJi5.6 formed on the waveguide 3B and facing each other. /
Set the length to 3.

In this way, there is a Mach-Zender type waveguide M consisting of waveguides 2A, 2B and 5, 6 whose opposing portions have a length of t, and a Mach-Zender type waveguide M whose opposing portions have a length of 12. Mach −Zcnde with ε・,6
R-type equal wave path noise is formed.

Light is introduced into such a modulator from, for example, a laser light source via an optical fiber. In this way, the light introduced through the optical fiber is not in a linearly polarized state but in a square polarized state, and this introduced light is converted into TE by a branching element.
mode light and 7M mode light (separated. Here 'r
E mode light travels straight ahead
Entering the r-type introduction path M2, the 1゛I4 mote light curves and advances Mach-Zender type introduction path rv1,
There were 244 people. Then °r'rE mode light is 1via
In the ch -Zender type introduction path rvJ2, the length of the gap t2 is -11-, and the 1'fvl7-t light is reversed by Mach-Zend.
It is modulated by the upper poles 5 and 6 having a length of tl in the er-type introduction path 1.

Here, 61 pieces of Li Nb tJ are cut on a plane perpendicular to the C axis.
In the crystal of No. 3, the electro-optic constant r13 for 1゛E mode light and the electro-optic constant r for TPA mode light
It is different from x+ and has a relationship of 3r in r33, 3, so 4 and 4 are '2/4 = 3 in r33/r+3
(In other words, the length t2 of the opposing T4L pole where the 1゛E mode light is modulated is approximately three times the length tl of the opposing electrode where the 7M card light is modulated.) .

Thereafter, the 7M mode light applied with VMl is further bent by the merging element, and the TE mode light is made to proceed straight, so that the combined light of both modes is outputted to the outside of the modulator.

In this way, it is possible to obtain a polarization-independent optical modulator that is unaffected by both the TE mode and TM mode of light, and by using such a modulator, linearly polarized light can be Since not only light but also circularly polarized light can be modulated without polarization dependence, it is extremely effective for use in optical communication systems and the like.

(g> Effects of the Invention As described above, according to the optical modulator of the present invention, an optical modulator that is not affected by the polarization plane of introduced light can be obtained, and the modulator can be used in optical communication systems, etc. It has great effects, such as enabling highly efficient and highly reliable optical communications.

[Brief explanation of drawings]

FIG. 1 is a plan view showing the structure of the optical modulator of the present invention, and FIG. 2 is a sectional view taken along the line AA'. Figure [Here, l is Li Nb O3 substrate, 2.2A,
2B. 3. 3A and 3B are waveguides, 4 is an insulating film, 5 and 6 are electrodes,
rcl, , M2 are Mach-Zendar type waveguides, B and C are intersection points, and Ll + 4 is the length of the electrode. Figure 1 Figure 2

Claims (1)

[Claims] (υ Two waveguides are provided on an electro-optic crystal substrate, the waveguides are crossed, and the TE mode light of the light introduced is 1M
A mode light separating element is formed, and then the two waveguides are branched to form an electro-optic optical system for each mode light of the crystal when TE mode light and 1M mode light are introduced and modulated. A pine-no-Zender type modulation element having an electrode with a length corresponding to the ratio of the constant values is formed, and then the branched waveguides are merged to form a merging element that merges the TE mode light and the 1M mode light. (2) Lithium Niobay as the electro-optic crystal.
(1-i Nb 03).