JPH05264937A - Light control device - Google Patents

Abstract

PURPOSE:To provide the light control device of extremely high speed without receiving the restriction of the operating speed by the mismatching of the phase velocity of a modulation signal wave and the phase velocity of the light transmitted in an optical waveguide. CONSTITUTION:A reinforcing plate 12 consisting of a dielectric substrate is disposed in the upper part of the central electrode 14 and grounding electrode 15 of an optical modulator constituted of an LiNbO3 substrate 10 having thermal diffusing optical waveguides 11 near the surface and having an electrooptical effect, the central electrode 14 and grounding electrode 15 disposed on the surface of the LiNbO3 substrate 10. The dielectric constant of the reinforcing plate 12 and the thickness h of the LiNbO3 substrate 10 are so set that the effective refractive index nm of the central electrode 14 to the microwaves impressed to the central electrode 14 and the effective refractive index no to the light of the thermally diffusing optical waveguides 11 are nearly equaled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical control device such as an optical modulator or an optical switch which operates at an extremely high speed.

[0002]

2. Description of the Related Art At present, in an optical communication system, a ferroelectric material, a semiconductor material, an organic material or the like having an electro-optical effect is used to modulate or switch light by an electric signal, or control polarization. Optical control devices such as optical modulators, optical switches, and polarization controllers are often used.

Here, FIG. 8 and FIG. 9 show an example of the configuration of a conventional optical modulator which is composed of a LiNbO ₃ substrate on which traveling wave electrodes are formed and which has a high operation speed. In these figures,
8 is a plan view of the optical modulator, and FIG. 9 is a sectional view taken along the line AA ′ of FIG. In this optical modulator, Ti is formed on a z-plate LiNbO ₃ substrate (thickness h = 1 mm) 1 having an electro-optical effect.
The Mach-Zehnder type heat diffusion optical waveguide 2 is formed by heat-diffusing.

On the LiNbO ₃ substrate 1, as shown in FIG. 9, in order to suppress the propagation loss of light due to the traveling wave electrode composed of the center electrode 4 and the earth electrode 5,
A buffer layer (a clad layer of an optical waveguide) 3 made of an insulator such as SiO ₂ is formed with a thickness d (about 1 μm), and a center electrode 4 and a ground electrode 5 are formed on the buffer layer 3 as traveling wave electrodes. A Co-Planer Waveguide (CPW electrode) is formed.

The center electrode 4 and the ground electrode 5 are usually
Are each of a thickness of 1 to 10 [mu] m, a width w _e of the center electrode 4 is 10 [mu] m, a gap g _e between the center electrode 4 and the ground electrode 5 is about 15 [mu] m. The length L of the coupling portion between the center electrode 4 and the thermal diffusion optical waveguide 2 is usually several meters.
It is about m to several cm. For the center electrode 4 and the ground electrode 5, a microwave line such as an asymmetrical coplanar strip line or a microstrip line is used in addition to the CPW electrode.

Further, 6 is a terminating resistor, and 7 is a feed for inputting a microwave modulation signal generated by modulating the DC voltage of the DC bias power supply 9 by the microwave generated in the microwave signal source 8 to the center electrode 4. It is an electric wire.

[0007]

By the way, in the above-mentioned conventional optical modulator, since the effective refractive index n _e for the microwave modulation signal and the effective refractive index n _{o of the} heat diffusion optical waveguide 2 are different, The phase velocity of the wave modulation signal and the phase velocity of the light transmitted through the thermal diffusion optical waveguide 2 are different.
For example, in an optical modulator made of a LiNbO ₃ substrate, both the microwave modulation signal and the light wavelength λ are 1.
If it is 5μm band, while the effective refractive index n _e is 3-4 for a microwave modulation signal, since the effective refractive index n _o of the thermal diffusion optical waveguide 2 is to 2.2, the microwave modulation The phase velocity of the signal and the phase velocity of the light propagating through the thermal diffusion optical waveguide 2 differ by about twice. Therefore, there is a drawback that the mismatch of the phase velocities limits the upper limit of the operating speed or the operating band of the optical modulator. The present invention has been made under such a background,
An object of the present invention is to provide an optical control device capable of high-speed operation by removing the limitation of the operation speed due to the mismatch between the phase speed of the microwave modulation signal and the phase speed of the light transmitted through the optical waveguide.

[0008]

The invention according to claim 1 is
In a light control device comprising a substrate having at least one optical waveguide near the surface and having an electro-optical effect, and an electrode arranged on the surface of the substrate, a reinforcing plate made of a dielectric material is provided above the electrode. And the dielectric constant of the reinforcing plate and the thickness of the substrate are set so that the effective refractive index of the electrode with respect to the modulated signal wave applied to the electrode is substantially equal to the effective refractive index of the light of the optical waveguide. It is characterized by doing.

According to a second aspect of the present invention, in the first aspect of the invention, the dielectric constant of the reinforcing plate is smaller than the dielectric constant of the substrate. The invention according to claim 3 is characterized in that, in the invention according to claim 1 or 2, a buffer layer having a dielectric constant lower than that of the substrate is formed between the substrate and the electrode.

The invention according to claim 4 is the invention according to claims 1 to 3.
In any one of the inventions, the vicinity of the optical waveguide formed on the substrate is formed into a ridge shape. According to a fifth aspect of the invention, in the invention according to any one of the first to fourth aspects, a loading material having a refractive index higher than that of the substrate is used for the core portion of the optical waveguide.

[0011]

According to the above construction, since the reinforcing plate is arranged on the electrode arranged on the surface of the substrate having the electro-optical effect,
The degree of freedom with respect to the thickness of the substrate is increased. Further, by appropriately setting the dielectric constant of the reinforcing plate and the thickness of the substrate, the effective refractive index of the electrode with respect to the modulated signal wave applied to the electrode can be made substantially equal to the effective refractive index of the optical waveguide with respect to the light. The optical control device operates at an extremely high speed without being restricted by the operation speed due to the mismatch between the phase speed of the modulated signal wave and the phase speed of the light transmitted through the optical waveguide.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing the structure of an optical modulator including an x-plate LiNbO ₃ substrate 10 according to the first embodiment of the present invention. The plan view of the optical modulator is the same as that of FIG. 8 and is the same in other embodiments described later. Also in this optical modulator, the thermal diffusion optical waveguide 11 is formed by thermally diffusing Ti on the LiNbO ₃ substrate 10.

A CPW electrode composed of the center electrode 14 and the ground electrode 15 is directly formed on the LiNbO ₃ substrate 10. Since the thickness h of the LiNbO ₃ substrate 10 is thinner than that of the conventional example, in order to maintain its mechanical strength, for example, an optical adhesive 13 or the like is added to the reinforcing plate 12 made of a dielectric material such as SiO _2. Pasted together by.

2 and 3 are diagrams for explaining the principle of the present invention. In the first embodiment of FIG. 1, the optical adhesive 13 and the reinforcing plate 12 both have the same dielectric constant. In the case of 3.8, LiNbO ₃ with the width w _{e of} the center electrode 14, the gap g _e between the center electrode 14 and the ground electrode 15, and the thickness t _e of the center electrode 14 and the ground electrode 15 as parameters. The result of having calculated the microwave characteristic of the center electrode 14 and the earth electrode 15 with respect to the thickness h of the board | substrate 10 by the conformal mapping method is shown. Figure 2 is related to the characteristic impedance Z, FIG. 3 relates to a microwave effective index n _m.

[0015] As can be seen from FIG. 3, the thinner the thickness h of the LiNbO ₃ substrate 10, since the effective microwave refractive index n _m is smaller than choosing the thickness h of the LiNbO ₃ substrate 10 appropriately substantially equal to the effective microwave refractive index n _m of the light of the effective refractive index n _o (~2.2) (0.5
Double to double). Moreover, as can be seen from FIG. 2, the dielectric constant of the reinforcing plate 12, the width w _{e of} the center electrode 14, the gap g _e between the center electrode 14 and the ground electrode 15, the thickness of the center electrode 14 and the ground electrode 15. By selecting t _e , it is possible to set the magnitude of the characteristic impedance Z to an appropriate magnitude (for example, 50Ω).

2 and 3, for example, the width w _e of the center electrode 14 is 8 μm, the gap g _e between the center electrode 14 and the ground electrode 15 is 30 μm, and the thicknesses of the center electrode 14 and the ground electrode 15 are 30 μm. t _e is 20 μm, LiNbO ₃
By setting the thickness h of the substrate 10 to about 3 μm,
It becomes substantially the same size as the microwave effective index n _m is the effective refractive index n _o of the light, about also the magnitude of the characteristic impedance Z 5
Since it becomes 0Ω, the problem of mismatch between the conventional phase velocity of the microwave modulation signal and the phase velocity of the light transmitted through the optical waveguide is solved, and a high-speed optical modulator can be realized.

According to the calculation result by the conformal mapping method, when gold is used for the center electrode 14 and the ground electrode 15, 1G
The microwave attenuation constant α at Hz is ~ 0.3 dB /
Since it is about cm, the center electrode 14 and the thermal diffusion optical waveguide 1
When the length L of the coupling portion with 1 is 1 cm, the operating band (3 dB band) of this optical modulator is 400 GHz or more, which is an extremely wide band characteristic.

Next, a second embodiment of the present invention will be described. FIG. 4 is a sectional view showing the structure of the optical modulator according to the second embodiment of the present invention. 4, parts corresponding to the respective parts in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. In the optical modulator shown in FIG. 4, a loading material 17 having a higher refractive index than the LiNbO ₃ substrate 10 is newly provided as the core portion of the optical waveguide 16 instead of the thermal diffusion optical waveguide 11 of FIG.

With such a configuration, by appropriately selecting the material of the loading material 17, the width w _o and the thickness t _o , the first embodiment using the thermal diffusion waveguide 11 shown in FIG. In comparison, since the optical waveguide 16 with strong confinement can be manufactured,
Even when the thickness h of the LiNbO ₃ substrate 10 is about several μm, single mode (zero-order mode) propagation is good, and it is possible to obtain characteristics with small mode conversion loss.

Moreover, the reinforcing plate 12, the optical adhesive 13, L
When the refractive indices of the iNbO ₃ substrate 10 and the loading material 17 are n _ro , n _ao , n _o , and n _co , respectively, n _ro , n _ao <n _o <
By selecting the respective materials so that they have the relationship of n _co , most of the optical power in the optical waveguide 16 is LiNbO ₃
Since it is possible to distribute in the substrate 10, LiN
The effective electro-optical effect of the bO ₃ substrate 10 is hardly reduced. Further, since the mode size can be reduced, the drive voltage can be relatively reduced.

Next, a third embodiment of the present invention will be described. FIG. 5 is a sectional view showing the structure of the optical modulator according to the third embodiment of the present invention. 5, parts corresponding to the respective parts in FIG. 1 are designated by the same reference numerals, and description thereof will be omitted. In the optical modulator shown in FIG. 5, the LiNbO of FIG.
_The −z plate LiNbO ₃ substrate 18 is newly used instead of the ₃ substrate 10, and the thermal diffusion optical waveguide 11 is formed by thermally diffusing Ti on the LiNbO ₃ substrate 18.
Further, on the LiNbO ₃ substrate 18, a buffer layer 19 made of SiO ₂ or an insulator such as Teflon having a dielectric constant lower than that of the LiNbO ₃ substrate 18 is formed.

According to this structure, the first unit shown in FIG.
The dimensions of the center electrode 14 and the ground electrode 15, that is, the width w _{e of} the center electrode 14, the gap g _e between the center electrode 14 and the ground electrode 15, the center electrode 14 and the ground, as compared with the embodiment of FIG. When the thickness t _e of the electrode 15 is the same, the thickness h of the LiNbO ₃ substrate 18 can be made relatively large in order to make the microwave effective refractive index n _{m the} same value, so that the manufacturability is good. ..

Next, a fourth embodiment of the present invention will be described. FIG. 6 is a sectional view showing the structure of an optical modulator according to the fourth embodiment of the present invention. 6, parts corresponding to the respective parts in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted. In the optical modulator shown in FIG. 6, a loading material 21 having a higher refractive index than the LiNbO ₃ substrate 18 is newly provided as a core portion of the optical waveguide 20 in place of the thermal diffusion optical waveguide 11 of FIG. Regarding the effect of this embodiment,
Since it is the same as the above-mentioned second embodiment, its explanation is omitted.

Next, a fifth embodiment of the present invention will be described. FIG. 7 is a sectional view showing the arrangement of an optical modulator according to the fifth embodiment of the present invention. 7, parts corresponding to the respective parts in FIG. 6 are designated by the same reference numerals, and the description thereof will be omitted. In the optical modulator shown in FIG. 7, the LiNbO ₃ substrate 18 has a ridge shape. According to such a configuration, since the low-dielectric constant optical adhesive 13 and the buffer layer 22 are arranged around the ridge portion, the microwave effective refractive index _nm is set to the same value as in the other embodiments described above. In addition, the thickness h of the LiNbO ₃ substrate 18 can be made relatively large as compared with the other embodiments. In addition, the dielectric constant of the reinforcing plate 12 is L
By smaller than the dielectric constant of the LiNbO ₃ substrate 18, while the same value as the other embodiments microwave effective index n _m is described above, since the thickness h of the LiNbO ₃ substrate 18 can be further increased, manufacturability Will get better.

In each of the above-mentioned embodiments, an example in which a traveling waveform electrode made of a CPW electrode is used as the center electrode 14 and the ground electrode 15 has been described. Even if microwave lines such as the above are used, the same effects as those of the above-described respective embodiments can be obtained.

Further, also in the light control device in which the electrodes are formed in the lumped constant type, the capacitance per unit length of the electrodes can be reduced by reducing the thickness of the substrate.
Needless to say, the operation speed can be increased. Furthermore, in each of the above-described embodiments, the principle and effect of the present invention have been described by taking an optical modulator using a LiNbO ₃ substrate as an example. However, in addition to this, a ferroelectric substance, a semiconductor, or an organic substance having an electro-optical effect is used. Utilizing materials etc., such as optical modulators, optical switches, polarization controllers, etc.
Of course, the present invention can be applied to any optical control device that controls an optical output by an electric signal.

[0027]

As described above, according to the present invention,
Adjust the effective refractive index of the modulated signal wave to the effective refractive index of the light by setting the dielectric constant of the reinforcing plate on the electrode and the thickness of the substrate that has the electro-optical effect to appropriate values. Therefore, there is an effect that an extremely high-speed optical control device can be realized without being restricted by the operation speed due to the mismatch between the phase speed of the modulated signal wave and the phase speed of the light transmitted through the optical waveguide.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of an optical modulator according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an example of the characteristic impedance Z of the center electrode 14 and the ground electrode 15 with respect to the thickness h of the LiNbO ₃ substrate 10.

FIG. 3 shows the effective microwave refractive index n of the center electrode 14 and the ground electrode 15 with respect to the thickness h of the LiNbO ₃ substrate 10.
It is a figure which shows an example of the characteristic of _m .

FIG. 4 is a sectional view showing a configuration of an optical modulator according to a second embodiment of the present invention.

FIG. 5 is a sectional view showing a configuration of an optical modulator according to a third embodiment of the present invention.

FIG. 6 is a sectional view showing a configuration of an optical modulator according to a fourth embodiment of the present invention.

FIG. 7 is a sectional view showing a configuration of an optical modulator according to a fifth embodiment of the present invention.

FIG. 8 is a plan view showing a configuration example of a conventional optical modulator.

9 is a cross-sectional view taken along the line A-A ′ of FIG.

[Explanation of symbols]

1,10,18 LiNbO ₃ substrate 2,11 thermal diffusion optical waveguide 3,19 buffer layer 4,14 center electrode 5,15 earth electrode 6 termination resistance 7 feeder line 8 microwave signal source 9 DC bias power supply 12 reinforcing plate 13 optics Adhesive 16,20 Optical waveguide 17,21 Loading material

Continuation of the front page (72) Inventor Mitsuaki Yanagibashi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (5)

[Claims] 1. A light control device comprising a substrate having at least one optical waveguide near the surface and having an electro-optical effect, and an electrode arranged on the surface of the substrate, wherein a dielectric is provided on the electrode. And a dielectric constant of the reinforcing plate and the substrate such that the effective refractive index of the electrode with respect to the modulated signal wave applied to the electrode is substantially equal to the effective refractive index of the optical waveguide with respect to the light. A light control device characterized in that the thickness of the light control device is set. 2. The light control device according to claim 1, wherein the dielectric constant of the reinforcing plate is smaller than that of the substrate. 3. The light control device according to claim 1, wherein a buffer layer having a dielectric constant lower than that of the substrate is formed between the substrate and the electrode. 4. The ridge shape is formed in the vicinity of the optical waveguide formed on the substrate.
The light control device according to any one of the claims. 5. The light control device according to claim 1, wherein a loading material having a refractive index higher than that of the substrate is used for a core portion of the optical waveguide.