JP2001174766A - Waveguide type optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simultaneously achieve both of prevention of a crosstalk and higher density in a waveguide type optical modulator of multiple channels. SOLUTION: By forming grooves 6-1 and 6-2 in the parts where signal electrodes 4-1 and 402 exist on a main surface 1B of a substrate 1 having an electro-optical effect, thin parts 7-1 and 7-2 are formed in such parts. The thickness t1 and t2 of the thin parts 7-1 and 7-2 are set smaller than the cut-off wavelength of a parasitic mode derived from the modulation signals generated from the signal electrodes 4-1 and 4-2.

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 modulator, and more particularly, to a multi-channel waveguide type optical modulator capable of simultaneously transmitting and modulating a plurality of information using a plurality of signal waves. About the vessel.

[0002]

2. Description of the Related Art With the recent progress in high-speed, large-capacity optical fiber communication systems, niobium acid has been used instead of the conventional direct modulation of a laser diode for reasons such as broadband characteristics, low chirp characteristics, and small propagation loss. The practical use of a waveguide type external modulator using lithium (LiNbO3: hereinafter sometimes abbreviated as LN) has been promoted. In order to respond to the demand for higher capacity of the optical fiber communication system, a plurality of optical waveguides are formed on a single substrate, and a plurality of modulation electrodes for modulating light waves guided through the plurality of optical waveguides are provided. Research and development of a multi-channel waveguide-type optical modulator in which a so-called single waveguide-type optical modulator is integrated are being advanced.

FIG. 1 is a sectional view of a part of a conventional multi-channel waveguide type optical modulator. The multi-channel waveguide type optical modulator 10 shown in FIG.
A substrate 1 having A and 1B and made of a Z-cut plate made of a material having an electro-optical effect, and optical waveguides 2-1 to 2-4. The buffer layer 3 is formed on the main surface 1A of the substrate 1, and the signal electrode 4-1 is formed on the buffer layer 3.
4-2, and ground electrodes 5-1 to 5-3.

The optical waveguides 2-1 and 2-2 and the optical waveguides 2-3 and 2-4 respectively constitute a pair of Mach-Zehnder type optical waveguides. Then, the signal electrode 4-1
And the ground electrodes 5-1 and 5-2 form a set of modulation electrodes, and the signal electrode 4-2 and the ground electrodes 5-2 and 5-3.
These form a set of modulation electrodes. These modulation electrodes independently apply a modulation signal to a light wave guided in the optical waveguides 2-2 and 2-4 to change the phase of the light wave, thereby changing the optical waveguides 2-1 and 2--2. 2, and 2-
The extinction / non-extinction of a light wave in the entire Mach-Zehnder type optical waveguide composed of 3 and 2-4, whereby a predetermined external signal superimposed on the light wave is turned on / off independently. It is.

[0005] The buffer layer 3 is used to prevent light waves guided in the respective optical waveguides from being absorbed by the signal electrode and the ground electrode, and to prevent the light waves guided in the optical waveguide from being absorbed by the signal electrodes. It is formed for the purpose of, for example, increasing the speed matching with the modulation signal applied from the controller.

That is, in the multi-channel waveguide type optical modulator 10 shown in FIG.
, Optical waveguides 2-1 and 2-2, signal electrode 4-1,
A single waveguide type optical modulator is constituted by the ground electrodes 5-1 and 5-2, and the substrate 1, the buffer layer 3, and the optical waveguide 2
-3 and 2-4, the signal electrode 4-2, and the ground electrode 5-2.
And 5-3 constitute one single waveguide type optical modulator. The single waveguide type optical modulator is integrated.

[0007]

In such a multi-channel waveguide optical modulator, a single waveguide optical modulator is integrated to increase the density and reduce the size of the entire modulator. Is required. The purpose of this is, for example, in FIG.
Is achieved by reducing the distance d between the optical waveguides 2-2 and 2-4 constituting each of the adjacent single waveguide optical modulators.
However, if an attempt is made to reduce the distance between the adjacent single waveguide type optical modulators in this way and to integrate them, FIG.
In the case of the waveguide type optical modulator 10 shown in FIG. 1, if the optical waveguides 2-2 and 2-4 are too close to each other, for example, a light wave guided through the optical waveguide 2-2 is transmitted from the signal electrode 4-2. A parasitic mode derived from the modulation signal has an effect, and a problem of so-called electrical crosstalk occurs.

In order to prevent this crosstalk, means for reducing the thickness of the substrate have been taken. However, in order to secure the intensity of the entire modulator and to have a level of intensity that does not cause any practical problem, it is necessary to increase the thickness of the substrate to some extent. Therefore, the electric crosstalk cannot be sufficiently reduced by this means. For this reason, at present, the density of the multi-channel waveguide type optical modulator is sacrificed to some extent in order to prevent electrical crosstalk.

SUMMARY OF THE INVENTION It is an object of the present invention to simultaneously prevent both electrical crosstalk and increase the density of a multichannel waveguide type optical modulator.

[0010]

In order to achieve the above object,
The waveguide type optical modulator according to the present invention is a multi-channel waveguide type optical modulator, wherein a single substrate optical waveguide and a modulation electrode are not formed on the main surface of the modulator. A groove is formed in a portion where the electrode is located, and a thin portion is formed in a portion of the single substrate where the modulation electrode is located. Further, the thickness of the thin portion is set to be equal to or less than the cutoff wavelength of the parasitic mode derived from the modulation signal from the modulation electrode.

The present inventors simulate various propagation states of a parasitic mode derived from a modulation signal emitted from a modulation electrode in a multi-channel waveguide type optical modulator, and furthermore, the multi-channel waveguide based on the simulation. Many optical modulators have been fabricated. As a result, only the portion where the modulation electrode of the single substrate constituting the multi-channel waveguide type optical modulator according to the present invention is thinned, and the thickness of such a portion is reduced by the parasitic mode generated from the modulation signal. It has been found that the propagation of the parasitic mode to an adjacent electrode can be prevented by setting the cutoff wavelength or less. Therefore, even when a single waveguide type optical modulator is integrated and a high-density multi-channel waveguide type optical modulator is manufactured, propagation of a parasitic mode to an adjacent electrode can be prevented. Electrical crosstalk can be effectively prevented.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings, according to embodiments of the present invention. FIG.
FIG. 3 is a cross-sectional view showing a part of the waveguide type optical modulator according to a preferred embodiment of the present invention, and FIG. 3 is a plan view showing a part of the waveguide type optical modulator shown in FIG. The sectional view shown in FIG. 2 is taken along line II of the plan view shown in FIG. In the waveguide type optical modulator shown in FIGS. 2 and 3, the same parts as those in FIG. 1 are denoted by the same reference numerals. FIG.
The waveguide type optical modulator 20 shown in FIGS.
Substrate 1, optical waveguides 2-1 to 2-4, buffer layer 3
, The signal electrodes 4-1 and 4-2, and the ground electrodes 5-1 to 5
-3. The substrate 1, the buffer layer 3, the optical waveguides 2-1 and 2-2, the signal electrode 4-1 and the ground electrodes 5-1 and 5-2 constitute one single waveguide type modulator. . Similarly, the substrate 1, the buffer layer 3, and the optical waveguide 2
-3 and 2-4, the signal electrode 4-2, and the ground electrodes 5-2 and 5-3 constitute another single waveguide type optical modulator.
Then, the signal electrodes 4-1 and 4-
The groove portions 6-1 and 6-2 are formed in the portion where the substrate 2 is located, thereby forming the thin portions 7-1 and 7-2 in such a portion of the substrate 1.

In the present invention, the groove is formed over an entire region where a light wave guided in an optical waveguide and a modulation signal applied from the modulation electrode to the light wave substantially interact with each other. Preferably, a thin portion is formed over the whole of the substantial interaction region. This makes it possible to effectively prevent the occurrence of the parasitic mode over the entire modulation region of the light wave, and to prevent the occurrence of crosstalk even when a multi-channel waveguide type optical modulator is manufactured at high density. be able to.

In the substantial interaction region, the modulation electrode and the optical waveguide are juxtaposed substantially in parallel, and a modulation signal from the modulation signal is actually applied to a light wave guided in the optical waveguide. Say the area. For example, in the case of the waveguide type optical modulator 20 shown in FIGS. 2 and 3, the signal electrode 4-1, the ground electrodes 5-1 and 5-2 and the optical waveguide 2-2, and the signal electrode 4-2 and the ground electrode 5- 2 and 5-3 and an optical waveguide 2-4 is a region where they are juxtaposed in parallel in the longitudinal direction.

The waveguide type optical modulator 2 shown in FIGS.
In the case of 0, the grooves 6-1 and 6-2 are formed only under the signal electrodes 4-1 and 4-2, so that thin portions are formed only in these portions. In general,
When a predetermined high-frequency voltage is applied to a signal electrode from an external power supply, a modulation signal is emitted from the signal electrode.
The modulated signal passes through the optical waveguide, applies a predetermined modulation to the light wave, and then enters the ground electrode. Then, the behavior of the parasitic mode emitted from the modulation signal was simulated based on the property of the modulation signal, and the behavior of the parasitic mode was confirmed by actually fabricating a waveguide type modulator. As a result, it was found that the parasitic mode occurred at the highest density below the signal electrode, and that the propagation of the parasitic mode to the adjacent electrode could be effectively prevented by reducing the thickness of this portion. In this case, since the thin portion may be formed only under the signal electrode constituting the modulation electrode, the strength of the entire substrate is increased, and practically sufficient strength can be given to the waveguide type optical modulator. it can.

In the case where the thin portion is formed only below the signal electrode, for example, in the waveguide type optical modulator 20 shown in FIG. 2, the width W1 of the thin portion 7-1 (groove 6-1) and the thin portion 7- Preferably, the width W2 of the groove 2 (groove 6-2) is larger than the distance D1 between the adjacent ground electrodes 5-1 and 5-2 and the distance D2 between the ground electrodes 5-2 and 5-3. Thereby, propagation of the parasitic mode can be more effectively prevented.

According to the present invention, the thickness t1 of the thin portion 7-1 and the thickness t2 of the thin portion 7-2 of the waveguide type optical modulator 20 need to be equal to or less than the cutoff wavelength of the parasitic mode. is there. Specifically, it differs depending on the modulation band frequency used. When the modulation band frequency is 2.5 GHz, the maximum values allowed for t1 and t2 are about 2.2 m.
m. Further, when the modulation band frequency is 10 GHz, the maximum value allowed as t1 and t2 is 0.5 m
m. Similarly, when the modulation band frequency is 30 GHz, the maximum values allowed for t1 and t2 are 0.2 m
m.

The lower limits of t1 and t2 are not particularly limited. However, in order to provide a certain amount of strength and facilitate handling, the lower limits of t1 and t2 are as follows:
Preferably it is about 0.05 mm.

The size of the thin portion, ie, the thickness and width, depends on the state of the modulation electrode and the optical waveguide corresponding to each single waveguide type optical modulator constituting the multi-channel waveguide type optical modulator. Thus, it is possible to control each modulation electrode independently. For example, in the case of the waveguide type optical modulator 20 shown in FIG. 2, regarding the widths W1 and W2 and the thicknesses t1 and t2 of the thin portions 7-1 and 7-2, the state of the signal electrodes 4-1 and 4-2, It can be the same or different depending on the state of the optical waveguides 2-2 and 2-4.

The thin portion is formed by forming a groove on the main surface of the substrate by a known dry etching method or a wet etching method using a predetermined mask.

Although not shown in FIG. 2, for example, the inner surfaces 6-1A and 6-2A of the grooves 6-1 and 6-2 have
A radio wave absorbing layer can also be formed. Thereby, the parasitic mode is absorbed by the radio wave absorbing layer, so that the above-described electric crosstalk can be more effectively prevented. This radio wave absorbing layer has inner surfaces 6-1A and 6-2A.
May be formed over the entire back surface 1B of the substrate 1. The substrate 1 is made of a ferroelectric single crystal X-cut plate,
Either a cut plate or a Z-cut plate can be used.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Example In this example, a waveguide type optical modulator 20 as shown in FIGS. 2 and 3 was manufactured. For the sake of simplicity, the waveguide type optical modulator has a configuration composed of two sets of single waveguide type optical modulators, that is, a configuration in which the embodiments shown in FIGS. 2 and 3 show the entire waveguide type optical modulator. Things were made.

A lithium-niobate Z-cut plate is used as the substrate 1. A photoresist is formed to a thickness of 0.5 μm on the main surface 1A of the substrate 1 by using a spin coater, and then exposed and developed. Thus, an optical waveguide pattern having a development width of 7 μm was formed. Next, a titanium layer having a thickness of 800 .ANG.
And a heat treatment at 1000 ° C. for 10 hours in an electric furnace to diffuse the titanium into the substrate 1, thereby forming optical waveguides 2-1 to 2-4 having a width of 9 μm. Next, a buffer layer 3 made of SiO ₂ was formed to a thickness of 1 μm on the main surface 1A of the substrate 1 by a vacuum evaporation method.

Thereafter, a titanium layer was formed to a thickness of 0.05 μm on the buffer layer 3 by an evaporation method. Next, the signal electrode 4-
1 and 4-2 and the ground electrodes 5-1 to 5-3 have a thickness of 2
It was formed to 0 μm. Next, grooves 6-1 and 6-2 are formed on the main surface 1B side of the substrate 1 by dicing, and the width W
Thin portions 7-1 and 7-2 each having a thickness of 1 μm and W2 of 57 μm and a thickness of t1 and t2 of 0.5 mm were formed.
The distance d between the signal electrodes 4-1 and 4-2 is 250 μm.
And

Next, an optical fiber was connected to each optical waveguide of the thus obtained waveguide type optical modulator, and the crosstalk characteristics and the transmission characteristics were examined. The results shown in FIGS. 4 and 5 were obtained. .

(Comparative Example) A waveguide type optical modulator was manufactured in the same manner as in Example except that the thin portion was not formed. When the crosstalk characteristics and the transmission characteristics were examined in the same manner as in the example, the results shown in FIGS. 6 and 7 were obtained.

As is clear from the graphs shown in FIGS. 4 and 6, in the waveguide type optical modulator in which the thin portion was not formed in the comparative example, loss dip occurs at a specific modulation signal frequency, and crosstalk occurs. You can see that it is.
On the other hand, in the waveguide type optical modulator having the thin portion formed according to the present invention, no loss dip is observed in the frequency band of the measured modulation signal, and it can be seen that no crosstalk occurs. Further, as is apparent from FIGS. 5 and 7, the transmission characteristics of the waveguide type optical modulator in which crosstalk has occurred indicate that the intensity of the optical modulation signal increases as the frequency of the modulation signal increases. . On the other hand, the transmission characteristics of a waveguide type optical modulator in which no crosstalk occurs are as follows.
It can be seen that the light modulation signal intensity is substantially constant over the entire measured modulation signal frequency. That is, it is understood that the waveguide type optical modulator in which the thin portion is formed according to the present invention exhibits excellent transmission characteristics.

As described above, the present invention has been described in detail based on the embodiments of the present invention with specific examples. However, the present invention is not limited to the above-described contents, and may be implemented in any form without departing from the scope of the present invention. Deformation and modification are possible.

[0029]

As described above, the waveguide type optical modulator of the present invention has a multi-channel structure, and has a single substrate constituting the modulator, and a modulator constituting the single waveguide type optical modulator. A thin portion is formed in a portion where the use electrode is located, and the thickness of the thin portion is made smaller than a cutoff wavelength of a parasitic mode derived from a modulation signal. Therefore, it is possible to prevent the parasitic mode from propagating to the adjacent electrode, thereby preventing the occurrence of crosstalk even when a single waveguide type optical modulator is integrated at a high density, and having good transmission characteristics. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a part of a conventional (multi-channel) waveguide type optical modulator.

FIG. 2 is a sectional view showing a part of a (multi-channel) waveguide type optical modulator according to a preferred embodiment of the present invention.

FIG. 3 is a plan view showing a part of the waveguide type optical modulator shown in FIG. 2;

FIG. 4 is a graph showing an example of crosstalk characteristics in the (multi-channel) waveguide type optical modulator of the present invention.

FIG. 5 is a graph showing an example of transmission characteristics in the (multi-channel) waveguide type optical modulator of the present invention.

FIG. 6 is a graph showing an example of crosstalk characteristics in a conventional (multi-channel) waveguide type optical modulator.

FIG. 7 is a graph showing an example of transmission characteristics in a conventional (multi-channel) waveguide type optical modulator.

[Explanation of symbols]

 1 Substrate 1A, 1B Main surface of substrate 2-1 2-2, 2-3, 2-4 Optical waveguide 3 Buffer layer 4-1 4-2 Signal electrode 5-1 5-2, 5-3 Ground Electrodes 6-1 and 6-2 Grooves 7-1 and 7-2 Thin portion 10, 20 (Multi-channel) waveguide type optical modulator t1, t2 Thickness of thin portion W1, W2 Width of thin portion

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tetsu Oikawa 585 Tomiichi-cho, Funabashi-shi, Chiba Prefecture Opto-Electronics Laboratory (72) Inventor Tokuichi Miyazaki 585 Tomi-cho, Funabashi-shi, Chiba Prefecture Opto-Electronics Laboratory F-term (reference) 2H079 AA02 AA12 BA01 BA03 CA05 DA03 DA22 EA05 EA08 EB05 EB15

Claims (5)

[Claims] 1. A single substrate having a pair of opposing main surfaces and made of a material having an electro-optical effect is provided on one main surface of a single substrate.
A plurality of optical waveguides for guiding the light wave are formed, and a modulation signal is applied to the light wave guided through the plurality of light waveguides to form a plurality of modulation electrodes for modulating the light wave. A multi-channel waveguide optical modulator, wherein a groove is formed in a portion of the other main surface of the single substrate where each of the plurality of modulation electrodes is located, A waveguide portion, wherein a thin portion is formed in a portion where each of the plurality of modulation electrodes is located, and the thickness of the thin portion is set to be equal to or less than a cutoff wavelength of a parasitic mode generated from the modulation signal. Light modulator. 2. The waveguide type optical modulator according to claim 1, wherein the thin portion is formed over a substantial interaction region between the light wave and the modulation signal. . 3. The method according to claim 2, wherein the modulation electrode comprises a signal electrode and a ground electrode, the groove is formed at a portion where the signal electrode is located, and the thin portion is formed only at a portion where the signal electrode is located. The waveguide type optical modulator according to claim 1, wherein: 4. The waveguide type optical modulator according to claim 3, wherein a width of the thin portion is equal to or larger than a distance between the adjacent ground electrodes. 5. The waveguide type according to claim 1, wherein a radio wave absorbing layer is formed on at least the inner surface of the groove on the other main surface of the single substrate. Light modulator.